Symposium F.NM06—Spin Dynamics in Materials for Quantum Sensing, Optoelectronics and Spintronics

It is a fundamental question how material properties of heterostructures can control spin transfer. This work investigates how spin can be transferred in a heterosystem of a hybrid metal-halide perovskite and a 2D material, with focus on transition metal dichalcogenide (TMDC) layers. Metal halide perovskites and TMDC have already been combined in type II heterostructure devices such as photodetectors [1,2]. However, spin transfer has not yet been reported to the best of our knowledge, which would open up opportunities for optical control in spintronics applications.

Following our study of spin-exciton interactions and spin depolarization times in hybrid perovskites, we aim to investigate how the properties of the heterostructures control spin transfer. The most suitable 2D materials for this approach are semiconducting TMDCs due to their reported controllable valley degree of freedom [3,4]. To benefit from the coupling between spins and valleys in TMD layers, the components in the investigated heterosystem require not only a suitable combination of bandgaps and appropriate band alignment but also positions in k-space that fulfill the optical selection rules [3].

We analyze the obtained heterostructures with ultrafast time resolved photoluminescence spectroscopy, circularly polarized transient absorption spectroscopy and Faraday rotation to resolve the excitation and spin dynamics.

From our fundamental insights into the spin transfer in TMDC-perovskite-heterosystems, we anticipate to discover avenues and design guidelines for application of our for emerging spintronic technologies.

[1] Ma, C. et al., Adv. Mater. 2016, 28, 3683–3689
[2] Shi et al., Chem. Soc. Rev., 2018, 47, 6046
[3] Mak, K. et al. Nature Photonics, 2018, 12, 451-460
[4] Ciccarino, C., et al. Nano Lett., 2018, 18, 57809-5715

Chemical synthesis creates materials that are atomically precise, customizable, and reproducible. By bringing this toolkit to bear on the challenge of creating materials for quantum information science, we can design custom materials, each molecule geared towards a specific purpose. With these "designer qubits" it may be possible to imbue a qubit with water solubility for biological quantum sensing, create quantum memories with the careful placement of nuclear spins, or tune the emission frequency for quantum communication. Results will be presented on creating and understanding molecular qubits, with an emphasis on strategies to create qubits that can be integrated with established read-out approaches.

Metal complexes will be proposed to acting as active quantum units for Quantum Computing (QC). We report on the implementation of metal complexes into nanometre-sized (single-)molecular spintronic devices by a combination of bottom-up self-assembly and top-down lithography techniques. The controlled generation of magnetic molecular nanostructures on conducting surfaces/electrodes will be shown and persistence of their magnetic properties under confinement in Supramolecular Quantum Devices (SMQD) will be proven. The quantum properties (e.g.. superposition entanglement) of the metal complexes will be addressed at the single molecule level^1-9 to finally implement a quantum algorithm on a TbPc₂ Qudit performing quantum computing operations.¹⁰

References:
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Recent Reviews:
Molecular spin qudits for quantum algorithms.
E. Moreno-Pineda, C. Godfrin, F. Balestro, W. Wernsdorfer, M. Ruben. Chem. Soc. Rev. 2018, 47, 501.

Synthetic Engineering oft he Hilbert Sapce of Molecular Qudits: Isotopoloque Chemistry
W. Wernsdorfer, M. Ruben. Adv. Mat. 2019, doi.org/10.1002/adma.201806687.

A common approach to spin-based technologies is to manipulate the magnetic order of materials using ultrashort laser pulses, promising operation speeds that are orders of magnitude faster than in conventional spin-based devices. The electromagnetic field components of the radiation change electronic correlations and induce motions of the atoms that produce effective magnetic fields, which couple to the orbital and spin angular momentum of the electrons. A central challenge is to induce large fields that are able to quantitatively change the magnetic order, and usually they range on the order of a milli to a few tesla [1,2].

Here, we show that the extraordinarily strong spin-phonon coupling in the rare-earth trihalides allows the generation of effective magnetic fields of more than 100 tesla by driving circular lattice vibrations (phonons) in the material [3]. At these fields, the paramagnetic spins polarize, nearly reaching saturation, and the magnetization can be reversed by changing the handedness of circular excitation. Specifically, we use a combination of phenomenological modeling and first-principles calculations to predict the effective magnetic fields generated by the excitation of phonons and the subsequently induced magnetization for experimentally accessible pulse energies and temperatures. The phonon-induced magnetizations potentially enable entirely novel ways of ferroelectric or ferromagnetic switching.

[1] Nova et al., Nat. Phys. 13, 132 (2017)
[2] Juraschek, Narang, and Spaldin, arXiv:1912.00129 (2019)
[3] Juraschek and Narang, Under Review (2020)

A central goal of antiferromagnetic spintronics is the ability to sense and control the magnetic state of an antiferromagnet. While the magnetic state of a metallic antiferromagnet (AF) may be inferred using magnetoresistance measurements, this is not an option for insulating AF materials. The spin Seebeck effect (SSE), where thermal gradients are used to drive magnon spin currents, has proven to be an effective probe [i] of AF order. Substantial spin currents have been measured using the SSE in AF insulators Cr₂O₃ [ii] and MnF₂ [iii], and even in geometrically frustrated antiferromagnets [iv]. In this talk, I will briefly review the antiferromagnetic spin Seebeck effect and present our latest results on epitaxial Pt/Cr₂O₃ (0001)/Pt heterostructures. In our devices, the spin currents generated by thermal gradients are detected using the inverse spin Hall effect (ISHE) in thin films of Pt, grown both as an epitaxial underlayer beneath and as an overlayer on top of the Cr₂O₃ film. Cr₂O₃ is unique in that the spins on the top and bottom surfaces of the (0001) oriented film are uncompensated, and belong to separate magnetic sublattices, even in the presence of surface roughness. Using the ISHE in both top and bottom Pt layers [v], we find that the SSE is strongly sensitive to the respective sublattice magnetization. As a result, we are able to track the exact orientation of both sublattices upon application of a magnetic field, including during spin-flop processes. Furthermore, Cr₂O₃ is a classic linear magnetoelectric, where an electric field applied in the [0001] direction can be used to tune the net magnetization [vii]. In this context, I will discuss how the application of electric fields in Pt/Cr₂O₃/Pt heterostructures can be used to tune magnon spin currents, generated using thermal gradients [viii].


[i] S. M. Rezende et al., Phys. Rev. B 93, 014425 (2016).
[ii] S. Seki et al., Phys. Rev. Lett. 115, 266601 (2015)
[iii] S. M. Wu et al., Phys. Rev. Lett. 116, 097204 (2016)
[iv] S. M. Wu et al., Phys. Rev. Lett. 116, 097204 (2015); C. Liu et al...
[v] Y. Luo et al., arXive:1910.10340
[vii] D. N. Astrov, Sov. Phys. JETP 11, 780 (1960).
[viii] C. Liu et al., in preparation (2020).

Antiferromagnetic (AF) materials are candidates for next generation spintronic devices due to terahertz AF spin dynamics, high packing densities, and insensitivity to external magnetic fields. In AF insulators (AFIs), long spin diffusion lengths and predicted superfluid transport of spin currents are attractive for low-power device operation. The electrical control of AF domains has been attempted in AFIs with an adjacent heavy metal (HM) as a source of spin-orbit torque (SOT). Electrical detection of AF order via spin Hall magnetoresistance (SMR) has been developed, but due to heating artifacts in the HM, electrical detection methods must be supplemented by direct probing techniques. Hematite (α-Fe₂O₃), an easy-plane AFI with triaxial anisotropy, is at the forefront of this effort, spurring discoveries of magnon-mediated long-distance spin transport, understanding of anisotropy-driven AF phase transitions, and an unambiguous control of the AF order by electrical current. Contrary to ferromagnets, where domain structure is governed by long-range demagnetizing fields, the domain structure in AFMs is controlled by “destressing fields” of magnetoelastic origin. Here, we demonstrate that the destressing energy, rather than the anisotropy energy, governs the final state of the AF order in a low anisotropy AFI after the application of an external stimulus (applied current or magnetic field). We report the purely transient effect of electrical current and magnetic field application to the rotation of the AF vector in epitaxial α-Fe₂O₃/Pt bilayers. Implementing a combination of angle-dependent SMR and current pulsing methods, we probe, respectively, the full and partial re-orientation of AF domains. The current-induced rotation of the AF order within the α-Fe₂O₃ domains is confirmed with full rotation to a monodomain state via external magnetic field (due to the low spin-flop transition). We find that approximately 20% of the maximal SMR signal is retained after the external magnetic field has been turned off and follows the angular symmetry of the saturated SMR (rather than the triaxial anisotropy). Meanwhile, a remanent SMR switching signal (~5% of the expected maximal SMR signal) remains after current pulsing along two of the three easy axes to rotate the AF order along each one. Therefore, a small fraction of the re-oriented domains retains their AF order even after the magnetic field (or, analogously, a current pulse) is turned off and the rest of the domains relax toward an equilibrium state. This finding is crucial to developing and engineering AFIs properties which would prevent such a relaxation following external stimulus—a key limitation to the stability of switched states in spintronic devices.

Nanostructured material is important because of low dimension fundamental physics and device application. The oldest magnetic material, magnetite (Fe₃O₄) undergoes a sharp Verwey transition with an abrupt change in its structure and electrical conductivity at about 120 K. To date, a variety of fabrication technique to realize high-quality nanostructures. However, the thin film and nanostructured configuration of Fe₃O₄ are unable to show bulk−like Verwey transition due to a large number of unavoidable defects in them [1]. Three-dimensional (3D) Fe₃O₄ epitaxial nanowires of 10 nm length scale have been constructed on a 3D-MgO nanotemplate using an original nanofabrication technique [2]. In spite of the high density of inevitable nanoscale defects, the ultrasmall Fe₃O₄ nanowires exhibit a prominent Verwey transition at about 112 K with a maximum relative change in resistance of 9.5, which is six times larger than that of the thin-film configuration. Numerous measurements on a large number of Fe₃O₄ nanowires grown concurrently on the same 3D-MgO nanotemplate reveal a dramatic difference in their electrical transport property with the presence/absence of the Verwey transition. A comparative study of Fe₃O₄ wires of increasing volume and a thin film reveals that a profound change in the Verwey transition is observed only for wires with a volume of 10 nm³ order. Moreover, a significant decrease in the sharpness of the resistance jump and the transition temperature (T_MI) of the Verwey response is noticed with increasing volume of Fe₃O₄. This indicates the potency of the 3D nanofabrication technique in controlling nanoscale defects which is further reconfirmed through magnetoresistance (MR) measurement. A feature of the MR curve identifies the antiphase boundaries as a major source of defects. The occurrence of the smallest MR in the ultrasmall nanowire with the highest T_MI and resistance change ratio proves that 3D isotropic spatial confinement into a length scale comparable to the average spacing between two antiphase boundaries enables the favourable control over nanoscale defects. Comparison of the Verwey transition properties of Fe₃O₄ wires with increasing volume and thin film revealed that the efficient isolation of defects is only possible through isotropic 3D nanoconfinement at a length scale comparable to the average distance between two defects and also indicates the dependence of the Verwey response on the defect concentration. A simple statistical model satisfactorily illustrates the dependence of electrical transport properties on the volume of Fe₃O₄ from the macroscale down to the nanoscale. Finally, an ultrasmall nanowire with a low defect concentration allows the estimation of the true coherent length of the fundamental quasiparticle, the trimeron, responsible for the Verwey transition.
[1] M. S. Senn et al., Nat. Lett., 481, 173 (2012), [2] R. Rakshit et al., Nano Lett., 19, 5003 (2019)

The concept of Ferromagnetic resonance (FMR) based hydrogen gas sensing is a potential candidate for future hydrogen gas sensing technology. However, improvements of the FMR signal strength, response time, and stability of the employed sensing medium are required to make this novel concept competitive with established sensing platforms. Here, we proposed a novel cubic phased-Fe/Pd alloy film based hydrogen gas sensor (HGS), fabricated in a microwave waveguide like geometry. The fabricated devices show a superior FMR signal strength and a faster response time in ambient and H₂ environments in comparison with previous HGS based on continuous magnetic films. Moreover, upon annealing, the as sputtered Fe/Pd patterned devices undergo a structural phase transition from a tetragonal to a cubic phase. The transition is confirmed by careful analysis and interpretation of collected XRD and TEM data. Our findings suggest that the cubic phase of the Fe/Pd alloy film is thermally stable and offers high response times when compared to the tetragonal Fe/Pd film-based devices. This progress in the understanding of the fundamental principles behind the sensing process and the achieved enhancement in device performance represents a step forward towards the practical implementation of the FMR based sensing concept.

Antiferromagnetic insulators are a ubiquitous class of magnetic materials, holding the promise of low-dissipation spin-based computing devices that can display ultra-fast switching and are robust against stray fields. However, their imperviousness to magnetic fields also makes them difficult to control in a reversible and scalable manner. Here we demonstrate a novel ionic pathway to drive a 90° spin-reorientation reversibly in the most common antiferromagnetic insulator α-Fe₂O₃ (hematite) – now an emerging spintronic material that has recently been shown to host topological AFM textures (eg: half-skyrmions and bimerons) and long magnon-diffusion lengths. Our low-temperature catalytic-spillover process involves the incorporation (or removal) of hydrogen from α-Fe₂O₃, post-growth, driving pronounced changes in its magnetic anisotropy, Néel-vector orientation and canted magnetism via electron injection and localized strains. We explain these effects with a detailed magnetic anisotropy model and first-principles calculations. Tailoring our work for future applications, we demonstrate control of the spin-state at room-temperature by reversibly doping hydrogen in Rh-substituted α-Fe₂O₃ and suggest pathways to build electric-field-controlled antiferromagnetic devices by deploying ionotronics.

Manipulating spins in a ferromagnet spin channel using electrical current or voltage has received substantial attention in pursuit of more compact and energy-efficient memory and spintronic devices. As the technology focuses on nanoscale device applications, understanding the relationship between the dimensionality of the magnetic channel and magnetic and structural properties is crucial. We characterize the magnetic and structural properties of ferromagnetic thin films of α-Fe (body-centered cubic, bcc) as a function of thickness, which are grown epitaxially on MgO substrates using molecular beam expitaxy (MBE). The out-of-plane dimensions of the Fe layers, measured with x-ray reflectivity, range from sub-nanometer to ~30nm. The MgO capping layer prevents the Fe layer from oxidation, as measured using x-ray absorption spectroscopy, and all films show saturation magnetization of ~2.1u_B/Fe at 100K, consistent with unoxidized bulk Fe. The lattice constant mismatch between the α-Fe and the bulk MgO imposes a 3.7% tensile strain on Fe, resulting in a tetragonal distortion of the films for all thicknesses. Measurements of crystal truncation rods (CTRs) show strain relaxation of thicker Fe thin films while 3 unit cell thick Fe is coherently strained to the substrate. The synchrotron CTR measurements also show an unexpected expansion of the unit cell for ultra-thin Fe films, which does not strongly affect the magnetic properties. This work lays the groundwork for understanding how to control the spin excitations in ferromagnetic systems.

Work at Yale is supported by a grant from the Department of Energy, Basic Energy Sciences under grant number DE-SC0019211. This research used beamline 2-ID of the National Synchrotron Light Source II, a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Brookhaven National Laboratory under Contract No. DE-SC0012704.

Light-matter coupling in the so-called ultrastrong coupling (USC) regime [1] has become an important general subject of modern science and technology. Many next-generation quantum technologies that exploit the extremely efficient light-matter interactions in the USC regime have been proposed. Further, a wealth of exotic phenomena has been predicted in the USC regime, such as quantum vacuum radiation or nonclassical ground states known as two-mode squeezed vacuums. The source of these phenomena is the so-called counter-rotating terms in the Hamiltonian, which are typically negligible compared to the so-called co-rotating terms for weaker coupling strengths. USC has now been achieved in diverse platforms, but the exotic nature of the ground state due to the counter-rotating terms has not been explored. We have recently observed unusual matter-matter USC phenomena in rare-earth orthoferrites. First, we observed cooperative, Dicke-type coupling between a paramagnetic Er³⁺ spin ensemble and a single mode of terahertz magnons of ordered Fe³⁺ spins in ErFeO₃ [2]. The data exhibited vacuum Rabi splitting values in the USC regime, indicating that rare-earth orthoferrites provide a rich playground in which to explore USC phenomena without light. Furthermore, we observed that magnons in YFeO₃ display USC with a dominant role played by the counter-rotating terms. Our system is described by an unusual Hamiltonian, where: (i) the coupling strengths are tunable, (ii) the counter-rotating interactions dominate the co-rotating interactions, and (iii) the ground state is a magnonic two-mode squeezed vacuum with quantum fluctuation suppression up to 5.9 dB. These observations demonstrate that YFeO₃ provides an extraordinary platform for both applied and fundamental studies of USC beyond the reaches of photonic platforms.
1. P. Forn-Diaz, L. Lamata, E. Rico, J. Kono, and E. Solano, “Ultrastrong Coupling Regimes of Light-Matter Interaction,” Rev. Mod. Phys. 91, 025005 (2019).
2. X. Li et al., “Observation of Dicke Cooperativity in Magnetic Interactions,” Science 361, 794 (2018).

Over the past two decades there has been growing interest in organic magnetic materials, due to their potential applications in the field of magnonics and spintronics.[1,2,3,4] Vanadium tetracyanoethylene, V(TCNE)_x≈2, is a room temperature ferrimagnetic semiconductor with a T_c ~ 600 K [5] which has very low loss ferromagnetic resonance and spin-wave propagation[1, 2]. Previous first principles calculations of the electronic structure have indicated a substantially larger band gap (0.8 eV) than experimentally inferred (0.5 eV) [6,7], and that the band gap itself is an indirect gap with the valence maximum located at the (0,0,0.5) point and the conduction minimum at the (0,0.5,0) point. Our crystal axes are defined so that an equatorial TCNE ligand lies in the plane (0 0 1) and an apical ligand lies in (1 1 0) in a unit cell. The study of Ref. 6 used a local-orbital calculation with B3LYP hybrid functional. Here we explore the electronic structure using a plane-wave code VASP[8,9,10]. We have tried the following functionals: Perdew-Burke-Ernzerhof (PBE), PBE0, Becke, 3-parameter, Lee-Yang-Parr (B3LYP), Heyd-Scuseria-Ernzerhof (HSE06). The ferrimagnetic structure is studied using both collinear magnetic structure calculations and non-collinear calculations. We confirm that the structure of VTCNE has a triclinic unit cell with each V atom surrounded by 6 organic ligands, as found in Ref. 6. However, in contrast to the previous study we find a direct band gap of 0.4 eV located at the (0,0.5,0.5) point. This band gap better agrees with the experimental inference from the conductivity activation energy [7]. Further studies will explore the optical properties of this material and its magnetic dynamics, including anisotropy and magnetoelastic properties.
We acknowledge funding from NSF Grant No. DMR-1808742.
[1] H. Yu, M. Harberts, R. Adur, Y. Lu, P. C. Hammel, E. Johnston-Halperin, and A. J. Epstein, Appl. Phys. Lett. 105, 012407 (2014).
[2] N. Zhu, X. Zhang, I. H. Froning, M. E. Flatté, E. Johnston-Halperin, and H. X. Tang, Appl. Phys. Lett. 109, 082402 (2016).
[3] H. Liu, C. Zhang, H. Malissa, M. Groesbeck, M. Kavand, R. McLaughlin, S. Jamali, J. Hao, D. Sun, R. A. Davidson, L. Wojcik, J. S. Miller, C. Boehme, and X. V. Vardeny, Nat. Mater. 17, 308–312 (2018).
[4] A. Franson, N. Zhu, S. Kurfman, M. Chilcote, D. R. Candido, K. S. Buchanan, M. E. Flatté, H. X. Tang, and E. Johnston-Halperin, APL Materials 7, 121113 (2019).
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The ability to move beyond the study of isolated qubits to explore hybrid systems, i.e. where two or more component qubits coherently couple to yield new functionality, and to transduce and transmit quantum information is a key challenge for the emerging field of quantum information science and technology (QIST). Magnons (or spin-waves) provide an appealing avenue toward realizing this functionality as they can couple to a wide variety of spin-qubits using either exchange or dipole coupling, have coherence lengths from microns to millimeters, and can coherently transduce between spins, microwave photons, and phonons.

However, in order to fulfill this potential, materials with high quality-factor (high-Q), or low-loss, magnons are required. For almost half a century, we have been limited to yttrium iron garnet (YIG) despite substantial efforts to identify alternative materials. The molecule-based ferrimagnet vanadium tetracyanoethylene (V[TCNE]x; x ~ 2) has recently emerged as a compelling alternative to YIG. In contrast to other molecule-based materials, V[TCNE]x has a single-peaked, narrow magnetic resonance feature (less than 1 G at 10 GHz) and has a Curie temperature of over 600 K with sharp hysteresis switching to full saturation at room temperature. In addition to being magnetically robust, V[TCNE]x is a promising material for integration into solid-state quantum information systems due to its low damping (α = (3.98 ± 0.22) × 10^-5), high quality factor (Q up to 8,000) microwave resonance, ease of patterning, and compatibility with a wide variety of substrates.

Here we will discuss how V[TCNE]x can be exploited for applications in quantum information science. We report anomalous temperature dependent anisotropy in V[TCNE]_x thin films, where we observe switching of magnetic easy-axis from in-plane at 300 K to out of plane at 25 K with a clear absence of anisotropy at 100 K. Further, we observe a reversion to room temperature anisotropy and linewidth values at a temperature of 5 K, validating V[TCNE]_x for low temperature quantum applications. We will discuss applications of patterned V[TCNE]x magnon-resonators in a hybrid quantum device.

An organic semiconducting ferrimagnet, vanadium tetracyanoethylene (V[TCNE]_x~2) has a low magnetic damping and narrow ferromagnetic resonance linewidth[1-3], similar to a more commonly explored magnonic material, yttrium iron garnet (YIG)[4,5]. However, YIG requires high-temperature growth on a specific substrate, gadolinium gallium garnet, for high performance[6]. On the other hand, V[TCNE]_x~2 can be deposited on various rigid substrates at low temperature[1] without a substantial increase of damping. This allows integration of V[TCNE]_x~2 with other interesting materials.
Here we describe the spin wave dynamics in quasi one-dimensional patterened magnonic crystals of V[TCNE]_x~2 that can be formed by deposition of a uniform thickness film on a patterned substrate with ridges. The dispersion relations, linewidths and magnetization profiles are calculated by using the Landau-Lifshitz-Gilbert formalism[7,8]. For the structure, we considered alternatively aligned gratings on top and bottom of a thin film. Furthermore, we studied the symmetry of the magnon modes of this magnonic crystals under the influence of an arbitrarily oriented external magnetic field.
We acknowledge support from NSF EFRI NewLAW under Award No. EFMA-1741666.

[1] H. Yu, M. Harberts, R. Adur, Y. Lu, P. C. Hammel, E. Johnston-Halperin and A. J. Epstein, Appl. Phys. Lett. 105, 012407 (2014).
[2] N. Zhu, X. Zhang, I. H. Froning, M. E. Flatté, E. Johnston-Halperin and H. X. Tang, Appl. Phys. Lett. 109, 082402 (2016).
[3] H. Liu et al. Nat. Mater. 17, 308-311 (2018).
[4] E. G. Spencer, R. C. LeCraw, and R. C. Linares, Jr, Phys Rev. 123, 1937 (1961).
[5] C. Hauser et al. Sci. Rep. 6, 20827 (2016).
[6] Y. Y. Sun, Y. Y. Song and M. Z. Wu, Appl. Phys. Lett. 101 082405 (2012).
[7] G. Sietsema, T. Liu, M. E. Flatté, SPIN, 7 1740012 (2017).
[8] R. A. Gallardo et al. Phys. Rev. B, 97, 144405 (2018).

The anomalous Hall effect has been observed in non-magnetic metallic films with strong spin-orbit scattering that are in intimate contact with a magnetic insulator [1]. Two mechanisms have been proposed to explain this observation: the magnetic proximity effect (MPE) and a combination of the spin Hall effect and inverse spin Hall effect. Direct measurements of the local magnetization of the non-magnetic metal using spectroscopic techniques that are sensitive to local magnetic moments have been inconclusive. To definitively determine whether there is an MPE, point contact Andreev reflection (PCAR) measurements [2] were used to detect an induced transport spin polarization in Pt films deposited on yttrium iron garnet and manganese aluminum ferrite films. The results will be compared to density functional theory calculations and can be used in the interpretation of measurements of spin currents.

The authors acknowledge the support of the Basic Research Office of the Assistant Secretary of Defense for Reserch and Engineering under the Vannevar Bush Faculty Fellowship (funded by the Office of Naval Research through grant N00014-15-1-0045).

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Silicon carbide (SiC) is a host to a variety of robust, optically active quantum systems. Recent studies have shown that basally oriented kh divacancy defects in SiC possess a robust optical interface as well as exceptional spin coherence properties even in commercially available, naturally abundant SiC. We show that these excellent intrinsic properties can be further augmented and tuned by applying a periodic drive, giving rise to emergent phenomena unique to non-equilibrium quantum systems.

First, we demonstrate electrically driven coherent quantum interference in the optical transition of single kh divacancies [1]. By applying microwave frequency electric fields, we coherently drive the divacancy’s excited-state orbitals and induce Landau-Zener-Stückelberg interference fringes in the resonant optical absorption spectrum. This result shows that the optical absorption spectrum of these divacancy defects can be rapidly reconfigured using localized electric field control, and reveals a highly responsive mechanism to potentially coherently interact with other electrically active quantum systems.

We then construct a robust qubit embedded in a decoherence-protected subspace, obtained by hybridizing an applied microwave drive with the ground-state electron spin of the kh divacancy [2]. This qubit is protected from magnetic, electric, and temperature fluctuations, which account for nearly all relevant decoherence channels in the solid state. This culminates in an increase of the qubit's inhomogeneous dephasing time by over four orders of magnitude (to >22 milliseconds), while its Hahn-echo coherence time approaches 64 milliseconds. Such an extension of inhomogeneous dephasing time opens up pathways to a vast array of hybrid quantum system applications. Furthermore, since this protocol requires few key platform-independent components, this result suggests that substantial coherence improvements can be achieved in a wide selection of quantum architectures.

This work was done in collaboration with Alexander L. Crook, Gary Wolfowicz, Joseph P. Blanton, Samuel J. Whiteley, Sam L. Bayliss, Hiroshi Abe, Takeshi Ohshima, Gergo Thiering, Péter Udvarhelyi, Viktor Ivády, and Ádam Gali.

[1] K. C. Miao, A. Bourassa, C. P. Anderson, S. J. Whiteley, A. L. Crook, S. L. Bayliss, G. Wolfowicz, G. Thiering, P. Udvarhelyi, V. Ivády, H. Abe, T. Ohshima, Á. Gali, D. D. Awschalom, “Electrically driven optical interferometry with spins in silicon carbide,” Sci. Adv. 5 (11), eaay0527 (2019)

[2] K. C. Miao, J. P. Blanton, C. P. Anderson, A. Bourassa, A. L. Crook, G. Wolfowicz, H. Abe, T. Ohshima, D. D. Awschalom, “Universal coherence protection in a solid-state spin qubit,” arXiv 2005.06082 (2020)

The neutral divacancy in 4H-SiC has a spin-1 ground state that is highly sensitive to electrical and mechanical perturbations [1]. The long room temperature coherence times of these deep centers suggest that they are robust candidates for nanoscale quantum sensing [2]. Within the past five years optical readout of single divacancies in SiC has been demonstrated [3]. Electrical readout of single spins typically relies on Boltzmann distributed spin polarizations which necessitate operation at low temperatures and/or high magnetic fields to improve the quality of the single-shot protocol [4]. Recently ambient electrical readout has been demonstrated on SiC defects using photocurrent detected magnetic resonance [5], relying on optical excitation to generate current. In contrast, we propose a time-averaged projective measurement technique that derives from local influences on the ground state coherent dynamics of the neutral divacancy. This provides resolution of processes corresponding to energy scales far below the thermal noise floor of the system without the need for optical hardware, AC fields, or large DC fields. We calculate the defect mediated current through a spin-polarized contact under a small (~mT) external magnetic field similar to previous calculations done on a spin-1/2 ground state [6]. We predict that measurement of the spin relaxation time is possible from analysis of the zero-field current and that spin-decoherence is discernible from the zero-field width. Magnetoresistive signatures also provide information on local electric and strain fields suggesting a path toward scalable, all-electric, room temperature quantum technology based on SiC. We acknowledge support for this work from DOE BES through grant number DE-SC0016379.

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[5] M. Niethammer, et al. Nat. Commun. 10, 5569 (2019)
[6] S. R. McMillan, et al., arXiv:1907.05509 [cond-mat.mes-hall]

Advances in localised tip-induced lithography mean that the process of doping a semiconductor need no longer be random: instead, dopant atoms can be located in well-defined positions relative to one another. This has already been demonstrated for donors in silicon [1,2] and may in future be possible for other types of dopant. This opens the possibility of controlling the interactions between the dopants to form new deterministic devices, which in turn elevates the spin degree of freedom from a spectator into an important variable. As well as the established potential for spin-based quantum gates [3] a number of other applications can now be foreseen.

We have computed the excitation spectra of donor pairs and larger clusters [4] in silicon, using a combination of ab initio band-structure simulations and quantum chemistry techniques. The results show strong and characteristic changes in the character of the dominant excitation as a function of separation and spin state, raising the possibility optical detection and initialisation of the multi-donor spin state. We have also studied the application of controlled donor arrays as quantum simulators that can be probed by their transport properties [5]; in particular we have identified topological edge states and shown how their properties vary across the metal-insulator transition [6].

Finally, for acceptor clusters we have studied the consequences of the richer spin-orbit coupling present in the silicon valence band [7]. We find a complex interplay between the relative strengths of hopping and Coulomb interactions, the arrangement of the bond lengths, and the spin-orbit coupling. This can result in the emergence of novel kinds of topological state.

[1] Schofield, S. R., et al (2003). Atomically precise placement of single dopants in Si. PHYSICAL REVIEW LETTERS, 91 (13), ARTN 136104. doi:10.1103/PhysRevLett.91.136104
[2] Stock, T. J. Z., et al. (2020). Atomic-Scale Patterning of Arsenic in Silicon by Scanning Tunneling Microscopy. ACS Nano, acsnano.9b08943. doi:10.1021/acsnano.9b08943
[3] He, Y et al. (2019). A two-qubit gate between phosphorus donor electrons in silicon. Nature 571 p 371 doi:10.1038/s41586-019-1381-2
[4] Wu, W., et al. (2018). Excited states of defect linear arrays in silicon: A first-principles study based on hydrogen cluster analogs. Physical Review B, 97 (3), ARTN 035205. doi:10.1103/PhysRevB.97.035205
[5] Le, N. H., et al. (2017). Extended Hubbard model for mesoscopic transport in donor arrays in silicon. Physical Review B, 96 (24), ARTN 245406. doi:10.1103/PhysRevB.96.245406
[6] Le, N. H., et al. (2020). Topological phases of a dimerized Fermi–Hubbard model for semiconductor nano-lattices. npj Quantum Information, 6 (1), 24. doi:10.1038/s41534-020-0253-9
[7] Zhu, J., et al. (2020). Linear combination of atomic orbitals model for deterministically placed acceptor arrays in silicon. Physical Review B, 101 (8), ARTN 085303. doi:10.1103/PhysRevB.101.085303

Magnetoelectric coupling is the coupling between magnetic order and the electric polarization in an insulator. To date it has been largely studied in inorganic oxides that show various types of ferromagnetic or antiferromagnetic order. Metal-organic frameworks and molecular compounds are a new and fertile direction to search for new magnetoelectric coupling mechanisms. I will talk about several recent results in which we couple spin crossovers to structure and electric polarization. I will also highlight multiferroic behavior in metal-organic frameworks. The experimental part of the work takes advantage of pulsed magnetic fields to perform high-sensitivity measurements and reach extended regions of the magnetic field-temperature phase diagram.

Among the main trends for spintronic applications, narrow gap semiconductors provide a fertile ground to engineering the electron effective g factor, because of its strong dependency on the energy gap of the bulk material. Furthermore, it is well-known that in semiconductor heterostructures, the mesoscopic confinement also renormalizes the effective g factor by introducing extra anisotropies, transforming scalar g factors into tensors. In particular, for narrow-gap semiconductor layered structures, the electronic g-tensor is expressed as a diagonal matrix, where the diagonal elements correspond to configurations of magnetic-field longitudinal and transversal to the growth direction. Considering bound electrons in both, InAs- and InSb-based multilayer structures, we use an analytical and transparent envelope-function solution based on the Kane model for the bulk, to investigate the effective g tensor and the corresponding anisotropy, i.e., the difference between the longitudinal and transversal components. Following the prescription abovementioned, we 1) study the dependency between the obtained results and the bulk-band parameters, 2) study the fine-tunning of the effective g-factors (longitudinal and transversal) with the structural parameters, i.e., the thicknesses of the active layer and the tunneling barrier, in particular, we 3) explore the strongly interacting regime for thin active layers, for which the structure inversion asymmetry plays a fundamental role by inverting the signal of the g-factor anisotropy. Such results provide a clear understanding of the mechanisms of the g-factor renormalization in semiconductor nanostructures and can be useful in electron g-factor engineering.
This work was partially supported by the Office of Naval Research (Global office in South America, NICOP) Grant No.N62909-18-1-2121, FAPESP Grant No. 2014/03085-0, CAPES (No. PNPD 88882.306206/2018-01),CNPq Grant No.306145/2018-9, FAPESB Grants No. PNX 0007/2011 and INT 0003/2015.

The spin-orbit interaction (SOI) is the relativistic correction, caused by the coupling between a spin σ and an effective magnetic field (EMF) of a charged carrier. Despite the necessity of understanding the characteristics in the SOI phenomena for holes as well as for electrons, the complexity of the valence band structure prevents us from our deep understanding of the SOI for holes. The SOIs are generally caused by symmetry breakings, such as the bulk-inversion-asymmetry (BIA) and the structure-inversion-asymmetry (SIA). Dresselhaus has studied intensively the BIA (Dresselhaus) SOI and revealed the characteristics for holes. Rashba has theoretically proposed that an electric field externally applied to quantum systems has potential to cause the SIA (Rashba) SOI.
In this presentation, we apply the Rashba external field to the two-dimensional (2D) SiGe mixed alloy quantum well (QW) system, and then study how the coexistence of the BIA and SIA SOIs influences the spin textures of holes. We particularly focus on the spin texture and Berry curvature of the heavy-mass holes (HHs), because it is our one more purpose how the inter-subband interaction influences them via the strongly anisotropic and non-parabolic eigenstates. Taking into account the in-planar crystal momentum as well as the particle one, we extend the kp approach including the 2^nd-order terms crossing with those SIA and BIA SOIs and solve the kp perturbation equation under the virtual crystal approximation. By employing the solved Bloch periodic parts, we calculate the k-distribution of the in-plane spin of HHs.
The SIA-BIA coexistence system changes the symmetry of the present SiGe 2D QW system characteristically. The electric-field application perpendicular to the 2D (001) plane causes the SIA whereas the alternating arrangement of Si and Ge heteroatoms causes the BIA. Eventually, the SIA and BIA SOIs strengthen the axial vectors cooperatively in the first and third quadrants of the Brillouin zone (BZ) whereas they compete and suppress themselves in the second and fourth quadrants.
The present SIA and BIA SOIs resolve the spin degeneracy of HHs into the stabilized (HH+) and destabilized (HH-) one. Each spin texture exhibits the inequivalence between the [110] and [-110] directions, and both planes of (110) and (-110) are the mirror plane for an axial-vector spin. For the <110> direction, the HH spin orients to the tangent line of the equi-energy surface. However, except for this direction, the spin does not orient toward the tangential line. HHs basically orient their spins parallel to their own EMF. However, the EMF has a remarkable term to cause the directional discrepancy from the tangential line. In addition to this EMF characteristics, the EMF and spin correlate via the ls coupling. Consequently, the spin texture shows the characteristic k-distribution, significantly different from that found in 2D electron gas (2DEG).
We further study the Berry curvature B⁺ for HH+ and B^- for HH- influenced by the coexistence of SIA and BIA SOIs. HHs are energetically separated from other holes and the pair relation of B⁺=-B^- results. The coexistence of SIA and BIA SOIs multiply changes the sign of the Berry curvatures. Furthermore, the strong anisotropy and significant non-parabolicity of HHs distribute the Berry curvatures complicatedly. Nevertheless, the pair relation B⁺=-B^- is valid. We carry out the integration of the Berry curvature surrounded by the equi-energy contour E, and estimate the Berry phase γ(E) at an energy E. The resulting γ⁺(E) causes a plateau of a half-integer 1/2 until E~16meV; γ^-(E) does that of -1/2. However, it suddenly changes the sign and an opposite plateau of -1/2 (+1/2) results in the higher energy region. This characteristic feature is peculiar to HHs not found in the 2DEG because the sign and k-distribution of the Berry curvature in the integral area are crucial for determining γ(E). This feature influences the Chern number and Hall conductivity.

Following the second law of thermodynamics, the Landauer’s principle dictates a fundamental physical limitation for the switching energy of complementary metal-oxide-semiconductor (CMOS). A primary solution in breaking this limitation is to utilize the spin-orbit coupling (SOC) effect, as it allows easy manipulation of spin currents. However, this SOC effect is often quite weak, especially in the absence of external voltage biases. We report a four-fold SOC enhancement at zero bias voltage and pronounced SOC evolution in correlated LaAlO₃-SrTiO₃ heterostructures buffered by a carrier modulating LaFeO₃ layer. We use an entirely new approach to provide evidence of generating Rashba SOC. Correlating the magnetotransport data with first-principles calculations and high-resolution electron microscopy, we reveal that its origin lies in the asymmetric hybridization of the interfacial wavefunctions. Our results open hitherto unexplored avenues of generating and controlling Rashba coupling to design next-generation two-dimensional electron system based spin-orbitronic devices.

Recently, the physicists and material scientists have experienced a new wave of interest in spin effects in solid-state systems. On the one hand, semiconductors and semiconductor nanosystems, allow one to perform benchtop studies of quantum and relativistic phenomena. On the other hand, the interest is supported by the prospects of realizing spin-based electronics where the electron or nuclear spins can play the role of quantum or classical information carriers. In my tutorial lecture, I will present the key aspects of multifaceted physics of interacting electron and nuclear spins in semiconductor-based low-dimensional structures. The hyperfine interaction of the charge carrier and nuclear spins increases in nanosystems compared with bulk materials. The enhanced coupling results in the efficient spin exchange between these two systems. It gives rise to beautiful and complex physics occurring in the ensemble of electrons and nuclei in semiconductors.

I will address the hyperfine interaction, nuclear magnetic resonance, dynamical nuclear polarization, spin-Faraday and -Kerr effects, processes of electron spin decoherence and relaxation, effects of electron spin precession mode-locking and frequency focusing, as well as fluctuations of electron and nuclear spins.

One of the possible hybrid realizations of a qubit is the spin of an electron confined in a semiconductor quantum dot (QD), which is interacting with the surrounding nuclear spins. The prominent advantage of QDs is their strong optical dipole moment, which allows efficient coupling of photons to the confined electron spins according to the optical selection rules. The electron, on the other hand, is coupled to the nuclei of the QD crystal matrix by the hyperfine interaction. These couplings allow one to design schemes where the angular momentum of the photon is transferred to the nuclear spins using the electron as an auxiliary spin state. This may have significant advantages, as the electron spin coherence is limited to several microseconds at low temperatures, while the nuclear spin coherence can reach milliseconds.
To test the degree of control between the electron and the nuclear spin system, we present a model system of an ensemble of singly charged QDs. The use of an ensemble is justified in our case by the spectroscopic signal strength, which allows us to observe an averaged and integral response of many electron spins at the same time. It is also of interest for quantum memory applications. On the other hand, such an ensemble is usually strongly inhomogeneous, which leads to a strong variation of the intrinsic parameters, like optical dipole moments, electron g-factor values, and strength of electron-nuclear spin interactions. This inhomogeneity can, however, be overcome in designed experiments, enabling effects such as spin mode-locking and nuclear-induced frequency focusing.
In previous studies on nuclei-induced frequency focusing in singly charged (In,Ga)As/GaAs QDs multiple precession modes were covered by the ensemble, as the nuclear fluctuations were of the same order as the mode separation in low external fields. Here we explore the degree of possible control of the nuclear surrounding using an inhomogeneous QD ensemble excited with a high repetition laser operated at 1 GHz. The GHz laser repetition frequency allows us to investigate the interaction of nuclear spin polarization and nuclear spin fluctuations with the electrons as the mode separation is an order of magnitude larger than the fluctuating nuclear field. It further allows us to use the effects of the strong electron-nuclear feedback and drive the inhomogeneous ensemble of electron spins to a single frequency precession. Additionally, we demonstrate the discretization of the total magnetic fields seen by the electron ensemble and show the effect of the reduction of the nuclear field fluctuations. While for fields lower than 0.6 T nuclear spin fluctuations inhibit the formation of a significant nuclear polarization, above the nuclear spin relaxation time becomes elongated such that the electron spins precess on the closest preferred frequency over a range of 128 mT. This leads to the observation of plateaus in the precession frequency as a function of the external magnetic field. The nuclear spin bath is constantly tuned, such that the phase synchronization condition is fulfilled while nuclear fluctuations are suppressed.

Designing new quantum materials with long-lived electron spin states urgently requires a general theoretical formalism and computational technique to reliably predict intrinsic spin relaxation times. We present a new, accurate and universal first-principles methodology based on Lindbladian dynamics of density matrices to calculate spin-phonon relaxation time of solids with arbitrary spin mixing and crystal symmetry¹. This method describes contributions of Elliott-Yafet and D’yakonov-Perel’ mechanisms to spin relaxation for systems with and without inversion symmetry on an equal footing. We show that intrinsic spin and momentum relaxation times both decrease with increasing temperature; however, for the D’yakonov-Perel’ mechanism, spin relaxation time varies inversely with extrinsic scattering time. We predict large anisotropy of spin lifetime in transition metal dichalcogenides. The excellent agreement with experiments for a broad range of materials underscores the predictive capability of our method for properties critical to quantum information science. [1] J. Xu et al, Nature Communications 11, 2780 (2020)

Antiferromagnetic tetragonal Fe₂As attracts attention due to its easy-plane magnetism which might enable future usage in magnetic memory devices. Moreover, the possibility for electrical switching of the Néel vector is expected because of its metallicity. To understand the mechanism of switching for easy-plane magnetism, it is essential to know the magnetocrystalline anisotropy energy of antiferromagnetic Fe₂As, which acts as an energy barrier. Using first-principles density functional theory, we study the contributions of spin-orbit interaction (SOI) and magnetic dipole-dipole interaction (MDD) to magnetocrystalline anisotropy. The tetragonal structure exhibits out-of-plane and in-plane anisotropy energy terms. The out-of-plane anisotropy, computed as a |K₁| coefficient of 830 kJ/m³, originates from both SOI and MDD. The contribution of SOI is two times larger than that of MDD, and both show the same two-fold symmetry. Conversely, the in-plane MDD contribution is almost negligible and most of the anisotropy energy comes from SOI, which shows four-fold symmetry and a |K₂₂| coefficient of 280 J/m³. This closely corresponds to the value of 150 J/m³, measured using torque magnetometry. Based on the calculated anisotropy energy, antiferromagnetic Fe₂As shows potential for the switching process to occur, however, the results also indicate low thermal stability of the magnetic structure.

** The Illinois Materials Research Science and Engineering Center is supported by the National Science Foundation MRSEC program under NSF Award Number DMR-1720633.

The ability to strongly and deterministically manipulate magnetic order is crucial for applications ranging from spintronics to information storage. For example, strain engineering enables tuning of magnetic phases in a variety of complex systems. Combining this tunability with optical methods would enable novel optomagnetic technologies operating on ultrafast time scales. In this work, we demonstrate a route to dynamically mimic strain and create non-equilibrium magnetic properties by engineering the crystal structure with light. [1]. In particular, in the antiferromagnet CoF₂, we induce a ferrimagnetic phase transition through selective THz excitation of optical phonons. The effect takes advantage of the coupling of the local magnetic moment to the crystal field environment, which statically leads to a net magnetization under uniaxial strain (piezomagnetism). Rather than strain, we exploit nonlinear, three-phonon coupling to drive the desired crystal field distortions [2]. Optical measurements reveal that, as a result, a bi-directional ferrimagnetic moment is generated on a time scale of few to hundreds of picoseconds. The generated moment is nearly three orders of magnitude larger than previously achieved by strain, demonstrating phonon-driven control of magnetic order beyond the elastic limit.

References
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In optical spectroscopy experiments, it is crucial to excite an emitter with a laser very close to its transition energy. Experiments under resonant excitation are essential for accessing the intrinsic optical and spin-polarization properties of large class of emitters ^1–6.Using linear cross-polarization in a confocal setup has been successfully employed as a dark-field method to carry out resonant fluorescence experiments to suppress scattered laser light, with the added benefit of high spatial resolution. Resonant fluorescence experiments allow crucial insights into light-matter coupling, such as the interaction of a single photon emitter with its environment, with optical cavities and also studying single defects in atomically thin materials such asWSe2. Dark-field confocal techniques allow developing single photon sources with high degrees of photon indistinguishability and longer coherence. In practice dark-field laser suppression is not just a spectroscopy tool, it is also a key part of more matured quantum technology systems. Despite many advances based on experiments in confocal microscopes with cross-polarization laser rejection, the physical mechanisms that make these experiments possible are not understood, hampering further progress in this field. The key figure of merit is the suppression of the excitation laser background by at least six orders of magnitude. Indeed a suppression by a factor of 10⁸ up to 10¹¹ (this work) have been measured. But this is very surprising as mirrors and beam-splitter in such a system reduce the theoretical extinction limit to the 10³ to 10⁴ range. In this work we explain the physics behind the giant enhancement of the extinction ratio by up to eight orders of magnitude that make microscopy based on dark-field laser suppression possible. The measurements of resonant fluorescence are typically performed in an epifluorescence geometry, for which laser excitation and fluorescence collection are obtained through the same focusing lens. This involves necessarily the use of a beam-splitter orienting the back-reflected light containing the fluorescence towards a detection channel. In our work we identify two key ingredients that explain the giant amplification of the cross-polarization extinction ratio : (i) a reflecting surface (i.e. the beam-splitter) placed between a polarizer and analyzer, and (ii) a confocal arrangement. We demonstrate giant extinction ratios in our experiments for different mirrors (silver, gold, dielectric, beam-splitter cubes) and polarizers (Glan-Taylor, nanoparticle thin films). We demonstrate that behind this general observation lies the intriguing physics of the Imbert-Fedorov effect, which deviates a reflected light beam depending on its polarization helicity. We discover that a confocal arrangement not only amplifies the visibility of the Imbert-Fedorov effect dramatically, taking it from the nanometer to the micrometer scale, but also exploits conveniently the symmetry of the newly observed Imbert-Fedorov modes to insure that the cross-polarized laser beam is not coupled, explaining the near complete suppression of the laser background signal. In other words, we cannot treat the spatial (i.e. modal) and polarization properties of light separately in our dark-field confocal microscope analysis. In addition to new developments in dark-field microscopy our experiments provide powerful tools to understand spin-orbit coupling of light, in the broader context of topological photonics. Funding: European Union’s Horizon 2020 research and innovation program, Marie Sklodowska-Curie grant agreement No 721394 ITN 4PHOTON.

¹ Aharonovich, I., et.al. M. Nat. Phot. 10, 631 (2016)
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Photon correlation spectroscopy is a powerful technique for measurement and analysis of optical dynamics. Historically, analysis of photon detection times enabled early studies of fundamental quantum mechanical phenomena in trapped atoms and ions and was used to probe dynamics in single-molecule spectroscopy. More recently, it has been applied to solid-state emitters to enable their characterization for quantum technological applications. The predominant use of photon statistics in solid-state systems is as an indicator of single-photon emission, a desirable property for applications such as quantum key distribution and quantum repeaters. However, a more comprehensive analysis of photon statistics can reveal even more characteristics of a system such as the electronic structure, multi-level optical dynamics, and responses to external fields. We discuss photon correlation spectroscopy as a measurement tool for solid-state emitters and outline important methods and considerations for proper acquisition and analysis of photon statistics. In particular, we explore the differences between the two most common measurement techniques, start-stop histogram and time-tagged-time-resolved mode, and we highlight the technical considerations in accounting for background fluorescence and detector jitter for robust characterization of single-photon emission. We also discuss how both an emitter’s internal dynamics and experimental nonidealities must be taken into account in order to guarantee reliable measurements. Finally, we illustrate how the photon statistics framework can be used to characterize quantum emitters by applying it to known emitters like the diamond nitrogen-vacancy center and unknown emitters in hexagonal boron nitride. Through comparison of the measurements to optical-dynamics simulations, we posit an optical level diagram for the unknown emitters, advancing understanding of their electronic structure and optical dynamics.

Magnetic nanoparticles (MNPs) are an important class of low-dimensional materials displaying size-dependent magnetic behavior. The controllable tunablility of magnetic anisotropy introduced at nanoscale size regimes allows for nanoparticles of ferro/ferrimagnetic materials to be dispersed in a medium, with the addition of colloidal stabilizers, and still produce a significant collective response to a dynamic applied magnetic field. This strong response to external applied fields, while still maintaining colloidal dispersity, is the basis for employing these magnetic materials in biomedical applications such as imaging, sensing, drug delivery, and hyperthermia.

Magnetic nanothermometry is an emerging application of MNPs. While conventional remote thermometry can measure the temperature of a microscale spot only on an object’s surface or in a small local region of an optically transparent material, magnetic nanothermometry is being developed to accurately measure temperature throughout a three-dimensional volume. This metrology is based on the temperature-dependent nature of the spin interaction within each particle, and the ability of applied AC magnetic fields to penetrate through many materials to probe the magnetization therein as a function of position. However, commercially available MNPs display only modest magnetic thermosensitivity, especially near room temperature; this response must be strengthened as a prerequisite to practical remote volumetric nanothermometry. Carefully designed doped or multi-domain nanoparticles, with a strong AC magnetic response and selective control of thermosensitive temperature ranges arising from magnetic exchange interactions, represent a potential route to overcome these limitations.

Here, we engineer MNPs specifically for room-temperature thermometry applications. Their synthesis relies on reproducible, easily scalable, solution chemistry routes and is tunable to afford colloidal particles of controlled size, structure, and composition. We exploit compositional doping and exchange coupling between core-shell magnetic heterostructures as means of controlling their temperature-dependent spin interaction. We find that these structurally complex MNPs based on ferrites show improved temperature-dependent change in their magnetization. Analysis conducted using X-ray scattering and diffraction, Raman spectroscopy, and high-resolution electron microscopies provide correlations between the nanoscale structure of these particles with their magnetic thermosensitivity. Finally, we use an AC magnetic particle spectrometer to measure the magnetization-dependence of these nanothermometers as a function of temperature in their local environment. Our results demonstrate that complex ferrite MNPs are a promising material platform for remote thermometric sensing based on temperature-dependent spin ordering under an applied magnetic field.

The localized electron spin of a single impurity in a semiconductor is a promising system to realize quantum information schemes [1]. Coherent control of this spin depends on understanding the structure of the magnetic moment that couples the system with external fields. In this work we investigate the orbital contribution to the magnetic moment originated from the spin-orbit induced circulating current associated with the ground state of a single magnetic impurity in zincblende III-V semiconductor. This circulating current is dissipationless and represents an electron moving in a closed trajectory [2]. The orbital moment associated with the circulating current could be directly measured by the dc-magnetic field it produces through nanoscale magnetometry techniques provided by NV-centers in diamond [3].
In this project we developed a formalism employing Green's functions obtained by the Koster-Slater technique [4] with a sp³d⁵s^* empirical tight-binding Hamiltonian [5,6] to describe the host material. We calculated the circulating current and orbital moments of a single Mn dopant in GaAs. The spin-correlated orbital moments originates from the hybridization between the Mn(d⁵) spin-polarized electrons and the As dangling bonds leading to t₂-symmetric triplet acceptor states in the band-gap above the valence band edge.

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[6] Kortan, V.R. and Sahin, C. and Flatté, M.E., PRB 93, 220402(R) (2016).

* This project has received funding from the European Union's Horizon 2020 research
and innovation programme under the Marie Sklodowska-Curie grant agreement No 721394

Given its unrivaled potential of integration and scalability, silicon is likely to become a key platform for large-scale quantum technologies. Individual electron-encoded artificial atoms either formed by impurities [1] or quantum dots [2, 3] have emerged as a promising solution for silicon-based integrated quantum circuits. However, single qubits featuring an optical interface needed for large-distance exchange of information [4] have not yet been isolated in such a prevailing semiconductor.
We show the first isolation of single optically-active point defects in a commercial silicon-on-insulator wafer implanted with carbon atoms [5]. These artificial atoms exhibit a bright, linearly polarized single-photon emission at telecom wavelengths suitable for long-distance propagation in optical fibers. Our results demonstrate that despite its small bandgap (~1.1 eV) a priori unfavorable towards such observation [6], silicon can accommodate point defects optically isolable at single scale, like in wide-bandgap semiconductors [7]. This work opens numerous perspectives for silicon-based quantum technologies, from integrated quantum photonics to quantum communications [8] and metrology.

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[5] Redjem*, W., Durand*, A. et al., arXiv : 2001.02136 (2020).
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[7] Aharonovich, I., Englund, D. & Toth, M., Nature Photonics 10, 631–641 (2016).
[8] Wehner, S., Elkouss, D. & Hanson, R., Science 362, eaam9288 (2018).

Mass imaging technique for multiple length-scales from single cells to tissues is necessary for a wide range of applications such as the development of anti-cancer agents with fewer side effects[1,2]. Theoretical calculation demonstrated a hybrid sensor using nitrogen-vacancy (NV) centers combined with giant magnetostrictive(GMS) material as a promising platform for such application [3,4]. For the realization of the hybrid sensor, deposition of GMS material and vector imaging of GMS material with NV center is necessary. In this study, we performed imaging of stray magnetic field vectors produced by deposited GMS material using NV centers. Perpendicular magnetic anisotropy (PMA) in SmFe2 was confirmed from both vibrating sample magnetometer (VSM) and vector imaging using NV center. SmFe2 also could make sensitivity higher than GMS material with in-plane magnetic anisotropy due to its smaller anisotropic magnetic field.
SmFe2 thin film with a thickness of 100 nm was deposited on a quartz substrate and Si/SiO2 substrate with the facing target sputtering. M-H hysteresis curve was measured on SmFe2 thin film using a VSM for testing PMA. Magnetostriction constant λ, directly related to the sensitivity of the hybrid sensor, was calculated from the change of the magnetic anisotropic energy density with bending stress of ± 10 MPa. Ensemble NV centers were fabricated on a IIa(100) diamond substrate by nitrogen ion implantation with implantation energy of 6 keV and 2×10¹³ cm^-2 concentration. SmFe₂ thin film was placed on top of the ensemble NV center for vector magnetic field measurement. The stray magnetic field was imaged by pulsed-optically detected magnetic resonance (ODMR) with a uniform bias magnetic field of approximately 5 mT solving degeneracy of NV energies.
M-H hysteresis measurements using VSM indicated the PMA of the deposited SmFe2 thin-film and large magnetostriction constant of λ= -920 ppm which is the same order as general GMS materials. ODMR measurements using NV centers revealed a magnetic field vector profile produced by the SmFe2 thin-film with a spatial resolution of ~500 nm, a measurement field-of-view of 64 µm x 64 µm, and a magnetic field sensitivity of 50 µT/Hz^1/2. The stray field measured by NV centers was perpendicular to the surface, indicating that the SmFe₂ layer had PMA consistent with the VSM result. We established essential techniques for constructing the hybrid system through deposition of SmFe₂ and magnetic field vector imaging using NV centers. We aim a mass sensitivity of ~1 ng/Hz^1/2 (1% change in cell mass (~1 ng) can be detected every 60 minutes) by optimizing the GMS thin film and the magnetic field sensitivity of NV center.
Acknowledgements
This study was supported in part by MEXT Q-LEAP JPMXS0118067395.

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We propose [1] an approach to realize a hybrid quantum system composed of a diamond nitrogen-vacancy (NV) center spin coupled to a magnon mode of the low-damping, low-moment organic ferrimagnet vanadium tetracyanoethylene [2,3]. We derive an analytical expression for the spin-magnon cooperativity as a function of NV position under a micron-scale perpendicularly magnetized disk, and show that, surprisingly, the cooperativity will be higher using this magnetic material than in more conventional materials with larger magnetic moments, due to in part to the reduced demagnetization field. For reasonable experimental parameters, we predict that the spin-magnon-mode coupling strength is g~10 kHz. For isotopically pure ¹²C diamond we predict strong coupling of an NV spin to the unoccupied magnon mode, with cooperativity C~6 for a wide range of NV spin locations within the diamond, well within the spatial precision of NV center implantation. Thus our proposal describes a practical pathway for single-spin-state-to-single-magnon-occupancy transduction and for entangling NV centers over micron length scales.

The material is based on work supported by the U.S. Department of Energy, Office of Basic Energy Sciences, under Award Number DE-SC0019250.

[1] Denis R. Candido, Gregory D. Fuchs, Ezekiel Johnston-Halperin, Michael E. Flatté, arXiv:2003.04341 (Material for Quantum Technology -- https://iopscience.iop.org/article/10.1088/2633-4356/ab9a55)
[2] Na Zhu, Xufeng Zhang, I. H. Froning, Michael E. Flatté and Hong X. Tang., Applied Physics Letters 109, 082402 (2016)
[3] A. Franson, et. al., APL Materials 7, 121113 (2019)

<div style="direction: ltr;">The discussed work deals with a control and characterization of spin degrees of freedom of photo-generated carriers in colloidal seeded nanorods (sNRs) - via magnetic doping. The material under consideration is CdSe/CdSe:Mn sNRs, including diluted concentration of Mn²⁺ ions at the rod region. Magnetic doping was implemented extensively in colloidal quantum dots, but with alimited way in anisotropic structures, wherein the latter posesses other significant properties (e.g., anisotropy and reduced Auger rates).
Our work reports a thorough investigation of spin degrees of freedom in the mentioned materials, monitored by an optically detected magnetic resonance (ODMR) spectroscopy tool, which provide a significant information on exact location of host carriers and dopants, as well as examine interactions between them.
More specifically, the comparison between the pristine structures (CdSe/CdS) and the magnetically doped derivative, CdSe/CdS:Mn, indicated that both include trapping of the photo-generated carriers at the interface between the seed and the rod. The trapped carriers are already unpaired spins that endow selective magneto-optical properties with extended radiative and spin-relaxation times. The dominant interaction with the magnetic dopant takes place along the seed/rod interface, leading to further enhancement of spin helicity and the radiative lifetime. Most importantly, the carrier-dopant interaction bestows an order of magnitude extension of spin-lattice relaxation time in doped sNRs with respect to that of the pristine material, with an extreme importance for practical applications. The extracted physical parameters from the ODMR experiments included: g-factors and their anisotropy, spin exchange interactions, angular momentum, carrier-dopant coupling constants, radiative and spin-lattice relaxation times.
The spin-properties in confined systems as mentioned here ,undoubtedly can play an important role in the development of new spin-based technologies.
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The recently discovered atomically-thin magnetic crystals provide a unique playground to develop new approaches to manipulate magnetism. Rapid progresses have been made that demonstrate the potentials of utilizing 2D magnets to construct novel spintronics devices. However, their spin dynamics, which are crucial for microscopic understanding and determine the fundamental limit of spin manipulation, still remain elusive due to the difficulty of characterizing these micron-sized samples with conventional microwave techniques. In this talk, we will show how we can access and probe the collective spin-wave excitations in an antiferromagnetic bilayer CrI₃, which allows us to extract magnetic anisotropy and exchange energy. In particular, we will demonstrate the gate tunability of magnon frequencies, which is unique for the 2D magnet system.

Spin Hall effect (SHE), a mechanism by which materials convert a charge current into a spin current, has attracted significant attention owing to its novel physics and potential applications to spintronics, magnetic memory and spin logic devices. However, there have remained fundamental gaps in our understanding of the essential factors that control SHE in materials. Here we answer this important question, presenting a comprehensive theory of five rational design principles for achieving giant intrinsic SHE in transition metal oxides. Stemming from the inherently geometric nature of SHE, two of these design principles are weak crystal fields and the presence of structural distortions. Additionally, we discover that novel materials such as anti-perovskites are a highly promising class of materials for achieving giant SHE, with predicted SHE values reaching an order of magnitude larger than that reported for any oxide. We further examine the SHE values found in a variety of anti-perovskite oxides. Moreover, we derive three other design principles for enhancing SHE. Our findings shed light on the physics driving SHE, uncover technologically important SHE materials, and could help enhance and externally control SHE values.

Semiconducting transition metal dichalcogenides (TMDCs) have gained a great attention because of their fascinating electronic and optical properties when downing the thickness to one monolayer. In the monolayer form, due to the breaking of inversion symmetry and strong spin–orbit coupling, the carriers in TMDCs carry a valley index noted K+ or K−. This valley degree of freedom refers to local extrema in the band structure of TMDC monolayers at the K points of the two-dimensional Brillouin zone and it can be exploited to develop novel valleytronic devices. In MX₂ (M = Mo, W and X = S, Se) monolayers, the broken inversion symmetry and strong spin–orbit coupling originating from the d-orbitals of the transition metal atom align the electron spins near opposite valleys K+ and K− to opposite out-of-plane directions thus preserving time reversal symmetry. The broken spatial inversion symmetry also generates large Berry curvatures near the K valleys, the Berry curvatures being opposite at opposite valleys. This effect gives rise to unconventional electronic properties such as the valley Hall effect. An interesting growing field of investigation consists in coupling the valley physics with thermoelectrics. Several spin/valley-dependent thermoelectric effects have already been predicted theoretically in monolayer TMDCs such as the spin Nernst effect (thermoelectric counterpart of the spin Hall effect) and the valley Nernst effect (VNE). Hence, the experimental demonstration of those effects could give rise to a new field called valley caloritronics (by analogy with spin caloritronics) with the advantage that the coupled valley-spin degree of freedom is more robust than the spin with respect to external magnetic fields due to the large intrinsic spin–orbit coupling.
In this presentation, I will first review our recent results on the MBE growth of transition metal dichalcogenides on various substrates over large areas and explain the mechanisms underlying the van der Waals epitaxy regime [1]. Using this growth technique, we could study the interplay between thermoelectricity and the valley degree of freedom in monolayers of WSe₂. For this purpose, high quality WSe₂ mono and multilayers were grown on epitaxial graphene on SiC. Using millimeter-sized samples, we were able to apply well-defined temperature gradients and demonstrate the very strong Seebeck response of this material. In a second step, we used the ferromagnetic resonance-spin pumping technique (FMR-SP) to (i) apply the temperature gradient by off centering the sample in the radio frequency (RF) cavity and (ii) address a single valley and break time reversal symmetry using the spin pumping through spin-valley coupling. For FMR-SP, we deposited in-situ the Al/Ni₈₁Fe₁₉/Al magnetic stack on top of the WSe₂ layer. The combination of a temperature gradient and the valley polarization lead to the valley Nernst effect in WSe₂ that we could detect electrically in the RF cavity. The Nernst coefficient we measured is in very good quantitative agreement with the predicted value [2]. This effect could be exploited to generate large transverse valley currents for valleytronics applications.

References
[1] M.-T. Dau et al., ACS Nano 12, 2319 (2018).
[2] M.-T. Dau et al., Nat. Commun. 10, 5796 (2019).

The pyrene derivatives have been widely investigated in the field of organic electronics and spintronics. However, the fundamental understanding of their excited state dynamics and spin physics remains elusive. Here, we present the temperature-dependent photoluminescence (PL) and electron spin resonance (ESR) study of a model fluorescent organic dilute semiconductor where organic fluorophore pyrene is incorporated in the insulating Poly-methyl methacrylate (PMMA) polymeric backbone as pendant unit (py-PMMA). We observe that lowering the temperature from 300K to 140K, the monomer fluorescence emission from py-PMMA gets blue-shifted, and their intensity enhances, which is attributed to less charge carrier scattering by lattice sites at low temperature. Interestingly, we observe additional higher energy peaks at 368 nm (~3.36 eV) below 70K in fluorescence emission, which is explained by the self-confined exciton-phonon coupling. The strong spin-orbit coupling was observed in our system by temperature-dependent electron spin resonance (TDESR) spectroscopy, and spin relaxation time was calculated in order of picoseconds. The temperature-dependent intensity and bizarre linewidth behavior of the ESR signal are attributed to the phonon bottleneck effect. Our study provides the direction to examine and understand the electron-phonon interaction and spin-orbit coupling at low temperatures in organic polymers, which helps to design new organic devices in the field of spintronics.

Rhenium-based double perovskites possess a rich array of properties including magnetic ordering at room temperature and an intricate interplay between structure, orbital ordering, and metal-insulator transitions (MIT). In particular, Sr₂CrReO₆ (SCRO) possesses a high Curie temperature of Tc=620K, a large strain-tunable magnetocrystalline anisotropy, and an insulating ground state with a bandgap of 0.21 eV. To determine the energy scale for the electronic interactions, we used resonant inelastic X-ray scattering (RIXS) at the Re L₃ edge for two strain states of SCRO thin films, compressive -1% on a (LaAlO₃)_0.3(Sr₂AlTaO₆)_0.7 (LSAT) substrate and tensile +1% on a relaxed SrCr_0.5Nb_0.5O₃ buffer layer on LSAT. We observe three Raman-like excitations around 150, 600 and 1000 meV on both samples and a broad fluorescence above 8 eV. All the Raman-like peaks show a strong dependence on strain, and only the lowest energy peak shows a temperature dependence. Using the exact diagonalization RIXS toolkit (EDRIXS) [2], we are able to assign the Raman-like features to Re d orbitals excitations. Implications of the observed experimental strain and temperature dependences on the physics of SCRO are discussed. .
[1] Hauser, A. J. et al. Fully ordered Sr₂CrReO₆ epitaxial films: A high-temperature ferrimagnetic semiconductor. Phys. Rev. B 85, 161201 (2012).
[2] Wang, Y. et al. EDRIXS : An open source toolkit for simulating spectra of resonant inelastic X-ray scattering. Comp. Phys. Comm. 243, 151 (2019).
Work at Yale is supported by a grant from the Department of Energy, Basic Energy Sciences under grant number DE-SC0019211. Work was done at the Advanced Photon Source, a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under Contract No. DE-AC02-06CH11357.

Electrons bound to donors in silicon are electrically-controlled qubits with exceptional spin coherence properties. Here we seek to find analogous donor systems in direct band gap semiconductors which will enable an efficient optical interface to the donor electron spin qubit. We show that ZnO donors, optically coupled to donor-bound excitons, are particularly promising candidates [1]. Due to low spin-orbit coupling, we find donors exhibit longitudinal spin relaxation times up to 0.5 s at 1.5 K with relaxation fundamentally-limited by phonon interactions. Both lambda and near-cycling optical transitions exist which are suitable for optical spin initialization and readout. Spin-echo coherence times, which we determine via both optical and microwave control, are in the tens of microseconds and likely limited by nuclear spins in natural isotopic ZnO. These characteristics are all extracted from measurements on an ensemble of Ga donors in a commercial substrate. We will present ongoing efforts and challenges toward single donor isolation in nanowires, dual optical-microwave control of donors and two-laser coherent spin-spectroscopy in this emerging qubit platform.

[1] X. Linpeng, M.L.K Viitaniemi, A. Vishnuradhan, Y. Kozuka, C.Johnson, M. Kawasaki, K.-M.C. Fu, Phys. Rev. Applied 10, 064061 (2018)

In spintronics the generation and manipulation of pure spin currents is in the focus of research activities. Amongst the utilized fundamental effects is spin pumping where a precessing magnetization of a ferromagnet being at ferromagnetic resonance (FMR) transfers angular momentum to an adjacent nonferromagnetic layer [1]. The transfer of angular momentum into the nonferromagnetic layer can be described as a spin-current. The pumped spin current has a dc and an ac component corresponding to the reduction of the projection of the magnetization at FMR as well as the dynamic high-frequency magnetization, respectively. Spin pumping in a ferromagnet/nonferromagnet heterostructure is directly imaged with spatial resolution as well as element selectivity. The time-resolved detection in scanning transmission x-ray microscopy [2] allows to directly probe the spatial extent of the ac spin polarization in Co-doped ZnO which is generated by spin pumping from an adjacent permalloy microstrip complementing earlier results in Py/Co:ZnO heterostructures [3]. Comparing the relative phases of the dynamic magnetization component of the two constituents is possible and found to be antiphase. The correlation between the distribution of the magnetic excitation in the permalloy and the Co-doped ZnO reveals that laterally there is no one-to-one correlation. The observed distribution is rather complex, but integrating over larger areas clearly demonstrates that the spin polarization in the nonferromagnet extends laterally beyond the region of the ferromagnetic microstrip. Therefore the observations are better explained by a local spin pumping efficieny and a lateral propagation of the ac spin polarization in the nonferromagnet over the range of a few micrometers [4].
[1] Y. Tserkovnyak, A. Brataas, and G. E. W. Bauer, Phys. Rev. Lett. 88 (2002) 117601
[2] S. Bonetti et al., Rev. Sci. Instrum. 86 (2015) 093703; T. Schaffers et al. Rev. Sci. Instrum. 88 (2017) 093703
[3] M. Buchner et al. J. Appl. Phys. 127 (2020) 043901 (2020)
[4] S. Pile et al. arXiv 2005.08728

Wide bandgap semiconductors are essential to numerous commercial devices and have been extensively investigated due to their high optical transparency, controllable transport properties, carrier concentrations, and mobilities. Transition metal impurities and impurity-vacancy complexes in wide-gap semiconductors have also attracted both theoretical and experimental interest due to possible applications in single-photon emission with minimal decoherence [1], the possibility of robust exchange interactions of spin centers [2], and inducing also enhancing of the ferromagnetic order by doping [3]. Among various wide-gap semiconductors, zinc sulfide (ZnS) has been a platform for numerous applications such as display technologies, luminescent devices, nonlinear optical devices, solar cells, and other optoelectronic applications [4,5,6]. Furthermore, certain defects and defect-vacancy centers may provide a coherent, room-temperature interface between spins and photons in this host material.

Here we carry out the calculation of the electronic and structural properties of transition metal - vacancy complexes in ZnS within the density functional theory framework as implemented in the Vienna ab initio simulation package and projector-augmented wave approach. We simulate the host material ZnS as a 64 atom supercell and use a 2x2x2 Monkhorst-Pack k-point mesh for the sampling of the Brillouin zone. Perdew–Burke-Ernzerhof class of generalized gradient approximation is used for the treatment of the exchange-correlation potential. Both lattice constants and atomic positions are optimized with a convergence criterion of 1 meV. To fix the well-known band-gap problem of the density functional theory method, we use hybrid functionals resulting in a 3.65 eV gap, which agrees excellently with the experimental value.

Transition metal dopants are substitutionally replaced with a zinc atom, and the vacancy is formed at the neighboring sulfur atom. We then perform the ionic relaxations with a fixed lattice constant using the conjugate gradient algorithm. The defect formation energies are calculated following the formula of Ref. [7]. We have also calculated different charged states of the impurities in Zn-rich and S-rich conditions to establish the possibility of forming these impurities as a function of the Fermi level in the bandgap region. The charge transition levels are also computed from the relaxed final energies for different charged states. We have also included the charge corrections due to finite-size effects originating from the Columb interaction between impurities in the neighboring supercells using the Freysoldt-Neugebaure-Van de Walle scheme. [8]. Finally, we calculate the density of states, orbital projected densities, and electronic band structures of the system to identify states located in the bandgap and determine the characteristics of the impurities, such as deep vs. shallow. We conclude that ZnS is a strong candidate for a system in which impurity spin-light interfaces can be realized similarly to the nitrogen-vacancy color centers in diamond and divacancies in silicon carbide.

We acknowledge support from NSF DMREF Award No. DMR-1921877.

[1] M. Koperski, K. Nogajewski, A. Arora, V. Cherkez, P. Mallet, J.-Y. Veuillen, J. Marcus, P. Kossacki, and M. Potemski, Nature Nanotechnology, 10, 503-506 (2015)
[2] V. R. Kortan, C. Sahin, and M. E. Flatté, Phys. Rev.B 93, 220402(R) (2016)
[3] K. Sato and H. Katayama-Yoshida, Jpn. J. Appl. Phys., 39, L555 (2000)
[4] X. Ma, J. Song, and Z. Yu, Thin Solid Films, 519(15),5043-5045 (2011)
[5] Y. Lun, Y. Lin, Y. Meng, and , and Y. Wang, Ceramics International, 40(6):8157–8163 (2014)
[6] X. Xu, S. Li, J. Chen, S. Cai, Z. Long, and X. Fang, Adv. Funct. Mater. 28 (36) 1802029 (2018)
[7] C. G. Van de Walle and J. Neugebauer, J. Appl. Phys. 95, 3851 (2004)
[8] C. Freysoldt, J. Neugebauer, and C. G. Van de Walle, Phys. Rev. Lett. 102, 016402 (2009)

Chemical synthesis creates materials that are atomically precise, customizable, and reproducible. By bringing this toolkit to bear on the challenge of creating materials for quantum information science, we can design custom materials, each molecule geared towards a specific purpose. With these "designer qubits" it may be possible to imbue a qubit with water solubility for biological quantum sensing, create quantum memories with the careful placement of nuclear spins, or tune the emission frequency for quantum communication. Results will be presented on creating and understanding molecular qubits, with an emphasis on strategies to create qubits that can be integrated with established read-out approaches.

Metal complexes will be proposed to acting as active quantum units for Quantum Computing (QC). We report on the implementation of metal complexes into nanometre-sized (single-)molecular spintronic devices by a combination of bottom-up self-assembly and top-down lithography techniques. The controlled generation of magnetic molecular nanostructures on conducting surfaces/electrodes will be shown and persistence of their magnetic properties under confinement in Supramolecular Quantum Devices (SMQD) will be proven. The quantum properties (e.g.. superposition entanglement) of the metal complexes will be addressed at the single molecule level^1-9 to finally implement a quantum algorithm on a TbPc₂ Qudit performing quantum computing operations.¹⁰

References:
[1] S. Kyatskaya et. al. J. Am. Chem. Soc. 2009, 131, 15143-15151.
[2] M. Urdampilleta et al. Nature Mater. 2011, 10, 502-506.
[3] J. Schwöbel et. al. Nature Comms. 2012, 3, 953-956.
[4] R. Vincent et al. Nature 2012, 488, 357-360.
[5] M. Ganzhorn et al. Nature Nano. 2013, 8, 165–169.
[6] M. Ruben et. al. Nature Nano. 2013, 8, 377–389.
[7] S. Wagner et. al. Nature Nano. 2013, 8, 575–579.
[8] S. Thiele, et al. Science 2014, 344, 1135-1138.
[9] M. Ganzhorn, et. al. Nature Comms 2016, 11443.
[10] C. Godfrin et al. PRL 2017, 119, 187702 (perspective article by A. Morello Nature Nano 2018, 13, 9-10).


Recent Reviews:
Molecular spin qudits for quantum algorithms.
E. Moreno-Pineda, C. Godfrin, F. Balestro, W. Wernsdorfer, M. Ruben. Chem. Soc. Rev. 2018, 47, 501.

Synthetic Engineering oft he Hilbert Sapce of Molecular Qudits: Isotopoloque Chemistry
W. Wernsdorfer, M. Ruben. Adv. Mat. 2019, doi.org/10.1002/adma.201806687.

A central goal of antiferromagnetic spintronics is the ability to sense and control the magnetic state of an antiferromagnet. While the magnetic state of a metallic antiferromagnet (AF) may be inferred using magnetoresistance measurements, this is not an option for insulating AF materials. The spin Seebeck effect (SSE), where thermal gradients are used to drive magnon spin currents, has proven to be an effective probe [i] of AF order. Substantial spin currents have been measured using the SSE in AF insulators Cr₂O₃ [ii] and MnF₂ [iii], and even in geometrically frustrated antiferromagnets [iv]. In this talk, I will briefly review the antiferromagnetic spin Seebeck effect and present our latest results on epitaxial Pt/Cr₂O₃ (0001)/Pt heterostructures. In our devices, the spin currents generated by thermal gradients are detected using the inverse spin Hall effect (ISHE) in thin films of Pt, grown both as an epitaxial underlayer beneath and as an overlayer on top of the Cr₂O₃ film. Cr₂O₃ is unique in that the spins on the top and bottom surfaces of the (0001) oriented film are uncompensated, and belong to separate magnetic sublattices, even in the presence of surface roughness. Using the ISHE in both top and bottom Pt layers [v], we find that the SSE is strongly sensitive to the respective sublattice magnetization. As a result, we are able to track the exact orientation of both sublattices upon application of a magnetic field, including during spin-flop processes. Furthermore, Cr₂O₃ is a classic linear magnetoelectric, where an electric field applied in the [0001] direction can be used to tune the net magnetization [vii]. In this context, I will discuss how the application of electric fields in Pt/Cr₂O₃/Pt heterostructures can be used to tune magnon spin currents, generated using thermal gradients [viii].


[i] S. M. Rezende et al., Phys. Rev. B 93, 014425 (2016).
[ii] S. Seki et al., Phys. Rev. Lett. 115, 266601 (2015)
[iii] S. M. Wu et al., Phys. Rev. Lett. 116, 097204 (2016)
[iv] S. M. Wu et al., Phys. Rev. Lett. 116, 097204 (2015); C. Liu et al...
[v] Y. Luo et al., arXive:1910.10340
[vii] D. N. Astrov, Sov. Phys. JETP 11, 780 (1960).
[viii] C. Liu et al., in preparation (2020).

Silicon carbide (SiC) is a host to a variety of robust, optically active quantum systems. Recent studies have shown that basally oriented kh divacancy defects in SiC possess a robust optical interface as well as exceptional spin coherence properties even in commercially available, naturally abundant SiC. We show that these excellent intrinsic properties can be further augmented and tuned by applying a periodic drive, giving rise to emergent phenomena unique to non-equilibrium quantum systems.

First, we demonstrate electrically driven coherent quantum interference in the optical transition of single kh divacancies [1]. By applying microwave frequency electric fields, we coherently drive the divacancy’s excited-state orbitals and induce Landau-Zener-Stückelberg interference fringes in the resonant optical absorption spectrum. This result shows that the optical absorption spectrum of these divacancy defects can be rapidly reconfigured using localized electric field control, and reveals a highly responsive mechanism to potentially coherently interact with other electrically active quantum systems.

We then construct a robust qubit embedded in a decoherence-protected subspace, obtained by hybridizing an applied microwave drive with the ground-state electron spin of the kh divacancy [2]. This qubit is protected from magnetic, electric, and temperature fluctuations, which account for nearly all relevant decoherence channels in the solid state. This culminates in an increase of the qubit's inhomogeneous dephasing time by over four orders of magnitude (to >22 milliseconds), while its Hahn-echo coherence time approaches 64 milliseconds. Such an extension of inhomogeneous dephasing time opens up pathways to a vast array of hybrid quantum system applications. Furthermore, since this protocol requires few key platform-independent components, this result suggests that substantial coherence improvements can be achieved in a wide selection of quantum architectures.

This work was done in collaboration with Alexander L. Crook, Gary Wolfowicz, Joseph P. Blanton, Samuel J. Whiteley, Sam L. Bayliss, Hiroshi Abe, Takeshi Ohshima, Gergo Thiering, Péter Udvarhelyi, Viktor Ivády, and Ádam Gali.

[1] K. C. Miao, A. Bourassa, C. P. Anderson, S. J. Whiteley, A. L. Crook, S. L. Bayliss, G. Wolfowicz, G. Thiering, P. Udvarhelyi, V. Ivády, H. Abe, T. Ohshima, Á. Gali, D. D. Awschalom, “Electrically driven optical interferometry with spins in silicon carbide,” Sci. Adv. 5 (11), eaay0527 (2019)

[2] K. C. Miao, J. P. Blanton, C. P. Anderson, A. Bourassa, A. L. Crook, G. Wolfowicz, H. Abe, T. Ohshima, D. D. Awschalom, “Universal coherence protection in a solid-state spin qubit,” arXiv 2005.06082 (2020)

Advances in localised tip-induced lithography mean that the process of doping a semiconductor need no longer be random: instead, dopant atoms can be located in well-defined positions relative to one another. This has already been demonstrated for donors in silicon [1,2] and may in future be possible for other types of dopant. This opens the possibility of controlling the interactions between the dopants to form new deterministic devices, which in turn elevates the spin degree of freedom from a spectator into an important variable. As well as the established potential for spin-based quantum gates [3] a number of other applications can now be foreseen.

We have computed the excitation spectra of donor pairs and larger clusters [4] in silicon, using a combination of ab initio band-structure simulations and quantum chemistry techniques. The results show strong and characteristic changes in the character of the dominant excitation as a function of separation and spin state, raising the possibility optical detection and initialisation of the multi-donor spin state. We have also studied the application of controlled donor arrays as quantum simulators that can be probed by their transport properties [5]; in particular we have identified topological edge states and shown how their properties vary across the metal-insulator transition [6].

Finally, for acceptor clusters we have studied the consequences of the richer spin-orbit coupling present in the silicon valence band [7]. We find a complex interplay between the relative strengths of hopping and Coulomb interactions, the arrangement of the bond lengths, and the spin-orbit coupling. This can result in the emergence of novel kinds of topological state.

[1] Schofield, S. R., et al (2003). Atomically precise placement of single dopants in Si. PHYSICAL REVIEW LETTERS, 91 (13), ARTN 136104. doi:10.1103/PhysRevLett.91.136104
[2] Stock, T. J. Z., et al. (2020). Atomic-Scale Patterning of Arsenic in Silicon by Scanning Tunneling Microscopy. ACS Nano, acsnano.9b08943. doi:10.1021/acsnano.9b08943
[3] He, Y et al. (2019). A two-qubit gate between phosphorus donor electrons in silicon. Nature 571 p 371 doi:10.1038/s41586-019-1381-2
[4] Wu, W., et al. (2018). Excited states of defect linear arrays in silicon: A first-principles study based on hydrogen cluster analogs. Physical Review B, 97 (3), ARTN 035205. doi:10.1103/PhysRevB.97.035205
[5] Le, N. H., et al. (2017). Extended Hubbard model for mesoscopic transport in donor arrays in silicon. Physical Review B, 96 (24), ARTN 245406. doi:10.1103/PhysRevB.96.245406
[6] Le, N. H., et al. (2020). Topological phases of a dimerized Fermi–Hubbard model for semiconductor nano-lattices. npj Quantum Information, 6 (1), 24. doi:10.1038/s41534-020-0253-9
[7] Zhu, J., et al. (2020). Linear combination of atomic orbitals model for deterministically placed acceptor arrays in silicon. Physical Review B, 101 (8), ARTN 085303. doi:10.1103/PhysRevB.101.085303

Magnetoelectric coupling is the coupling between magnetic order and the electric polarization in an insulator. To date it has been largely studied in inorganic oxides that show various types of ferromagnetic or antiferromagnetic order. Metal-organic frameworks and molecular compounds are a new and fertile direction to search for new magnetoelectric coupling mechanisms. I will talk about several recent results in which we couple spin crossovers to structure and electric polarization. I will also highlight multiferroic behavior in metal-organic frameworks. The experimental part of the work takes advantage of pulsed magnetic fields to perform high-sensitivity measurements and reach extended regions of the magnetic field-temperature phase diagram.

Light-matter coupling in the so-called ultrastrong coupling (USC) regime [1] has become an important general subject of modern science and technology. Many next-generation quantum technologies that exploit the extremely efficient light-matter interactions in the USC regime have been proposed. Further, a wealth of exotic phenomena has been predicted in the USC regime, such as quantum vacuum radiation or nonclassical ground states known as two-mode squeezed vacuums. The source of these phenomena is the so-called counter-rotating terms in the Hamiltonian, which are typically negligible compared to the so-called co-rotating terms for weaker coupling strengths. USC has now been achieved in diverse platforms, but the exotic nature of the ground state due to the counter-rotating terms has not been explored. We have recently observed unusual matter-matter USC phenomena in rare-earth orthoferrites. First, we observed cooperative, Dicke-type coupling between a paramagnetic Er³⁺ spin ensemble and a single mode of terahertz magnons of ordered Fe³⁺ spins in ErFeO₃ [2]. The data exhibited vacuum Rabi splitting values in the USC regime, indicating that rare-earth orthoferrites provide a rich playground in which to explore USC phenomena without light. Furthermore, we observed that magnons in YFeO₃ display USC with a dominant role played by the counter-rotating terms. Our system is described by an unusual Hamiltonian, where: (i) the coupling strengths are tunable, (ii) the counter-rotating interactions dominate the co-rotating interactions, and (iii) the ground state is a magnonic two-mode squeezed vacuum with quantum fluctuation suppression up to 5.9 dB. These observations demonstrate that YFeO₃ provides an extraordinary platform for both applied and fundamental studies of USC beyond the reaches of photonic platforms.
1. P. Forn-Diaz, L. Lamata, E. Rico, J. Kono, and E. Solano, “Ultrastrong Coupling Regimes of Light-Matter Interaction,” Rev. Mod. Phys. 91, 025005 (2019).
2. X. Li et al., “Observation of Dicke Cooperativity in Magnetic Interactions,” Science 361, 794 (2018).

Recently, the physicists and material scientists have experienced a new wave of interest in spin effects in solid-state systems. On the one hand, semiconductors and semiconductor nanosystems, allow one to perform benchtop studies of quantum and relativistic phenomena. On the other hand, the interest is supported by the prospects of realizing spin-based electronics where the electron or nuclear spins can play the role of quantum or classical information carriers. In my tutorial lecture, I will present the key aspects of multifaceted physics of interacting electron and nuclear spins in semiconductor-based low-dimensional structures. The hyperfine interaction of the charge carrier and nuclear spins increases in nanosystems compared with bulk materials. The enhanced coupling results in the efficient spin exchange between these two systems. It gives rise to beautiful and complex physics occurring in the ensemble of electrons and nuclei in semiconductors.

I will address the hyperfine interaction, nuclear magnetic resonance, dynamical nuclear polarization, spin-Faraday and -Kerr effects, processes of electron spin decoherence and relaxation, effects of electron spin precession mode-locking and frequency focusing, as well as fluctuations of electron and nuclear spins.

One of the possible hybrid realizations of a qubit is the spin of an electron confined in a semiconductor quantum dot (QD), which is interacting with the surrounding nuclear spins. The prominent advantage of QDs is their strong optical dipole moment, which allows efficient coupling of photons to the confined electron spins according to the optical selection rules. The electron, on the other hand, is coupled to the nuclei of the QD crystal matrix by the hyperfine interaction. These couplings allow one to design schemes where the angular momentum of the photon is transferred to the nuclear spins using the electron as an auxiliary spin state. This may have significant advantages, as the electron spin coherence is limited to several microseconds at low temperatures, while the nuclear spin coherence can reach milliseconds.
To test the degree of control between the electron and the nuclear spin system, we present a model system of an ensemble of singly charged QDs. The use of an ensemble is justified in our case by the spectroscopic signal strength, which allows us to observe an averaged and integral response of many electron spins at the same time. It is also of interest for quantum memory applications. On the other hand, such an ensemble is usually strongly inhomogeneous, which leads to a strong variation of the intrinsic parameters, like optical dipole moments, electron g-factor values, and strength of electron-nuclear spin interactions. This inhomogeneity can, however, be overcome in designed experiments, enabling effects such as spin mode-locking and nuclear-induced frequency focusing.
In previous studies on nuclei-induced frequency focusing in singly charged (In,Ga)As/GaAs QDs multiple precession modes were covered by the ensemble, as the nuclear fluctuations were of the same order as the mode separation in low external fields. Here we explore the degree of possible control of the nuclear surrounding using an inhomogeneous QD ensemble excited with a high repetition laser operated at 1 GHz. The GHz laser repetition frequency allows us to investigate the interaction of nuclear spin polarization and nuclear spin fluctuations with the electrons as the mode separation is an order of magnitude larger than the fluctuating nuclear field. It further allows us to use the effects of the strong electron-nuclear feedback and drive the inhomogeneous ensemble of electron spins to a single frequency precession. Additionally, we demonstrate the discretization of the total magnetic fields seen by the electron ensemble and show the effect of the reduction of the nuclear field fluctuations. While for fields lower than 0.6 T nuclear spin fluctuations inhibit the formation of a significant nuclear polarization, above the nuclear spin relaxation time becomes elongated such that the electron spins precess on the closest preferred frequency over a range of 128 mT. This leads to the observation of plateaus in the precession frequency as a function of the external magnetic field. The nuclear spin bath is constantly tuned, such that the phase synchronization condition is fulfilled while nuclear fluctuations are suppressed.

In optical spectroscopy experiments, it is crucial to excite an emitter with a laser very close to its transition energy. Experiments under resonant excitation are essential for accessing the intrinsic optical and spin-polarization properties of large class of emitters ^1–6.Using linear cross-polarization in a confocal setup has been successfully employed as a dark-field method to carry out resonant fluorescence experiments to suppress scattered laser light, with the added benefit of high spatial resolution. Resonant fluorescence experiments allow crucial insights into light-matter coupling, such as the interaction of a single photon emitter with its environment, with optical cavities and also studying single defects in atomically thin materials such asWSe2. Dark-field confocal techniques allow developing single photon sources with high degrees of photon indistinguishability and longer coherence. In practice dark-field laser suppression is not just a spectroscopy tool, it is also a key part of more matured quantum technology systems. Despite many advances based on experiments in confocal microscopes with cross-polarization laser rejection, the physical mechanisms that make these experiments possible are not understood, hampering further progress in this field. The key figure of merit is the suppression of the excitation laser background by at least six orders of magnitude. Indeed a suppression by a factor of 10⁸ up to 10¹¹ (this work) have been measured. But this is very surprising as mirrors and beam-splitter in such a system reduce the theoretical extinction limit to the 10³ to 10⁴ range. In this work we explain the physics behind the giant enhancement of the extinction ratio by up to eight orders of magnitude that make microscopy based on dark-field laser suppression possible. The measurements of resonant fluorescence are typically performed in an epifluorescence geometry, for which laser excitation and fluorescence collection are obtained through the same focusing lens. This involves necessarily the use of a beam-splitter orienting the back-reflected light containing the fluorescence towards a detection channel. In our work we identify two key ingredients that explain the giant amplification of the cross-polarization extinction ratio : (i) a reflecting surface (i.e. the beam-splitter) placed between a polarizer and analyzer, and (ii) a confocal arrangement. We demonstrate giant extinction ratios in our experiments for different mirrors (silver, gold, dielectric, beam-splitter cubes) and polarizers (Glan-Taylor, nanoparticle thin films). We demonstrate that behind this general observation lies the intriguing physics of the Imbert-Fedorov effect, which deviates a reflected light beam depending on its polarization helicity. We discover that a confocal arrangement not only amplifies the visibility of the Imbert-Fedorov effect dramatically, taking it from the nanometer to the micrometer scale, but also exploits conveniently the symmetry of the newly observed Imbert-Fedorov modes to insure that the cross-polarized laser beam is not coupled, explaining the near complete suppression of the laser background signal. In other words, we cannot treat the spatial (i.e. modal) and polarization properties of light separately in our dark-field confocal microscope analysis. In addition to new developments in dark-field microscopy our experiments provide powerful tools to understand spin-orbit coupling of light, in the broader context of topological photonics. Funding: European Union’s Horizon 2020 research and innovation program, Marie Sklodowska-Curie grant agreement No 721394 ITN 4PHOTON.

¹ Aharonovich, I., et.al. M. Nat. Phot. 10, 631 (2016)
² Högele, A. et. al. Phys. Rev. Lett. 108, 197403 (2012)
³ Paillard, M. et. al. Phys. Rev. Lett. 86, 1634 (2001)
⁴ Högele, A. et. al. Phys. Rev. Lett. 93, 217401 (2004)
⁵ Vamivakas, et.al. Nat. Phys. 5, 198 (2009)
⁶ Kaldewey, T. et al. Nat. Phot. 12, 68 (2018)

Given its unrivaled potential of integration and scalability, silicon is likely to become a key platform for large-scale quantum technologies. Individual electron-encoded artificial atoms either formed by impurities [1] or quantum dots [2, 3] have emerged as a promising solution for silicon-based integrated quantum circuits. However, single qubits featuring an optical interface needed for large-distance exchange of information [4] have not yet been isolated in such a prevailing semiconductor.
We show the first isolation of single optically-active point defects in a commercial silicon-on-insulator wafer implanted with carbon atoms [5]. These artificial atoms exhibit a bright, linearly polarized single-photon emission at telecom wavelengths suitable for long-distance propagation in optical fibers. Our results demonstrate that despite its small bandgap (~1.1 eV) a priori unfavorable towards such observation [6], silicon can accommodate point defects optically isolable at single scale, like in wide-bandgap semiconductors [7]. This work opens numerous perspectives for silicon-based quantum technologies, from integrated quantum photonics to quantum communications [8] and metrology.

[1] He, Y. et al., Nature 571, 371 (2019).
[2] Watson, T. F. et al., Nature 555, 633–637 (2018).
[3] Maurand, R. et al., Nature Comm. 7, 13575 (2016).
[4] Hensen, B. et al., Nature 526, 682–686 (2015).
[5] Redjem*, W., Durand*, A. et al., arXiv : 2001.02136 (2020).
[6] Weber, J. R. et al., PNAS 107, 8513–8518 (2010).
[7] Aharonovich, I., Englund, D. & Toth, M., Nature Photonics 10, 631–641 (2016).
[8] Wehner, S., Elkouss, D. & Hanson, R., Science 362, eaam9288 (2018).

The recently discovered atomically-thin magnetic crystals provide a unique playground to develop new approaches to manipulate magnetism. Rapid progresses have been made that demonstrate the potentials of utilizing 2D magnets to construct novel spintronics devices. However, their spin dynamics, which are crucial for microscopic understanding and determine the fundamental limit of spin manipulation, still remain elusive due to the difficulty of characterizing these micron-sized samples with conventional microwave techniques. In this talk, we will show how we can access and probe the collective spin-wave excitations in an antiferromagnetic bilayer CrI₃, which allows us to extract magnetic anisotropy and exchange energy. In particular, we will demonstrate the gate tunability of magnon frequencies, which is unique for the 2D magnet system.

Semiconducting transition metal dichalcogenides (TMDCs) have gained a great attention because of their fascinating electronic and optical properties when downing the thickness to one monolayer. In the monolayer form, due to the breaking of inversion symmetry and strong spin–orbit coupling, the carriers in TMDCs carry a valley index noted K+ or K−. This valley degree of freedom refers to local extrema in the band structure of TMDC monolayers at the K points of the two-dimensional Brillouin zone and it can be exploited to develop novel valleytronic devices. In MX₂ (M = Mo, W and X = S, Se) monolayers, the broken inversion symmetry and strong spin–orbit coupling originating from the d-orbitals of the transition metal atom align the electron spins near opposite valleys K+ and K− to opposite out-of-plane directions thus preserving time reversal symmetry. The broken spatial inversion symmetry also generates large Berry curvatures near the K valleys, the Berry curvatures being opposite at opposite valleys. This effect gives rise to unconventional electronic properties such as the valley Hall effect. An interesting growing field of investigation consists in coupling the valley physics with thermoelectrics. Several spin/valley-dependent thermoelectric effects have already been predicted theoretically in monolayer TMDCs such as the spin Nernst effect (thermoelectric counterpart of the spin Hall effect) and the valley Nernst effect (VNE). Hence, the experimental demonstration of those effects could give rise to a new field called valley caloritronics (by analogy with spin caloritronics) with the advantage that the coupled valley-spin degree of freedom is more robust than the spin with respect to external magnetic fields due to the large intrinsic spin–orbit coupling.
In this presentation, I will first review our recent results on the MBE growth of transition metal dichalcogenides on various substrates over large areas and explain the mechanisms underlying the van der Waals epitaxy regime [1]. Using this growth technique, we could study the interplay between thermoelectricity and the valley degree of freedom in monolayers of WSe₂. For this purpose, high quality WSe₂ mono and multilayers were grown on epitaxial graphene on SiC. Using millimeter-sized samples, we were able to apply well-defined temperature gradients and demonstrate the very strong Seebeck response of this material. In a second step, we used the ferromagnetic resonance-spin pumping technique (FMR-SP) to (i) apply the temperature gradient by off centering the sample in the radio frequency (RF) cavity and (ii) address a single valley and break time reversal symmetry using the spin pumping through spin-valley coupling. For FMR-SP, we deposited in-situ the Al/Ni₈₁Fe₁₉/Al magnetic stack on top of the WSe₂ layer. The combination of a temperature gradient and the valley polarization lead to the valley Nernst effect in WSe₂ that we could detect electrically in the RF cavity. The Nernst coefficient we measured is in very good quantitative agreement with the predicted value [2]. This effect could be exploited to generate large transverse valley currents for valleytronics applications.

References
[1] M.-T. Dau et al., ACS Nano 12, 2319 (2018).
[2] M.-T. Dau et al., Nat. Commun. 10, 5796 (2019).

Electrons bound to donors in silicon are electrically-controlled qubits with exceptional spin coherence properties. Here we seek to find analogous donor systems in direct band gap semiconductors which will enable an efficient optical interface to the donor electron spin qubit. We show that ZnO donors, optically coupled to donor-bound excitons, are particularly promising candidates [1]. Due to low spin-orbit coupling, we find donors exhibit longitudinal spin relaxation times up to 0.5 s at 1.5 K with relaxation fundamentally-limited by phonon interactions. Both lambda and near-cycling optical transitions exist which are suitable for optical spin initialization and readout. Spin-echo coherence times, which we determine via both optical and microwave control, are in the tens of microseconds and likely limited by nuclear spins in natural isotopic ZnO. These characteristics are all extracted from measurements on an ensemble of Ga donors in a commercial substrate. We will present ongoing efforts and challenges toward single donor isolation in nanowires, dual optical-microwave control of donors and two-laser coherent spin-spectroscopy in this emerging qubit platform.

[1] X. Linpeng, M.L.K Viitaniemi, A. Vishnuradhan, Y. Kozuka, C.Johnson, M. Kawasaki, K.-M.C. Fu, Phys. Rev. Applied 10, 064061 (2018)