Session S59: Transport, Probes, and Structure of Topological Insulators and Topological Crystalline Insulators

Can the Boltzmann-tryanny be overcome via topological field effect switching? - insignts into mechanisms and fundamental limits

Electric field-driven topological phase transitions in quantum spin Hall (QSH) materials with buckled honeycomb lattice in the presence of Rashba spin-orbit interactions are considered to be an interesting playground for implementing Topological Quantum Field-Effect Transistors (TQFETs). However, defying the wisdom  in conventional transistors being ideally limited to 60mV/decade, we demonstrate that for a topological transistor the thermionic limit of subthreshold slope is actually twice as much.  Also, by employing the non-equilibrium Green's function (NEGF) approach, we demonstrate that the mere introduction of Rashba interaction [1] does not help to reduce the subthreshold swing. As a probable solution, we propose that the introduction of anti-ferromagnetic exchange interaction, enabling a transition between the quantum spin valley Hall (QSVH) phase and the quantum anomalous Hall (QAH) phase, can help us go past not only the 120mV/decade limit but also the 60mV/decade limit for an appropriately chosen set of parameters.   

Role of dephasing on electric field induced topological transition in 2D Xenes

We analyze the electric field driven topological field effect transition on 2D-xene materials with the addition of momentum relaxation effects, in order to account for dephasing processes. The topological field effect transition between the quantum spin Hall phase and the quantum valley Hall phase is analyzed in detail using the Keldysh non-equilibrium Green's function technique with the inclusion of momentum and phase relaxation, within the self-consistent Born approximation. Details of the transition with applied electric field are elucidated for the ON-OFF characteristics with emphasis on the transport properties along with the tomography of the current carrying edge states. We note that for moderate momentum relaxation, the current carrying quantum spin Hall edge states are still pristine and show moderate decay with propagation. To facilitate our analysis, we introduce two metrics in our calculations, the coherent transmission and the effective transmission. In elucidating the physics clearly, we show that the effective transmission, which is derived rigorously from the quantum mechanical current operator is indeed the right quantity to analyze topological stability against dephasing. Exploring further, we show that the insulating quantum valley Hall phase, as a result of dephasing carries band-tails which potentially activates parasitic OFF currents, thereby degrading the ON-OFF ratios. Our analysis sets the stage for realistic modeling of topological field effect devices for various applications, with the inclusion of scattering effects and analyzing their role in the optimization of the device performance.    

Twist-resilient and robust ferroelectric quantum spin Hall insulators driven by van der Waals interactions

Quantum spin Hall insulators have been proposed to power a number of applications, many of which rely on the possibility to switch on and off the non-trivial topology. Typically this control is achieved through strain or external electric fields, which require energy consumption to be maintained. On the contrary, a non-volatile mechanism would be highly beneficial and could be realized through ferroelectricity if opposite polarizations are associated with different topological phases. While this is not possible in a single ferroelectric material owing to symmetry constraints, the necessary asymmetry could be introduced  by combining a ferroelectric layer with another 2D trivial insulator. Here, by means of first-principles simulations, not only we propose that this is a promising strategy to engineer non-volatile ferroelectric control of topological order in 2D heterostructures, but also that the effect is robust and can survive up to room temperature. Remarkably, the topological band gap is mediated by the interlayer hybridization and allows to maximise the effect of on-site spin-orbit coupling, promoting a robust ferroelectric topological phase that could not exist in monolayer materials and is resilient against relative orientation and lattice matching between the layers.   

Copper migration in intercalated topological insulator CuxBi2Se3

Cu_xBi₂Se₃ is a topological insulator (TI) material in which Cu intercalation between the Bi₂Se₃ quintuple layers can yield superconductivity. Using ambient-pressure X-ray photoelectron spectroscopy (AP-XPS), we report copper migration from the bulk towards the surface in this material in controlled oxidizing environments as well as ambient conditions. The migration occurs on a timescale of hours to weeks after initial cleaving, and proceeds along with the oxidation of the sample surface. These results demonstrate dynamic surface chemistry, relevant for tailoring the topological surface states for utilization in ambient environments.   

Magneto-optical Third Harmonic Generation in Topological Insulator Films

The discovery of materials where interactions with spin, lattice, and orbital degrees of freedom are linked with the electronic kinetic energy has led to a heightened interest in their nonlinear optical (NLO) response. Examples of these materials in three dimensions are topological insulators (TIs), which have a gapped bulk spectrum while hosting metallic helical states on surfaces. They exhibit strong magneto-optical responses due to the topologically nontrivial bulk and surface states, making them attractive candidates for probing enhanced NLO responses in a magnetic field. In this talk, we present our theory for magneto-optical third-harmonic generation (THG) in topological insulator films in a quantizing magnetic field. We obtain the THG NLO conductivity arising from one-, two-, and three-photon processes and investigate the effects of changing magnetic field, chemical potential, and film thickness. Comparing to the zero-field case, the THG nonlinear conductivity exhibits strong enhancements due to multiphoton cyclotron resonances and bulk states contribution. We address the key differences of these enhancements in TI from the THG in graphene.   

Probing the density of states in monolayer WTe2 with time domain capacitance spectroscopy

Time domain capacitance spectroscopy (TDCS) probes the density of states in 2D systems without electrical contact. The technique generates a purely out-of-plane tunneling current and thus eliminates the effects of in-plane transport, maintaining sensitivity over a wide range of compressibility, from metallic to highly insulating states. We use TDCS to measure the density of states in electrostatically gated WTe₂ embedded in a vertical tunnel structure and observed an asymmetric and “hard” (with very low density of states out to the gap edge) gap pinned to the Fermi level. The gap occurs over a range of electron densities and thus does not appear related to WTe2 band structure but instead may arise from electron-electron interactions within the WTe₂ layer.   

Doping dependence and Fermi surface studies of the topological crystalline insulators SnxPb1-xTe/Se

This work presents Fermi surface studies of the topological crystalline insulator Sn_xPb_1-xTe/Se. Magnetic torque measured at higher fields up to 35 T and temperatures down to 0.32 K shows clear de Haas-van Alphen (dHvA) oscillations. The dHvA oscillations are well-defined and consist of two major frequencies (F₁ ~ 46 T, F₂ ~ 230 T) for the x = 0.35 sample. To understand the Fermi surface properties, we have measured dHvA oscillations at different tilt angles with respect to the applied field and at different temperatures up to 60 K. The temperature dependent data was analyzed using the Lifshitz-Kosevich (LK) formula, and we estimated several parameters characterizing the Fermi surface.

 

Work at West Texas A&M University is supported by the Killgore Faculty Research program, the KRC Undergraduate Student Research Grant, and the Welch Foundation (Grant No. AE-0025); work at TcSUH was supported in part by the U.S. Air Force Office of Scientific Research Grant FA9550-15-1-0236, the T. L. L. Temple Foundation, the John J. and Rebecca Moores Endowment, and the State of Texas through the Texas Center for Superconductivity at the University of Houston; work at the National High Magnetic Field Laboratory was supported by NSF/DMR-1644779 and the State of Florida.   

Low-energy effective theory and anomalous Hall effect in monolayer WTe2

Monolayer WTe₂ in its 1T’ phase has been realized as a quantum spin Hall insulator with various intriguing properties such as superconductivity and non-linear Hall effect. We develop a symmetry-based effective low-energy theory for monolayer WTe₂ in its 1T’ phase. We find that to the leading order, the spin-orbit coupling in the k.p-theory near the gamma-point in the Brillouin zone is independent of the quasimomentum, and describes four fully spin-polarized bands, which are represented by two Kramers-partner pairs. Using the low-energy model, we study the spin susceptibility and time-dependent anomalous Hall effect induced by injected or equilibrium spin polarization in this system. We show that these measurements can help to determine the orientation of the conserved spin projection, the dimensionless tilt of band dispersion and the strength of the spin-orbit coupling of the system.   

Observation of a Smoothly Tunable Dirac Point in Ge(BixSb1-x)2Te4

Next-generation topological devices use topologically protected surface states to drive electronic transport. In this study, we examine Ge(Bi_xSb_1-x)₂Te₄, a tunable topological system, for a range of x values from 0 to 1 using a combination of Fourier-Transform Scanning Tunneling Spectroscopy (FT-STS) and Angle-Resolved Photoemission Spectroscopy (ARPES). We show that the Dirac point changes linearly with x, crossing the Fermi energy at x = 0.7. This novel observation of a smoothly tunable, isolated Dirac point traversing the topological transport regime and having strong linear dependence with substitution can be critical in designing new topological spintronics applications.   

Chiral Hall effect in the kink states in topological insulators with magnetic domain walls

We consider the chiral Hall effect due to topologically protected kink states formed in topological insulators at boundaries between domains with differing topological invariants. Such systems include the surfaces of three-dimensional topological insulators magnetically doped or in proximity to ferromagnets, as well as certain two-dimensional topological insulators. We analyze the equilibrium charge current along the domain wall and show that it is equal to the sum of counterpropagating equilibrium currents flowing along the external boundaries of the domains. In addition, we also calculate the current along the domain wall when an external voltage is applied perpendicularly to the wall.   

Quantum phase transitions in the disordered chalcogenide topological insulators (Bi1-xSbx)2Y3 and X2(TeySe1-y)3, (X = Bi/Sb, Y = Te/Se)

Topological systems with various Bi/Sb and Te/Se concentrations have been studied with a view towards tuning topological phase transitions, and we try to clarify this by varying the lattice parameters in this family of materials and driving electronic topological-to-trivial phase transitions. We first investigate disorder on each site and perform first-principles calculations on (Bi_1-xSb_x)₂Y₃ and X₂(Te_ySe_1-y)₃ (X=Bi/Sb, Y=Te/Se) in which we systematically vary the c/a ratio of lattice constants to study the topological phase transition, before studying alloying on all sites. To model substitutional disorder, we have constructed supercells, alloying on one sublattice, where the atomic pair correlations are zero up to the third nearest neighbor shell, and we vary the impurity concentration across the phase diagram with the goal of identifying the influence of each atomic species on the transition. We determine the bulk electronic structure and pay special attention to the band inversion near the topological transition.   

Time-Domain THz studies of topological insulator waveguides

Topological insulators (TI) subject to time-reversal symmetry breaking perturbations are well known to exhibit quantized magneto-elecric effect. In conventional transmission experiments, light being transmitted through such TI's will experience Faraday and Kerr rotations of polarization.  However, these effects are on the order of fine structure constant α which makes it challenging to observe experimentally. In this work, we utilize TI's at the interface between guiding and cladding layers of a parallel-plate waveguide to induce coupling between guided TE and TM modes. It was previously predicted that this coupling is dependent on the number of filled Landau levels in the TI surface plus an axionic term that is measure of the topological invariant. Here we present the working principle of the waveguide device and provide experimental results of time-domain terahertz (THz) spectroscopy studies on it.    

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Recent advances reveal that the surfaces of three-dimensional (3D) topological insulators can host massless (Dirac) and massive surface states. The surface of a 3D topological insulator is a closed 2D manifold. Therefore, the interplay of top and bottom surfaces is expected to be prominent via sides, especially as the edge states are predominant in quantum Hall regime. Here we demonstrate the formation of counter-propagating edge states, originating from opposite surfaces. Additionally, our measurements show that the counter-propagating edge states are intensively scattered into each other, evidenced by a deviation from the non-interscatering Landauer-Büttiker formalism. In this case, a scattering model helps to quantify the inter-scattering strength among counter-propagating edge states, which is later shown to be tuable by gate voltages and temperature. The modelling for muliple edge states suggests a selection rule of inter-scattering among edge states, which only allows for prominent inter-scattering between paried edge states with the same Landau index N. Our research provides a platform to further investigate the microscopic mechanism of interaction of edge channels in quantum Hall regime.   

Controlling Current in Topological Insulators with Magnetic Impurities

Topological insulators are theoretical materials that allow for an ideal conductivity of electricity with minimal loss of energy. This proves to be of great use in the field of electronics, as it greatly increases the energy efficiency of electronic components. Local conductive properties of the material are dependent on the location and physical properties of impurities placed within the material, and this allows for control of direction and strength of the current traveling through this material. We implemented a discrete lattice model of a topological insulator known as the BHZ model and constructed its Hamiltonian matrix to predict the behavior of electrons in this lattice. We numerically explored the relationship between conductance and the location and energy of magnetic impurities in the BHZ model. We found that conductance is largely dependent on the dimensions of the lattice used in the model as well as the configuration of the coupling between the impurity and the lattice. This conductance was indirectly quantified by the transmission coefficient of incoming electrons. We observed a significant decrease of the transmission coefficient after altering the impurity coupling, yielding a greater control of the current.   

High-throughput first-principles exploration of 2D-tin structures and their topological properties

Stanene (2D Sn) is a promising material for nanoelectronics or quantum computing because it is a proposed topological insulator (TI) at the room temperature. Because electrons can travel on the edges of these TIs without heat dissipation, this could greatly increase the efficiency and performance of electronic devices. We explored the (meta)stable structures of 2D Sn using our automated high-throughput workflow that combines first-principles calculations and particle swarm optimization and identified new structures of 2D Sn with different electronic and topological properties. We also found that substrates play a critical role in the stability of 2D Sn structures with versatile properties. Our findings should be instrumental for the experimental development of new 2D Sn with desirable properties.