Unveiling the hidden charge density wave spectral signature in 2H- NbSe2

Abstract

Although 2H-NbSe2 has been discussed as a prototypical charge density wave (CDW) material with strong electron-phonon coupling, its energy gap structure, such as its position and size, has been elusive in spectroscopic measurements. We carefully reinvestigate the spectral change in the scanning tunneling microscopy and spectroscopy (STM/STS) when crossing the CDW transition in temperature and space. In contradiction to previous assignments, our spectroscopy measurements exhibit no significant spectral change across the CDW transition. Density functional theory calculations attribute the absence of the spectral signature to the CDW gap opening at the M point and the Γ-point-sensitive tunneling in the STS measurement. As an alternative CDW signature, we identify a strong bias-dependent modulation of local charge density across the Fermi energy which originates from the bonding/antibonding character of bands involved in the CDW gap at the M point. Our findings provide important insights for understanding partially gapped mutliband CDW materials.