Tunneling spectroscopy around the boundary of a small impurity phase on the surface of 2H-NbSe2

The structure and tunneling spectra of a cleaved surface of a 2H-NbSe2 single crystal were studied at 4.2 K in an ultra-high vacuum using scanning tunneling microscopy. Small domains with root 13 x root 13 surface modulation were found adjacent to the flat 2H-type surface with the 3x3 intrinsic charge density wave (CDW). Using tunneling spectroscopy, density of states around the Fermi energy at the domains was found to be lower than that at the 2H-type surface. The root 13 x root 13 modulation is considered to originate from another type of CDW. The observed proximity of super conductivity of 2H-crystal to the highly modulated domain was very small. These results suggest that the crystal structure of the highly-modulated domain is the 4Hd polytype.