Symposium NM06—Manipulation and Detection of Physical Properties of Two-Dimensional Quantum Materials

Two-dimensional (2D) transition metal carbides and nitrides, known as MXenes, have shown great promise in applications such as electromagnetic interference shielding, electrode and current collector in energy storage devices, transparent conductive electrodes, and optoelectronics. These applications are enabled by MXenes’ high electronic conductivity, specifically Ti₃C₂T_x (where T_x stands for surface termination such as O, F, and Cl) with metallic conductivity reaching 20,000 S/cm. Benefiting from a large variety of transition metals, C and/or N in the X layers, and versatile surface terminations, MXenes provide for multi-level tuning of electronic and transport properties. In this talk, we show that electronic and transport properties of MXenes can be manipulated by careful tuning of their chemical compositions. For instance, having a mix of C and N in the X layer of Ti₃CNT_x leads to a lower electrical conductivity, carrier concentration, and mobility compared to Ti₃C₂T_x. Moreover, the work function of Ti₃C₂T_x thin films can be continuously tuned from ~4 to ~5 eV upon desorption of surface terminations via thermal annealing in vacuum. In addition, we also show that the intercalants, governing MXene interlayer spacing, play a crucial role in electronic transport properties in MXene free-standing films. This multi-level control of electronic properties is a unique advantage of MXenes compared to other 2D materials. This work is supported by the U.S. Department of Energy (DOE), Office of Science, Office of Basic Energy Sciences, grant #DESC0018618.

Optical spectroscopy is a powerful tool to study nanoscale materials, and in particular atomically thin 2D materials that exhibit extraordinarily strong light matter coupling in 2D materials. In this tutorial I will introduce the optical spectroscopy of 2D transition metal dichalcogenides that are layered semiconductors, and the concept of excitonic Rydberg states to non-invasively understand electronic processes in these materials. In particular I will discuss how excitonic states can be a probe of local strain, doping, dielectric screening and interfacial magnetic exchange. Technical aspects of micro-spectroscopy will be covered along with an overview of advanced capabilities available at national labs that can enable unique insights through correlated, multimodal measurements.

Pressure is known to control the physical properties of quantum materials. This tutorial will focus on the hydrostatic and uniaxial pressure spanning from general principles to pressure cells design. This includes hydrostaticity, pressure media, experimental techniques under pressure/strain. In particular, this tutorial covers the properties of quantum materials such as superconductors, fragile magnets, novel semimetals under pressure.

Many layered materials have been explored as electrocatalysts for various energy-relevant reactions, most notably for hydrogen evolution reaction (HER). This tutorial will provide basic information on electrochemically driven HER, such as the reaction pathways, mechanisms, and standard experimental set-ups, and present our current understanding of the active sites of layered materials, mostly focusing on transition metal dichalcogenides. The tutorial will discuss the effects of hydrogen adsorption energy, density of catalytically active sites, and catalyst-support interface on the overall activity of HER. Moreover, the tutorial will highlight in situ characterization techniques and HER studies based on nanoscale devices, which can provide detailed information on the effects of microstructure and transport properties on HER during reaction.

MnBi₂Te₄ and MnBi₄Te₇ are intrinsic antiferromagnetic topological insulators, offering a promising materials platform for realizing exotic topological quantum states. However, high densities of intrinsic defects in these materials not only cause bulk metallic conductivity, preventing the measurement of quantum transport in surface states, but may also affect magnetism and topological properties. In this paper, systematic density functional theory calculations reveal specific material chemistry and growth conditions that determine the defect formation and dopant incorporation in MnBi₂Te₄ and MnBi₄Te₇. The large strain induced by the internal heterostructure promotes the formation of large-size-mismatched antisite defects and substitutional dopants. Our results show that the abundance of antisite defects is responsible for the observed n-type metallic conductivity. We predict that a Te-rich growth condition should reduce the bulk free electron density, which is confirmed by experimental synthesis and transport measurements in MnBi₂Te₄. Furthermore, Na doping is proposed to be an effective acceptor dopant to pin the Fermi level within the bulk band gap to enable the observation of surface quantum transport.

M. -H. Du, J. -Q. Yan, V. R. Cooper, M. Eisenbach, Adv. Func. Mater. DOI: 10.1002/adfm.202006516.

A main thrust of condensed matter physics concerns the discovery of new electronic states in emerging quantum materials. One example is the rapidly expanding class of topological materials, which are posited to enable realization of non-abelian particles and topological quantum computing. In this talk, I will discuss how exotic electronic phenomena can arise from the interplay of topology and magnetism in epitaxially-grown topological insulator thin films. Our experiments employ low-temperature microwave impedance microscopy (MIM), which characterizes the local complex conductivity of a material, to directly image chiral edge modes and phase transitions in a magnetic topological insulator. Finally, I will outline how MIM could be used in the future to visualize and manipulate Majorana modes, an emerging platform for topological quantum information processing.

A discovery of unconventional superconductivity in a twisted bilayer graphene (tBLG) system has spurred intensive discussions about the importance of interlayer coupling in the tBLG. Even though the electronic valence band structure of this system has been thoroughly studied, much of its macroscale properties, such as conducting electron behavior and light-matter interactions remain uncertain. A 30° twisted bilayer graphene (30°-tBLG) is another material system of interest that was recently enabled experimentally. Here, I present the results of the investigations of the interlayer coupling effects in the twisted 30° bilayer graphene system using surface sensitive low energy electron microscopy (LEEM), micro-spot low energy electron diffraction (µLEED), x-ray photoemission electron microscopy (XPEEM), Raman scattering spectroscopy and scattering-type scanning near-field optical microscopy (sSNOM). Two types of samples were studied in detail – as-grown graphene structures on a copper foil and the graphene transferred to a TiO_x/SiO₂/Si substrate. Strong interlayer coupling between two individual graphene sheets rotated by 30^o was revealed by a sharp 12-fold symmetrical LEED pattern acquired in the LEEM microscope. Distinct electron reflectivity contrasts in the LEEM experiment observed between 30°-tBLG and AB-stacked bilayer graphene yielded differences in the conduction band electronic structure between these two systems. Enhancement of interlayer light scattering was observed by the Raman scattering spectroscopy. Most surprisingly, the plasmonic interaction was observed to be sharply quenched on 30°-tBLG, in contrast to enhanced plasmonic interaction observed on chemically similar, AB-stacked bilayer graphene. The plasmonic interactions and sharp contrast in the 30°-tBLG on TiO_x/SiO₂/Si was directly captured in real-space in the sSNOM measurements. We conclude that the 30°-tBLG interface features a unique interlayer resonant interaction in which the interlayer scattering is greatly enhanced. This enhanced interlayer scattering promotes nearly energy-free charge carrier exchange in between two twisted atomic planes.

This research used resources of the Center for Functional Nanomaterials and National Synchrotron Light Source II, which are U.S. DOE Office of Science Facilities, at Brookhaven National Laboratory under Contract No. DE-SC0012704.

The control of magnetic ordering and magnetic anisotropy in ultrathin complex oxide films plays a critical role in advancing the development of novel spin-based sensors, storage devices and magnetic tunnel junctions. Perovskite complex oxide interfaces provide an indeal platform for manipulating magnetic ground states due to the stronge coupling of spin, electronic, orbital and structural degrees of freedom at these interfaces. Using a combination temperature-dependent magnetometry and element-specific X-ray magnetic circular dichroism, we investigate the competition between epitaxial strain in interfacial magnetic exchange interactions on the magnetic anisotropy in ferromagnetic LaSrCrO3(LSCO)/LaSrMnO3(LSMO) heterostructures grown by molecular beam epitaxy. Here, we observe an in-plane easy axis for 2 unit cell LSCO/2 unit cell LSMO heterostructures independent of the substrate-induced epitaxial strain and an antiferromagnetic Mn-Cr exchange across the LSMO/LSCO interface. These results highlight the critical role of interfacial magnetic exchange interactions in tuning magnetic anisotropy at complex oxide interfaces.

*This work was support by the US National Science Foundation under Grant No. NSF DMR1751455

Two-dimensional (2-D) magnetic materials are of great interest for fundamental science and advanced applications while dilute magnetic semiconductors (DMS) have captured worldwide interest in the hope of combining magnetism with electronic properties. At the intersection of these fields, MnGaN-2D is discovered as an ultimately-thin, 2D-DMS material, consisting of a densely concentrated, hexagonal-like arrangement of Mn, Ga, and N, but with a crystal structure unique from its wurtzite GaN substrate. MnGaN-2D shows room-temperature ferromagnetism as demonstrated using spin-polarized scanning tunneling microscopy and as a function of applied magnetic field. This is also predicted by first-principles theoretical calculations which reveal the origins of the ferromagnetism through its highly spin-split and spin-polarized manganese states.[1]

The SP-STM results have been furthermore confirmed by SQUID magnetometry which reveals a high spin-polarization of ∼79% at room temperature and perpendicular magnetic anisotropy.[2] New results for this novel system also suggest the possibility of switching the magnetic anisotropy using strain. Theoretical calculations reveal sensitivity of the filled states magnetic Mn peak to compressive versus tensile strain. Spin-orbit coupling is included in the calculations, which indicate either out-of-plane or in-plane anisotropy, dependent on the type of strain.

Clear evidence for both compressive and tensile local lattice strains is found by detailed analysis of atomic resolution STM images which reveal a wide, non-Gaussian range of lattice spacings (-18% to + 18% variations), unlike normal materials. Furthermore, scanning tunneling spectroscopy measurements, showing the clear spin-polarized, filled states manganese peak, find fluctuations in the electronic position of this peak. The filled-states Mn peak energy is also found to fluctuate depending on lattice strain in the theoretical calculations, thus indicating a connection between electronic states and magnetic anisotropy in this RT ferromagnetic 2D layer.

[1] Yingqiao Ma, Abhijit V. Chinchore, Arthur R. Smith, María Andrea Barral, and Valeria Ferrari, A Two-Dimensional Manganese Gallium Nitride Surface Structure Showing Ferromagnetism at Room Temperature, Nano Letters, Vol. 18, p. 158 (2018)

[2] Yingqiao Ma, Diego Hunt, Kengyuan Meng, Tyler Erickson, Fengyuan Yang, María Andrea Barral, Valeria Ferrari, and Arthur R. Smith, Local strain-dependent electronic structure and perpendicular magnetic anisotropy of a MnGaN 2D magnetic monolayer, Physical Review Materials, Vol. 4, p. 064006 (2020)

In this presentation I will review our work on different 2D magnetic materials. I will mainly focus on tunnelling magnetoresistance measurements, in which thin layers of different semiconducting 2D magnetic materials are used as tunnel barrier, whose magnetoresistance exhibit new and interesting effects. I will concentrate on three different systems and of the evolution of their magnetic properties with thickness. The first is CrI3, a ferromagnet in bulk form that exhibit layered antiferromagnetism when exfoliated in thin multilayer form. When used as tunnel barriers, these multilayers exhibit a 10'000% magnetoresistance that originates from a spin-valvbe effect in the transmission probability of the barrier. The second is CrCl3, also a layered antiferromagnet in multilayer form, that contrary to CrI3 (which is an easy axis material), has an easy plane with only weak anisotropy. I will discuss in detail how the evolution of the tunneling magnetoresistance as a function of magnetic field, number of layers and temperature allows the full magnetic phase diagram of these atomically thin crystals to be determined experimentally, and reproduced theoretically in terms of a simple "linear antiferromagnetic chain" model. Finally, I will address the case of MnPS3, which contrary to the other two materials exhibit antiferromagnetic ordering within the individual layers of the the material. I will show that the tunneling magnetoresistance of sufficiently thick multilayers (i.e., 6-layer) allows the bulk magnetic phase boundaries to be traced. As the thickness is reduced to mono or bilayer, a large magnetoresistance persists, indicating the persistence of magnetism, event though its functional dependence differs from that observed in thicker multilayers. Overall, these examples show how tunneling magnetoresistance is a powerful technique to extract information about the magnetic state of 2D magnetic materials, which would not be easy to obtain otherwise.

Applied strain has proven to be one of the most powerful tools in the study of semiconductors, enabling control of fundamental material properties such as the bandgap and electron-phonon coupling. In the context of the 2D semiconductors such as WSe2, inhomogeneous strain in particular has received significant scientific attention with the discovery of localized excitons (LXs) associated with structural features such as nanobubbles, wrinkles, and nanotents. Such LX states have been shown to become single photon sources at cryogenic temperatures, potentially allowing for deterministic placement of single photon sources in-plane by depositing single 2D layers over designer structures, e.g. nano-pillars, nano-pits, and nano-antennas. While deposition over fabricated structures allows for high spatial precision in LX location, important parameters such as the strain distribution and resulting emitter emission energy are fixed, and can only approximately controlled during deposition of the 2D layer.
In this presentation, I will show a novel approach that allows for the dynamic tuning of the LX emission energy and coupling to a plasmonic cavity. Using a metallized nano-optical probe as an indentor and, we show that nanobubbles in monolayer WSe2 can be controllably deformed, tuning the local strain profile, and leading to controllable redshift in the emission energy. High resolution nano-Raman spectroscopy shows that high strains of order 1 - 2 % can be induced, consistent with naturally formed wrinkles that have been shown to host strongly LXs. Finally dramatic line splitting of the LX luminescence is seen, indicating possible strong-coupling with the gap mode plasmon formed between the metallized nano-optical probe and the gold substrate. Our results show the power of the combined plasmonic cavity-nanobubble approach to control the optical coupling and emission localized excitons in WSe2, and provide greater insight for developing excitonic light sources in 2D materials.

Integration of 3D bulk materials with 2D quantum materials for physical coupling and applications has been a relatively new area of research. Interfacing of these materials, excitingly, leads to drastic modulations in optoelectronic properties beyond those of the individual materials, thereby leading to an entire genre of innovation and technologies. In this research, DFT studies of 2D WSe₂ coupled with Cs₂AgBiBr₆, a non-Pb based perovskite structure, provide a suite of exciting modulations in electronic properties including higher density of states, and conversion of bandgap from indirect to direct type. Additionally, the charge contribution to the HOMO level of the coupled heterostructure is shown to be predominantly by the p orbital of Br atoms, whereas the LUMO level is seen to receive electrons from the d orbital of W, showcasing the critical transition of the excited electron from the bulk Cs₂AgBiBr₆ to WSe₂ monolayer upon irradiation. Moreover, the energy levels of the surface states are located just above the top of the valence bands in the bulk AgBiBr₂, indicating that the energy loss through charge transfer via surface states is expected to be considerably small. Lastly no mid-gap surface states are seen from density of states plots, indicating that electron-hole recombination hardly occurs even at the surfaces, and thus, long lifetimes of the photoexcited carriers can be realized. Optical properties show enhanced absorption and reflectivity of the 2D-3D heterostructure over each system by itself, together with critically modulated refractive index, dielectric constant (both real and imaginary) and loss term, and extinction index. Thermal and transport properties show similar modulations in the Seeback coefficient and thermal conductivity. These results provide motivation for the application of such a 2D-3D heterostructure for a novel non-invasive glucose monitoring device.Such a device depends on a highly accurate detection of a signal (from an LED light source) that has undergone a change in intensity after passing through the sugar solution. The enhanced electronic, optical, thermal and transport modulations that the 2D-3D heterostructure showcases upon irradation across a range of frequenceis offers multiple modalities for amplifying and detecting such a signal. Importantly, these modalities can be utilized to detect not only changes in blood sugar level, but other markers in the blood (all non-invasively) that indicate changing health conditions.

Machine learning force field has attracted more and more attentions because those powerful function-fitting schemes are capable to reach first principle accuracy and simulate large scale systems that are prohibited for quantum molecular dynamics (QMD). Gaussian process (GP) regression is one promising technique among them, whose remarkable competitive advantage over empirical interatomic force fields is the built-in uncertainty quantification based on Bayesian posterior inference, which can be used to monitor the quality of GP predictions. A current limitation of existing GP force fields is that accurate predictions are only enabled by large training data set. However, the prediction cost grows linearly with the size of the training data set, making accurate GP predictions slow and defying the competitive advantage over more accurate ab-initio techniques.

In this work, we present a way to dramatically accelerate Gaussian process models for interatomic force fields based on many-body kernels by mapping both forces and uncertainties onto functions of low-dimensional features. This allows for automated active learning of models combining near-quantum accuracy, built-in uncertainty, and constant cost of evaluation that is comparable to classical analytical models, capable of simulating millions of atoms.

Using this approach, we train force fields and perform large scale molecular dynamics simulations of stanene monolayer. The monolayer dynamics reveals an unusual phase transformation mechanism of 2D stanene, where ripples lead to nucleation of bilayer defects, densification into a disordered multilayer structure, followed by formation of bulk liquid at high temperature or nucleation and growth of the 3D bcc crystal at low temperature.

The presented method opens possibilities for rapid development of fast accurate uncertainty-aware models for simulating long-time large-scale dynamics of complex materials.

Named after the two faced Roman God Janus, 2D Janus layers contain two different atomic types on its top and bottom faces. Previous theoretical studies have shown that broken mirror symmetry together with large change transfer across the top and bottom face opens up completely new quantum properties including Rashba effect, colossal Janus field, dipolar excitons, and Skyrmion formation. Despite the theoretical advances in the field, experimental results are still limited due to limitations in high quality 2D Janus layer synthesis. In this talk, I will introduce recent discoveries made at Arizona State University towards different types of Janus layers. The growth process relies on Plasma enhanced low pressure chemical vapor deposition (PE-LPCVD). With this all room temperature technique, our team can synthesize different Janus layers as well as their vertical / lateral heterojunctions, and Janus nanoscrolls. Further studies from our team will introduce on-demand fabrication of 2D Janus layers with unique in-situ growth capabilities that allows us to collect spectroscopy data during the course of Janus material growth. Results are presented along with microscopy, spectroscopy, high -pressure studies, and electronic transport datasets for complete understanding of these systems.

The interplay between topology and broken time reversal symmetry in a solid-state system can create a Chern insulator. The non-trivial property of its band structure enables quantized Hall transport with dissipation-free chiral edge states. The recently-discovered topological magnet, MnBi₂Te₄, provides a desirable platform to study tunable Chern insulator phase. In this talk, I will describe our recent progress in understanding this new material system by the combination of transport, magneto-optical Kerr effect, and microwave impedance microscopy measurement. These probes reveal the formation of the Chern insulator gap, image the associated chiral edge states, and visualize the band-crossing in atomically thin MnBi₂Te₄ devices. Our results offer insights into the topological character of the band structure in this new Chern insulator.

We present density functional theory (DFT) calculations of the reconstruction in MoSe₂/WSe₂ hetero-bilayers twisted a small angle away from 180-degree stacking. The bilayer reconstructs to increase the area of the domains with the lowest energy stacking. The overall twist is concentrated in the domain walls and vertices, where the energetic cost of twisting away from 180 degrees is reduced. We consider the local stacking, order parameter, and strain in the relaxed twisted bilayer.

We use neutron scattering to study magnetic order and spin excitations in honeycomb lattice van der Waals ferromagnet CrI$_3$ and CrGeTe$_3$. In the case of CrI$_3$, we show that ferromagnetic phase transition is first order in nature, and controlled by spin-orbit coupling (SOC) induced magnetic anisotropy, instead of magnetic exchange coupling as in a conventional ferromagnet. We discuss the role of the next-nearest-neighbor Dzyaloshinskii-Moriya interaction and Kitaev’s interaction plays on the spin-wave spectra of CrI$_3$. For CrGeTe$_3$, we show evidence of a novel spin-lattice coupling that induce a very large spin-wave broadening over a wide energy and momentum space. We discuss the possible presence of topological spin excitations in CrI$_3$ and CrGeTe$_3$.

Layered van der Waals magnets, such as CrI3, show enormous potential for using light to drive phonons and to control magnetic exchange interactions. We present a numerical investigation of such coupled spin-phonon dynamics in model Hamiltonians which incorporate Heisenberg exchange as well as bond-anisotropic Kitaev couplings or Dzyaloshinskii-Moriya interactions. We discuss our results in the context of recent optical pump-probe experiments which observe an oscillatory time-resolved polarization rotation below Tc which may stem from coherent oscillations in the Kerr angle.

The electronic properties of transition metal dichalcogenide (TMD) bilayers vary according to the local atomic structure [1]. In minimally twisted bilayers, there is significant atomic reconstruction away from the rigid moiré pattern [2]. We present the results of density functional theory (DFT) calculations that model the electronic properties of MoSe₂/WSe₂ bilayers with a small twist away from commensurate 180-degree stacking. We present band structures and local densities of states that exhibit flat bands in the gap near the band edges.
[1] M. Phillips and C. S. Hellberg, arXiv:1909.02495
[2] S. Carr, et al, Phys. Rev. B 98, 224102

Two-dimensional (2D) transition metal dichalcogenide (TMD) crystals exhibit electronic, magnetic, and optical properties which can be manipulated through control of crystal dimensionality, edge character, and strain state. We recently introduced a rational chemical synthesis strategy, based on gas-phase growth of TMD crystals on designer surfaces, that substantially improves control over the dimensions, morphology, and crystalline edges of 2D TMDs without top-down fabrication. TMD nanoribbons prepared in this manner exhibit atomically sharp edges, readily controllable dimensions, and anomalous photoluminescence features. This talk will focus on our recent efforts to better understand some of these optical anomalies. A mixed-dimensional architecture comprised of a 2D TMD crystal carefully interfaced with a Si nanowire enables the introduction of localized strain fields. Detailed structural and optical characterization of these architectures reveals unique edge- and strain-mediated photoluminescence that is localized and significantly red-shifted in energy. Our results highlight how rational design of low-dimensional materials can enable future optoelectronic and quantum device goals.

It has in recent years become clear that optical pumping of specific solid state modes using ultrashort pulses of light can provide highly targeted access to magnetic order. Resonant excitation of phonons or of electronic orbitals in magnetic ions has been shown to drive non-equilibrium magnetic dynamics, modify magnetic interactions and induce magnetic phase transitions. Such demonstrations have been mainly focused on three-dimensional crystals, and it is of interest to expand this approach to two-dimensional magnets where the magnetic order is intrinsically fragile and is expected to be more sensitive to perturbation. Here, using time-resolved optical second harmonic generation, we study picosecond dynamics of the antiferromagnetic order in the layered van der Waals antiferromagnet manganese thiosulfate (MnPS3) in response to excitation of various sub-gap transitions. We demonstrate that resonant pumping of an orbital d-d transition within the Mn²⁺ electronic multiplet launches coherent antiferromagnetic spin precession, while excitation of high-energy phonons leads to quasithermal incoherent magnetic melting.

Magnetic order in topologically non-trivial materials gives rise to new, unconventional topological phases. These phases exhibit unique properties such as quantized electronic transport and magnetoelectric coupling, which can result in potentially groundbreaking applications in error-tolerant quantum computing and spintronics. The key to harnessing these properties lies in being able to control magnetism in these materials. In this work, we demonstrate that ultrafast optical pulses can be used to coherently control magnetism in the topological insulator MnBi₂Te₄, through coupling to optical phonons. First, we find using Raman spectroscopy that optical phonons strongly modulate the exchange interaction, resulting in large changes in phonon energies and linewidths across the magnetic transition in MnBi₂Te₄. The spin-induced phonon renormalization observed here is the largest reported in any two-dimensional magnet to date. We exploit this strong spin-phonon coupling to optically engineer the spin Hamiltonian through ultrafast modulation of the exchange interaction through displacively excited coherent phonons. Using a time-resolved Kerr effect probe, we find that this results in a coherent modulation of the magnetization at femtosecond timescales. Our work thus unlocks a new mode of control of magnetism in topological materials.

Two-dimensional (2D), van der Waals crystals allow the creation of arbitrary, atomically precise heterostructures simply by stacking disparate monolayers without the constraints of covalent bonding or epitaxy. Charge and energy transfer processes at these novel junctions is an area of burgeoning interest both from the fundamental and application points of view. At a type II heterojunction between two 2D semiconductors, ultrafast charge transfer has been previously determined to occur on the order of 10’s of femtoseconds after photoexcitation. However, the coupling between the lattice degrees of freedom of the photoexcited monolayers remains less understood. We use ultrafast electron diffraction to directly visualize lattice dynamics in the individual monolayers of the van der Waals heterojunction. We are able to track the transfer of energy from one layer to another by following the change in intensity of the Bragg peaks after photoexcitation. With the aid of first principles calculations, we are able to shed light on the role of lattice dynamics during ultrafast electronic processes at 2D van der Waals heterojunctions.

The ability to achieve noninvasive detection and efficient control of electric and magnetic orders in 2D quantum materials is of great importance to the development of ultrathin quantum electronics, optics, spintronics. Here we present our recent theoretical predictions of nonlinear response and sensing of 2D topological materials [1, 2, 3] and electrostatic doping induced switching of magnetic orders in 2D high Curie temperature ferromagnetic semiconductors CrSBr and CrSeBr [4]. First, we will show that time-reversal invariant few-layer WTe₂ holds out-of-plane polarization which can be switched via slight interlayer sliding under vertical electric field. Moreover, ferroicity-driven switching of nonreciprocal nonlinear photocurrent [5] such as ferroelectric nonlinear anomalous Hall effect can be achieved in few-layer WTe₂ by utilizing the intrinsic coupling among nonlinear susceptibility, crystalline symmetry, and quantum geometry of electronic states [1], paving theoretical foundation for nonlinear quantum memory such as Berry curvature memory. We will discuss the very recent experimental demonstration of ferroelectric nonlinear anomalous Hall effect in tri-layer WTe₂ [2]. We will also introduce our recent work on nonlinear photocurrent in PT-symmetry magnetic topological quantum materials [3], where magnetic shift photocurrent can be magnetically switched between two antiferromagnetic states in bilayer antiferromagnetic MnBi₂Te₄. Moreover, external electric field can break PT-symmetry and enable normal shift photocurrent that are electrically switchable and tunable down to a few THz regime, suggesting bilayer antiferromagnetic MnBi₂Te₄ as a tunable platform with rich THz and magneto-optoelectronic applications. Finally, we will briefly present our recent theoretical prediction of long-range two-dimensional ferromagnetism in monolayer semiconductors CrSBr and CrSeBr with high Curie temperature of ~150K [4] which was also experimentally demonstrated recently, offering potential alternatives to dilute magnetic semiconductors. We show that electrostatic doping can switch magnetization easy axis, providing unprecedented opportunities for 2D spintronics such as spin valves and spin FETs [4].

References: [1] npj Computational Materials 5, 119 (2019). [2] Nature Physics 16, 1028-1034 (2020). [3] arXiv Preprint, arXiv:2006.13573 (2020). [4] Applied Physics Letters 117, 083102 (2020). [5] Science Advances 5, eaav9743 (2019).

The core requirement and challenge for integrated hybrid photonic platforms is effective coupling between the guided mode of a photon-routing structure and a photo-active material. In traditional (e.g. silicon nitride) platforms, the mode center is inaccessible which significantly limits the interaction between material and platform. Here, we overcome this limitation through integration of an atomically thin 2D material (WS₂ ) on hBN into the mode center of a novel elastomeric single-mode rib waveguide. This is achieved through transfer of the emitter onto a precured polymer layer and its subsequent bonding to the supporting polymer matrix by oxygen-plasma activation. We verify waveguide-emitter coupling by various means: observing polarization-dependent excitation of the integrated emitter when excited through the waveguide, collection of emission from the guide’s output face, as well as polarization-dependent extinction in transmission measurements. Representative for room-temperature curing polymers, this work opens up new ways for the integration of (single-photon) emitters into polymer-based optical circuits.

Heterostructures of transition metal dichalcogenides (TMDs) have been extensively studied as the alignment of the bands in the constituent materials allow for manipulation of optoelectronic and transport properties. The band offset between the bands is usually the most important parameter in determining the physical properties of these structures. However, as evidenced in the so-called ‘magic-angle graphene’ [1], the twist angle between the crystallographic directions of the two layers is an important parameter that affect the physical properties. As the twist angle between two layers of a given set of materials is varied, the phonon spectrum as well as the electronic band structure and optoelectronic properties change systematically. Furthermore, at very small twist angles, atomic-scale lattice reconstruction [2] is observed and should be accounted for in describing the physical properties of heterostructures. In this talk, I will present some of the latest experimental data on the phonon spectra and the band structures from twisted heterostructures of TMDs.
References
[1] Cao Y., et al., Nature (2018); 556, 43; Ibid. 80.
[2] Yoo H., et al., Nature Materials (2019), 18, 448.

For the last two decades, graphene's impressive prospect as a material to revolutionize technologies from electronics to spintronics and quantum computing has prompted numerous studies of its spin dynamics. One of the puzzling phenomena in these studies has been a large discrepancy, at best by two orders of magnitude, between the observed and the predicted spin relaxation lifetimes. Several theoretical studies based on various models have proposed that external factors including but not limited to the substrate and externally applied electric field can influence relaxation reducing spin lifetimes from the predicted microseconds to nanoseconds in measurements. In particular, both these factors are expected to break space-inversion symmetry enhancing spin-orbit coupling strength significantly and consequently increasing spin relaxation rates in graphene. However, past theoretical models do not capture the effects of real substrates and hence do not explain the spin lifetime anisotropy at a graphene/hBN interface with an applied electric field. Consequently, a first-principles approach to explain these relaxation channels is necessary.

Here, we present our investigations of spin dynamics and in particular spin-phonon relaxation fully from first-principles in single layer graphene in the presence of an external electric field and/or hBN substrate. Using our ab initio computational method based on density matrices[1], we evolve electron(hole) spins for microseconds after an initial perturbation by a small magnetic field all the while fully accounting for their scattering against phonons. From our results, we predict that room-temperature spin lifetimes in graphene range from hundreds of picoseconds to a few nanoseconds in the presence of an external electric field or hBN. We find that the out-of-plane vs. in-plane spin lifetime anisotropy is greater than 0.5 in graphene on hBN, in a remarkable agreement with experimental observations but deviates from the general consensus in model-based theoretical studies. We report spin anisotropy of 0.5 only as a result of external applied electric field on free-standing graphene confirming its origin to the Bychkov-Rashba type spin-orbit field as widely accepted in theoretical studies. Our findings suggest that both substrate-induced spin-orbit fields and substrate phonons influence spin relaxation substantially and reduce lifetimes to the orders observed in experiments. [1] J. Xu et al, Nat. Commun. 11, 2780 (2020)

This work is funded by the National Science Foundation under grant number 1904547, and Adela Habib acknowledges support from the American Association of University Women(AAUW) fellowship program.

Two-dimensional (2D) materials and heterostructures provide excellent platforms for exploring and understanding chiral and helical edge transport. Here we present our works on magnetotransport in high mobility 2D heterostructures. In few-layer black phosphorus devices, we use p-p’-p junctions to study the equilibration and transmission of edge states at the interfaces of regions with different charge densities. We observe both full equilibration, where all edge channels equilibrate and are equally partitioned at the interfaces, and partial equilibration, where only equilibration only takes place among modes of the same spin polarization. Furthermore, the inner p'-region with low-doping level in the junction can function as a filter for highly doped p-regions which demonstrates gate-tunable transmission of edge channels. In Bernal-stacked tetralayer graphene, we observe two topologically distinct phases supporting helical edge states at the charge neutrality point. As the magnetic field and out-of-plane displacement field D are varied, we observe a phase diagram consisting of an insulating phase and two metallic phases, with 0, 1 and 2 helical edge states, respectively. These phases are accounted for by a theoretical model that relates their conductance to spin-polarization plateaus. Transitions between them arise from a competition among inter-layer hopping, electrostatic and exchange interaction energies.

In van der Waals layered materials, the symmetries and functionalities could be controlled by modifying the stacking order through rotation and translation between the layers. Whereas most previous work has focused on the electronic and optical properties associated with the van der Waals stacking, the recent discovery of magnetism in 2D materials, achieved through both mechanical exfoliation and molecular beam epitaxy (MBE), provides an exciting opportunity to explore the effects of stacking order on a material’s magnetic properties. In this talk, I will present two of our recent studies in this direction. 1) By discovering a giant nonreciprocal second harmonic generation from layered antiferromagnetism in mechanically exfoliated bilayer CrI3, we revealed the underlying C2h symmetry, and thus monoclinic stacking order in bilayer CrI3, providing crucial structural information for the microscopic origin of layered antiferromagnetism. 2) By using the molecular beam epitaxy to grow monolayer and bilayer CrBr3 and in situ characterization with a spin-polarized scanning tunneling microscopy and spectroscopy, we observed that while individual CrBr3 monolayer is ferromagnetic, the interlayer coupling in bilayer depends strongly on the stacking structure and can be either ferromagnetic or antiferromagnetic. Thus, the direct correlation between stacking order and interlayer magnetism pave the way for manipulating 2D magnetism.

Broadening the knowledge and understanding on the magnetic correlations in van der Waals layered magnets is critical in realizing their potential next-generation applications. In this study, we employ high frequency (240 GHz) electron spin resonance (ESR) spectroscopy on plate-like CrBr₃ to gain insight into the magnetic interactions as a function of temperature (200 – 4 K) and the angle of rotation θ. We find that the temperature dependence of ESR linewidth is well described by the Ginzberg-Landau critical model as well as Berezinskii-Kosterlitz-Thouless (BKT) transition model, indicative of the presence of two-dimensional (2D) correlations. Furthermore, our findings show that the resonance field follows a (3cos²θ - 1)-like angular dependence, while the linewidth follows a (3cos²θ - 1)²-like angular dependence. This observed angular dependence of the resonance field and linewidth further confirm an unanticipated 2D magnetic behavior in CrBr₃. This behavior is likely due to the interaction of the external magnetic field applied during the ESR experiment that allows for the mediation of long-range vortex-like correlations between the spin clusters that possibly formed due to magnetic phase separation. This study demonstrates the significance of employing spin sensitive techniques such as ESR to better understand the magnetic correlations in similar van der Waals magnets and suggests the need to study the magnetic correlations in CrBr₃ more closely.

Magnetic two-dimensional (2D) materials [1, 2] opened up unprecedented opportunities for nanoscale spintronic devices in the miniaturized form factor. With the aggressive pursuit for dimensional downscaling is the other paramount endeavor for high energy efficiency [3]. Under the umbrella of the pursuit for the low energy consumption, the realization of high spin polarization via minimal amount of electrical input is a critical prerequisite. In this talk, I will introduce two schemes [4, 5] to create half-metallicity in 2D layered material systems. In scheme I [4], electric field will be applied across the bilayer A-type antiferromagnetic 2D materials. In scheme II [5], a multiferroic superlattice consisting of alternative ferroelectrics and A-type antiferromagnets is constructed for the ferroelectric control of half-metallicity. Both schemes are experimentally relevant, and will provide valuable contributions to the developments of miniaturized high-efficiency spintronic devices.

[1] C. Gong et al., Discovery of intrinsic ferromagnetism in two-dimensional van der Waals crystals, Nature 546, 265–269 (2017).
[2] C. Gong, X. Zhang, Two-dimensional magnetic crystals and emergent heterostructure devices, Science 363, eaav4450 (2019).
[3] C. Gong et al., Multiferroicity in atomic van der Waals heterostructures, Nat. Commun. 10, 2657 (2019).
[4] S.-J. Gong, et al., Electrically induced 2D half-metallic antiferromagnets and spin field effect transistors, Proc. Natl. Acad. Sci. 115, 8511–8516 (2018).
[5] E.-W. Du, et al., Ferroelectric switching of pure spin polarization in two-dimensional electron gas, Nano Lett. 20, 7230–7236 (2020).

Controlling magnetic states by a small current is essential for the next-generation of energy-efficient spintronic devices. The emerging spin-orbit-torque-Magnetic-Random-Access-Memory (SOT-MRAM) is the most promising commercial magnetic-RAM in the near future[1]. Unfortunately, the switching current density and power dissipation are still too high for conventional magnet/heavy-metal based SOT-MRAM[1], prompting the development of new large-SOT systems. On the other hand, very recently, van-der-Waals (vdW) magnetic materials have rapidly emerged as key members of the field of two-dimensional materials and device physics [2-7]. Among all the magnetic vdW materials, Fe3GeTe2 received special attention because it is the only topological ferromagnetic vdW metal[8].
Here we report that surprisingly an in-plane current can tune the magnetic state of nm-thin vdW ferromagnet Fe3GeTe2 from a hard magnetic state to a soft magnetic state, through substantial reduction of the coercive field. This surprising finding is possible because the in-plane current produces a highly unusual type of gigantic spin-orbit torque for Fe3GeTe2, which is directly related to the large Berry curvature and so its band topology. And we further demonstrate a working model of a new nonvolatile magnetic memory based on the principle of our discovery in Fe3GeTe2, controlled by a tiny current. Our findings open up a new window of exciting opportunities for magnetic vdW materials with potentially huge impacts on the future development of spintronic.


[1] M. Cubukcu, O. Boulle, M. Drouard, K. Garello, C. O. Avci, I. M. Miron, J. Langer, B. Ocker, P. Gambardella, G. Gaudin, Appl. Phys. Lett. 2014, 104, 042406.
[2] J. G. Park, J. Phys. Condens. Matter 2016, 28, 301001.
[3] J. U. Lee, S. Lee, J. H. Ryoo, S. Kang, T. Y. Kim, P. Kim, C. H. Park, J. G. Park, H. Cheong, Nano Lett. 2016, 16, 7433.
[4] B. Huang, G. Clark, E. Navarro-Moratalla, D. R. Klein, R. Cheng, K. L. Seyler, D. Zhong, E. Schmidgall, M. A. McGuire, D. H. Cobden, W. Yao, D. Xiao, P. Jarillo-Herrero, X. Xu, Nature 2017, 546, 270.
[5] C. Gong, L. Li, Z. Li, H. Ji, A. Stern, Y. Xia, T. Cao, W. Bao, C. Wang, Y. Wang, Z. Q. Qiu, R. J. Cava, S. G. Louie, J. Xia, X. Zhang, Nature 2017, 546, 265.
[6] K. S. Burch, D. Mandrus, J. G. Park, Nature 2018, 563, 47.
[7] S. Kang, K. Kim, B. H. Kim, J. Kim, K. I. Sim, J.-U. Lee, S. Lee, K. Park, S. Yun, T. Kim, A. Nag, A. Walters, M. Garcia-Fernandez, J. Li, L. Chapon, K.-J. Zhou, Y.-W. Son, J. H. Kim, H. Cheong, J.-G. Park, Nature 2020, 583, 785.
[8] K. Kim, J. Seo, E. Lee, K. T. Ko, B. S. Kim, B. G. Jang, J. M. Ok, J. Lee, Y. J. Jo, W. Kang, J. H. Shim, C. Kim, H. W. Yeom, B. Il Min, B. J. Yang, J. S. Kim, Nat. Mater. 2018, 17, 794.

We report the observation of a giant c-axis nonlinear anomalous Hall effect in the non-centrosymmetric T_d phase of MoTe₂, where application of an in-plane current generates a Hall field perpendicular to the layers. By measuring samples across different thicknesses and temperatures, we find that the nonlinear susceptibility obeys a universal scaling with sample conductivity. Application of higher bias yields an extremely large anomalous Hall ratio and conductivity.

The study of spin transport through semiconductor quantum dots is experiencing a renaissance due to recent advances in nano-fabrication and the realization of quantum dots as candidates for quantum computing. In this work, we combine atomistic electronic structure calculations with quantum master equation methods to study the transport of electrons and holes through quantum dots coupled to two leads with a voltage bias. We find that a competition between the energy spacing between the two lowest quasiparticle energy levels and the strength of the exchange interaction determines if a spin singlet or triplet is the lowest energy two quasiparticle state. Specifically, the low density of electron states results in a spin singlet being the lowest energy two electron state whereas, in contrast, a spin triplet is the lowest energy two hole state due to the high density of states and significant exchange interaction. The exchange interaction is also responsible for spin blockades that our transport calculations suggest persist up to temperatures as high as 77 K for colloidal quantum dots with small core diameters. Lastly, we relate the findings of this study to both optical experiments and the preparation and manipulation of singlet and triplet spin states in quantum dots using voltage biases.

This work is partially supported by the Army Research Office MURI (Ab-Initio Solid-State Quantum Materials) under Grant No. W911NF-18-1-0431. JP is a Ziff Fellow at the Harvard University Center for the Environment.

Colloidal CdSe nanoplatelets (NPLs) can be made with a thickness of atomic precision in the range of about one to a few nanometers only. The lateral sizes are of the order of several to tens of nanometers. The thickness is less than the bulk exciton bohr radius and consequently leads to strong effects of spatial confinement on the internal energy of an exciton. Variation of the thickness of a NPL thus allows one to tune the photoluminescence (PL) and optical absorption spectra.
Interestingly, however, the experimental shape of PL and absorption spectra also depends on the lateral sizes of a NPL. To date, the latter has not received much attention, with the exception of a few (mainly theoretical) studies and the origin of this effect has been inconclusive.
We measured the PL and absorption spectra for a series of NPLs with different lateral sizes and find that the dependence of the optical spectra on the lateral size is fully explained by taking into account the quantum-confinement effects on the translational motion of excitons in the plane of the NPLs. The spectra of all samples considered can be reproduced very accurately by a theoretical description of exciton energies and oscillator strengths based on the quantum mechanical particle-in-a-box model and the known size-distribution of the NPLs.

Magnetic van der Waals (vdW) materials are considered as promising candidates for low-dimensional magnetism since their vdW nature suggests the possibility of exfoliation down to the monolayer level and their low crystalline symmetry implies an intrinsic magnetic anisotropy. An understanding of the details of their bulk magnetic structure and the tunability of their properties is an important step towards potential applications.
In this talk, a brief overview of the effects of hydrostatic pressure magnetic vdW materials will be presented with a particular emphasis on the recent magnetization and heat capacity results under pressure for VI₃ and CrPS₄, where a complex interplay between different phases is observed at moderate pressure values.
Work at the Ames Laboratory was supported by the US Department of Energy, Office of Science, Basic Energy Sciences, Materials Sciences and Engineering Division. Ames Laboratory is operated for the US Department of Energy by Iowa State University under Contract No. DE-AC02-07CH11358.

Low dimensional magnetic lattices are being studied recently owing to the possibility of realizing flatbands in the magnon spectrum which can then lead to dissipation-less spin transport and associated magnon Hall effect. One could expect to find a magnon insulator, similar to a topological insulator. In the present work we present a rather less-studied honeycomb material Na₂Ni₂TeO₆. Our samples of Na₂Ni₂TeO₆ conform to hexagonal P6₃/mcm space group with refined lattice parameters, a = 5.2023(1) Å and c = 11.1552(8) Å. The physical properties of the present sample are characterized using magnetic susceptibility and specific heat, both of which confirm a phase transition at 28 K. Application of 8 T magnetic field only slightly polarizes the moment at the transition. We obtain a Curie-Weiss temperature of -9.7(2) K and effective paramagnetic moment of 2.24(4) m_B/Ni. This matches well with the spin-only moment of Ni²⁺. Neutron diffraction experiments are used to determine the static magnetic structure and inelastic neutron scattering reveals a rather flat spin wave excitation at 5 meV. Combining neutron diffraction with density functional theory computations, we obtain a handle on the magnetism of honeycomb layers of Ni in Na₂Ni₂TeO₆.

Finding energy efficient, earth-abundant catalysts to aid in the production of hydrogen has been the subject of intense research for several decades. Characterizing new catalysts is an important step in replacing rare-earth Pt-based catalysts for the hydrogen evolution reaction (HER). To the best of our knowledge, the earth-abundant catalyst MoS₂ has not been studied under HER reaction conditions using spin sensitive techniques. Therefore, our objective is to study the relationship between crystal defects and reactivity on the surface and edge sites of hydrothermally grown nanocrystalline MoS₂ using in situ Electron Spin Resonance (ESR). For this purpose, we have developed an in situ ESR cell to characterize MoS₂ during HER. This technique is useful for tracing the dynamics of conduction electrons and ferromagnetic/paramagnetic species, which can be used to understand reaction mechanisms and charge transfer processes. The charge transfer mechanisms and defect – reactivity relationships in MoS₂ during HER will be discussed in terms of this highly sensitive in situ technique.

The emergence of novel properties in layered van der Waals magnets subject to external stimuli, such as exfoliation or doping, creates opportunites to favorably adjust their characteristics for the production of functional devices [Nat Rev Phys 1, 646–661 (2019)]. In particular, Mn₃Si₂Te₆, a vdW ferrimagnet, has shown enhanced magnetization after irradiation [Appl. Phys. Lett. 116, 172404 (2020)]. Therefore, we present the effects of proton irradiation on the magnetic properties of CrSiTe₃ (CST), a similar compound that has garnered attention for its potential use in atomically thin devices. Bulk CST is a layered van der Waals semiconducting ferromagnet with a previously reported Curie temperature (T_C) of 32 K [Phys. Rev. Materials 3, 014001]. In this study, bulk CST crystals were irradiated at several fluences ranging from 10¹⁵ – 10¹⁸ protons/cm². Low-temperature SQUID magnetometry measurements reveal changes in the reported behavior of the magnetic phases of CST; for example, we find a T_C of up to 37 K in the sample irradiated with 10¹⁸ protons/cm². The evolution of the temperature and magnetic field dependence of the magnetization provides further insight into the differences between pristine and irradiated CST. Furthermore, the magnetic properties of CST will be described by our experimental findings on saturation magnetization, coercivity, T_C, and magnetocrystalline anisotropy.

Understanding the intrinsic stability, electronic and magnetic properties of functional materials, and their response to external chemical environments is crucial for their application in devices. Two-dimensional (2D) transition metal dichalcogenides (TMDCs) draw great attention due to their unusual electronic properties promising for applications in quantum information sciences (QIS). In particular, edges in some TMDCs exhibit non-trivial topologic states that enable spin-momentum locking critical for realization of topological qubits. Utilization of these states in practical devices depends on our ability to control the details of their properties upon integrating them into larger materials systems. It is therefore critical to understand how the structure, stability, and electronic properties of these edges are affected by their interfaces with other materials and by intrinsic and extrinsic defects.

In this study, we use first-principles simulations to quantify the stability of several types of edges in pristine and functionalized 1T’-phase WTe₂ and describe their electronic properties. We consider one-dimensional (1D) edges along {010}, {100}, {110} and {310} lattice directions and investigate the effects of the interactions with Cu atoms and clusters depending on the distribution and concentration of the adsorbed Cu species. We find that Cu atoms prefer to either cluster or form a quasi-1D chain along (010) edge, depending on the details of its atomic termination. Our simulations also predict that Cu clusters introduce magnetization and break the non-trivial topological order of WTe₂ protected by time-reversal symmetry. In contrast, the quasi-1D chain of the Cu atoms introduces a deformation that shifts the energy levels of the edge states, most of which remain intrinsic to WTe₂ and support quantum spin Hall effect. Our study suggests avenues for a controlled modification of the critical properties of WTe₂ through the control of its external chemical environments, which can be potentially generalized to a broader class of TMDCs and provide guidance for the design of new devices and manufacturing process.

Magnetic ordering in the two-dimensional (2D) limit has been one of the most important research topics in condensed matter physics for the past couple of years. Layered quasi 2D materials such as MnPS₃, FePS₃ and NiPS₃ form a unique class of compounds. All three compounds are isostructural with the magnetic lattice being the two-dimensional honeycomb, the spin dimensionalities for the three compounds are different. While MnPS₃ is best described by the isotropic Heisenberg Hamiltonian, FePS₃ is most effectively described by the Ising model and NiPS₃ by the anisotropic Heisenberg Hamiltonian. The materials order antiferromagnetically at low temperatures, the Neel temperatures being 78,123 and 155K, respectively. Our main objective of this study is to investigate the magnetic properties of the above compounds upon proton irradiation, which is the versatile strategy in tuning the magnetic properties¹. These crystals were subjected to proton irradiation at fluences ranging from unirradiated, 1x10¹⁵, 5x10¹⁵, 1x10¹⁶ and 5x10¹⁶ protons/cm². We performed Raman, electron spin resonance, and magnetization measurements on all the proton irradiated samples, including the pristine crystals. We will present and discuss the comprehensive experimental findings, and offer the possible explanation for the observed trends.

The van der Waals class of materials offer a new approach for two-dimensional magnetism allowing spin fluctuations to be tuned upon exfoliation of layers. Moreover, it has recently been shown that spin lattice coupling and long-range magnetic order can be modified with pressure in van der Waals materials. In this study we look to understand the magnetic structure and properties of CrBr₃, a member of the highly anisotropic MX₃ (M = Cr, V; X = Cl, Br, I) compounds. As research on pressure induced magnetic properties of CrX₃ is at its nascent stage, we seek to gain further insight into whether pressure increases or decreases the long-range magnetic ordering on bulk CrBr₃ crystals. The application of pressure (0 - 0.8 GPa) shows a decrease in saturation magnetization with small decrease in the Curie temperature from 32.8 K to 29.2 K at 0.8 GPa. Density functional theory calculations with pressure from 0 - 1 GPa show a reduction in volume and interplanar distance as pressure increases. To further understand magnetic properties with applied pressure we calculate the magnetocrystalline anisotropy energy (MAE) and exchange coupling parameter (J). There is minimal decrease in MAE and the first nearest neighbor interaction (J₁) with a Hubbard U parameter, U = 0 eV, shows a reduction in J₁ with respect to pressure. Overall, CrBr₃ displays ferromagnetic interlayer coupling and the calculated parameters match well with experiment. We also explore the magnetocaloric effect as a function of pressure for this compound and in the future, further insight will be collected on the local electronic and magnetic structure of this material by employing x-ray magnetic circular dichroism (XMCD) (Cr L2,3 edge) measurements at high pressure (0-10 GPa).

The Shastry-Sutherland lattice-based compounds have perennial interest in the condensed matter physics community owing to the quantum phases that it realizes due to frustration. The R₂T₂X intermetallic compounds (R = rare earth, T = transition metal, X =main group), have been reinvestigated recently owing to the spin liquid state in the underlying Shastry-Sutherland lattice (SSL) formed by the rare earth [1, 2]. These compounds crystallizing in the Mo₂FeB₂ structure type display an interplay of frustration and quantum criticality. For the current study, we have selected the less-investigated Er₂Pd₂In and Er₂Pd₂Sn. X-ray powder diffraction and subsequent Rietveld refinements confirmed that the compounds were phase-pure and crystallize with tetragonal Mo₂FeB₂ structure. Both the compounds obeyed Curie-Weiss law in the paramagnetic regime, as judged from magnetic susceptibility data, which indicated anti-ferromagnetism. Specific heat data on both the compounds revealed a double peak indicating complex magnetic structure and phase transitions close to 6 K. We will present a detailed analysis of the magnetization and specific heat on both Er₂Pd₂(Sn/In) and the microscopic magnetic structure determined from neutron powder diffraction which indicates an incommensurate wave vector, k (0.09 0.09 0.5).

[1] Shastry B. S. and Sutherland B., Physica B 108B, 1069, 1981
[2] Kim M. S., Aronson M. C., Phys. Rev. Lett. 110, 017201, 2013

Magnetic van der Waals materials provide an ideal playground for exploring the fundamentals of low-dimensional magnetism and open new opportunities for ultrathin spin-processing devices. The Mermin-Wagner theorem dictates that as in reduced dimensions isotropic spin interactions cannot retain long-range correlations; the order is stabilized by magnetic anisotropy. Here, using ultrashort pulses of light, we demonstrate all-optical control of magnetic anisotropy in the two-dimensional van der Waals antiferromagnet NiPS₃. Tuning the photon energy in resonance with an orbital transition between crystal-field split levels of the nickel ions, we demonstrate the selective activation of a sub-THz magnon mode with markedly two-dimensional behaviour. The pump polarization control of the magnon amplitude confirms that the activation is governed by the instantaneous magnetic anisotropy axis emergent in response to photoexcitation of electronic orbitals with a lowered symmetry. Our results establish pumping of orbital resonances as a universal route for manipulating magnetic order in low-dimensional (anti)ferromagnets.

The recent discovery of long-range magnetic order in atomically thin ``van der Waals’’ (vdW) crystals [1] has attracted significant attention due to their fundamental and technological interest, including predictions of exotic magnetic phases and unique opportunities to control magnetism at the atomic scale.

I will present recent experiments, where we employ single-spin-based quantum sensors for quantitative, nanoscale probing of atomically thin vdW magnets [2]. Specifically, I will describe experiments, where we employ this technology to address magnetism in the prototypical vdW magnet CrI₃ [3], down to the level of atomic monolayers. Our approach enabled nanoscale imaging of magnetic domains, quantitative determination of CrI₃‘s layer-dependent magnetization, and revealed a delicate interplay between magnetic and crystalline order in CrI₃.

Further, I will present recent results where we employ our quantum sensors to study two-dimensional systems of uncompensated magnetic moments that arise on surfaces of specific antiferromagnets [4]. On the use-case of a single-crystal of the prototypical, magnetoelectric antiferromagnet of Cr₂O₃, we establish how these spin systems can act as reports for the underlying antiferromagnetic order parameter and use this principle to gain new insight into the behavior of domain-walls in this antiferromagnet. Our results show remarkably pristine domain wall behavior in this material, and offer new routes to engineered domain-wall behavior and its application in the field of antiferromagnetic spintronics [5].

I will conclude with an outlook of future engineering challenges [6][7] for nanoscale quantum sensors and our ongoing developments of single spin magnetometers for extreme conditions, such as high magnetic fields, millikelvin temperatures or for high-frequency sensors to probe the dynamics of nanomagnetic systems.

B. Huang et al., Nature 546, 270; C. Gong et al, ibid 265 (2017)
L. Thiel et al., Science 364, 973 (2019)
M. Gibertini et al., Nature Nanotechnology 14, 408 (2019)
P. Appel et al., Nano Letters 19, 1682 (2019)
N. Hedrich et al., arXiv: 2009.08986 (2020)
P. Appel et al., Review of Scientific Instruments 87, 063703 (2016)
N. Hedrich et al. arXiv:2003.01733 (2020)

This talk will discuss recent magneto-absorption optical spectroscopy studies of dual-gated WSe2 monolayers in high magnetic fields up to 60 T. When doped with a 2D Fermi sea of mobile holes, well-resolved sequences of optical transitions are observed in both left- and right- circular polarization, which unambiguously and separately indicate the number of filled Landau levels (LLs) in both K and K' valleys. This reveals the interaction-enhanced valley Zeeman energy, which is found to be highly tunable with hole density. We exploit this tunability to align the LLs in K and K', and find that the 2D hole gas becomes unstable against small changes in LL filling and can spontaneously valley polarize [1]. These results cannot be understood within a single-particle picture, highlighting the importance of exchange interactions in determining the ground state of 2D carriers in monolayer semiconductors. [1] Jing Li, Mateusz Goryca, Nathan Wilson, Andreas Stier, Xiaodong Xu, & Scott A. Crooker, Phys. Rev. Lett. 125, 147602 (2020).

Scalable synthesis of heterostructures is recently realized through the confinement heteroepitaxy (CHet) technique. In this technique, atoms intercalate the epitaxial graphene (EG) interface and silicon carbide (SiC) substrates, which facilitates large-area environmentally stable 2D Nitrides, 2D Oxides, and 2D metals. One of the recently advanced 2D metals is gallium (2D Ga or Gallenene), which is characterized as a half-van der Waal 2D metal heterostructure due to the bonding variation across the interface. Interestingly, 2D Ga exhibits exotic optical and electronic properties, such as enormous non-linear optical response, superconductivity, and near-zero index. In prior work, the spectroscopic ellipsometry technique reports that 2D Ga is mainly grown with bilayer thickness. However, the heterostructure’s thickness, uniformity, and area coverage are not quantitatively understood with high spatial resolution techniques.

Here, we unlock the contrast from scanning electron microscope (SEM) images as a function of the 2D Ga position and the EG thicknesses. The SEM contrast is directly related to the local surface potential. We applied multiple correlative electron microscopy techniques, including the SEM and surface chemical maps in plan view for understanding the secondary electron (SE) emission contrast in SEM images. We also verified the 2D Ga location and the number of layers by correlating SEM with Scanning-Transmission electron microscopy (STEM) imaging and spectroscopy obtained from multiple site-specific cross-sections. Furthermore, we stitched hundreds of atomic-resolution STEM images over 7 µm FIB lamella to confirm that the 2D Ga grows with uniform bilayer Ga covered by EG with thicknesses that vary from 2-layers and up.

Besides, locating the uniform 2D Ga heterostructure positions enables us to segment SEM images that cover few millimeters in area. This segmentation process can be done manually from the image histogram or un-supervised by machine learning techniques. We form a montage view of ~ 2 mm x 1.5 mm area by stitching low-magnification and pixel-dense SEM images. This panorama retains lateral resolution controlled by the SEM resolution (few nm) but preserves atomic-resolution as resolved by correlative microscopy of the heterostructure’s thicknesses.

Unlike spectroscopic techniques (ellipsometry or XPS), the spectroscopic imaging in SEM allows high-spatial-resolution for tracking intercalants, identify relative surface potentials, or determining the number of 2D layers. This could be further employed to characterize wafers’ scalability and uniformity of 2D materials and 2D Van der Waals heterostructures.

MXenes (carbides and/or nitrides of early transition metals) are a very large family of 2D materials. They have a chemical formula of M_n+1X_nT_x, where M represents a transition metal (Ti, Mo, Nb, V, Cr, etc.), X is either carbon and/or nitrogen (n=1, 2, 3 or 4), and T_x represents surface terminations. The diversity in compositions (>30 stoichiometric MXenes have already been reported, >100 predicted), availability of solid solutions on M and X sites, and control of surface terminations, such as O, OH, F, Cl, S, etc., offers a plethora of structures and chemistries to investigate.¹
Combining the plasmonic properties with ease in processing, high electronic conductivity (up to 20 S/cm) and mechanical properties exceeding other solution-processable 2D materials, MXenes have the characteristics necessary to develop as optical and electronic materials. Optical, electronic and transport properties of MXenes can be manipulated by tuning of their chemical composition. This presentation will provide insight into control of the optoelectronic properties (plasmon resonance frequency, electrical conductivity, carrier concentration, mobility and work function) and describe potential applications of MXenes as well as spectroscopic information, which can be applied to designing photonic and optoelectronic devices, such as electron transport layers for solar cells, optical sensors, electrochromic devices, metamaterials, EMI shields, antennas, etc.

Surface plasmons enable routing and manipulating of light on sub-diffraction limit length scales. Surface plasmons are commonly excited by coupling to an electromagnetic field leading to confined local field enhancements effect that be used for strengthening the sub-diffraction light-matter interaction and potentially offer a transformative solution for a wide range of high impact applications such as ultra-efficient nanostructured solar cells, high speed nano-devices, and photothermal biomedical therapeutic applications. Various conventional noble metals (gold and silver) based nanostructures have been extensively studied for their well-known plasmonic responses in the visible spectral range due to their long relaxation time. However, there are strong motivations in exploring alternative plasmonic structures that exhibit lower resistive losses at optical frequencies and more functionally diverse properties. Novel plasmonic building blocks such as topological insulators are considered to be a promising path for exploring low-loss plasmonic alternatives with exotic physical properties at optical frequencies in compare to conventional noble metals-based nanostructures.

Recently, two dimensional materials and group V-VI chalcogenides, known for their excellent thermoelectric and topological insulator (TI) properties, attracted a lot of attention for their exotic plasmonic phenomena. From the group of V-VI chalcogenides, this study will focus on low-dimensional Bi₂Te₃-Sb₂Te₃ in-plane heterostructures as low-band gap semiconductors and topological insulators providing gapless metallic surface states. Bi₂Te₃-Sb₂Te₃ in-plane heterostructure flakes contain a heterointerface between Bi₂Te₃ and Sb₂Te₃ that is formed under multi-step solvothermal synthesis process Bi₂Te₃ and Sb₂Te₃ have a rhombohedral crystal structure with space group of Rm. The bulk structure consists of alternating hexagonal monatomic crystal planes of Bi and Te arranging in ABC order along c-axis.

Monochromated electron energy loss spectroscopy (Mono-EELS) in conjunction with aberration corrected scanning transmission electron microscopy (STEM), in-situ transmission electron microscopy (TEM) and x-ray energy dispersive (XEDS) combined with singular value decomposition (SVD) analysis was used to identify and spatially resolve various surface plasmon excitation modes in these nanostructures. Furthermore, the effect of edge, heterointerface and defects (intrinsic and extrinsic) as potential paths to tailor the plasmonic response in low-dimensional Bi₂Te₃-Sb₂Te₃ in-plane heterostructures was explored. This work is also combined e-DDA calculations to further elaborate the experimental EELS measurements. These findings open a new path to design tunable plasmonic nanostructures with unique functionalities beyond noble metals.

a-RuCl3 is a layered antiferromagnetic Mott insulator exhibiting phenomena related to the Kitaev quantum spin liquid. It can be readily exfoliated down to monolayer thicknesses and incorporated into van der Waals heterostructures along with graphene and myriad other atomically thin materials. In such devices, proximity of a-RuCl3 to graphene leads to a significant charge transfer between the two that, surprisingly, persists even when a thin insulating layer is inserted between them, a phenomenon analogous to modulation doping in epitaxially-grown semiconductors. The effect can be observed in other materials including WSe2, bilayer graphene, and EuS. This enables a new method of control over the charge distribution in van der Waals stacks, for instance by patterning RuCl to create local charge variations. As but one application, we describe the potential to create sharp pn junctions in graphene.

In this communication we present results of highly effective gating of multilayer graphene in structure containing GaN epilayer with undoped GaN spacer. Bias switchable G band splitting showed great perspectives in characterization of devices based on graphene/GaN heterostructures and gives new opportunities in construction of nanosensors.

A 100 nm thick layer of undoped GaN preceded by a 1.3 μm thick layer of highly conductive GaN (n-doped) was grown on a sapphire substrate by MOCVD technique. Four-layer graphene (4-LGr) was used as a top contact to the GaN structures and was prepared in three steps. Firstly, a graphene monolayer grown on a copper substrate was transferred onto another graphene monolayer on a copper substrate by high-speed electrochemical delamination method. Then, a third graphene monolayer was transferred on the two layers of graphene obtained in the previous step. Finally, a fourth graphene monolayer was transferred on the graphene trilayer obtained in the previous step. 4-LGr was transferred on GaN heterostructure by polymer frame method [1]. So fabricated multi-layer graphene has an inhomogeneous and uncontrolled layer stacking.

A detailed analysis of Raman spectra showed three different types of spectra with G-band splitting into one, two or three subbands. No splitting of 2D band was identified. The presence of G band splitting is related to the turbostratic order and suggest different carrier concentrations of subsequent graphene layers in the 4-LGr structure. Therefore, high potential gradient near graphene/GaN junction is present even without bias. Additionally a strong shift of the G bands as a function of gate bias was observed. G band energy shift trends suggest n-doping of graphene. Interestingly, the most significant G band energy shift up to 8.5 cm^-1 occurs for the lowest bias, in the range between 1 V and -1 V. Further G band energy shift up to 1.4 cm^-1 in the range between -1 V and -5 V was observed. Raman micromapping showed that strong gating effect as well as band splitting is present over large surface area. Electroreflectance as well as band alignment analysis showed that undoped GaN spacer at reverse bias behaves like capacitor. Current-voltage characteristics exhibit Schottky-like behaviour. Graphene Fermi level shift calculated from current-voltage characteristic and electroreflectance measurements agree with Raman results. Fermi level shifts from 88 meV at 0 V to 200 mV at -5 V. For forward bias, a rapid decrease of electron concentration in graphene occurs (by 9x10¹² cm^-2), the carrier concentration in across subsequent layers becomes more uniform so band splitting is less significant.

A substantial Raman position shifts occurs in our Schottky diode structures at one order of magnitude smaller voltages than for graphene deposited on SiO₂ substrates in capacitator type devices [2]. Obtained gating efficiency is comparable to the results for solution-gated graphene. This strongly suggest that the diode junction is located directly near the graphene layers [3]. Thus, graphene gating is effective at very low voltages which is highly promising for further electron-phonon studies. Moreover, bias switchable G band splitting in turbostratic graphene enable to construct sensors based not on precise fitting of G band position shift but on tracing more significant band splitting.
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The research on layered van der Waals (vdW) magnets is rapidly progressing owing to exciting fundamental science and potential applications. In bulk crystal form, CrCl₃ is a vdW antiferromagnet with in-plane ferromagnetic ordering below 17 K, and CrI₃ is a vdW ferromagnet below 61 K. Here, we report on the electron spin resonance (ESR) properties of CrCl₃ and CrI₃ single crystals upon photo-excitation in the visible range. We noticed remarkable changes in the ESR spectra upon illumination. In the case of CrCl₃, at 10 K, the ESR signal is shifted from g = 1.492 (dark) to 1.661 (light), line width increased from 376 to 506 Oe, and the signal intensity is reduced by 1.5 times. Most interestingly, the observed change in the signal intensity is reversible when the light is cycled on/off. We observed almost no change in the ESR spectral parameters in the paramagnetic phase (>20 K) upon illumination. Upon photo-excitation of CrI₃, the ESR signal intensity is reduced by 1.9 times; the g-value increased from 1.956 to 1.990; the linewidth increased from 1170 to 1260 Oe at 60 K. These findings are discussed by taking into account the skin depth, the slow relaxation mechanism and the appearance of low-symmetry fields at the photo-generated Cr²⁺ Jahn-Teller centers. Such an increase in the g-value as a result of photo-generated Cr²⁺ ions is further supported by our many-body wavefunction calculations. This work has the potential to extend to monolayer vdWs magnets by combining ESR spectroscopy with optical excitation and detection.

Measurements of thermodynamic properties of few-layer van der Waals superconductors are scarce due to the inherently small volume of mechanically exfoliated samples. Here we measure the magnetic susceptibility of a van der Waals superconductor by using local magnetic measurements enabled by scanning SQUID microscopy. Specifically, we study transition metal dichalcogenide MoS2 which exhibits a superconducting dome with a maximum critical temperature of nearly 10 K in ionic-gated few-layer devices. We combine the magnetic measurements with simultaneous electrical transport and map out changes in superfluid stiffness as a function of temperature and carrier density across the dome. We observe magnetic signatures consistent with a Kosterlitz-Thouless transition, and report our progress towards extracting absolute values of the superfluid stiffness and density from these measurements.

Nanoplatelets (NPLs), the colloidal analogues of more conventional two-dimensional materials such as transition metal dichalcogenide (TMD) layers and graphene, likewise exhibit modified electronic structure that arises from quantum confinement in only one dimension. It is often assumed that the lack of quantum confinement in the lateral plane yields a spatially extended exciton; however, studies in monolayer TMDs have shown that this is not the case. For colloidal NPLs, smaller exciton binding energies potentially challenge this picture, and reduced dielectric screening at the nanoparticle surface imposed by organic ligands provides additional considerations.

In this work, we implement absorption spectroscopy of CdSe NPLs in pulsed magnetic fields up to 60 T at the National High Magnetic Field Laboratory. The shift of the exciton energy as a function of magnetic field is probed for three different thicknesses and three different lateral areas of CdSe NPLs. Application of magnetic fields up to 60 T allows for the determination of the diamagnetic shift, the (small) second-order modification of the exciton energy. The diamagnetic shift provides insight into exciton lateral extent, as larger spatially extended excitons experience greater perturbations in a magnetic field. We calculate exciton lateral extents based on diamagnetic shift coefficients and find that exciton lateral extent is comparable to NPL thickness, indicating that the quantum confinement and reduced dielectric screening concomitant with few-monolayer thickness strongly reduces the exciton lateral extent. Atomistic electronic structure calculations of the exciton size for varying lengths, widths, and thicknesses support the substantially smaller in-plane exciton extent. This work follows similar studies of exciton spatial extent in monolayer TMDs and provides clear insight into the spatial extent of excitons in these promising, highly processable and scalable, two-dimensional materials.

The presence of magnetocrystalline anisotropy in Fe_2.7GeTe₂ has previously been predicted by DFT calculations and confirmed through bulk magnetization measurements. This feature of Fe_2.7GeTe₂ makes it potentially useful for magnetic storage applications. For this material to be utilized in next-generation spintronic applications, the critical behavior, dimensionality of interaction, and magnetic exchange distance of Fe_2.7GeTe₂ must be fully understood. Here the hard-axis critical behavior was studied using a modified asymptotic analysis from bulk magnetization measurements for the first time. While much work has been previously done to study the critical behavior of Fe_2.7GeTe₂, we have found that the critical exponents and exchange distance vary between H//ab and H//c. We have characterized the orientation-dependent critical phenomena of Fe_2.7GeTe₂, thus establishing a basis for the magnetocrystalline anisotropy from the perspective of competing universality classes.

Abstract. In any commercial production of semiconductor wafers the required final step is characterization, producing the information about density of carriers (density of doping), and electron /holes mobilities. These characterization parameters are vital for design of any type of semiconductor electronics. The common, widely used characterization method consists of conductivity measurements combined with Hall effect testing. This common characterization technology requires to produce repeatedly the Van-der-Paw pattern with 6 contacts at testing pattern. The hetero-structured devices and thus hetero-structured wafers are common nowadays in mass production. Some of these layers are intentionally two-dimensional. For these wafers, the photolithography of Van-der-Paw pattern repeated for each epitaxial layer grown one above the other is complicated and costly by itself. Additional equipment, i.e., DC magnet positioned below the tested wafer is needed to finish Hall effect measurements. We developed a new characterization method to be used for testing of one after the other layers, or selected test of only given layer by optical pulses of controlled wavelength. The optical pulses of selected wavelengths (colors) are produced by wide spectrum source of light and controlled by monochromator. Two ohmic probes are positioned on tested epilayer with distance of few millimeters between them. Focused light spot generates optical pulses with controllable amplitude and frequency near one contact. The second ohmic contact collects pulses of the photocurrent. At the end of this photocurrent stripe the amplitude and shape of moving pulses of carriers are analyzed by oscilloscope. The high amplitude of box-like pulse is registered at first contact. As the pulse proceeds, its amplitude decays and the pulse widens. Changes of pulse width, recorded by the scope, help to calculate diffusivity coefficient of generated photo carriers (D_n or D_p). Voltage bias at probes and direction of current defines the type (electrons or holes) of carriers. Einstein ratio (D/μ=kT/q) is used to calculate the mobility of photo carriers. Simple measurement of current and voltage drop between two ohmic contacts, when the light is switched off, generates information about density of free carriers (density of doping). We discuss experimental setup of the new wafer characterization method [1] and how selection of light wavelengths defines selection of the tested layer.
1. S. Mil’shtein, B. Keating, “Novel Characterization Method for Semiconductor Wafers” Patent Application, December,2021

The breaking of inversion symmetry at the surface of a magnet significantly alters its properties and can induce a uniaxial anisotropy with its symmetry axis along the surface normal. In a complex oxide, such as ferrimagnetic garnet, this surface anisotropy is easy-plane in nature and can be enhanced by deposition of a non-magnetic material such as a heavy metal or a topological insulator, a phenomenon attributed to the enhanced Rashba spin orbit coupling. In striking contrast, we show here that if low symmetry, semi-metallic WTe₂ is placed in contact with Y₃Fe₅O₁₂ (YIG), an additional interfacial perpendicular magnetic anisotropy (iPMA) results. We probe the magnetism of the micron-sized YIG/WTe₂/hBN heterostructure using a high sensitivity ferromagnetic resonance force microscope, that can measure local ferromagnetic resonance with spatial resolution of 100nm and spectral resolution of 1 Oe. The magnitude of iPMA remains constant down to monolayer WTe₂, indicating its interfacial origin. In addition, we probe the spin pumping effect from YIG to WTe₂ and find no WTe₂ thickness dependence, suggesting that interfacial spin relaxation plays the dominant role or that the spin diffusion length along the WTe₂ c axis is extremely short. This demonstrates the power of this approach in enabling studies of non-traditional heterostructures such as stacks of two-dimensional materials on epitaxially grown thin films. Our result further suggests that interfacial magnetic anisotropy in complex oxide ferro- or ferrimagnetic insulators is highly tunable by choice of non-magnetic overlayers, and could play an essential role in the development of next generation magnetic solid state devices.

Many efforts to control the electronic properties of atomically thin two-dimensional materials (2D), either by twisted stacks or by deposition on a patterned substrate, are an intense line of research nowadays. Tuning the electronic properties of graphene by periodic potentials has resulted in many interesting properties such as Fermi velocity renormalization, extra Dirac points and tunable bandgap. However, it is still challenging to couple graphene to a lithographic structure with a periodicity of few nanometers. A very promising option consists of growing graphene on an appropriate periodical substrate, a natural route to induce periodic potentials in graphene [1]. However, the interaction of graphene with the substrate is still challenging [2,3]. Graphene on Ir(111) and Pt(111) substrates are particularly interesting because it exhibits a very weak graphene-substrate interaction, so the effect on the band structure as a function of the periodicity could exhibit a transition from mild coupling towards no coupling at all. In our recent work, we have investigated the influence of the one-dimensional periodic nanostructured substrate on graphene electronic properties. We have shown that graphene is extremely sensitive to periodic arrays, as demonstrated on two different nanostructured substrates, namely Ir (332) and Pt (111)[4,5]. The observed minigaps in graphene band structure by angle-resolved photoemission spectroscopy (ARPES) is related to the spatial periodicity. In this talk, I will present, the successful preparation of graphene on periodically nanostructured Pt (111)substrate and discuss how it is possible to control the superperiodicity and thus the bandgap by tuning the vicinality of the substrate, a simple way to engineer the electronic properties of graphene and other 2D materials.

1. I. Palacio et al., Nano Lett. 15,182 (2015)
2. M.S.Nevius et al., Phy. Rev. Lett.115,136802 (2015)
3. M. N. Nair et al., Nano Lett. 17, 2681 (2017)
4. A.Celis et al., Phy. Rev. B 97,195410 (2018)
5. A. Celis et al., Nanotechnology 31, 285601 (2020)

Photoluminescence (PL) quantum yield (QY) is a key metric of optoelectronic performance that directly dictates the maximum optoelectronic device efficiency. In conventional semiconductors, defects act as a trapping center for nonradiative Shockley-Read-Hall or Auger recombination mechanism and drastically reduces PL QY at all generation rates. Two-dimensional transition-metal dichalcogenides (TMDCs), such as monolayer MoS₂, often exhibit low PL QY for as-processed samples, which has typically been attributed to a large native defect density. However, we show that the PLQY of as-processed MoS₂ and WS₂ monolayers reaches near-unity when they are made intrinsic through either by electrostatic or chemical doping. In the presence of background charge carries, photogenerated excitons create positive or negative trions, which mostly recombine nonradiatively, leading to a low PL QY. This could be a result of a defect-assisted decay and/or a geminate Auger-like process in which the electron provides the third particle required for momentum conservation. On the other hand, if the semiconductor is intrinsic, the photogenerated exciton stays neutral and near unity PL QY is observed. Surprisingly, neutral exciton recombination is entirely radiative even in the presence of a high native defect density. This finding enables TMDC monolayers for optoelectronic device applications as the stringent requirement of low defect density is eased. Furthermore, the observed PL QY as a function of doping and generation rate in all four major TMDC monolayers (WS₂, WSe₂, MoS₂, MoSe₂) can be understood by considering the interaction of neutral excitons, charged trions, and free charge carriers and their subsequent recombination channels. This raises the intriguing possibility that the principle that excitonic systems show maximum PL QY when they are intrinsic is general enough to be applied to other material systems without the need for material-specific passivation techniques.

Quantum spin liquids (QSLs) form an extremely unusual magnetic state in which the spins are highly correlated and fluctuate coherently down to the lowest temperatures, but without symmetry breaking and without the formation of any static long-range-ordered magnetism. Such intriguing phenomena are not only of great fundamental relevance in themselves, but also hold promise for quantum computing and quantum information. Among different types of QSLs, the exactly solvable Kitaev model is attracting much attention, with most proposed candidate materials, e.g., RuCl3 and Na2IrO3, having an effective S=1/2 spin value with 4d/5d heavy transition metal. Here in this talk, I will first report the prediction of nonnegligible Kitaev interaction in 3d Cr-based 2D materials, e.g. CrI3 and CrGeTe3, from first-principles; then, via extensive first-principles-based simulations, I will further report the investigation of the Kitaev physics and possible Kitaev QSL state in epitaxially strained Cr-based monolayers, such as CrSiTe3, that rather possess a S=3/2 spin value. Our study thus extends the playground of Kitaev physics and QSLs to 3d transition metal compounds.

Owing to the small energy differences between the polymorphs, tellurium-based transition metal dichalcogenides, such as MoTe₂ and WTe₂, can access a full spectrum of electronic states, from the 2H semiconducting state to the 1T’ semimetallic state, and from the T_d topological semimetallic state to the superconducting state in the 1T’ and T_d phase at low temperature. Thus, it is a model system for phase transformation studies and applications as well as for studies of quantum phenomena such as the quantum spin Hall effect and potential topological superconductivity.

In this talk, I will first discuss our synthesis strategy to obtain cm²-scale thin films of 2H and 1T’ MoTe₂ with layer control and large grains, which was achieved by tellurizing MoO_x thin films deposited by atomic layer deposition. Despite the van der Waals epitaxy, we show that the precursor-substrate interface critically determines the uniformity in thickness and grain size of the resulting MoTe₂ films: MoTe₂ grown on sapphire show uniform films while MoTe₂ grown on amorphous SiO₂ substrates form islands.

Phase transition from the 1T’ phase to the T_d phase of MoTe₂ is explored using in situ cryogenic transmission electron microscopy, in which flakes of MoTe₂ are cooled from room temperature to liquid nitrogen temperature. Distinct changes in the electron diffraction patterns as a function of temperature demonstrate the phase change. Thus, in situ cryo-TEM enables studies of phase transitions as a function of thickness and morphology of 2D flakes in real space, providing important information to study the nucleation and growth of the new phase during phase transition. Finally, I will discuss the effects of heterointerface and mechanical strain on lithium-induced phase transition of MoS₂ and 2D heterostructures during lithium intercalation, which was investigated by combining field-effect transistor platforms with in situ probes during electrochemically controlled intercalation of lithium.