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

Yuki Nakata,Takashi Takahashi,Keiji Ueno,Katsuaki Sugawara,Shuji Hasegawa,Takashi Koretsune,Takafumi Sato,Yoshinori Okada,Taro Hitosugi,Satoru Ichinokura

doi:10.1038/s41699-018-0057-3

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 NbSe2 film with a trigonal prismatic structure by molecular beam epitaxy, and the study of its electronic properties by scanning tunneling microscopy, angle-resolved 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.

Highlights

Layered transition-metal dichalcogenides (TMDs) MX2 (M = transition metal, X = chalcogen) have been extensively studied since they exhibit a variety of physical properties, such as superconductivity and charge density waves (CDWs).[1]
This behavior is well reproduced by the band-structure shows the energy distribution curves (EDCs) measured along the calculation in Fig. 2b and is interpreted as a consequence of the spin–orbit coupling (SOC) and inversion symmetry breaking
All these results suggest a spinpolarized nature of the Fermi surface (FS) in monolayer NbSe2.34 It is noted that band splitting has been observed or predicted in some semiconducting TMDs with a trigonal prismatic structure,[34,35,36,37,38,39]

Summary

Introduction

This behavior is well reproduced by the band-structure shows the energy distribution curves (EDCs) measured along the calculation in Fig. 2b (see area enclosed by blue rectangle) and is interpreted as a consequence of the SOC and inversion symmetry breaking.

Results

Conclusion