Session L36: 2D Materials - Topological States

Imaging Conductive Edge States in Monolayer WTe2

The quantum spin Hall (QSH) effect occurs in two-dimensional topological insulators where helical edge states circulate the insulating bulk. Recent experiments have reported QSH signatures in monolayer WTe2. In our experiment, we use microwave impedance microscopy (MIM), a scanning probe technique that senses materials’ local electrical properties, to directly image the edge states in monolayer WTe2, and correlate their behaviors with electrical transport. Conductive states are clearly resolved at edges of the monolayer WTe2. They remain conductive when the bulk carrier density is tuned through the bandgap, but become only slightly weaker at high magnetic field while the conductance measured by transport is strongly suppressed. The edge conduction can be resolved up to 100 K in temperature when the bulk becomes conductive. In addition, we also observe conduction features in the bulk of the WTe2 flake. We will discuss the implications of these observations on the topological properties of monolayer WTe2.   

Raman Signatures of Broken Inversion Symmetry and In-plane Anisotropy in Type-II Weyl Semimetal Candidate TaIrTe4

The layered ternary compound TaIrTe₄ is an important candidate to host the recently predicted type-II Weyl Fermions. However, a direct and definitive proof of the absence of inversion symmetry in this material, a prerequisite for the existence of Weyl Fermions, has not yet been obtained. Herein, an unambiguous identification of the broken inversion symmetry in TaIrTe₄ is established using angle-resolved polarized Raman spectroscopy and first-principle calculations. We also report strong optical anisotropy in TaIrTe₄ crystals, which can be exploited to efficiently and nondestructively determine the exact crystallographic orientation of the material. Such technique could be extended to the fast identification and characterization of other type-II Weyl Fermions candidates. A surprisingly strong in-plane electrical anisotropy in TaIrTe₄ thin flakes is also revealed, up to 200% at 10K. Such strong electrical anisotropy, together with the high carrier density and the environmental stability of the material, may enable new architectures in electrical interconnects in future integrated circuits.   

Differential resistance at the topological insulator-superconductor interface in a cross-bar geometry

We have studied the low temperature electronic transport properties of an interface between a topological insulator bismuth selenide and a superconducting niobium strip in a cross-bar geometry. In the regime where niobium is superconducting, we find that the zero-bias differential resistance increases with increasing magnetic field or temperature, which turns into a negative differential resistance just above the superconducting transition. We will discuss the possible origins of this phenomenon.

  

Two-dimensional topological insulator behavior in monolayer WTe2

The van der Waals layered material WTe₂ is a semimetal, with closely matched electron and hole pockets and very strong spin-orbit coupling, which exhibits gigantic magnetoresistance at low temperatures and has topological features its single-particle band structure. Studying WTe₂ down to the monolayer limit by exfoliation and encapsulation in hexagonal boron nitride, we find that it exhibits a diverse range of phenomena. In particular, monolayers show edge conduction with properties matching those expected for the helical boundary modes of a two-dimensional topological insulator (producing the quantum spin Hall effect). These include no band gap at zero magnetic field, suppression by an in-plane magnetic field, absence in bilayers, and conductance per edge not exceeding e²/h. However, the conductance never actually reaches e²/h, shows large mesoscopic fluctuations, and falls at dilution fridge temperatures, showing a sub-meV bias threshold for current flow of unknown origin. We also discuss the rather peculiar conductivity behavior of the bulk insulating state, in which electron-hole correlations may be an important factor.   

Quantum spin Hall state in monolayer 1T’ TMDcs

QSH state or 2D topological state is a topologically nontrivial quantum state that features quantized Hall conductance in the absence of a magnetic field. We successfully synthesized stable 2D van der Waals materials that host quantum spin Hall insulators, monolayer 1T’-TMDcs, using molecular-beam- epitaxy (MBE). Together with the theoretical calculation, subsequent in situ ARPES measurements reveals the characteristic signatures of a QSH state in the electronic structure, i.e., a band inversion and a band gap opening in monolayer 1T’-WTe 2 . Another essential evidence of QSH phase came from a STM measurements, which observed confined edge states. Similar cases are also achieved in 1T’-WSe 2 with subtle differences in the details of electronic structure and larger nontrivial bulk gap. We have also found that 1T’-MoTe 2 is a semimetal with a significant overlap between inverted valence and conduction bands rather than a QSH insulator.   

Quantum Spin Hall State in Monolayer 1T'-WTe2

Using molecular beam epitaxy (MBE), we successfully synthesized monolayer 1T’-WTe₂ on graphitized SiC substrate. High-resolution angle-resolved photoemission spectroscopy (ARPES) and scanning tunneling spectroscopy (STS) measurement are performed to investigate the electronic structure of monolayer 1T’-WTe₂. Together with theoretical calculation and ARPES measurement, the band inversion which alters the band topology has been confirmed. The bulk gap opening due to the strong spin orbital coupling is clearly observed by ARPES and STS measurement. STS further provides evidences for a robust and modified electronic structure near the edge that is consistent with the expectations for a quantum spin Hall insulator.
  

Layer-Dependent Electronic structure of exfoliated 2H-MoTe2 revealed by Nanospot Angle-Resolved photoemission Spectroscopy

Transition metal dichalcogenides exhibit strong quantum confinement effects and the electronic structure is strongly dependent on the number of layers. Resolving the electronic structure of atomically thin exfoliated samples is critical, yet it is a challenge for angle-resolved photoemission spectroscopy (ARPES) measurements due to the small sample size. Here I will present our recent experimental progress on the electronic structure of exfoliated 2H-MoTe2 flakes with different thickness by using nanospot ARPES (NanoARPES).   

MoTe2 Monolayers by Molecular-Beam Epitaxy – Phase Tuning by Changing the Growth Conditions

Monolayer transition-metal dichalocogenides (TMDs) have been under extensive research attention in recent years. Many of the studied TMDs are semiconductors or semimetals for their attractive physical properties and application promises. MoTe₂ is an interesting material that can exist in both the hexagonal (2H) and monoclinic (1T’) phases at experimentally accessible conditions, which show respectively semiconducting and metallic properties. Controlled growth and tuning of its structural phases can thus lead to new applications such as phase-change electronics. In this work, we epitaxially grow monolayer MoTe₂ on highly oriented pyrolytic graphite by molecular-beam epitaxy and obtain both 2H and 1T’ phases. The ratio between the two domain areas is derived by reflection high-energy electron diffraction measurements and is found to vary with the MBE conditions. Low-temperature and high Te flux both lead to increasing 1T’ domain size, which is explained by an effect of Te adsorption on surface. The 1T’ and 2H domain interface forms a metal-semiconductor junction and the property is probed by scanning tunneling spectroscopy.   

A Study of the Phase Change Material Mo1-xWxTe2

Structural polymorphism in the transition metal dichalcogenide (TMD) MoTe₂ is ideal for enabling ultrathin phase change memories. Under ambient conditions, MoTe₂ crystallizes in the 2H semiconducting phase, but can be grown in the 1T’ semimetallic phase that transitions into the T_d phase below ~250 K. Alloying MoTe₂ with WTe₂ is theorized to reduce the barrier between the 2H and 1T’ phases, which may improve the efficiency of TMD-based memories and provide on-demand topological phase transitions. Contradictory reports of the Mo_1-xW_xTe₂ phase diagram [1,2] demonstrate the need for new investigations of this alloy system. Here, we combine Raman spectroscopy, x-ray diffraction, scanning transmission electron microscopy, and density-functional theory to explore the phase boundaries of bulk Mo_1-xW_xTe₂ alloys. Our work confirms the existence of 2H, 1T’, and T_d regions, as well as a two-phase 1T’-T_d region in the MoTe₂-WTe₂ system [3]. We extract the phonon correlation length through the phonon confinement model, identify disorder-enhanced two-phonon scattering, and observe disorder-activation of infrared modes. These results provide a foundation for future applications of the Mo_1-xW_xTe₂ alloys.

[1] Rhodes et al. Nano Lett. 2017 [2] Lv et al. Sci. Rep. 2017 [3] Oliver et al. 2D Mat. 2017   

First-principles studies of Rashba effect on a two-dimensional topological insulator.

Finding giant Rashba effect materials is of great importance to realize new spin-driven devices. Using first-principles DFT calculations, we report a giant Rashba effect, and possibly switchable spin textures with external electric field found on one of two-dimensional topological insulators in group-11 chalcogenide compounds. The stable structure of this material is polar with broken inversion symmetry along c axis with calculated dipole moment of 5.1 pC/m. The strong spin-orbit coupling responsible for the band inversion is calculated to be about 130 meV, which is significantly larger than other widely celebrated topological insulators. The calculated band structure shows a giant Rashba type spin splitting in the valence band. The Rashba coefficient is about 3.7 eVÅ, comparable with that of the bulk Rashba material BiTeI. We also present the switchable energy barrier (~0.1eV/f.u.) between the two polar states, and accordingly spin texture changes, providing an effective way to control the magnetic structure using external electric fields.   

Controlling structural deformations in WTe2 using THz pulses

Tungsten ditelluride is a layered transition-metal dichalcogenide that crystalizes in a distorted hexagonal net with an orthorhombic unit cell. The lack of inversion symmetry in this phase leads to a predicted new topological semimetal hosting the so-called type-II Weyl points. Here, we use intense THz pulses to trigger a structural deformation in WTe2, and probe its dynamics using an ultrafast electron diffraction (UED) technique. We observe large amplitude interlayer shear oscillations that occur along the in-plane motion between the orthorhombic and monoclinic phases of the material. We will discuss the driving mechanism that can lead to such structural deformation and its implication toward ultrafast THz field control over the topological properties in solids.