NEW SUPERSTRUCTURE ON THE SURFACE OF 2H-NBSE2 AND TUNNELING SPECTRA AT 4.2 K

The atomic structure and tunneling spectra of a cleaved surface of a 2 H-NbSe2 single crystal were studied by scanning tunneling microscopy a t 4.2 K. Small domains with root 13 x root 13 modulation were found ad jacent to the flat surface with a 3 x 3 intrinsic charge density wave (CDW). The surface atom arrangement in the domains is the same as that in the 2H-type surface. The density of states around the Fermi energy was measured at the domains by tunneling spectroscopy, and found to b e lower than that at the 2H-type surface. The root 13 x root 13 modula tion is considered to originate from another CDW, which can be expecte d to occur in 4H-NbSe2 clusters buried in a 2H-type crystal. The chang e of the tunneling spectrum across the boundary between the 2H-type su rface and a root 13 x root 13 modulated domain was also measured, and the proximity of superconductivity of 2H type to the modulated domain is discussed.