Symposium NM03—Topological and Quantum Phenomena in Intermetallic Compounds and Heterostructures

With the steady rise of topological materials, attention of the community is now shifting to magnetic incarnations that host a largely unexplored territory from both a theoretical and experimental perspective. Magnetic topological materials allow for the formation of new
structures (as the spin can introduce new periodicities) and are yet to be mapped out in generality and resultantly harbor a lot of potential for novel effects. Hence, having a stable material platform is of tremendous interest to underpin both theory and experimental progress in this direction. So far, several materials are reported as magnetic Weyl semimetal though a material with a single pair of Weyl points that readily offers tuning of its topological states is yet to be found. EuCd2As2 is a magnetic semimetal that has the potential of manifesting nontrivial electronic states, depending on its low temperature magnetic ordering. By discovering and taking advantage of the chemical tunability of EuCd2As2, we report the successful growths of single crystals of EuCd2As2 with the ferromagnetic and antiferromagnetic ground state. In addition, we will discuss a detailed temperature-dependent Angle-resolved photoemission spectroscopy (ARPES) study on the ferromagnetic EuCd2As2.
This work was supported by the Center for Advancement of Topological semimetals, an Energy Frontier Research Center funded by the U.S.DOE, Office of Basic Energy Sciences. Work at the Ames Laboratory was supported by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, Materials Sciences and Engineering Division. The Ames Laboratory is operated for the U.S. DOE by Iowa State University under Contract No. DEAC0207CH11358.

Novel physics discoveries heavily rely on the design and synthesis of new materials, which, in turn, depends a lot on the growth method. I will use the backdrop of my group’s discoveries of several Kondo systems, to emphasize the importance of the interplay between chemistry, materials science and physics in unveiling new materials’ properties. In particular, I will focus on YbT3M7 where the transition metal T is either Rh or Ir, and the metal M is Si or Ge. These are rhombohedral compounds, chemically and structurally very similar, with some similar physical properties and substantive differences. They all show evidence for strong electronic correlations and Kondo physics; one orders ferromagnetically (YbIr3Ge7) while the other two have antiferromagnetic ground states. Most remarkable, YbIr3Si7 shows long range antiferromagnetic order and non-Fermi liquid behavior even within the ordered state, while the electrical resistivity indicates bulk insulating behavior with ARPES and DFT calculations pointing to a conductive surface. We reconcile the observed Kondo physics, long range magnetic order and the insulating-to-metal crossover from bulk to surface with a proposal of Kondo exhaustion and evience for a Yb valence transition from magnetic (3+ in the bulk) to non-magnetic (2+ on the surface).

Asymmetric Weyl semimetals, which possess an inherently chiral structure, have different energies and dispersion relations for left- and right-handed fermions. They exhibit certain effects not found in symmetric Weyl semimetals, such as the quantized circular photogalvanic effect and the helical magnetic effect. In this work, we derive the conditions required for breaking chiral symmetry by applying an external field in symmetric Weyl semimetals. We explicitly demonstrate that in certain materials with the T_d point group, magnetic fields along low symmetry directions break the symmetry between left- and right-handed fermions; the symmetry breaking can be tuned by changing the direction and magnitude of the magnetic field. In some cases, we find an imbalance between the number of type I left- and right-handed Weyl cones (which is compensated by the number of type II cones of each chirality.)

Ref arXiv:2011.00970

In this work, we study the non-dissipative viscoelastic response of three dimensional crystals. We show that for systems with tetrahedral symmetries, there exist new, intrinsically three-dimensional Hall viscosity coefficients that cannot be obtained via a reduction to a quasi-two-dimensional system. To study these coefficients, we specialize to a tight binding model for a chiral magentic metal inspace group P2₁3 (198), which features a threefold degenerate “spin-1” fermion at the Fermi level. Using the Kubo formula for viscosity, we compute the non-dissipative Hall viscosity for the spin-1 fermion in two ways. First we use an electron-phonon coupling ansatz to derive the “phonon” strain coupling and associated phonon Hall viscosity. Second we use a momentum continuity equation to derive the viscosity corresponding to the conserved momentum density. We conclude by discussing the implication of our results for hydrodynamic transport in three-dimensional magnetic metals,and discuss some candidate materials in which these effects may be observed.

Since the discovery of the existence and manipulation possibilities of individual nanoscale skyrmions in ultrathin transition metal films on heavy-element substrates [1] by spin-polarized scanning tunneling microscopy (SP-STM) [2], the field of magnetic skyrmions [3,4] has gained significant interest due to their great potential for future magnetic memory and logic devices [5,6]. In particular, the small size, enhanced stability and unidirectional current-driven movement of chiral magnetic skyrmions make them interesting for novel types of magnetic device concepts. These unique properties of skyrmions are intimately linked to interfacial Dzyaloshinskii-Moriya (DM) interactions [7,8] which recently have been studied directly at the level of individual magnetic atoms on surfaces of heavy-element substrates [9]. Based on these fundamental investigations of the distance- and material dependencies of interfacial DM interactions, it has become possible to design non-collinear spin textures in low-dimensional systems based on the powerful combination of SP-STM and single-atom manipulation techniques [10,11]. Proximity-coupling of such well-defined low-dimensional non-collinear spin-states with elemental superconductors allows the design of yet another type of interesting topological states, i.e. Majorana zero modes, which offer great potential for topological quantum computation [11,12]. Besides the emergence of Majorana states in quasi-1D hybrid systems consisting of spin-spirals interacting with elemental superconductors, we will also discuss most recent progress towards the observation of Majorana states in quasi-2D skyrmion-superconductor hybrid systems [13].

References
[1] N. Romming et al., Science 341, 6146 (2013).
[2] R. Wiesendanger, Rev. Mod. Phys. 81, 1495 (2009).
[3] A.N. Bogdanov and Ch. Panagopoulos, Nature Reviews Physics 2, 492 (2020).
[4] A.N. Bogdanov and Ch. Panagopoulos, Physics Today 73(3), 44 (2020).
[5] R. Wiesendanger, Nature Reviews Materials 1, 16044 (2016).
[6] A. Fert, N. Reyren, V. Cros, Nature Reviews Materials 2, 17031 (2017).
[7] M. Bode et al., Nature 447, 190 (2007).
[8] S. Heinze et al., Nature Physics 7, 713 (2011).
[9] A. A. Khajetoorians et al., Nature Commun. 7, 10620 (2016).
[10] M. Steinbrecher et al., Nature Commun. 9, 2853 (2018).
[11] H. Kim et al., Science Advances 4, eaar5251 (2018).
[12] A. Palacio-Morales et al., Science Advances 5, eaav6600 (2019).
[13] A. Kubetzka et al., Phys. Rev. Mater. 4, 081401(R) (2020).

Magnetic domain structures are fundamentally altered by the presence of the Dzyaloshinskii-Moriya interaction (DMI), which introduces a preference for the twisting of neighboring spins. This produces various chiral spin textures including topological skyrmion bubbles, helical phases, and chiral domain walls. Two important directions for the field are to (1) explore and visualize the complex magnetic phases down to the atomic scale and (2) develop small skyrmions at room temperature for potential high-density memory applications. For this, we are investigating skyrmions and chiral spin textures in thin films of FeGe, MnGe, and Fe-rich FeGe grown by molecular beam epitaxy. These materials have a B20 structure with broken inversion symmetry to generate a bulk-like DMI, which yields skyrmions that are often studied by the topological Hall effect or other macroscopic probes. In our work, we visualize the spin textures from the submicron-scale down to the atomic-scale using spin-polarized scanning tunneling microscopy, magnetic force microscopy, and Lorentz transmission electron microscopy. I will discuss our team’s advances in atomic-layer engineering for FeGe to boost the Curie temperature above room temperature while shrinking the skyrmion size down to < 20 nm, the atomic-scale visualization of helical spin textures with topological defects in MnGe, and initial work toward skyrmions in epitaxial 2D magnets.

The work was done in collaboration with Tao Liu, Camelia Selcu, Jake Repicky, Jay Gupta, David McComb, Mohit Randeria, Prasanna Balachandran, Nuria Bagues Salguero, Binbin Wang, Po-Kuan Wu, Shuyu Cheng, Tim Hartnett, Denis Pelekhov, Perry Corbett, Brendan McCullian, Adam Ahmed, Chris Hammel.

Topology a mathematical concept became recently a hot topic in condensed matter physics and materials science, a complete classification of all non-magnetic inorganic materials are availbale [1,2]. Recently the focus have shifted to magnetic materials, first antiferromagnetic materials are classified, too [3]. In magnetic materials the Berry curvature in real and reciprocal space lead to new topological properties such as Skyrmions [4] and Antiskyrmions [5] and in magnetic Weyl semimetals to giant responses in antiferromagnetic [6] and ferromagnetic compounds [7]. Heusler compounds as tunable materials are interesting for both categories, they might even allow for the investigation the relation between Berry curvatures in real and reciprocal space. In materials hosting Antikyrmions the Dzyaloshinskii Moriya-Exchange and the dipol-dipol interaction is important. The dipol-dipol interactions allows for interesting mesoscopic effects. New materials can be designed by changing the symmetry, magnetic moments (ferro, ferri, antiferro, compensated ferri) and magnetic crystalline anisotropy [8].

[1] Bradlyn et al., Nature 547, 298, (2017)
[2] Vergniory, etal., Nature 566, 480 (2019)
[3] Xu, et al., Nature (2020) accepted, preprint arXiv:2003.00012
[4] Mühlbauer, et al. Science 323, 915 (2009)
[5] Nayak, et al., Nature 548, 561 (2017)
[6] Nayak, et al., Science Advances 2 e1501870 (2016)
[7] Liu, et al. Nature Physics 14, 1125 (2018)
[8] Manna, et al., Nature Reviews Materials 3, 244 (2018)

Heusler compounds are a large family of compounds which exhibit a wide range of properties. The inverse tetragonal Heusler compounds exhibit unidirectional magnetic anisotropy. Some of these compounds, in particular those containing heavy elements, exhibit non-collinear spin structures, that are derived from a Dzyaloshinskii-Moriya (DMI) vector exchange interaction. We show that such materials can exhibit a range of spin textures including both anti-skyrmions and elliptical Bloch skyrmions in the same compound, as well as helical spin textures that propagate along particular crystal directions. Another fascinating aspect of these structures is that the size of the anti-skyrmion and the wavelength of the helices can be varied from sub 100 nm to more than a micron by varying the thickness of the lamella in which the structures are observed. This is due to the important role of long-range magneto-dipole interactions that are more much important in these Heusler compounds than, for example, in the widely studied cubic B20 compounds in which skyrmions have been extensively explored. Chiral spin textures in ferro-, ferri- and anti-ferrimagnetic materials and thin film heterostructures are of fundamental interest with great potential for spintronic applications especially Racetrack Memory.

Two-dimensional (2d) nano-electronics, plasmonics, and emergent phases require clean and local charge control, calling for layered, crystalline acceptors or donors. Here I will describe how the Relativistic Mott Insulating state of RuCl3 provides a new opportunity to introduce modulation doping into 2D materials. Specifically, we demonstrate and optimize this charge transfer with extensive Raman, photovoltage, and electrical conductance measurements combined with ab initio calculations. Also, we find the doping is exceptionally local, can occur through hBN, works with various exfoliated, CVD, and MBE materials. Time permitting, I will discuss new opportunities this opens for nanoplasmonic, optoelectronics, and correlated phases.

Transition metal-based carbides that belong to the family of MAX phases are an intriguing class of materials – particularly in terms of their mechanical properties – as they combine characteristics of metals and ceramics. More recently, some of their members have also been discussed in terms of their magnetic behavior, whereas most studies are conducted on thin film samples and the synthesis of bulk materials with later transition metals (e.g. Mn) is still a real challenge. Besides, MAX phases are used as precursors for a relatively new class of 2D materials, the so-called MXenes that are investigated in the context of a plethora of potential applications, for example in catalysis, battery and biomedical research.
Our group utilizes a diverse set of synthesis techniques to prepare new MAX phases as well as known ones with unique morphologies. In this talk, I will focus on the following two examples: (i) The synthesis of MAX phase Cr₂GaC by an initially wet chemical approach that allows a new type of processability of the soluble precursors. (ii) Design of a “smart” hybrid MXene with switchable conductivity and temperature as an external stimulus. We employ X-ray powder diffraction and electron microscopy techniques to study the structure and microstructure of the products, respectively. In the case of the wet chemical-based synthesis of Cr₂GaC, we conducted detailed thermal analysis and ex-situ Neutron diffraction to propose a formation mechanism of the MAX phase particles.

Ternary ABC intermetallic compounds exhibit a rich variety of crystal structures and electronic properties. In this work, we study the structural energetics and band structures of real and hypothetical ABC intermetallic phases with structures obtained by stacking binary honeycomb layers with single layers of interstitial atoms in various ways, using first principles calculations to determine the structural parameters and the bands in each phase. We use this dataset to analyze and model the bands near the Fermi level to classify the systems considered and to extract a set of rules that allows us to to predict and design hexagonal ABC intermetallic materials with targeted transport and optical properties, connecting to experimental measurements on known hexagonal ABC phases. in particular, we investigate the rich physics of polar metals in this family of materials, which offer the promise of functional properties switchable by appropriate applied fields and stresses.

We investigate the anomalous thermoelectric transport in bilayer WSe₂ with broken inversion symmetry, due to a gate electric field, regardless of time-reversal symmetry. We compare the cases in which the spin-orbit coupling (SOC) is absent or present. In the presence of SOC and of a valley-contrasting Berry curvature, anomalous spin and valley Nernst responses are generated. The Nernst signals exhibit peaks and dips, as the chemical potential is varied, that have the signs of the Berry curvatures of the bands and are proportional to their magnitudes. In the absence of SOC but with an electric field present, the Nernst responses are the same in the conduction and valence bands due to particle-hole symmetry. The anomalous valley Nernst coefficient is enhanced by increasing the electric field strength. When time-reversal symmetry is violated, e.g., upon using an insulating magnetic substrate, the total Nernst coefficient is finite and exhibits a dip-peak feature. We also analyze the orbital magnetization and the orbital magnetic moment. In the absence of a gate electric field the magnetization vanishes due to the spin degeneracy of the bands. In the presence of electric field, the magnetization and its two contributions, one due to the magnetic moment and one due to the Berry curvature, are calculated and interpreted in terms of opposite circulating currents of the bands in the two layers. The results are pertinent to other transition metal dichalcogenides and future caloritronic applications.

Kagomé intermetallics with 2D lattices have served as an ideal platform to study the exotic quantum phenomenon associated with flat bands and Dirac-type dispersions. It is essential to explore the electronic properties of these structures for the experimental realization of 2D kagomé lattice in bulk materials. We explore a specific family of kagomé intermetallics with the formula MT₆X₆ compound where M = Mg, Lu, Y; T = Co, Fe, Cr; and X = Ge. Single crystals of MgCo₆Ge₆ were grown by laser Bridgman technique. X-ray precession images and electron diffraction measurements collected at T = 293(2) K show that the compound crystallizes in space group P6/mmm, with a = 5.06094(15) Å, c = 7.7271(2) Å. Residual electron maps provide evidence of columnar disorder along c axis while no defects were found in the electronic structures of flux-grown LuFe₆Ge₆ and YCr₆Ge₆ crystals. Further investigation of the crystal structure reveals spontaneous twisting of the Co kagomé layers in MgCo₆Ge₆. We look into crystal orbital hamiltonian population analysis to understand the cooperative twisting between layers within the Co-kagomé network and the interlayer tetragonal bonding. Despite the appearance of static tilting, no evidence of a phase transition was found in magnetization, resistivity, or specific heat measurements, implying that the twisting exists at all temperatures, but is thermally fluctuating at room temperature. This behavior is further supported by looking into related materials to understand the pairwise twisting and bonding in layered structures.
Funding:
This work was supported by the David and Lucile Packard Foundation. HKV, VJS and TTT acknowledge support of the Institute for Quantum Matter, an Energy Frontier Research Center funded by the United States Department of Energy, Office of Science, Office of Basic Energy Sciences, under Award DE-SC0019331. MKS, LAP, TB, and WAP acknowledge support of the Platform for the Accelerated Realization, Analysis, and Discovery of Interface Materials (DMR-1539918), a National Science Foundation Materials Innovation Platform. ZW, IJ and MC acknowledge support of Whiting School of Engineering, the Johns Hopkins University, and the NSF (NSF NSF-DMR-1804320). CW acknowledges the support of Hopkins Extreme Materials Institute (HEMI). Access to the Bruker 1172 instrument was also possible via the Hopkins Extreme Materials Institute (HEMI).

A characteristic feature of quantum materials is the presence of various lattice degrees of freedom manifesting themselves as local structural distortions leading to competing ground state phases of the electronic system and exotic behavior. More often than not, the distortions are not well expressed and/or perfectly periodic, making it difficult to identify and quantify them using traditional crystallographic techniques. We will demonstrate the advantages of total x-ray scattering and large-scale structure modeling in studying lattice instabilities in archetypal quantum materials such as Ta-based dichalcogenides. In particular, we will show that the low-temperature charge density wave state in trigonal prismatic 2H-TaSe2 emerges via a gradual buildup of locally correlated clusters of Ta atoms, and not via a spontaneous emerging of Ta superstructure at the transition temperature [1]. We will also show the presence of a hierarchical relationship among the crystal lattice, charge density wave and superconducting orders in ternary Ta-Te-Se solid solutions, where different degrees of crystal lattice order-disorder appear to promote and maintain the different orders to a different extent. The relationship may well explain the observed irregular evolution of the superconducting transition temperature with the relative Te to Se ratio [2].

1. V. Petkov et al. Phys. Rev. B 101, 121114(R) (2020).
2. V. Petkov et al. Phys. Rev. B 102, 134119 (2020).

Materials by design is the rational prediction and creation of functional materials with defined properties. Its goal is to meet current and future societal needs for better or more complex materials, from biocompatible materials in medicine to lightweight alloys for space applications and energy generation, storage, and transport. Unfortunately the chemistry underlying modern materials science and engineering has lagged other sub-fields in an extremely critical area: the ability to selectively make and break bonds in the solid state. This is due to limited synthetic methodology and method development. True materials by design cannot be achieved until reliable synthetic capabilities are developed that can actually produce the specified materials. In this talk, I will highlight the progress being made in such synthesis by design, with a particular focus on intermetallic quantum materials necessary to realize new electronic and magnetic states of matter including topological superconductivity and axion insulators. Efforts to build and maintain US leadership in the area of new materials discovery and synthesis – particularly via the JHU Institute for Quantum Matter and PARADIM, the Platform for Accelerated Realization, Analysis, and Discovery of Interface Materials (PARADIM), an NSF-MIP National User Facility, and opportunities for use of these capabilities by the community, will also be presented.

The room temperature ferromagnetic phase of the cubic antiperovskite Mn₃ZnC is known from from first-principles calculation to be a nodal line Weyl semimetal. Features in the electronic structure that are the hallmark of a nodal line Weyl state—a large density of linear band crossings near the Fermi level—can also be interpreted as signatures of a structural and/or magnetic instability. We examine this system carefully, and use it as a jumping off point to understand the possible implications of Weyl-like features in the electronic structures, and Peierls-like structural distortions and related structural and magnetic instabilities.

Dirac materials have recently been one of the most significant attractions of the scientific community. We present an ab initio study of electronics properties of IrGa and RhGa compounds, with the absence and presence of spin-orbit interaction using first-principles calculations. Linearly dispersed band crossings, which are characteristic of topological semimetals, were identified near Fermi energy. These include type I and II Dirac points and nodal lines. Additionally, the sensitivity of Dirac points to physical pressure was studied by applying compressive and tensile stress to the lattice axis. The unusual electronic structure of IrGa will be useful to search for novel Dirac Fermions and it is a good candidate for further research and experimental studies.

We theoretically study magnetism and superconductivity in UTe2, which is a recently discovered strong candidate for an odd-parity spin-triplet superconductor. Theoretical studies for this compound faced difficulty because first-principles calculations predict an insulating electronic state, incompatible with superconducting instability. To overcome this problem, we take into account electron correlation effects by a GGA+U method and show the insulator-metal transition by Coulomb interaction. Using Fermi surfaces obtained as a function of U, we clarify the topological properties of possible superconducting states. Fermi surface formulas for the three-dimensional winding number and three two-dimensional Z2 numbers indicate topological superconductivity at an intermediate U for all the odd-parity pairing symmetry in the Immm space group. Symmetry and topology of superconducting gap nodes are analyzed, and the gap structure of UTe2 is predicted. Topologically protected low-energy excitations are highlighted, and experiments by bulk and surface probes are proposed to link Fermi surfaces and pairing symmetry. We show that a recent ARPES experiment is consistent with the topological superconductivity.

Next, we provide and analyze a periodic Anderson model for UTe2. The 24-band tight-binding model reproduces the band structure obtained from a GGA+U calculation consistent with an ARPES experiment. The Coulomb interaction of f-electrons enhances Ising ferromagnetic fluctuation along the a-axis and stabilizes the spin-triplet superconductivity of either B3u or Au symmetry. When effects of pressure are taken into account in hopping integrals, the magnetic fluctuation changes to an antiferromagnetic one, and accordingly, the spin-singlet superconductivity of Ag symmetry is stabilized. Based on the results, we propose pressure-temperature and magnetic field-temperature phase diagrams revealing multiple superconducting phases. Interestingly, a mixed-parity superconducting state with spontaneous parity violation is predicted.

A fundamental property of a superconductor is its response to an applied magnetic field. In this talk, I will discuss how we use scanning superconducting quantum interferences device (SQUID) microscopy to study the local magnetic response in two different types of superconductors. First, I will discuss measurements on focus ion beam defined microstructures fabricated from single crystals of the heavy-fermion superconductor CeIrIn₅. By imaging the local diamagnetic response, we observe that the superconducting transition temperature, T_c, varies throughout the structure in a complex pattern. This pattern arises due to the interplay of a non-trivial strain field from the differential thermal contraction of the substrate and microstructure and the sensitivity of T_c in CeIrIn₅ to the strength and direction of strain. Devices with different geometry show that the spatial modulation of T_c can be tailored in agreement with predictions based on finite element simulations. These results offer a new approach to manipulate strain-sensitive electronic order on micrometer length scales in strongly correlated matter. Second, I will show how we use scanning SQUID to perform local magnetic measurements on few-layer van der Waals superconductors. We can directly probe the diamagnetic response as a function of temperature and other tuning parameters despite the extremely small sample volume. I will discuss how we can extract the superfluid stiffness and other characteristics of the superconducting state from our measurements.


Imaging of CeIrIn₅ microstructures was primarily supported by the Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering, under Award DE-SC0015947. Measurements of van der Waals superconductors were supported by the Cornell Center for Materials Research with funding from the NSF MRSEC program (DMR-1719875) and the NSF (DMR-2004864).

Spin triplet superconductivity is found in UTe₂ below 1.6 K. Remarkably, it coexists with strong spin fluctuations, has an extremely high upper critical field of 35 T, and breaks time reversal symmetry. I will describe how superconductivity in UTe₂ differs from conventional superconductivity and I will present evidence that this phase is topologically nontrivial. I will also discuss how in UTe₂ large magnetic fields give rise to a new, reentrant, superconducting phase, between 40 T and 65 T, the highest values reported for any material.

We report magnetic imaging of Cr-doped (Bi,Sb)₂Te₃ heterostructures in the quantum anomalous hall regime. We used a scanning superconducting quantum interference device (SQUID) microscope with micrometer scale spatial resolution to image the magnetic fields above current biased devices. Using these images we reconstructed the current density, allowing us to visualize where current flows in the devices. We also used top and back gates to study how the magnetization is affected by electrostatic gating. By performing these measurements as a function of current bias, gate voltage and magnetization direction we construct a comprehensive picture of electronic transport in our devices.

Double Dirac materials are a topological phase of matter that have an unprecedented eightfold electronic degeneracy. They display a host of exotic charge transport properties that are strongly impacted by magnetic fields. Quantum magnets are known to exhibit a variety of magnetic phases of matter with emergent quasiparticles. Here we report the discovery of quantum magnetism in the Dirac material EuPd₃S₄. The zero-field magnetic structure is found to be a 2-sublattice antiferromagnetic state with no magnetic coupling between sublattices at the mean-field level that results in an emergent degree of freedom corresponding to the relative orientation of the sublattice magnetizations. Application of a magnetic field drives domain ordering before collapsing to a fully ferromagnetic state. These magnetic states are found to dramatically impact the electronic structure of EuPd₃S₄, resulting in the destruction of the 8-fold degenerate state and formation of mostly likely Weyl states. Our findings open the door to the experimental and theoretical explanation of the interplay between novel magnetic states and topological phases of matter.

Multiple new forms of unconventional superconductivity have been discovered over the past four decades. In most cases, the strong electronic correlations and the interplay between superconductivity and other phases (magnetic ones, among others) leads to understanding that the simple frame of electron-phonon superconductivity breaks down. Because this implies that complex phase diagrams with different superconducting states are expected, it is striking that many materials only show a single-component superconducting phase diagram. Here, we report the discovery of two-phase unconventional superconductivity in CeRh₂As₂ with a transition temperature of 0.26 K. Using thermodynamic and magnetic probes, we establish that the superconducting critical field is as high as 14 T for magnetic fields along the c-axis. Furthermore, a c-axis field drives a transition between two different superconducting states. In spite of the fact that CeRh₂As₂ is globally centrosymmetric, we show that local inversion-symmetry breaking at the Ce sites enables Rashba spin-orbit coupling to play a key role in the underlying physics. More detailed analysis identifies the transition from the low- to high-field states to be associated with one between states of even and odd parity.

Single-crystalline membranes of functional materials enable the tuning of properties via extreme strain states; however, conventional routes for producing membranes require the use of sacrificial layers and chemical etchants, which can both damage and limit the ability to make membranes ultrathin. Here we demonstrate the epitaxial growth of the cubic Heusler compound GdPtSb on graphene-terminated Al2O3 substrates. The weak Van der Waals interactions of graphene enable the mechanical exfoliation to yield free-standing GdPtSb membranes. Despite the presence of the graphene interlayer, the Heusler films have epitaxial registry to the underlying sapphire, as revealed by x-ray diffraction, reflection high energy electron diffraction, and transmission electron microscopy. Whereas unstrained GdPtSb is antiferromagnetic, we show that the large strains or strain gradients in rippled membranes induce a spontaneous magnetic moment at room temperature, with a saturation magnetization of 5.2 bohr magneton per Gd atom, approaching the ~ 7 bohr magneton limit expected for ferromagnetic ordering. Our membranes provide a novel platform for tuning the magnetic properties of intermetallic compounds via strain (piezomagnetixm and magnetostriction) and strain gradients (flexomagnetism).

Magnetic semiconductors are attractive for memory devices and spintronic applications [1]. However, the practical utility of dilute magnetic semiconductors (DMS) such as Mn: GaAs, albeit having all key ingredients for these applications, are hindered due to low Curie temperature. On the other hand, narrow bandgap semiconductors often make excellent thermoelectric candidates. A conventional strategy for improving the thermoelectric figure of merit (ZT) is to decrease the phonon thermal conductivity via nano-structuring [2]. Recently, it has been suggested that magnon drag (spin wave) [3], spin fluctuations [4], and enhanced effective mass [5] are alternative strategies to enhance thermoelectric performance by enhancing the power factor. Here we demonstrate the controllable synthesis of a ferromagnet/semiconductor nanostructured system: Fe nanoparticles embedded epitaxially within a semiconducting FeVSb matrix. Transmission electron microscopy confirms that the nanoparticle formation is mediated by bulk segregation for all composition greater than 10% excess Fe in stoichiometric FeVSb. Our Fe: FeVSb thin films, grown by molecular beam epitaxy, display magnetic moment and anomalous Hall effect that scale with the volume fraction of Fe nanoparticles. The parent stoichiometric FeVSb is predicted to be diamagnetic, but our stoichiometric films also exhibits magnetic signatures in transport and magnetometry at 300K. This suggests that within the solubility limit of Fe, Fe_1+δVSb, where δ is the bound of solubility, is a potential dilute magnetic semiconductor with Curie temperature greater than room temperature. Additionally, the ferromagnetic nanoparticles can induce spin polarized charge carriers in the semiconducting matrix due to proximity effect, making this highly controllable and tunable system appealing for spintronics. Our angle-resolved photoemission spectroscopy (ARPES) measurements on the parent semiconductor FeVSb reveal an enhanced effective mass due to electronic correlations [6]. The combination of nanostructure precipitates, magnetic interaction, and electronic correlation in this highly tunable heterostructure presented here offers a promising route for new thermoelectric application for practical waste heat recovery.

References:

[1] Y. Ohno, D. K. Young, B. Beschoten, F. Matsukura, H. Ohno and D. D. Awschalom, Nature (London) 402, 790 (1999).
[2] H. Scherrer and S. Scherrer, in Handbook of Thermoelectrics, edited by D. M. Rowe (CRC Press, New York, 1994), pp. 211– 237.
[3] M. V. Costache, G. Bridoux, I. Neumann and S. O. Valenzuela, Magnon-drag thermopile, Nat. Mater. 11 (2012).
[4] N. Tsujii, A. Nishide, J. Hayakawa, and T. Mori, Science advances 5, eaat5935 (2019).
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[6] E. H. Shourov et al, arXiv preprint, arXiv:2009.11489 (2020).

Polar materials can host a variety of topologically significant magnetic phases, which often emerge from a modulated magnetic ground state. Relatively few noncentrosymmetric tetragonal materials have been shown to host topological spin textures and new candidate materials are necessary to expand the current theoretical models. This presentation will discuss the anisotropic magnetism in the polar, tetragonal material NdCoGe₃ via thermodynamic and neutron diffraction measurements. The H-T phase diagram shows several magnetic field-induced phases with an applied field in the plane. Neutron diffraction data reveal that NdCoGe₃ hosts complicated magnetic order derived from modulated magnetic moments, with the ground state characterized by the propagation vector k = (0.494, 0.0044, 0.385) at 1.8 K.

In recent years, much efforts have been made towards the discovery of topological superconductors. Topological superconductivity can be artificially engineered in hybrid structures combining topological materials with conventional superconductors, or it can exist intrinsically in certain unconventional superconductors. Odd-parity superconductors are candidates for intrinsic topological superconductivity because topologically non-trivial gap functions naturally show up in these materials. These superconductors can be discovered in the proximity of magnetic instabilities such as in the superconducting ferromagnets, or in structural families that lack inversion symmetry. In this presentation, I will present our experimental and computational results on a new superconductor that breaks-time reversal symmetry because of its intrinsic topological properties. The normal state band structure includes Dirac lines and a Dirac loop at the Fermi level resulting from non-symmorphic symmetry operations, as well as Dirac points robust against splitting from SOC. The topology of the Fermi surface provides a platform for inter-band pairing and time-reversal symmetry breaking superconductivity. Our results illustrate a new route to realize spin-triplet superconductivity in a centro-symmetric structure without the need for a nearby magnetic instability.

The Weyl points with opposite chiralities can be coupled by charge density wave (CDW) resulting in axion quasiparticle in condensed matter physics ^{1, 2}. Recently, the Weyl semimetal Ta₂Se₈I was reported to exhibit axions in the collective mode of CDW, the sliding CDW which is detected with nonlinear V-I curves ³. Here we investigate the electrical transport property and crystal structure under pressure. Upon applying pressure, the axionic CDW is suppressed, which is manifested by the decrease of transition temperature and corresponding single particle energy gap. A superconducting transition appears at the vicinity of the complete suppression of the CDW. The superconducting T_c is enhanced monotonically to 4.5 K with pressure up to 47 GPa. No main structural transition is detected from Raman spectra and synchrotron XRD patterns. However, partial amorphization is observed in which the stretching mode of Se-Se survives, and Ta₂Se₈I evolves into superconducting state at low temperature indicating that the TaSe₄ chains may play an important role in the pressure-induced superconductivity. Our investigations construct a complete phase diagram depicting the evolution of anionic CDW and superconductivity with respect to pressure, giving further understanding of correlated topological states.

1. Li R, Wang J, Qi X L, Zhang S C. Dynamical axion field in topological magnetic insulators. Nature Physics 6, 284-288 (2010).
2. Wang Z, Zhang S C. Chiral anomaly, charge density waves, and axion strings from Weyl semimetals. Physical Review B 87, (2013).
3. Gooth J, et al. Axionic charge-density wave in the Weyl semimetal (TaSe₄)₂I. Nature, (2019).

The layered antiferromagnetic MnBi₂Te₄ films have been proposed to be an intrinsic quantum anomalous Hall (QAH) insulator with a large gap. It is crucial to open a magnetic gap of surface states. However, recent experiments have observed gapless surface states, indicating the absence of out-of-plane surface magnetism, and thus, the quantized Hall resistance can only be achieved at the magnetic field above 6 T. We propose to induce out-of-plane surface magnetism of MnBi₂Te₄ films via the magnetic proximity with magnetic insulator CrI₃. A strong exchange bias of ∼40 meV originates from the long Cr-e_g orbital tails that hybridize strongly with Te p orbitals. By stabilizing surface magnetism, the QAH effect can be realized in the MnBi₂Te₄/CrI₃ heterostructure. Moreover, the high–Chern number QAH state can be achieved by controlling external electric gates. In addition, we have derived a simple electron-counting rule to predict the magnetic exchange coupling between general vdW layers for both homo- and hetero-vdW junctions.

Nonmagnetic topological materials have dominated the landscape of topological physics for the past two decades. These breakthroughs in nonmagnetic materials have not yet been matched by similar advances in magnetic compounds. Using magnetic band theory and topological indices obtained from Magnetic Topological Quantum Chemistry (MTQC), I will present a systematic way of identifying magnetic topological materials. I will then, focus on high order magnetic semimetals, and provide a topological classification of different fermions in these phases. Finally I will present new experimental realizations in materials. In particular I will focus on the pyrite compound CoS2, using complementary bulk- and surface-sensitive angleresolved photoelectron spectroscopy and ab-initio calculations we discovered Weyl-cones at the Fermi-level and we directly observed the topological Fermi-arc surface states that link the Weyl-nodes, which will influence the performance of CoS2 as a spin-injector by modifying its spin-polarization at interfaces.

Nowadays we know of many gapless electronic phases with conical bands, such as graphene, Dirac semimetals, and Weyl semimetals. Their low-energy excitations resemble truly relativistic particles.
To see those excitations, we have to capture the physics at a milli-electron-volt scale.
In our experiments, we access electronic structures at these low energies by combining Landau level spectroscopy, infrared spectroscopy, and effective Hamiltonian models.

Topological properties in quantum materials promise applications in lossless electronics and quantum information processing. Recently, dynamical control of the topological phase of the Weyl semimetal WTe₂ has been demonstrated, in which a terahertz-field induced charge current leads to interlayer shear strain that dynamically changes the Weyl nodes of the system [1]. Here, we theoretically describe an alternative route for topological phase control that is based on a nonlinear phonon coupling. The interlayer shear and breathing modes of layered topological materials couple to infrared-active phonon modes that can be resonantly excited with ultrashort terahertz pulses. The nonlinear coupling leads a transient distortion along the coordinate of the shear and breathing modes that in turn induce changes in the topological phase of the material. Using a combination of first-principles calculations and phenomenological modeling, we demonstrate that the Dirac and Weyl nodes in WTe₂ and ZrTe5 can be modulated through this excitation. Our results suggest that nonlinear phononic rectification of the crystal lattice is a powerful tool for dynamical control of topology in layered quantum materials.

[1] E. J. Sie et al., Nature 565, 61 (2019)
[2] D. M. Juraschek, N. B. Joseph, P. Narang, and A. Narayan, in preparation

This work is supported by the Swiss National Science Foundation (SNSF) under Project No. 184259, the DARPA DSO under the Driven Nonequilibrium Quantum Systems (DRINQS) program, Grant No. D18AC00014, and by the Department of Energy ‘Photonics at Thermodynamic Limits’ Energy Frontier Research Center under Grant No. DE-SC0019140. This research used resources of the National Energy Research Scientific Computing Center (NERSC) under Contract No. DE-AC02-05CH11231.

We derive symmetry indicator formulas for the filling anomaly on 2D square lattices with and without time reversal, inversion symmetry, or their product, in the presence of spin-orbit coupling. We go beyond previous work by considering lattices with atoms occupying multiple Wyckoff positions. We also provide an algorithm using the Smith normal form that systematizes the derivation.
The formulas determine the corner charge in 2D atomic or fragile topological insulators, as well as in 3D insulators and semimetals by studying their 2D slices. We apply our results to a 3D tight-binding model on a body-centered tetragonal lattice, whose projection into the 2D plane has two atoms in the unit cell. We apply these results to several antiperovskites as material examples.

Electrons organize in ways to give rise to distinct phases of matter such as insulators, metals, magnets or superconductors. In the last ten years or so, it has become increasingly clear that in addition to the symmetry-based classification of matter, topological consideration of wavefunctions plays a key role in determining distinct or new quantum phases of matter [see, for an introduction, Hasan & Kane, Reviews of Modern Physics 82, 3045 (2010)].
In this talk, I briefly introduce these new topological concepts in the context of their experimental realizations in magnetic materials. As examples, I present how tuning a topological insulator whose surface hosts an unpaired Dirac fermion can give rise to Weyl fermion (massless charged fermion) in nonmagnetic and magnetic semimetals with “fractional” Fermi surfaces, and in strongly correlated magnets such as the Heusler, kagome and related materials. These exotic topological matter harbor novel and unprecedented properties that may lead to the development of next generation quantum technologies including novel qubits.

The field of topological semimetals continues to reveal new insights. I will discuss recent developments starting with the classification of nodal fermions in both magnetic and non-magnetic space groups. I will then introduce higher order Fermi arcs as a bulk-edge correspondence for Dirac fermions, and discuss a refinement of the symmetry indicators that predict these hinge states. Finally, I will discuss some material predictions.

Interfaces and heterostructures with topological semimetals offer new opportunities to control and manipulate their unique electronic states and a wealth of associated phenomena, for example, via electric field effect, strain, or symmetry breaking. In this presentation, we will discuss recent progress in heterostructures of a prototype three-dimensional Dirac semimetal, cadmium arsenide (Cd₃As₂), which are grown by molecular beam epitaxy. We show that high-mobility, epitaxial Cd₃As₂ films can be grown in different crystallographic orientations. We discuss the nature of the quantum Hall effect of thin Cd₃As₂ films grown in different orientations and using different measurement geometries that uniquely identify the quantum Hall effect from single Dirac fermions on the surfaces. We will discuss other remarkable phenomena in the quantum limit of (001) films. We will also discuss pathway towards realizing novel topological systems.

Cd₃As₂ is an ideal system for investigating quantum transport in topological semimetals. In addition to its high electron mobility and long mean free path, its natural growth orientation is different from the rotational axis connecting the two Dirac nodes. This allows us to detect orbital motions in topological semimetal surfaces. We have successfully developed a growth technique realizing high mobility Cd₃As₂ films with excellent surface flatness, and first observed quantum Hall states induced by quantum confinement [1]. Related film techniques such as electric gating and chemical doping of Zn also enable systematic transport studies of the Dirac semimetal films, with controlling the bulk dimensionality [1, 2], Fermi energy [3, 4], and band topology [3, 4].
By carefully fabricating (Cd_1-xZn_x)₃As₂ films with three-dimensional and uniform thickness, we have observed surface quantum oscillations and their evolution into quantized states in films [5, 6]. This is evidenced by distinct differences in oscillation frequency, field angle dependence, and temperature change from the bulk ones. Moreover, we have found intrinsic coupling between two spatially-separated surface states in Weyl orbits by measuring dual-gate devices [7]. Independent scans of top- and back-gate voltages reveal concomitant modulation of doubly-degenerate quantum Hall states, which is not possible in conventional surface orbits as in topological insulators. Our results evidencing the unique spatial distribution of Weyl orbits provide new opportunities for controlling the novel quantized transport. In particular, fabrication of heterointerfaces for proximitizing the surface Fermi-arcs with ferromagnets [8] and superconductors will be promising, as well as another chemical doping of Sb for increasing band-inversion energy [9].

References
[1] M. Uchida et al., Nat. Commun. 8, 2274 (2017).
[2] Y. Nakazawa, M. Uchida et al., Sci. Rep. 8, 2244 (2018).
[3] S. Nishihaya, M. Uchida et al., Sci. Adv. 4, eaar5668 (2018).
[4] S. Nishihaya, M. Uchida et al., Phys. Rev. B 97, 245103 (2018).
[5] S. Nishihaya, M. Uchida et al., Nat. Commun. 10, 2564 (2019).
[6] Y. Nakazawa, M. Uchida et al., APL Mater. 7, 071109 (2019).
[7] S. Nishihaya, M. Uchida et al., submitted.
[8] M. Uchida et al., Phys. Rev. B 100, 245148 (2019).
[9] Y. Nakazawa, M. Uchida et al., Phys. Rev. B 103, 045109 (2021).

Controlling electronic properties via bandstructure engineering is at the heart of modern semiconductor devices. We have extended this concept to semimetals utilizing confined thin film geometries and hetero-epitaxial interfaces to engineer electronic structure in rare-earth monopnictide and half-Heusler semimetallic systems. In the case of the rare-earth monopnictide, LuSb, quantum confinement changes the carrier compensation and differentially affects the mobility of the electron and hole-like carriers resulting in a strong modification in its large, non-saturating magnetoresistance behavior. Bonding mismatch at the heteroepitaxial interface of the semi-metal (LuSb) and a semiconductor (GaSb) leads to the emergence of a novel, two-dimensional, interfacial hole gas and is accompanied by a charge transfer across the interface that provides additional avenues to modify the electronic structure and magnetotransport properties in the ultra-thin limit.

The prediction and observation of topological surface states in half-Heusler compounds raises exciting possibilities to realize exotic electronic states and novel devices by exploiting their multifunctional nature. However, their position with respect to the Fermi level and the high density of bulk carriers has made it difficult to detect them through transport measurements. Here, we introduce compensation doping in epitaxial PtLuSb thin films as an effective route to tune the chemical potential and simultaneously reduce the bulk carrier concentration by more than two orders of magnitude compared to the parent compound. Linear magnetoresistance is shown to appear as a precursor phase that transmutes into a quantum Hall phase arising from the topological surface states on further reduction of the coupling between the surface states and the bulk carriers. Our approach paves the way to both reveal and manipulate exotic properties of topological phases in Heusler compounds.

Intrinsic magnetic topological insulator MnBi₂Te₄ has been extensively studied due to the theoretical prediction that it will provide a platform for novel topological phases, such as the quantum anomalous Hall effects (QAHEs), the axion insulators (AIs), and the type-II magnetic Weyl semimetals depending on thickness. Recently, the QAHE and AI have been observed on MnBi2Te4 flakes with odd and even number of layers, respectively. However, further progress on this novel material is hindered by the difficulty in preparing its high-quality thin films with well-controlled composition and thickness. Here we report on the magnetic properties of MnBi₂Te₄ samples grown by molecular beam epitaxy (MBE) with various thicknesses. We observed two main results: magnetic phase transitions and the linear dependence between the Hall conductance and the magnetization. Both transport and SQUID measurements clearly show magnetic phase transitions between a ferromagnetic to an antiferromagnetic state and vice versa, which brings the possibility of hosting those new topological phases on MBE grown samples. In addition, we verify that the origin of the anomalous Hall effect of MnBi2Te4 is intrinsic, from its linear dependence of magnetization. Our results enable the realization of quantum anomalous Hall effects in the large area MnBi2Te4 films with thickness precisely controlled by MBE.

The Bi-chalcogenides are the most prominent topological materials to be studied to date. Alloying transition and alkali metals into the Bi-chalcogenides has enabled the discovery of topological superconductor candidates Sr-, Nb, and Cu- doped Bi₂Se₃. In this work, we synthesize Sr_xBi₂Se₃ thin film by molecular beam epitaxy on GaAs(111) substrates. While we do not succeed in obtaining superconductivity, our results provide insight on the mechanism by which Sr is alloyed into Bi₂Se₃. First, X-ray diffraction measurements indicate an increasing c-axis lattice parameter suggesting the formation of some Sr interstitials. Second, the mobility extracted from transport measurements and phonon linewidths extracted from Raman spectroscopy indicate that Sr likely alloys into the structure. Third, an increasing n-doping is seen with increasing Sr content. And lastly, a reduction of the electron coherence length with increasing Sr suggests that inelastic charged impurity or electron-electron scattering is enhanced. It is thus clear that Sr leads donor type impurities and more n-doping in Bi₂Se₃ despite. The thermodynamics of the MBE codeposition process seem to be generally unfavorable to the formation of Sr interstitials, thus making the synthesis of superconducting films highly challenging.

Topological nodal-line semimetals (TNLSMs) have attracted burgeoning attention because unprecedented quantum transport and magnetoelectric response originating from the Zak phase have been theoretically expected. Moreover, recent computational exploration has accelerated efforts to find magnetic TNLSM materials for controlling these topological phenomena by magnetic fields. In general, however, time-reversal symmetry breaking by magnetic ordering destroys robustness of the nodal lines, and thus only few magnetic TNLSM materials have been reported so far. In addition, quantum oscillations have never been observed in these magnetic topological nodal-line semimetals. Namely, discovery of a new magnetic TNLSM and fabrication of its high-mobility films have been strongly desired.
In order to introduce magnetic moments into typical nonmagnetic TNLSM CaSb₂, we prepare isostructural EuSb₂ single-crystalline films by molecular beam epitaxy (MBE). Our first-principles calculations demonstrate that EuSb₂ hosts topological nodal lines protected by nonsymmorphic symmetry even under antiferromagnetic ordering of the Eu²⁺ spins. An X-ray diffraction rocking curve is very sharp, ensuring high crystallinity of the obtained film. Observed Shubnikov-de Haas oscillations with multiple frequency components exhibit small effective masses and two-dimensional field-angle dependence even in a 250 nm thick film, suggesting possible contributions of surface states.
Our demonstration of the new magnetic TNLSM and first observation of quantum magnetotransport in its films will stimulate further investigation and control of exotic transport phenomena proposed for magnetic topological semimetals.