Observation of a superlattice in silver-intercalated NbSe2 by scanning tunneling microscopy.

Abstract

Scanning tunneling microscope measurements were performed on UHV cleaved samples of pure and silver intercalated 2H-${\mathrm{NbSe}}_{2}$ at room temperature. Samples intercalated to stage 2 showed a \ensuremath{\surd}13${\mathit{a}}_{0}$\ifmmode\times\else\texttimes\fi{}\ensuremath{\surd}13${\mathit{a}}_{0}$ hexagonal superlattice. We suggest that the superlattice arises from a surface charge density wave (CDW). Pure 2H-${\mathrm{NbSe}}_{2}$ does not display a CDW at room temperature but develops a 3${\mathit{a}}_{0}$\ifmmode\times\else\texttimes\fi{}3${\mathit{a}}_{0}$ CDW superlattice below 30 K. The superlattice in the intercalated samples is similar to the CDW lattices observed in many 1-T poly- morphs of the dichalcogenide materials. We suggest that the observed superlattice may result from a change in the stacking order of ${\mathrm{NbSe}}_{2}$ layers near the surface from 2H type to 1-T type in silver intercalated samples. \textcopyright{} 1996 The American Physical Society.