WTe2

We address the sliding thermodynamics of van der Waals-bonded bilayers by the continuum elasticity theory. We attribute the robustness of the ferroelectricity recently observed in h-BN and WTe2 bilayers to large monolayer in-plane stiffness. We compute the electric susceptibility and specific heat in the mean-field self-consistent phonon approximation. We compare critical temperatures and electric switching fields with the observations.

Published : "arXiv Mesoscale and Nanoscale Physics".

The anomalous Hall effect (AHE) has been extended into the nonlinear regime, where the Hall voltage shows higher-order response to the applied current. Nevertheless, the microscopic mechanism of the nonlinear AHE remains unclear. Here we report the orbital polarization and its induced third-order AHE in few-layer WTe2 flakes. Through angle-dependent electric measurements, it is found that the third-order AHE is quite consistent with the electric field induced polarization of orbital magnetic moment caused by the Berry connection polarizability tensor, which is further directly detected by polar reflective magnetic circular dichroism spectroscopy. The microscopic mechanisms of third-order AHE are analyzed through the scaling law, that is, the opposite orbital magnetic moments (up or down) deflect to opposite directions driven by electric field induced Berry curvature, forming the intrinsic contribution; driven by the Magnus effect of the self-rotating Bloch electrons, the opposite orbital magnetic moments are scattered towards opposite transverse directions, resulting in the skew scattering.

Published : "arXiv Mesoscale and Nanoscale Physics".

Interplay between the topological surface states and bulk states gives rise to diverse exotic transport phenomena in topological materials. The recently proposed Weyl orbit in topological semimetals in the presence of magnetic field is a remarkable example. This novel closed magnetic orbit consists of Fermi arcs on two spatially separated sample surfaces which are connected by the bulk chiral zero mode, which can contribute to transport. Here we report Shubnikov-de Haas (SdH) oscillation and its evolution into quantum Hall effect (QHE) in multilayered type-II Weyl semimetal WTe2. We observe both the three-dimensional (3D) QHE from bulk states by parallelly stacking of confined two-dimensional layers in the low magnetic field region and the 3D QHE in the quantized surface transport due to Weyl orbits in the high magnetic field region. Our study of the two types of novel QHEs controlled by magnetic field and our demonstration of the crossover between quantized bulk and surface transport provide an essential platform for the future quantized transport studies in topological semimetals.

Published : "arXiv Mesoscale and Nanoscale Physics".

Monolayer WTe2 stripes are quantum spin Hall (QSH) insulators. Density functional theory was used for investigating the electronic properties of the stripes and steps in bilayer Td-WTe2. For the stripes oriented along the dimer chains of W atoms (x direction), the hybridization between the two layers suppresses the QSH states. However, the QSH nature can be recovered by forming a step, depending on the atomic structure of the step. Conversely, the stripes and steps along the y direction maintain the QSH states. These findings can expand the application range of the QSH states in WTe2.

Published : "arXiv Mesoscale and Nanoscale Physics".

Highly transparent superconducting contacts to a topological insulator (TI) remain a persistent challenge on the route to engineer topological superconductivity. Recently, the higher-order TI WTe$_2$ was shown to turn superconducting when placed on palladium (Pd) bottom contacts, demonstrating a promising material system in perusing this goal. Here, we report the diffusion of Pd into WTe$_2$ and the formation of superconducting PdTe$_x$ as the origin of observed superconductivity. We find an atomically sharp interface between the diffusion layer and its host crystal, forming state-of-the-art superconducting contacts to a TI. The diffusion is discovered to be non-uniform along the width of the WTe$_2$ crystal, with a greater extend along the edges compared to the bulk. The potential of this contacting method is highlighted in transport measurements on Josephson junctions by employing external superconducting leads.

Published : "arXiv Mesoscale and Nanoscale Physics".

We report a detailed Shubnikov-de Haas (SdH) study on the Weyl type-II semimetal WTe2 in magnetic fields up to 29 T. By using the SdH results to guide our density functional theory calculations, we are able to accurately determine its Fermi surface by employing a moderate Hubbard U term, which is an essential step in explaining the unusual electronic properties of this much studied material. In addition to the fundamental orbits, we observe magnetic breakdown, which can consistently be explained within the model of a Russian-doll-nested Fermi surface of electron and hole pockets. The onset of magnetic breakdown in WTe2 is solely determined by impurity damping in contrast to magnetic breakdown scenarios in other metallic systems.

Published : "arXiv Mesoscale and Nanoscale Physics".

Photocurrent measurements are incisive probes of crystal symmetry, electronic band structure, and material interfaces. However, conventional scanning photocurrent microscopy (SPCM) convolves the processes for photocurrent generation and collection, which can obscure the intrinsic light-matter interaction. Here, by using ac magnetometry with a nitrogen-vacancy center spin ensemble, we demonstrate the high-sensitivity, sub-micron resolved imaging of vector photocurrent flow. Our imaging reveals that in anisotropic semimetals WTe2 and TaIrTe4, the photoexcited electron carriers propagate outward along the zigzag chains and inward perpendicular to the chains. This circulating pattern is explained by our theoretical modeling to emerge from an anisotropic photothermoelectric effect (APTE) caused by a direction-dependent thermopower. Through simultaneous SPCM and magnetic imaging, we directly visualize how local APTE photocurrents stimulate long-range photocurrents at symmetry-breaking edges and contacts. These results uniquely validate the Shockley-Ramo process for photocurrent collection in gapless materials and identify the overlooked APTE as the primary origin of robust photocurrents in anisotropic semimetal devices. Our work highlights quantum-enabled photocurrent flow microscopy as a clarifying perspective for complex optoelectronic phenomena.

Published : "arXiv Mesoscale and Nanoscale Physics".