Anisotropic band splitting in monolayer NbSe2: implications for superconductivity and charge density wave

Abstract

Realization of unconventional physical properties in two-dimensional (2D) transition-metal dichalcogenides (TMDs) is currently one of the key challenges in condensed-matter systems. However, the electronic properties of 2D TMDs remain largely unexplored compared to those of their bulk counterparts. Here, we report the fabrication of a high-quality monolayer NbSe 2 film with a trigonal prismatic structure by molecular beam epitaxy, and the study of its electronic properties by scanning tunneling microscopy, angleresolved photoemission spectroscopy, and electrical transport measurements, together with first-principles band-structure calculations. In addition to a charge density wave (CDW) with 3 × 3 periodicity and superconductivity below 1.5 K, we observed sizable (~0.1 eV) band splitting along the Γ-K cut in the Brillouin zone due to inversion symmetry breaking in the monolayer crystal. This splitting is highly anisotropic in k space, leading to a spin-split van-Hove singularity in the band structure. The present results suggest the importance of spin-orbit coupling and symmetry breaking for unconventional superconductivity and CDW properties in monolayer TMDs.

Ab initio phonon self-energies and fluctuation diagnostics of phonon anomalies: Lattice instabilities from Dirac pseudospin physics in transition metal dichalcogenides

“…1 (a)] are denoted by 2H-and 1H-M X 2 , where M stands for Nb or Ta and X for S or Se. Temperature-dependent phonon-mode softening and CDWs are ubiquitous in these materials but expla-nations have remained controversial for several decades and suggestions include strong-coupling arguments based on matrix elements [12,[24][25][26][27][28][29][30][31][32][33][34][35][36][37] or local chemical bonding [38][39][40][41][42][43] as well as weak-coupling arguments based on Fermi-surface nesting [13,[44][45][46][47][48][49] or Van Hove scenarios [14,50,51]. Our phonon-self-energy calculations and fluctuation diagnostics for monolayer 1H-TaS 2 reveal that coupling between the longitudinal-acoustic (LA) phonons and the electrons from an isolated low-energy metallic band [ Fig.…”

Ising superconductors: Interplay of magnetic field, triplet channels, and disorder

“…The large SOC enhances the critical in-plane field B c beyond the Pauli limit. This has been shown both theoretically [16,[19][20][21][22][23] and experimentally [1][2][3][4][5][6][7][8].…”

“…During the last decade, two-dimensional (2D) superconductivity became an active field of research. The renewed interest in the field is a result of technological advances in the fabrication of 2D materials comprised of one-to-several atomically thin monolayers exfoliated on substrates in a nearly perfect atomic registry [1][2][3][4][5][6][7][8]. Often such few-layer systems contain more than one chemical element.…”

Phase‐Engineered Weyl Semi‐Metallic MoxW1‐xTe2 Nanosheets as a Highly Efficient Electrocatalyst for Dye‐Sensitized Solar Cells

“…In particular, the type‐II TWS have an inverted band structure with broken inversion symmetry, leading to the existence of topological surface states, which are stable against defects, impurities, and other surface modifications . Moreover, the lattice distortion with spontaneous surface charge density wave on the basal plane could provide an additional advantage to allow a rapid charge exchange in electrochemical reactions, thus improving the catalytic activities . Ultimately, resolving the surface contamination in a corrosive electrolytic solution is one of the major concerns in catalysis; propitiously, this issue can be effectively addressed by using TWS as a catalyst, because the topologically protected surface states are robust and stable against contamination …”

“…Comparatively, chemical vapor deposition (CVD) is a facile bottom‐up synthesis approach with the advantages of growing high‐quality nanocrystals of large lateral sizes, controllable thickness, and high crystallinity on specific substrates . However, the CVD synthesis of telluride‐based few‐layered nanocrystals, such as MTe 2 ( M = Mo or W), remains challenging due to the very small difference in the electronegativities between Mo/W and Te (by ≈0.3–0.4 eV only) to form stable chemical bonds . Notwithstanding the structural instability of binary MTe 2 , the alloy system of Mo x W 1‐x Te 2 ( x = 2–40%) is stable at room temperature and exhibits a tunable TWS surface state.…”