Topological properties of bulk and bilayer 2M WS₂: a first-principles study

Abstract

Recently discovered 2M phase of bulk WS 2 was observed to exhibit superconductivity with a critical temperature of 8.8 K, the highest reported among superconducting transition metal dichalcogenides. Also predicted to support protected surface states, it could be a potential topological superconductor. In the present study, we perform a detailed first-principles analysis of bulk and bilayer 2M WS 2 . We report a comprehensive investigation of the bulk phase, comparing structural and electronic properties obtained from different exchange correlation functionals to the experimentally reported values. By calculation of the Z 2 invariant and surface states, we give support for its non-trivial band nature. Based on the insights gained from the analysis of the bulk phase, we predict bilayer 2M WS 2 as a new two-dimensional topological material. We demonstrate its dynamical stability from first-principles phonon computations and present its electronic properties, highlighting the band inversions between the W d and S p states. By means of Z 2 invariant computations and a calculation of the edge states, we show that bilayer 2M WS 2 exhibits protected, robust edge states. The broken inversion symmetry in this newly proposed bilayer also leads to the presence of Berry curvature dipole and resulting non-linear responses. We compute the Berry curvature distribution and the dipole as a function of Fermi energy. We propose that Berry curvature dipole signals, which are absent in the centrosymmetric bulk 2M WS 2 , can be signatures of the bilayer. We hope our predictions lead to the experimental realization of this as-yet-undiscovered two-dimensional topological material.

Superconducting properties in doped 2M-WS2 from first principles

“…The S p xy orbitals give almost half of the total DOS, while approximately 20% comes from the W d zy /d zx orbitals. In accordance with previous studies,19,27 the inclusion of spin-orbit coupling (SOC) is found to have only a minor effect. In particular, there is a small (33 meV) increase in the size of the gap formed between the valence and conduction bands just below the Fermi level at the G point (see Fig.S2(a) in the ESI †47 ).…”

“…5,18 The resulting compounds are both metallic, and 2M-WS 2 exhibits superconductivity with a critical temperature ( T c ) of 8.8 K, the highest T c among superconducting TMDs at ambient pressure. In addition to being superconducting, this phase has been predicted, 5,19 and later confirmed, 20 to display topologically protected surface states with a single Dirac cone at the Brillouin zone (BZ) center. On the other hand, in the sister compound 2M-WSe 2 , superconductivity only appears under pressure at 4.2 GPa, and the critical temperature reaches a maximum value of 7.3 K at 10.7 GPa.…”

“…Fig.3(a) shows the electronic band structure of 2M-WS 2 with atomic orbital decomposition. A characteristic feature of nontrivial topology is found just below the Fermi level (E F ) at the G point, where an inversion takes place between two bands made of S p xy and W d zy /d zx orbitals 5,19,22. The upper electron-like band of the pair crosses the Fermi level along the N-G-N 1 direction (see Fig.S1in the ESI †47 for the high-symmetry points in the BZ of 2M-WS 2 ).…”

Electrically switchable giant Berry curvature dipole in silicene, germanene and stanene

“…For example, a uniaxial strain reduces the symmetry of TMDs and gives rise to enhanced BCD [8][9][10]. Similar enhancements are also observed for few-layer TMDs [4][5][6]11], where the lowering of symmetry is the result of the stacking of monolayers. It has further been observed that the electric field can efficiently tune the anomalous NHE in lowsymmetry TMDs [12].…”

Tunable topology and berry curvature dipole in transition metal dichalcogenide Janus monolayers

“…Such a non-linear Hall effect can also be generated in systems preserving time reversal (TR) symmetry, in contrast to the linear Hall effect [34][35][36]. Diverse set of materials have been predicted to show finite BCD and the resulting second order Hall effect, including two-dimensional materials [37][38][39][40][41], and Dirac and Weyl semimetals [42][43][44][45][46], strained bilayer and monolayer graphene [47] and Rashba systems [48]. Significant advances have also been made in the experimental detection of such Hall signals in several material platforms, starting with few-layer WTe 2 [49][50][51][52][53][54][55].…”