Characterization of collective ground states in single-layer NbSe2

TL;DR

This study investigates the electronic properties of single-layer NbSe2, revealing persistent charge density wave order and superconductivity at reduced temperatures, providing insights into correlated electronic phases in two-dimensional materials.

Contribution

It provides the first detailed characterization of CDW and superconductivity in single-layer NbSe2 using STM/STS, ARPES, and transport measurements, highlighting their coexistence in 2D.

Findings

3x3 CDW order persists in 2D NbSe2

Superconductivity occurs at 1.9 K in single-layer NbSe2

STS reveals a 4 meV CDW gap at the Fermi level

Abstract

Layered transition metal dichalcogenides (TMDs) are ideal systems for exploring the effects of dimensionality on correlated electronic phases such as charge density wave (CDW) order and superconductivity. In bulk NbSe2 a CDW sets in at TCDW = 33 K and superconductivity sets in at Tc = 7.2 K. Below Tc these electronic states coexist but their microscopic formation mechanisms remain controversial. Here we present an electronic characterization study of a single 2D layer of NbSe2 by means of low temperature scanning tunneling microscopy/spectroscopy (STM/STS), angle-resolved photoemission spectroscopy (ARPES), and electrical transport measurements. We demonstrate that 3x3 CDW order in NbSe2 remains intact in 2D. Superconductivity also still remains in the 2D limit, but its onset temperature is depressed to 1.9 K. Our STS measurements at 5 K reveal a CDW gap of {\Delta} = 4 meV at the Fermi energy, which is accessible via STS due to the removal of bands crossing the Fermi level for a single layer. Our observations are consistent with the simplified (compared to bulk) electronic structure of single-layer NbSe2, thus providing new insight into CDW formation and superconductivity in this model strongly-correlated system.