Characterization of collective ground states in single-layer NbSe2

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  = 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.

Ising Superconductivity and Quantum Phase Transition in Macro-Size Monolayer NbSe2

“…[10][11][12][13] More interestingly, Zeeman-protected Ising superconductivity is expected in monolayer NbSe 2 due to the non-centrosymmetric structure with in-plane inversion symmetry breaking and strong spin−orbit coupling. Very recently, Ising superconductivity with the anomalous large in-plane critical magnetic field has become one important direction in crystalline 2D superconductors 7 .…”

Metallic Vanadium Disulfide Nanosheets as a Platform Material for Multifunctional Electrode Applications

“…However, the acclaimed 2D forms remain unattainable and almost unexplored to date, which is in stark contrast to semimetallic graphene 17,18 and their dichalcogenide analogues such as monolayer MoS2 19,20 , MoTe2 21,22 , and ReS2 23,24 . This is mainly attributed to the hysteretic research developments in the batch production of high-quality, large-domain-size MTMDC nanosheets 14,25 , and of paramount importance, with sub-10 nm thickness that approaches the 2D regime 26,27 .…”

“…Recently, there has been renewed research interest in these MTMDCs (TaS2, NbSe2, etc. ), as they have proven to be ideal systems for exploring collective electronic states down to the two-dimensional (2D) limit [11][12][13][14] . More intriguingly, the excellent electrical conductivity and lack of bandgaps in these layered materials also indicate a broad range of potential applications such as transparent electrodes 15 and energy conversion/storage 16 , if they can be thinned down to the nanoscale.…”

Layer-by-Layer Epitaxial Growth of Scalable WSe2 on Sapphire by Molecular Beam Epitaxy

“…Recent extensive researches on TMDC have revived the concept of van der Waals epitaxy after almost thirty years, and there is indeed a growing number of papers on MBE-grown high-quality TMDC thin films being reported for the last few years. [24][25][26][27][28] However, those TMDC thin films have been grown on conducting graphene layers formed on SiC substrates in most cases for spectroscopic studies including angle-resolved photoemission spectroscopy or scanning tunneling microscopy, and growth of large area TMDC epitaxial thin films on insulating substrates has been less successful despite its essential importance for fundamental transport studies as well as practical electronic and optoelectronic device applications.…”