Multiple charge density wave phases of monolayer VSe2 manifested by graphene substrates

Abstract

A combined study of scanning tunneling microscopy (STM) and angle-resolved photoemission spectroscopy (ARPES) is conducted to understand the multiple charge density wave (CDW) phases of monolayer (ML) VSe2 films manifested by graphene substrates. Submonolayer (similar to 0.8 ML) VSe2 films are prepared on two different substrates of single-layer graphene (SLG) and bi-layer graphene (BLG) on a 6H-SiC(0001). We find that ML VSe2 films are less coupled to the SLG substrate compared to that of ML VSe2/BLG. Then, ML VSe2 grown on SLG and BLG substrates reveals a very different topography in STM. While ML VSe2/BLG shows one unidirectional modulation of root 3 x 2 and root 3 x root 7 CDW in topography, ML VSe2/SLG presents a clear modulation of 4 x 1 CDW interfering with root 3 x 2 and root 3 x root 7 CDW which has not been previously observed. We explicitly show that the reciprocal vector of 4 x 1 CDW fits perfectly into the long parallel sections of cigar-shaped Fermi surfaces near the M point in ML VSe2, satisfying Fermi surface nesting. Since bulk VSe2 is also well-known for the 4 x 4 x 3 CDW formed by Fermi surface nesting, the 4 x 1 CDW in ML VSe2/SLG is attributed to the planar projection of 4 x 4 x 3 CDW in bulk. Our result clarifies the nature of the 4 x 1 CDW in ML VSe2 system and is a good example demonstrating the essential role of substrates in two-dimensional transition metal dichalcogenides.