Symposium QT02—Quantum and Topological Phenomena in Two-Dimensional Systems

Recently, high-quality thin films of cadmium arsenide (Cd₃As₂) have emerged as a promising platform for the observation of quantum transport phenomena and the realization of new topological states. In this talk, we will first discuss the nature of the topological insulator-like states of thin (001) Cd₃As₂ films and the specific features of the “half-integer” quantum Hall effect arising from these states. Next, we will discuss several new measurements relevant for quantum information devices, including the prospects for proximitized superconductivity. We will also discuss electronic quantum interference experiments that are a key ingredient in several approaches for topological quantum information processing. For example, we will show results from nanoscale p-n junctions fabricated on thin films of Cd₃As₂, which are in a topological insulator-like state, and report on conductance oscillations under small magnetic fields. We discuss their origins and connection to the junction properties. Our results show that high-quality thin films of cadmium arsenide Cd₃As₂ hold great promise for a number of applications in quantum devices.

The edge structure of hole-conjugate states in the fractional quantum Hall effect has long been the subject of extensive theoretical and experimental works. Originally proposed to host a backwards-flowing mode, carrying fractional charge, along with a forwards-flowing integer charge mode [1], the edge at the seminal hole-conjugate filling factor nu=2/3 was later understood to undergo a reconstruction due to disorder [2]. This reconstruction leads to the appearance of a backwards-flowing neutral mode which only carries heat, while a single fractional charge mode flows forwards. This peculiar structure turns out to raise further questions, in particular whether these two charge and neutral edge modes could exchange energy along their propagation, a phenomenon referred to as thermal equilibration. While the first experiments seemed to indicate a partial equilibration [3], very recent experiments, in both GaAs/Ga(Al)As and graphene, showed on the contrary that no equilibration takes place up to large distances (200 µm). Here we present the results of a heat transport experiment in graphene at the fractional filling factor 8/3, where we show that upon introducing a small amount of disorder in the same sample, a transition from non-equilibrated to fully-equilibrated edge modes is observed. This clear observation of the full thermal equilibration at the edge of a hole-conjugate fractional quantum Hall state allows us to set constrains to the different regimes of edge reconstruction.
[1] MacDonald, PRL 64, 220 (1990)
[2] Kane, Fisher, Polchinski, PRL 72, 4129 (1994)
[3] Banerjee et al., Nature 545, 75 (2017)
[4] Srivastav et al., Phys. Rev. Lett. 126, 216803 (2021)
[5] Melcer et al., arXiv:2106.12486 (2021)

Josephson junctions on 2D topological materials have been predicted to behave very differently from traditional Josephson junctions. In particular, if the topological material has broken inversion symmetry the magnitude of the critical Josephson currents in opposite directions at a fixed magnetic field could be unequal, and the symmetry relation |I^±_c (B)| = |I^±_c (−B)| could be violated. This predicted anomalous asymmetry in the junction’s critical current vs field (termed the asymmetric Josephson effect or AJE) is understood to be a result of different Fermi velocities at opposite current-carrying edges of the weak link material, and has been proposed to be a unique signature of inversion symmetry-breaking topological materials [1].

However, we suggest that under deliberately engineered circumstances, a Josephson junction on topologically trivial Bernal bilayer graphene could demonstrate similar asymmetric critical current vs. field behavior. Studies of Josephson junctions on gapped bilayer graphene have shown that the supercurrent is localized along sample edges [2][3]. Due to the nonlinear dispersion of bilayer graphene bands, the Fermi velocity of carriers at these edges can be explicitly tuned via electrostatic gating. By independently gating the bulk and edges of encapsulated bilayer graphene, such that the bulk is at charge neutrality and the edges have deliberately detuned Fermi velocities, we expect to create a SGS junction very similar in character to the junction proposed in Ref. [1].

Our simulations indicate that the proposed SGS junction should show AJE, and that this asymmetry will be explicitly tunable via electrostatic gating of each edge. Experimental confirmation of this could serve as a useful counterpoint to claims that AJE implies topologically non-trivial behavior in the weak link material. Strikingly, the simulations also suggest that the unequal Fermi velocities of the edges should yield nonzero current through the junction at zero superconducting phase difference. To test these ideas, we have fabricated an encapsulated bilayer graphene device with superconducting MoRe edge contacts and local gates to independently address each edge. We will present data on the critical current-field and current-phase behavior of this device as a function of edge detuning.

This work is primarily supported by the Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering, under contract DE-AC02-76SF00515. For early stages of the project, R.K. was supported by the National Science Foundation Graduate Research Fellowship Program under Grant No. DGE1656518. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation.

[1] Chen, C.-Z. et al. Asymmetric Josephson effect in inversion symmetry breaking topological materials. Phys. Rev. B 98, 075430 (2018).
[2] Allen, M., Shtanko, O., Fulga, I. et al. Spatially resolved edge currents and guided-wave electronic states in graphene. Nature Phys 12, 128–133 (2016)
[3] Zhu, M., Kretinin, A., Thompson, M. et al. Edge currents shunt the insulating bulk in gapped graphene. Nat Commun 8, 14552 (2017)

Kagome metals are fascinating materials capable of hosting electronic states hosting an interplay between topologically nontrivial electronic states and correlated electron phenomena. This interplay is born from the Dirac points, flatbands, and saddle-points endemic to the kagome lattice at select filling fractions. The discovery of a new class of kagome metals of the form AV₃Sb₅ with A=K, Cs, or Rb has recently provided a unique setting for exploring the interplay between Z₂ electronic topology and intertwined charge density wave and superconducting orders. These compounds host a kagome lattice of nonmagnetic vanadium ions with an electron-filling that places the Fermi level near the saddle-points and their corresponding van Hove singularities. Nesting effects due to this divergent density of states are predicted to stabilize a variety of unusual states, ranging from charge density wave order that breaks time reversal symmetry to unconventional superconductivity. Here I will present some of our recent work exploring the phase transitions and broken symmetries in these materials.

Transition metal dichalcogenides (TMDs) possess physical properties that make them potentially useful as both thermoelectric and topological insulator materials. Rapid, high-yield synthesis of TMDs like bismuth telluride, antimony telluride and bismuth selenide is possible through solution-liquid-solid (SLS) colloidal chemistry techniques. Subsequent edge modulation doping with noble metals (e.g. silver and copper) is known to enhance TMD thermoelectric parameters such as conductivity and the Seebeck coefficient. Iron and nickel may also play roles as useful dopants thanks to their magnetic properties and potential for spin injection at the chalcogenide-dopant interface.

In this study, bismuth telluride and antimony telluride were synthesized and then doped with different iron and nickel concentrations using SLS chemistry. Preliminary studies of magnetic and thermoelectric performance to assess the extent of doping and its effects on the chalcogenide’s physical properties are ongoing. Analysis of edge doping is undertaken through a variety of techniques including electron microscopy, X-ray Photoelectron Spectroscopy (XPS), Hall effect measurements, and thermoelectric measurements. If successful, the SLS chemistry approach represents a relatively low cost and straightforward strategy for making iron- and nickel-doped TMD materials that may achieve spin injection. Possible applications of spin-injected TMDs range from quantum computation to quantum sensors for gravitational wave detection to biomedicine.

Chiral crystals of elemental Te are Weyl semiconductors with the intriguing property of combining Weyl physics with a small semiconducting band gap, which enables the creation of gate-tunable devices to probe and utilize the topological properties of Te. Specifically, the formation of gate-defined quantum dots in Te would allow Coulomb blockade spectroscopy to provide information about the strength of exchange interaction, spin-orbit coupling, and g-factors associated with discrete quantum states in Te nanostructures. This talk presents our progress in this direction using low-pressure PVD-grown Te nanowires with local control of carrier density using electrostatic gates. While atomically flat hexagonal boron nitride (hBN) gate dielectrics have been widely used for high quality layered material devices, the relatively weak adhesion to Te nanowires makes hBN-insulated Te device assembly challenging. We compare different strategies involving more traditional dielectrics, as well as a hybrid approach that uses a global Si backgate and hBN-insulated local top gates for Weyl semiconductor devices.

Unprecedented advancement of technology over the past few decades, has raised the needs of having more efficient logic and memory devices. Magnetic Tunnel junction based Molecular Spintronic Devices (MTJMSD) can effectively combine ferromagnetic electrode (FME) with magnetic molecules and be potential candidates for making metamaterials and highly correlated systems. This work investigates the effect of FME’s length and thickness variation on MTJMSD’s molecule-induced correlated magnetic phases and spatial range of molecule impact. Our experimental transport studies show that in a strong FME-molecule coupling regime 10 to 20 nm change in thickness changed MTJMSD conductivity by>1000 fold. Magnetic Force Microscopy (MFM) showed the FME extending beyond the cross-junction area developed multiple room temperature stable magnetic phases. To explore other possible effects, we also conducted Monte Carlo Simulation (MCS) using Heisenberg atomic modeling. Our computational results agree with our experimental observations. According to the MCS study, increasing FME length produced multiple magnetic phases around MTJMSD. On the other hand, increasing FME thickness from 5 to 25 atoms reduced the penetration of molecule coupling impact along the thickness.

We have studied FME electrode including 1250, 2500, 5000,7500 and 10000 atoms. We studied correlation between molecule spin state and FME as a function of length and width. We also studied the spatial magnetic susceptibility of MTJMSD as a function of FME length and thickness. We observed that molecular coupling strength required to produce long range impact on MTJMSD with different FME lengths did not vary noticeably. However, variation in FME thickness caused critical molecular coupling strength required to produce strong coupling changed from 0.15 to ~0.65 as FME thickness changed from 5 to 25 atoms. MCS results matches with our experimental observation of thickness and length effect.

Nitrogen-Vacancy (NV) in graphene can be used for making qubits. The generation of ordered vacancies (V) during sustained tensile strain of graphene can be done in a controlled fashion if the V generation dynamics is clarified. We report on the dynamics of formation of single V and V arrays in pristine graphene sheet under tensile load. To that end, we performed Molecular Dynamics for uniaxial loads using LAMMPS and AIREBO potential. First, we investigated AIREBO potential capability to accurately capture the interactions of carbon atoms in graphene. Various computing environments were tested, including Solo supercomputer at Sandia National Labs, Intel compilers and multi-cores x86 servers using GNU compilers. In order to obtain accurate graphene properties, we used atomic systems of about one million atoms. The carbon-based interactions in AIREBO are parameterized with cut-off function that truncates the potential energy between certain inner and outer cut-off interatomic distances. These values appeared to have major effects on calculated mechanical properties. For instance, improper cut-off values resulted in under or over coordination between atoms, which led to non-physical interactions and properties. Runs on Solo supercomputer with AIREBO “as is”, produced stress-strain curves of graphene similar to brittle materials, and mechanical properties very close to experimental data. These accurate results were obtained without modifying the inner C-C cut-off value of the AIREBO potential, unlike most published work [1]. Tensile tests run on Solo, on large samples with the right compilers gave accurate results, whereas runs on X86 servers led to the spurious high bond forces and non-linear reversible behavior, which is unphysical. The “as defined” AIREBO potential can properly describe the elastic and plastic regimes of graphene, which refutes the approach used in other work, where the cutoff function in AIREBO is adjusted. A distinct advantage in using the Intel compiler implemented on Solo appeared due to the much improved vectorization through SSE and AVX instructions. These enable high degrees of vectorization hence, better performances and maximum accuracy through the size of its registers for floating point number representation that considerably minimizes the propagation of round-off errors.
For the first time, we could observe during the plastic regime, the complete dynamics of single V, V array and crack formation in pristine graphene. Note that published work on defect propagation in graphene started with defected graphene sheets [2]. MD simulation performed on Solo, allowed us to consistently observe the formation of single V then V array parallel to the pulling direction. Under the tensile load V cluster in vacancy lines (VL) defect. Each VL is bordered on one side with a single-atom Klein edge defect and on the opposite side with a zigzag edge. The first V appears at 4.2% strain as a result of stress of 35GPa in the pulling direction (// to the zigzag). The V line appears at 5.4% strain, at a stress of 44GPa with an average interval of 7 C-cycles (equal to 16A spacing). During subsequent loading a mirror defect image parallel to the original VL emerges and the graphene patch between two VLs keeps sliding through successive “zipping” and “unzipping” of the VLs. Subsequent load increase generates several vacancy line pairs.
Ordered VL(s) in graphene are useful since doped with N, they form an array of coupled quantum dots. DFT showed that NV is a stable complex in N doped graphene. We also found that NV complex behaves as binary system and we have obtained the electron energy spectrum. The complex exhibited localized and delocalized states.
[1] M. Dewapriya et al., Atomistic modeling of out-of-plane deformation of a propagating Griffith crack in graphene, Acta Mech. 228, 3063 (2017).
[2] X. Sun et al., Effects of vacancy defect on the tensile behavior of graphene, Theo. & Appl. Mech. Let. 4, 051002 (2014).

The interplay between superconductivity and magnetism is thought to play a role in a variety of unconventional superconductors, including cuprates, heavy fermions, and moire graphene. Here, I will describe a new venue for examining this interplay by tuning the chemical potential through a van Hove singularity in simple allotropes of graphene, in particular rhombohedral trilayer [1-2] and Bernal bilayer. In both systems, applying a perpendicular electric field gaps out a series of low-energy Dirac nodes, leading to large divergences in the density of states at low densities. Using both transport and compressibility measurements, we find that this regime is characterized by a cascade of phase transitions between states of differing fermi surface degeneracy. These include quarter- and half-metals with only one or two occupied (out of a possible four) combined spin- and valley flavors, as well as a variety of states showing partial polarization within the spin- and valley-isospin space. Most surprisingly, superconductivity arises near a number of phase boundaries. In the trilayer, we observe two superconducting states for hole doping; one arises from a normal state that preserves the spin and valley symmetry, and is suppressed by in-plane magnetic fields in accordance with the Clogston-Chandrasekhar limit, while the other arises from a full spin polarized half metallic state and is not affected by in plane magnetic fields. In bilayer graphene, superconductivity is not observed at B=0, but emerges only above a critical field in plane field, consistent with a magnetic field induced transition into a spin polarized ferromagnetic state with a superconducting ground state. I will lay out the many outstanding theoretical puzzles in these systems, as well as experimental opportunities enabled by the exceptionally high sample quality.
References
[1] Haoxin Zhou, Tian Xie, Areg Ghazaryan, Tobias Holder, James R. Ehrets, Eric M. Spanton,
Takashi Taniguchi, Kenji Watanabe, Erez Berg, Maksym Serbyn, & Andrea F. Young. “Half and quarter metals in rhombohedral trilayer graphene.” Nature (2021).
[2] Haoxin Zhou, Tian Xie, Takashi Taniguchi, Kenji Watanabe & Andrea F. Young. “Superconductivity in rhombohedral trilayer graphene,” Nature (2021).
[3] Haoxin Zhou, Yu Saito, Liam Cohen, William Huynh, Caitlin L. Patterson, Fangyuan Yang, Takashi Taniguchi, Kenji Watanabe, Andrea F. Young. “Isospin magnetism and spin-triplet superconductivity in Bernal bilayer graphene.” arXiv:2110.11317.

Engineering moire superlattices by twisting and stacking two layers of Van der Waals materials has proved to be an effective way to promote interaction effects and induce exotic phases of matter. After discovering superconductivity and correlated insulators in magic-angle twisted bilayer graphene (MA-TBG), several different two-dimensional materials have been utilized to create two-layer twisted systems and various novel phases. Here, the electronic properties of van der Waals moiré superlattices can further be tuned by adjusting the interlayer coupling or the band structure of constituent layers. In this talk, we will discuss emergent electronic states observed in various twisted multilayer graphene, including twisted double bilayer graphene (TDBG), twisted trilayer graphene (TTG) with alternative twisting angles. We also fabricate n-layer twisted graphene with alternating twisted angle, where $n \geq 4$. In these twisted multilayer graphene systems, we also demonstrate a flat electron band that is tunable by perpendicular electric fields in a range of twist angles. Various correlated behaviors, including superconductivity and flavor polarization, will be discussed in these twisted multilayer graphene systems, seeking the relationship between broken symmetry states and superconductivity that appeared in these systems.

We have explored magic angle twisted bilayer graphene’s (MATBG) electronic properties using variety of spectroscopic methods with the scanning tunneling microscope (STM). Beside unraveling signatures of strong correlations [1] driving a cascade of transitions [2] in the electronic properties of MATBG, we have shown that such correlations also drive formation of topological Chern insulators stabilized with weak magnetic fields[3]. Most recently, we have focused our attention on the nature of superconducting state that forms near half filling of MATBG’s valance flat band. [4] Our experiments show signatures of nodal superconductivity with tunneling gap to transition temperature ratio that far exceeds the BCS limit. Moreover, we find superconductivity emerges from a pseudogap phase, which might be indication of either preformed pairs or the formation of some ordered state that makes superconductivity possible. Remarkably, we find that samples in which MATBG is in perfect alignment with the hexagonal BN substrate, both the pesudogap phase and superconductivity are suppressed.

[1] Y. Xie. et al. Nature 572, 101 (2019).
[2] D. Wong, et al. Nature 582, 198 (2020).
[3] K. Nuckolls et al. Nature 588, 610-615 (2020).
[4] M. Oh et al. to appear in Nature (2021). https://doi.org/10.1038/s41586-021-04121-x

Magic-angle (θ=1.05○) twisted bilayer graphene (MATBG) has shown two seemingly contradictory characters: the localization and quantum-dot-like behavior in STM experiments, and delocalization in transport experiments. We construct a model, which naturally captures the two aspects, from the Bistritzer-MacDonald (BM) model in a first principle spirit. A set of local flat-band orbitals (f) centered at the AA-stacking regions are responsible to the localization. A set of extended topological conduction bands (c), which are at small energetic separation from the local orbitals, are responsible to the delocalization and transport. The topological flat bands of the BM model appear as a result of the hybridization of f- and c-electrons. This model then provides a new perspective for the strong correlation physics, which is now described as strongly correlated f-electrons coupled to nearly free topological semimetallic c-electrons - we hence name our model as the topological heavy fermion model. Using this model, we obtain the U(4) and U(4)×U(4) symmetries as well as the correlated insulator phases and their energies. Simple rules for the ground states and their Chern numbers are derived. Moreover, features such as the large dispersion of the charge ±1 excitations and the minima of the charge gap at the Γ point can now, for the first time, be understood both qualitatively and quantitatively in a simple physical picture. Our mapping opens the prospect of using heavy-fermion physics machinery to the superconducting physics of MATBG. We also show that the application of a unit flux magnetic field on the sample can give rise to re-entrant correlated insulator phases at integer filling of the TBG bands

Majorana zero modes (MZMs) can be arranged to be topologically distinct that may be used for topological qubits, where non-Abelian statistics is advantageous for error-free quantum computing. Experimental evidences of topological surface states and MZMs have been reported on the surface of superconducting iron-chalcogenide FeSe_1-xTe_x single crystals grown in our lab at Brookhaven National Laboratory (BNL). In this presentation, I will review some recent studies on the bulk and surface states in Fe_1+yTe_1−xSe_x^1-8, including the works at BNL using time-resolved and scanning angle-resolved photoemission spectroscopy (ARPES) and transport measurements. We established a phase diagram including superconductivity, antiferromagnetism and topological surface states. We found the inhomogeneity of superconducting and topological states in iron-chalcogenides, suggesting the compositional and electronic phase control is essential for use of MZMs in quantum computing.
References:
1) Zhang et al. Science 360,182 (2018); 2) Wang et al Science 362 333 (2018); 3) Zhang et al, Nature Physics 15, 41 (2019); 4) Zhu et al Science 367, 189 (2020); 5) Wang et al Science 367, 104 (2020); 6) Zaki et al PNAS 118 (3) e2007241118 (2021); 7) Li et al Nature Materials 20, 1221 (2021); 8) Nevola et al, (to be published).

We report the inelastic neutron spectrum of 2D triangular lattice KYbSe₂, and show proximity to a quantum spin liquid phase. We apply entanglement witnesses [1] to 2D KYbSe₂ neutron scattering, and show evidence of an entangled ground state. Fitting the high-temperature paramagnetic scattering shows the magnetic exchange Hamiltonian approximates the quantum triangular lattice J₁-J₂ model. Although the system orders at the lowest temperatures with 120° order, comparison to Schwinger boson and DMRG simulations shows the KYbSe₂ excitations strongly resemble a 2D quantum spin liquid. At finite temperatures we observe quantum critical scaling, which indicates proximity to a deconfined spin liquid phase, and also sheds light on the nearby quantum spin liquid state.
[1] Scheie, Laruell et al, Phys. Rev. B 103, 224434 (2021)
[2] Scheie et al, ArXiv:2109.11527 (2021)

Materials exhibiting reduced dimensionality and strongly interacting charge and lattice degrees of freedom can appear in various atomic structure states that harbor fascinating quantum phenomena. The complexity of the states, however, often makes it challenging to understand the nature of the phenomena, impeding their exploration for practical applications. Typical examples are the emergence of charge density waves (CDWs) and Wyel semimetal phases in transition metal dichalcogenides and 2D magnetism in transition metal thiophosphates. We will show that the problem can be solved by using high-energy x-ray diffraction coupled to atomic pair distribution function analysis. Examples will include results of our recent studies on the genesis of commensurate CDWs in 2H-TaSe₂ [1], chemically distinct CDWs in TaTe₂ [2], local structure memory effects in Wyel semimetal MoTe₂ [3] and TMPS₃ 2D antiferromagnets (TM=Mn, Fe, Ni) [4].
1. V. Petkov et al. PRB 101, 121114(R), (2020).
2. V. Petkov et al. PRB 102, 024111 (2020)
3. V. Petkov and Y. Ren PRB 103, 094101 (2021).
4. V. Petkov et al. (work in progress)

Topological insulators (TIs) are materials that have an inverted band structure. At the interface between a TI and a topologically-trivial material, linearly-dispersing surface states form. Electrons occupying these surface states are massless, two-dimensional, and spin-polarized. When light shines on a TI, Dirac plasmon polaritons (DPPs) can be excited from the surface state electrons. These DPPs are predicted to exist in the terahertz frequency range, show unusual dispersion relationships, and should be spin-polarized. In addition, by exciting DPPs, we can probe the properties of the TI film.
In this talk, I will review our work on exciting DPPs in TI thin films and multilayers. We first explored DPPs in single TI layers as a function of film thickness and stripe width. By mapping the dispersion relationship, we were able to show conclusively that we were exciting DPPs and that these excitations had record large mode indices and long lifetimes. We attribute the long lifetime to the reduction in scattering pathways due to the spin-momentum locking in the material. Next, we explored in-plane coupling between the TI stripes. In this experiment, we kept the stripe width and film thickness constant and changed the gap size between the stripes. The DPP frequency decreased as the gap size decreased, consistent with dipole-dipole coupling between the stripes. Finally, we explored coupling within TI heterostructures. In this case, we used structures comprising two TI layers separated by a trivially-insulating spacer layer. We show clear evidence for coupling across the spacer layer and explore how the coupling changes as a function of structure parameters. We now have an understanding of the TI plasmonic “bandstructure” and can start using these materials for terahertz applications.

Magnetic topological insulators have been greatly studied in the past decades to achieve and enhance the critical temperature of the Quantum anomalous Hall. One way to induce magnetization into TIs is by using the magnetic proximity effect (MPE). It will introduce a magnetic exchange gap which has never been directly measured. Pb_1-xSn_xSe (0.16<x<0.4) is a topological crystalline insulator (TCI) with mirror protected gapless surface states and is lattice match with EuSe a magnetic insulator. It is thus an ideal platform to study the MPE. Here, we investigate the MBE growth of Pb_1-xSn_xSe/EuSe heterostructures with x=0.1-0.3 and probe the exchange induced gap at the interface using magnetooptical spectroscopy. Landau level transitions can be extracted as of 4T and are fitted using a k●p quantum well model. The high mobility and low Fermi energy of these heterostructures will allow to directly probe the energy gap induced by the MPE.

Chalcogenide materials display a wide range of solid state phenomena, some of which are relevant for quantum and topological phenomena, including topological ground states, rich spin physics, and superconductivity. Thin-film synthesis of this materials class enables tuning of condensed matter properties using lattice strain and dimensional effects at surfaces and interfaces. It also facilitates the development of nanoscale devices for applications, and permits the integration of other functional materials via heterostructuring approaches, providing further opportunities for creating exotic quantum phases that result from proximity effects and quantum confinement. One obstacle to realizing these goals is a general lack of extremely high-quality thin films with very limited defects. For example, the synthesis of chalcogenide thin films is often accompanied by the formation of an inhomogeneous domain structure or stoichiometric imperfection, which can obscure novel quantum and topological states.

In this talk, we focus on the thin-film synthesis of chalcogenide materials by molecular beam epitaxy (MBE) and the application of characterization techniques to understand and control the structure-property relationships in thin film chalcogenides. The MBE technique allows precise stoichiometry control, film-thickness control at the sub-monolayer level, and kinetic control of defect formation. We describe advances in MBE techniques that enable epitaxial growth of chalcogenide thin films with atomically flat surfaces and interfaces, followed by a description of topological and electronic properties. We also introduce the possibility of inducing picoscale structural distortions and changing local stoichiometry at surfaces and interfaces, focusing on the effect of structural distortions by the substrate on electronic correlations and macroscopic quantum transport. The investigation of such effects requires methods of measuring electronic and physical structures that are sensitive to surface and buried interfaces, representing another challenge to investigating novel quantum phases of chalcogenide thin films. An iterative feedback approach, which combines synthesis with monolayer precision and high-resolution characterization techniques, provides new insights into novel quantum phases that arise in this class of materials.

The three-dimensional Dirac semimetal Cd₃As₂ has been shown to exhibit a variety of novel physics, providing a promising platform for their study. Thin film synthesis is enabling for scientific study as well as the realization of new devices, and growth has already been carried out on GaAs, GaSb, CdTe, SrTiO₃ and mica substrates. Due to its low energy (112) surface, however, the majority of thin film synthesis routes result in this orientation, while single crystals are limited by this cleave plane when performing studies requiring pristine surfaces. By expanding compatible substrate orientations, and ultimately the Cd₃As₂ orientation, much more of the band structure may be probed easily via photoemission, and a broader range of device structures may be integrated with it.
Here, we present the design of II-VI buffer layers to template high quality Cd₃As₂ in the (001) and (110) orientations on GaAs substrates. Lattice-matched Zn_xCd_1-xTe buffers are known to reduce defects in Cd₃As₂ epilayers grown on GaAs and improve their electron mobility [1]. We find that using ZnTe nucleation layers is critical for stabilizing Cd₃As₂(001) growth on GaAs (001) substrates, while Zn₃As₂ nucleation layers are required to remove tilt in the Zn_xCd_1-xTe buffer when growing on GaAs(110). These films have a much different morphology due to the higher surface energy, and also a much different dependence on arsenic incorporation compared to Cd₃As₂(112). However, we show that the Cd₃As₂ epilayers still exhibit room temperature electron mobilities greater than 18,000 cm²/V-s. Finally, we present a methodology for growing Cd₃As₂(112) epilayers on GaAs(001) substrates using an engineered CdTe interface to switch between orientations. Such schemes will allow for the design of Cd₃As₂ orientation for specific measurement and application needs.
[1] A. D. Rice, K. Park, E. T. Hughes, K. Mukherjee, K. Alberi. Phys. Rev. Mat. 3, 121201(R) (2019)

Two dimmensional metal-organic frameworks (2D MOFs) have applications in functional electronics. Recent experiments [1] has discovered that dicyanoanthracene-copper, a MOF with a kagome lattice, has magnetic moments which arise due to strong electronic correlations between dicyanoanthracene molecules when on some substrates. Using a combination of density functional theory calculations and a Hubbard model, we investigate how the substrate influences magnetism in this 2D MOF systems. We cosider a large variety of substrates ranging from semiconducting to metallic, and show why this interaction-induced magnetism appears on some substrates but not others. We demonstrate that MOF-substrate coupling, charge transfer, and strain are essential key variables that control the magnetism in 2D MOFs. We further show how strain and electric fields can activate and deactivate magnetic phases in these materials. Our model presented here can be generalised to other MOFs and help design new functional devices.

[1] D. Kumar et al. Adv. Funct. Mater. 2021, 2106474.

Nanoscale lateral p-n junctions in graphene provide a powerful platform for investigating fundamental quantum interfacial phenomena. In this work, we study graphene/α-RuCl3 intrinsic nanobubbles as a testbed for probing sharp p-n interfaces at the boundaries. In order to elucidate this, we deployed scanning tunneling microscopy (STM) and spectroscopy (STS), and scattering-type scanning nearfield optical microscopy (s-SNOM) as multi-messenger local probes to study both the electronic and plasmonic properties. Our differential conductivity (dI/dV) results indicate a substantial band offset of more than 0.6 eV due to work function mediated charge transfer between α-RuCl3 and graphene. Further, our findings show an abrupt junction along nanobubble boundaries separated by a lateral distance of ~ 3 nm and vertically by ~ 0.5 nm, giving rise to internal fields on the order of 10⁸ V/m. The lateral length scale is one order of magnitude smaller than previously reported junctions by lithographically defined gated devices. The rapid change in the graphene conductivity is also captured well in s- SNOM measurements validating these nano-junctions in plasmonically-active media. Our results are in agreement with density functional theory (DFT) calculations shedding light on the in-plane and out-of-plane charge transfer process. Our study provides insight for nano-scale device engineering using interfacial charge transfer in graphene.

A central goal in condensed matter physics is to understand and control the order parameter characterizing the collective state of electrons in quantum heterostructures. For example, new physical behaviors can emerge that are absent in the isolated constituent materials. With regards to superconductivity this has opened a whole new area of investigation in the form of topological superconductivity. This type of superconductivity is expected to host exotic quasi-particle excitations including Majorana bound states which hold promise for fault-tolerant quantum computing. In this talk, we first discuss the important role of epitaxial superconductor-semiconductor hybrid systems as an enabling materials platform. We present unprecedented values of transparency and induced gap that could allow us to reach into previously unexplored parameter regimes. In wide Josephson junctions exposed to magnetic field, we observe a minimum of critical current accompanied with a phase jump in the superconducting phase. We discuss this observation as a signature of a transition between trivial and topological superconductivity. These findings in addition to new directions in proximitizing edge modes reveal a versatile two-dimensional platform to explore mesoscopic and topological superconductivity.

Indium Arsenide (InAs) near surface quantum wells are ideal for the fabrication of semiconductor-superconductor heterostructures given that they allow for a strong hybridization between the two-dimensional states in the quantum well and the states in the superconductor. The key ingredients for realizing induced superconductivity in integer quantum Hall edge modes are 1) strong coupling of superconductor to the two-dimensional electron gases (2DEG) is confined near the surface. 2) superconductors that can tolerate large out-of-plane magnetic fields 3) 2DEG with high mobility and low density. We introduce NbTiN-InAs as a potential candidate which can satisfy all the above criteria necessary to achieve coupling of superconductor to quantum Hall edge. To achieve a transparent contact between NbTiN and InAs quantum well, we use molecular beam epitaxy (MBE) in combination with sputtering machine. We start with epitaxial Al-InAs heteroatructures and remove the Al right before deposition of NbTiN to achieve high transparency. The transport in quantum Hall regimes shows a negative resistance with a corresponding reduction of Hall resistance. We analyze the experimental data using the Landauer-Büttiker formalism, generalized to allow for Andreev reflection processes. Our analysis is consistent with a lower-bound for the averaged Andreev conversion of about 15%, a value that we estimate to be almost an order of magnitude larger than the values so far reported in the literature. We attribute the high efficiency of Andreev conversion in our devices to the large transparency of the InAs/NbTiN interface. We show that by introducing impurity at the interface the corrections to resistance disappear. This work was performed, in part, at the Center for Integrated Nanotechnologies, an Office of Science User Facility operated for the U.S. Department of Energy (DOE) Office of Science by Los Alamos National Laboratory (Contract 89233218CNA000001) and Sandia National Laboratories (Contract DE-NA-0003525). This work was supported partially by NSF EAGER under award number DMR-1836687, the MRSEC Program of the National Science Foundation under Award Number DMR-1420073, and ARO under award number W911NF-16-1-0387.

Jacutingate, a recently discovered Brazilian naturally occurring mineral, is the first experimental realization of the Kane-Mele topological model. Such material with atomic formula Pt2HgSe3 shares the structural backbone of the transition metal dichalcogenide PtSe2. Although the latter is a trivial insulator with an energy gap of 1.2 eV, the Hg incorporation leads to a topological phase. Such modification and induced topological phase are not exclusive to this material. We have theoretically explored 26 other minerals of the jacutingaite’s class and found materials surpassing its key features, i.e., high thermal/energetic stability and a robust topological phase [1]. Additionally to the Hg modification of the PtSe2 backbone, we show a mechanism of mediated interaction between Se vacancies inducing a topologically non-trivial phase, that is, a defect-induced topological phase. The vacancies states lead to a large topological gap of 180 meV neatly lying within the pristine system gap. The vacancy density required for such a phase to appear is already an experimental reality. We derive an effective model describing this picture in other transition metal dichalcogenide systems [2]. Such defect-induced topology in the trivial transition metal dichalcogenide was further explored in the disordered alloy system Pt(Hg1-xSex)2. Here for any Hg concentration that allows the interaction between its s states, a topological phase emerges.
REFERENCES
[1] F. Crasto de Lima, R. H. Miwa and A. Fazzio. Jacutingaite-family: A class of topological materials. PHYSICAL REVIEW B, v. 102, p. 235153, 2020.
[2] F. Crasto de Lima and A. Fazzio. At the verge of topology: vacancy-driven quantum spin Hall in trivial insulators. ArXiv:2109.11915 (2021)

It is critical to understand the laws of quantum mechanics in transformative new technologies for computation and quantum information science applications to enable the ongoing second quantum revolution calls. Recently, spin qubits based on point defects have gain great attention since these qubits can be initiated selectively controlled and readout with high precision at ambient temperatures. The major challenge in these systems is being able to controllably generate multiqubit systems while also properly coupling the defects. Transitional metal dichalcogenides (TMDs) have a lot of potential in this field because it's easy to controllably manufacture and alter defects for scalable samples. Defects can be created in 2D materials using a variety of methods. In this research we have used proton irradiation at various energies changing between 100 keV to 4 MeV to controllably create and modulate the antisite defects on CVD-grown monolayer MoS₂ and WS₂ materials. Here we report the experimental identification of antisite defects and their qubits via atomic scale morphological, physicochemical and optical studies in two TMD systems as novel 2D solid- state defect qubits. Our research is novel, timely, and full of intriguing findings about the influence of proton irradiation on the physicochemical properties of CVD-grown monolayer TMDS. Researchers will be able to engineer these materials with the help of these results and develop scalable, room temperature applications such as radiation detectors, spintronic devices, and space equipment that can operate in ambient settings.

Van der waal’s structures provide for many possibilities for tailoring magnetic, ferroelectric and superconducting properties. Firstly, we discuss the observation of topological chiral spin textures in thin lamellae prepared from single crystals of two vdw’s compounds, Fe₃GeTe₂¹ and and Cr_1+δTe₂ (δ ≈ 0.3)². We find direct evidence for Néel skyrmions using Lorentz transmission electron microscopy. We show that these topological magnetic structures are made possible by self-intercalation of Cr in the latter and by an asymmetric distribution of Fe vacancies in the former. These significant structural changes lower the symmetry of the respective crystals and, in particular, break the structural inversion symmetry, thereby, making possible the chiral spin textures that we observe. In both cases the skymion size increases significantly as the thickness of the lamella in which the skyrmions are hosted is increased. In the extreme monolayer limit, using molecular beam epitaxy, we have prepared single monolayers of CrCl₃ on graphene/6H-SiC(0001) substrates and observed robust nearly ideal easy-plane ferromagnetic ordering with critical scaling characteristic of a 2D-XY system³. Secondly, we discuss ferroelectric ordering of SnTe⁴ and SnSe⁵ in the monolayer limit and demonstrate greatly enhanced transition temperatures compared to the bulk materials related to significant structural changes. Finally, we discuss our recent work on proximity induced superconductivity in a Dirac semi-metal vdw’s layer and our observation of a very large Josephson diode effect.
1 Chakraborty, A. et al. Magnetic skyrmions in a thickness tunable 2D ferromagnet from a defect driven Dzyaloshinskii-Moriya interaction. under review (2021).
2 Saha, R. et al. Observation of Néel-type skyrmions in acentric self-intercalated Cr_1+δTe₂. submitted (2021).
3 Bedoya-Pinto, A. et al. Intrinsic 2D-XY ferromagnetism in a van der Waals monolayer. Science 374, 616-620 (2021).
4 Chang, K. et al. Enhanced Spontaneous Polarization in Ultrathin SnTe Films with Layered Antipolar Structure. Adv. Mater. 31, 1804428, doi:10.1002/adma.201804428 (2019).
5 Chang, K. et al. Microscopic manipulation of ferroelectric domains in SnSe monolayers at room temperature. Nano Lett. 20, 6590-6597 (2020).

One of the most successful strategies to generate strongly correlated electrons in materials is to design crystalline solids in which localized magnetic moments can entangle with itinerant electrons to produce a coherent narrow band of “heavy fermions”. Such heavy fermion bands are a playground for the observation of quantum phases driven by electron interactions. Confining heavy fermions in two dimensions offers unique opportunities to tune the Coulomb interactions in ways that are not possible in 3D crystals, and could provide new insights into the interplay of electron localization, magnetism, quantum criticality and superconductivity. In this talk, we will discuss our recent progress in developing a new class of 2D heavy fermion materials. These materials are conceptually related to traditional intermetallic heavy fermion compounds but their layered van der Waals (vdW) structures can be mechanically exfoliated to produce atomically thin flakes. We report heat capacity, magnetic, electrical transport and spectroscopic characterizations demonstrating that this compound is the first truly 2D heavy fermion material. In addition, the material features a triangular lattice of magnetic atoms that order antiferromagnetically below 7 K, producing a magnetically frustrated lattice with an incommensurate propagation vector. By varying the thickness of the material using mechanical exfoliation down to the monolayer, this new system offers the unique opportunity to interrogate the fundamental relationship between dimensionality, magnetism and heavy fermion.

The advent of long-range magnetic order in two-dimensional (2D) layered compound has stimulated interest in the possibility of topologically protected spin textures, such as skyrmions for applications in next-generation spintronic devices. However, stable skyrmion formation has only been achieved at cryogenic temperatures in a few van der Waals materials and heterostructures. This work reports a novel wurtzite-structure polar magnet, identified as AA’-stacked (Fe_0.5Co_0.5)_5-xGeTe₂, which exhibits a zero-field, Néel-type skyrmion lattice at room temperature. The static and dynamic spin textures were studied via electron microscopy, magnetic force microscopy, and magneto-transport measurements. This discovery reveals an unprecedented 2D magnetic system for investigating intriguing topological quantum phases in an ambient environment and ushers in a promising new framework for skyrmion-based spintronics.

We report a magnetic imaging study of the current distribution in the quantum anomalous Hall regime. We use a scanning superconducting quantum interference device (SQUID) microscope with micrometer scale spatial resolution to image the magnetic fields above Cr-doped (Bi,Sb)₂Te₃ samples. From these data we reconstruct the local current density, allowing us to visualize the current distribution in our devices. Surprisingly, we find that most current flows in the bulk when the transport coefficients are quantized. By combining this observation with images of the equilibrium magnetization, we construct a comprehensive picture of electronic transport in the quantum anomalous Hall regime.

Anyons are quasiparticles that, unlike fermions and bosons, show fractional statistics when two of them are exchanged. Here we report on the direct observation of anyonic braiding statistics for the ν=1/3 fractional quantum Hall state by using an electronic Fabry-Perot interferometer fabricated on a novel GaAs heterostructure hosting a two-dimensional electron gas. Strong Aharonov-Bohm interference of the edge mode is punctuated by discrete phase slips that indicate an anyonic phase θ =2π/3. The influence of anyonic statistics can be further probe by examination of interferometers in which key system parameters are varied. The interplay of material-specific parameters, heterostructure design, and device operation will be discussed. Finally we discuss how learnings from the GaAs system may be applicable to other material systems that may habor abelian and non-abelian quasiparticles.

Time reversal symmetry is central to the characterization of a wide variety of quantum materials as it places fundamental constraints on electronic properties. Breaking such symmetry often reflects the topological features of the underlying ground state, points to the emergence of internal magnetic fields and gives rise to Hall currents. Experimental probes which are sensitive to this symmetry breaking without relying on contacts are relatively rare, and, as a result, it is often challenging to unequivocally establish the presence and microscopic origin of time-reversal symmetry breaking. Here, we present an alternative approach to detect time-reversal symmetry breaking based on coupling between electromagnetic modes in a cross-waveguide geometry. Specifically, we consider a small dielectric sample at the intersection of two waveguides and show how the breaking of time-reversal symmetry manifests in the scattered electric field. We conclude by discussing various experimental platforms where this technique may be applied.

Photons play a significant role in quantum information and quantum communication technologies because they are largely immune to environmental perturbations and can be sent over long distances with minimal loss. However, photons do not interact with each other in linear media, making it difficult to perform two-qubit photonic gate operations or study many-body quantum physics with photons. As such, strong photon nonlinearities are indispensable. Rydberg electronic states with high principal quantum numbers can provide unprecedented single-photon nonlinearities via strong dipole-dipole and Van der Waals interactions facilitated by their extended wave functions. Rydberg excitons in solid-state materials are of particular interest, as they would marry the unparalleled interaction strengths of Rydberg states with the scalability and integrability of solid-state systems.

To accomplish this, a suitable platform for Rydberg excitons must be found. Due to their high binding energies, two-dimensional materials have been studied extensively in this regard, but they are limited by a short recombination time. Cuprite (Cu₂O), a semiconductor which also has a large binding energy, overcomes this barrier with its unique band structure and symmetric lattice. These properties allow it to foster long-lived Rydberg states while suppressing coupling to phonon modes. While states up to n = 30 have been observed in bulk cuprite crystals, it is often difficult to generate and control ordered excitonic states in bulk semiconductors. However, in synthetic thin-film samples, which are more suitable for this purpose, only states up to n = 6 have been observed. This makes it desirable to have a robust growth recipe for mass-producing high-quality synthetic cuprite with controlled quantum optical properties.
In this presentation, we will demonstrate excitonic Rydberg states in thin-films of synthetic cuprite fabricated using a CMOS compatible, single-step thermal oxidation process. In our synthesis, we began by depositing copper onto silicon wafers via a PVD e-beam evaporator with an intermediate Ti layer to help adhesion. We then oxidized the samples in a quartz tube furnace at elevated temperature and low pressure. As part of the characterization of our samples, we used X-ray diffraction to determine their crystal orientations and a scanning electron microscope to observe their surface morphologies. Finally, we used a high-resolution photoluminescence experiment to identify which Rydberg states were present in our samples. Using the results from this experiment, we will report the existence of highly excited Rydberg states in our synthetic cuprite samples. Our results will open the portal to a new paradigm of on-chip, scalable, Rydberg-based quantum technologies ranging from non-classical photon sources to long-lived paraexciton qubits, quantum simulators, and broadband quantum sensors.

Using ab initio methods, we systematically investigate the electronic structures and intrinsic magnetic properties in RMn₆Sn₆ with R = Gd, Tb, Dy, Ho, and Er. The calculations show that TbMn₆Sn₆ has an easy-axis anisotropy, DyMn₆Sn₆ and HoMn₆Sn₆ have easy-cone anisotropy, and ErMn₆Sn₆ has an easy-plane anisotropy, all agreeing well with experiments and explained by the Mn coordination of the rare-earth atoms. The Mn sublattice is found to have an easy-plane anisotropy of similar amplitude in all RMn₆Sn₆ compounds. Band structures of various RMn₆Sn₆ compounds share great similarities near the Fermi level as they mostly consist of non-4f bands. Multiple Dirac crossings occur at the BZ corners and are opened by spin-orbit coupling; most of them are strongly kz-dependent. The most prominent 2D-like (barely-kz-dependent) Dirac crossing at K lies 0.6–0.7 eV above the Fermi level. However, additional on-site correlations within Mn-d electrons can have profound effects on crossing energies and gap sizes. Finally, we discuss the effects of spin reorientation and surface on the topological band structures.

Metastable magnetic skyrmions are nano-scale magnetic states that could be used in various spintronics devices but a central issue is the mechanism and rate of various possible annihilation processes determining their lifetime. While most studies have focused on classical over-the-barrier mechanism for annihilation, it is also possible that quantum mechanical tunneling through the energy barrier takes place. Calculations of the lifetime of magnetic skyrmions in a two-dimensional lattice are presented and the rate of tunneling compared with the classical annihilation rate. A remarkably strong variation in the onset temperature for tunneling and the lifetime of the skyrmion is found as a function of the values of parameters in the extended Heisenberg Hamiltonian, i.e. the out-of-plane anisotropy, Dzyaloshinskii–Moriya interaction and applied magnetic field. Materials parameters and conditions are identified where the onset of tunneling could be observed on a laboratory time scale. In particular, it is predicted that skyrmion tunneling could be observed in the PdFe/Ir(111) system when an external magnetic field on the order of 6T is applied.
[1] S.M. Vlasov, P.F. Bessarab, V.M. Uzdin and H. Jónsson, Nanosystems: Physics, Chemistry, Mathematics, 8, 454 (2017)
[2] S. M. Vlasov, P.F. Bessarab, I. S. Lobanov, M. N. Potkina, V. M. Uzdin and H. Jónsson, New Journal of Physics 22, 083013 (2020).

Exotic topological quantum phases can be induced via the interplay between topology and magnetic ordering in magnetic topological quantum materials, however the characterization of these exotic phases are high nontrivial. Optical probes, especially nonlinear optical spectroscopy, provide a rich set of alternative tools to probe their topological nature and understand the inherent electronic structure in these quantum materials [1, 2]. Here we present a microscopic theory of two types of second-order nonlinear direct photocurrents, magnetic shift photocurrent (MSC) and magnetic injection photocurrent (MIC), as the counterparts of normal shift current (NSC) and normal injection current (NIC) in time-reversal symmetry and inversion symmetry broken systems [3] . We show that MSC is mainly governed by shift vector and interband Berry curvature, and MIC is dominated by absorption strength and asymmetry of the group velocity difference at time-reversed k-point pairs. We take parity-time symmetric magnetic topological quantum material bilayer antiferromagnetic (AFM) MnBi₂Te₄ as an example and predict the presence of large MIC in the terahertz (THz) frequency regime which can be switched between two AFM states with time-reversed spin orderings upon magnetic transition. In addition, external electric field breaks PT symmetry and enables large NSC response in bilayer AFM MnBi₂Te₄, which can be switched by external electric field. Remarkably, both MIC and NSC are highly tunable under varying electric field due to the field-induced large Rashba and Zeeman splitting, resulting in large nonlinear photocurrent response down to a few THz regime, suggesting bilayer AFM-z MnBi₂Te₄ as a tunable platform with rich THz and magneto-optoelectronic applications. Our results reveal that nonlinear photocurrent responses governed by NSC, NIC, MSC, and MIC provide a powerful tool for deciphering magnetic structures and interactions which could be particularly fruitful for probing and understanding magnetic topological quantum materials and their topological and ferroelectric phase transitions. This work was supported the National Science Foundation (NSF) under award number DMR-1753054. References: [1] npj Computational Materials 5, 119 (2019). [2] Nature Physics 16, 1028-1034 (2020). [3] npj Computational Materials 6, 199 (2020).

Topological photonics mimic topological condensed matter physics which has opened up possibilities to transport the photons efficiently and robustly. In this poster presentation, we present spin-valley locking of edge states through staggered chiral photonic crystals which have the two-dimensional honeycomb unit cell. We demonstrate the bulk band dispersion with a bandgap in the spin-valley locked edge states and by calculating with a coupled dipole method. And the spin-valley locking characteristics can be obtained when the staggered chirality breaks the inversion symmetry.

“Floquet engineering” of band structures through the application of coherent time-periodic drives has recently emerged as a powerful tool for creating new types of topological phases. In this work, we show that this tool can also be used to induce non-equilibrium correlated states with dynamical spontaneously broken symmetry. We study how such phenomena can arise in periodically driven systems featuring a ring-like minimum in their Floquet dispersion. In particular, we focus on lightly doped semiconductors driven by a resonant driving field. We show that such a system can spontaneously develop quantum liquid crystalline order featuring extreme anisotropy whose directionality rotates as a function of time. The phase transition to this correlated state occurs in the steady state of the system achieved due to the interplay between the coherent external drive, electron-electron interactions, and dissipative processes arising from the coupling to phonons and the electromagnetic environment. Our results demonstrate how Floquet engineering can be used to induce novel non-equilibrium phases exhibiting an interplay of topology and dynamical symmetry breaking.

In this work, we investigate the electronic transport properties of buckled bismuthene nanoribbons of realistic sizes using the full Hamiltonian in the orbital representation obtained from DFT calculations combined with recursive nonequilibrium Green’s functions (NEGFs). First, we explore the backscattering mechanism due to inter-edge hopping mediated by vacancy localized states in different nanoribbon widths and discuss the detrimental effects on transport properties caused by single vacancies in narrow nanoribbons. We proceed to investigate the robustness of the edge states for different ribbon widths, reaching extended lengths (157 nm) as a function of vacancy concentration, demonstrating distinct behaviors in the transport properties for bulk and edge states [1]. Additionally, we use a random bond-flip method combined with ab initio calculations to generate flat bismuthene amorphous structures. We characterize the structure by different statistical measures and find that the ring size distribution nicely captures the degree of amorphization of our structures. We find that the amorphization tends to suppress the bandgap but does not close it. We find that the survival of the QSH state through the amorphization process is associated with the SOC strength of the material and the size of the bulk bandgap [2]. A contrasting behavior appears on 3D systems. Crystalline Bi2Se3 is one of the most explored three-dimensional topological insulators. Its amorphous counterpart could bring to light new possibilities for large-scale synthesis and applications. Using ab initio molecular dynamics simulations, we have studied realistic amorphous Bi2Se3 phases generated by different pelting, quenching, and annealing processes. Extensive structural and electronic characterizations show that the melting process induces an energy gap decrease ruled by the growth of the defective local environments. Different from the crystalline phase, the realization of amorphous Bi2Se3 yields a trivial insulator characterized by a zero spin Bott index. By sampling different structures from annealing simulations, we study the structural phase transition from the crystalline to the amorphous phase and the topological phase transition induced by disorder. This behavior dictates the weak stability of the topological phase to disorder [3].


REFERENCES

[1] Armando Pezo, Bruno Focassio, Gabriel R. Schleder, Marcio Costa, Caio Lewenkopf, and Adalberto Fazzio. Disorder effects of vacancies on the electronic transport properties of realistic topological insulators nanoribbons: the case of bismuthene. Phys. Rev. Materials 5, 014204 (2021).

[2] Bruno Focassio, Gabriel R. Schleder, Marcio Costa, Adalberto Fazzio, and Caio
Lewenkopf. Structural and electronic properties of realistic two-dimensional amorphous topological
insulators. 2D Mater. 8 025032 (2021).

[3] Bruno Focassio, Gabriel R. Schleder, F. Crasto de Lima, Caio Lewenkopf, and Adalberto Fazzio. Amorphous Bi2Se3 structural, electronic, and topological nature by first-principles. ArXiv: 2109.10858 (2021)

In this talk, I will introduce a new type interesting band structure in 2D materials, consisting of a flat valence band and a flat conduction band around the Fermi level, named yin-yang flat bands characterized with opposite chirality (Chern number). I will first discuss the construction of the yin-yang flat bands based on line graph theorem and its extension to multiple atoms and orbitals per lattice site in 2D tight-binding lattice models. I will then present one exotic quantum phase, the excitonic insulator (EI) state that the yin-yang flat bands may host (1). The EI is a strongly correlated many-body ground state, arising from an instability in the band structure toward exciton formation. We show that the flat valence and conduction bands of a semiconducting diatomic Kagome lattice, as exemplified in a superatomic graphene lattice, can conspire to enable a triplet EI state, based on density-functional theory calculations combined with many-body GW and Bethe-Salpeter equation. Our results indicate that massive carriers in flat bands with highly localized electron and hole wave functions significantly reduce the screening and enhance the exchange interaction, leading to an unusually large triplet exciton binding energy (∼1.1 eV) exceeding the GW band gap by ∼0.2 eV and a large singlet-triplet splitting of ∼0.4 eV. I will end the talk discussing on-going studies and future directions to explore other possible exotic many-body quantum states associated with the yin-yang flat bands, such as excitonic spin-1 superfluid. [(1). G. Sethi, et. al., Phys. Rev. Lett. 126, 196403 (2021)]

In the present talk, I will first introduce how the non-interacting electronic structure of magic-angle twisted bilayer graphene (MABLG) can be approximately understood from the picture of pseudo-Landau level (PLL) states, which are formed near the AA stacking area and can be viewed as the “atomic states” for the Moire superstructure. Together with the plane wave components in the AB and BA region of MABLG, the exact band description can be constructed and the corresponding condition for the flat band can be derived. Next, I will show how the orbital magnetism in MABLG can be also understood based on the PLL picture and how it can be stabilized by the external magnetic field.

The kagome network is a generalized lattice model whose characteristic geometry enforces the coexistence of Dirac electrons and flat band. Bulk single crystal studies on quasi-layered antiferromagnetic metal FeSn (T_N = 365K), containing Fe-kagome network as a constituent layer, have experimentally verified the existence of the predicted band singularities in its bulk electronic structures, evincing the relevance of the original two-dimensional lattice model when inter-layer hybridization is sufficiently suppressed [1]. Here we report the synthesis and characterizations of FeSn epitaxial thin films. We employ electrical transport and magnetic torque measurements to confirm high electronic quality and robust antiferromagnetic order in our films (T_N,film = 364K) [2]. Quantum oscillations further reveal consistent Fermiology to that in bulk single crystal FeSn [2,3]. We also present planar tunneling spectroscopy across a Schottky heterointerface between FeSn films and n-type degenerate semiconductor substrates [3]. Based on these results, we present our current understanding of surface and bulk electronic structures of FeSn. We also discuss the implication of our works for potential spintronic device applications.
References:
[1] M. Kang, L. Ye et al., Nat. Mater. 19, 163 (2020)
[2] H. Inoue, M. Han et al., Appl. Phys. Lett. 115, 072403 (2019)
[3] M. Han, H. Inoue et al., Nat. Commun. 12, 5345 (2021)

Quasi 1D materials have recently received much interest due to their unique properties such as charge density waves, superconductivity, non-trivial topology, high current capacity, anisotropic behavior etc. The unique properties of these materials are attributed to the presence of 1D chains weakly bonded together while the plane of chains is separated by weaker van der Waals bonds. Much work has been done to understand the bulk form of these materials, but the study of extreme cross-sectional properties (layer and chain) is just beginning. In this work, we study the electronic, vibrational, and thermal properties of transition-metal trichalcogenide (TMT) quasi 1D material as the thicknesses of these materials are decreased (bulk to monolayer to single chain). We focus on TaSe₃ and ZrTe₃ due to their potential as interconnect and understanding their properties at extreme scales will help us to realize their performance under ultra-scaled cross-sections. Both TaSe₃ and ZrTe₃ are TMT materials with monoclinic configurations. The transition metal (M =Ta, Zr) of these materials sits at the center of a prism surrounded by chalcogen (X=Se, Te) atoms. Thus, a strong M-X covalently bonded chain is created by these prisms which are connected side-by-side via longer, weaker M-X bonds, and planarly stacked with even weaker van der Waals type bonds that makes them quasi 1D in nature. Using Density Functional Theory (DFT) calculations, we analyzed the vibrational properties of these materials. Our phonon calculation shows that, the monolayer structures are dynamically stable. Also, the spin orbit coupling (SOC) in monolayer TaSe₃ results in a small bandgap due to the anti-crossing of bands near the Fermi level, which is not present in its bulk counterpart.
Funding Acknowledgement: This work was funded in part by ONR award number N00014-21-1-2947 under the Vannevar Bush Faculty Fellowship awarded to A. A. Balandin. This work used the Extreme Science and Engineering Discovery Environment (XSEDE), which is supported by National Science Foundation Grant No. ACI-1548562 and allocation ID TGDMR130081.

Two-dimensional higher-order topological insulators can display a number of exotic phenomena, such as half-integer charges localized at both corners and disclination defects. Here, we analyze these phenomena in the continuum limit, and present a topological field theory description of the mixed geometry-charge responses. Our theory provides a unified description of the corner and disclination charges in terms of a physical geometry (which encodes disclinations), and an effective geometry (which encodes corners). We extend this analysis to interacting systems, and predict the existence of fractional quadrupole insulators, which exhibit charge e/2(2k+1) bound to corners and disclinations.

Monolayer FeSe_xTe_1-x is a high-temperature, unconventional, possibly topological superconductor thought to possesses a sign-changing s-wave order parameter. There is significant experimental evidence that the order parameter is of the s_+- type, but so far no experiments have been able to differentiate between the many variants of s_+- pairing, such as conventional s_+-, odd-parity s_+-, and orbital-antiphase s_+-. To address this shortcoming, we perform superconducting quasiparticle interference (QPI) measurements on FeSe_xTe_1-x grown on Bi₂Te₃, finding a striking antisymmetrized QPI response consisting of two peaks of opposite signs at the wavevector connecting the electron and hole pockets. According to the Hirschfeld-Altenfeld-Eremin-Mazin theory of QPI, a single peak of this kind indicates a sign change in the order parameter between the connected pockets and is an established experimental signature of s_+- pairing in bulk FeSe. We perform extensive QPI simulations using a detailed tight-binding model of FeSe_xTe_1-x to determine the origin of the dual peak structure and whether it is related to the sign structure of the order. By comparing simulated QPI responses for FeSe_xTe_1-x with s-wave, s_+-, odd-parity s_+-, and orbital-antiphase s_+- order parameters, we determine that a dual peak QPI response can occur for any order parameter with a sign change between the hole and electron pockets and that the additional sign changes of the odd-parity and orbital-antiphase order parameters are not necessary. Furthermore, we find that for the two peaks to be of opposite signs requires novel orbital-selective scattering, likely induced by substrate impurities. This indicates that consideration of the scattering mechanism will be key to determining the order parameters of iron-based superconductors via QPI experiments.

The Kagome antiferromagnet Mn₃Sn has become fascinating in the current times because it’s one of the rare antiferromagnets that exhibits large anomalous Hall and Nernst effects. This opens a new area of research using functional antiferromagnets¹, but this requires fabricating high-quality thin films. There are reports of the controlled growth of Mn₃Sn on substrates like m-plane sapphire,² MgO (111),³ Pt/Al₂O₃ (0001),⁴ and Si/SiO₂,⁵ and surfaces have been prepared by cleaving bulk samples.⁶ For the grown films, one thing common in these reports is that all the samples were sputter deposited which often results in polycrystalline films. In this work, the goal is to grow a crystalline high quality Mn₃Sn film using molecular beam epitaxy (MBE). Effusion cells are used for Mn and Sn sources which are calibrated using a quartz crystal thickness monitor. The growth is monitored in-situ using reflection high energy electron diffraction (RHEED) and measured ex-situ using X-ray diffraction and Rutherford backscattering. The samples are grown over a range of substrate temperatures (150-650 °C) and Mn:Sn flux ratios (1.0-4.0) and on different sapphire substrate orientations (c-plane, a-plane and m-plane) to develop an understanding of how to control the film orientation, crystalline quality, stoichiometry, and surface epitaxial smoothness. We also compare growth with and without using a 3 nm thick platinum buffer layer which was used recently by Cheng et al. to both achieve better epitaxial lattice match and to demonstrate a topological Hall effect.⁴ Our preliminary results show that highly crystalline thin films can be achieved via MBE growth at 650 °C with 3:1 stoichiometry and a highly streaky RHEED pattern showing 6-fold hexagonal crystal symmetry on c-plane sapphire substrates. Additional results pertaining to phase and crystallinity as a function of growth parameters will also be discussed.
¹ Songtian Sonia Zhang, Jia-Xin Yin, Muhammad Ikhlas, Hung-Ju Tien, Rui Wang, Nana Shumiya, Guoqing Chang, Stepan S. Tsirkin, Youguo Shi, Changjiang Yi, Zurab Guguchia, Hang Li,Wenhong Wang, Tay-Rong Chang, Ziqiang Wang, Yi-feng Yang, Titus Neupert, Satoru Nakatsuji, and M. Zahid Hasan, "Many body resonance in a correlated topological Kagome Antiferromagnet", Physical Review Letters 125, 046401 (2020).
² Seungjun Oh, Tadashi Morita, Tomoki Ikeda, Masakiyo Tsunoda, Mikihiko Oogane and Yasuo Ando,"Controlled growth and magnetic property of a-plane oriented Mn₃Sn thin film", AIP Advances 9, 035109 (2019).
³ Tomoki Ikeda, Masakiyo Tsunoda, Mikihiko Oogane, Seungjun Oh, Tadashi Morita and Yasuo Ando, “Fabrication and evaluation of highly c-plane oriented Mn₃Sn thin films ", AIP Advances 10, 015310 (2020)
⁴ Yang Cheng, Sisheng Yu, Menglin Zhu, Jinwoo Hwang, and Fengyuan Yang, "Tunable Topological Hall effects in noncollinear antiferromagnets Mn₃Sn/Pt bilayers", APL Materials 9,051121 (2021).
⁵ Tomoya Higo, Danru Qu, Yufan Li, C. L. Chien, Yoshichika Otani, and Satoru Nakatsuji, "Anomalous Hall effect in thin films of the Weyl antiferromagnet Mn₃Sn", Appl. Phys. Lett. 113, 202402 (2018).
⁶ Hung-Hsiang Yang, Chi-Cheng Lee, Yasuo Yoshida, Muhammad Ikhlas, Takahiro Tomita, Agustinus Nugroho, Taisuke Ozaki, Satoru Nakatsuji & Yukio Hasegawa, “Scanning tunneling microscopy on cleaved Mn₃Sn (0001) surface”, Scientific Reports 9, 1 (2019).
*The authors acknowledge support from the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering under Award No. DE-FG02-06ER46317.

Spin frustration in Kagome ferromagnets and antiferromagnets is of great scientific interest currently. Such spin frustrations can lead to non-collinear spin structures and fascinating effects such as the anomalous Hall effect and Nernst effect.¹ Furthermore, there is also great interest in possible topological effects with these materials and in combination with other metals. In addition, a recent paper discusses quantum excitations that result from fractionalization of conventional spin wave excitations present in Kagome-lattice antiferromagnetic material.² And finally, recent theoretical calculations demonstrate that generalized Kagome antiferromagnets support spin-liquid phases that are fractionalized.³ Recently, there have been quite a few reports on the cases of Mn₃Sn and Fe₃Sn₂. Motivated by the interesting effects already mentioned, here we explore the case with chromium. A 1994 paper by Wölpl et al. describes the crystal structure of CrSn2 as belonging to the class of transition metal with an orthorhombic structure.⁴ CrSn₂ crystals grown by heating a Cr/Sn melt and subsequently etching confirmed the existence of a possibly metastable Cr₂Sn₃ phase aside from the CrSn₂ phase and having a similar structure.⁵ Our goal then is to grow these materials using molecular beam epitaxy, and initially focus on the crystal structures, stoichiometries, and film orientations as functions of the growth parameters. Fluxes of Cr and Sn are provided by effusion cells which we calibrate using a quartz crystal thickness monitor. The growths are monitored in-situ using reflection high energy electron diffraction and are performed with a range of Cr:Sn flux ratios (0.5 - 2), substrate temperatures (250 °C – 650 °C), and type of substrate (c-plane Al₂O₃ and (001) MgO). X-ray diffraction (XRD) and Rutherford backscattering (RBS) are used to characterize the crystal structures and stoichiometric ratios ex-situ. Preliminary growths on c-plane sapphire reveal sharp peaks in XRD at 2theta angles of 37.5 and 79.9 degrees. However, for all growth temperatures in the stated range, so far we have only observed polycrystalline growth by means of the RHEED patterns, which may suggest the need for a cubic substrate instead. Ultimately, we plan to characterize the magnetic properties using a combination of SQUID and spin-polarized STM.

¹ L. A. Fenner, A. A. Dee, A. S. Wills, Non-collinearity and spin frustration in the itinerant Kagome ferromagnet Fe₃Sn₂, J. Phys.: Condens. Matter 21 (2009).
² T. H. Han, J. Helton, S. Chu et al., Fractionalized excitations in the spin-liquid state of a Kagome-lattice antiferromagnet. Nature 492, 406–410 (2012).
³ L. Balents, M. P. A. Fisher, S. M. Girvin, Fractionalization in an easy-axis Kagome antiferromagnet. Physical Review B 65, 224412 (2002).
⁴ Torsten Wölpl and Wolfgang Jeitschko, Crystal structures of VSn₂, NbSn₂ and CrSn₂ with Mg₂Cu-type structure and NbSnSb with CuAl₂-type structure, Journal of Alloys and Compounds 210, 185-190 (1994).
⁵ Ann-Kristin Larsson and Sven Lidin, The crystal structures of the closely related CrSn₂, NiCr₅Sn₁₂ and NiCr₃Sn₈ phases, Journal of Alloys and Compounds 221, 136-142 (1995).

This research has been supported by the US DOE, BES, DMSE under Award No. DE-FG02-06ER46317.

Two-dimensional transition metal dichalcogenides (TMDs) have recently gained interest due to their novel optical and electronic phenomena and potential for optoelectronic applications. Chemical vapor deposition (CVD) and chemical vapor transport (CVT) are two commonly used methods for synthesizing TMDs due to their fast growths and high yields. However, they produce highly defective TMDs, negatively impacting the novel properties [1]. One alternative synthesis approach is self-flux crystal growth, where inside of a vacuum sealed quartz ampule, the transition metal is dissolved in excess liquid chalcogen at an elevated temperature, then slowly cooled to produce high-quality crystals. Unlike CVD and CVT, self-flux uses high-purity elemental transition metal powders and chalcogen pellets as starting reagents, minimizing extrinsic impurity sources during synthesis. The self-flux growth method has been shown to yield TMDs with defect densities several orders of magnitude lower than those from CVD and CVT [2]. Yet, it is unknown how close the point defect density in self-flux TMDs is to the theoretical intrinsic equilibrium defect density. In this work, we perform density functional theory calculations for intrinsic defects and substitutional oxygen impurities in WSe₂. Ab initio modeling is used in conjunction with experimental thermodynamic data to calculate equilibrium formation energies and point defect densities as a function of growth temperature. Theoretical defect densities at the self-flux growth conditions were shown to be an order of magnitude less than experimental values determined by scanning tunneling microscopy images. The point defect density difference between experiment and theory suggests that equilibrium was not reached, and kinetic or impurity factors determine the point defect densities in self-flux TMDs.

[1] D. Rhodes, S. H. Chae, R. Pibeiro-Palau, J. Hone, Nat. Mater. 18 (2019), pp. 541-549, https://doi.org/10.1038/s41563-019-0366-8
[2] D. Edelberg, et. al. Nano Lett. 19 (2019), pp. 4371-4379, https://doi.org/10.1021/acs.nanolett.9b00985

Quantum materials host a vast array of emergent electronic phenomena, including high-temperature superconductivity, topological properties, and nanoscale charge / spin ordering. One of the challenges is to be able to precisely and deterministically manipulate their properties. To achieve this control, we employ molecular beam epitaxy (MBE) to synthesize artificial quantum materials with atomic layer precision, combined with angle-resolved photoemission spectroscopy (ARPES) which provides direct insights into the electronic structure. In particular, I will focus on the enhanced high-temperature superconductivity in monolayer FeSe grown on SrTiO₃ substrates, where superconductivity has previously been reported to exceed 60 K, nearly one order of magnitude higher than that of bulk FeSe (T_c = 8K). This dramatic enhancement remains the largest amongst known superconductors, positioning monolayer FeSe / SrTiO₃ as an ideal platform for investigating fundamental questions about interfacial superconductivity. In particular, the combination of its high T_c and inherently two-dimensional (2D) nature makes FeSe / SrTiO₃ suited for exploring the interplay between 2D phase fluctuations and the interfacial enhancement of superconductivity, a better understanding of which could enable the future design and engineering of artificial higher- T_c superconductors.