Locally modified charge-density waves in Na intercalatedVSe2studied by scanning tunneling microscopy and spectroscopy

Abstract

We have observed local effects of the charge-density wave (CDW) in in situ Na-intercalated $1T-{\mathrm{VSe}}_2$ using scanning tunneling microscopy and spectroscopy between 300 and 60 K. Na intercalates nonuniformly, and divides the sample into intercalated (I) and nonintercalated (NI) areas. Below the CDW transition temperature these areas displayed totally different CDW’s. The NI areas seemed unaffected by Na, and showed a $4\times 4$ CDW that did not differ from the CDW in pure ${\mathrm{VSe}}_2.$ The intercalated areas showed a strong octahedral CDW $({\lambda }_1=0.99\mathrm{}\mathrm{nm},$ ${\lambda }_2=0.68\mathrm{}\mathrm{nm},$ ∠70°, rotated 20° with respect to the atomic lattice) occurring in three different orientations. Additional spots in Fourier space at low temperatures were interpreted to originate from Na ordering in a $1.3\times 1.3$ $R45\mathrm{}°$ structure. Spectroscopy showed a shift of the V $3d$-derived state below the Fermi level ${(E}_F)$ upon intercalation. In the NI areas, a shift toward $E_F$ (-60 meV compared to -80 meV in pure ${\mathrm{VSe}}_2)$ was observed, while the I areas showed a shift away from $E_F$ (-150 meV). The CDW energy gap was enlarged in the I areas $(\Delta \approx 230\mathrm{meV}$ compared to $\Delta \approx 80\mathrm{meV}$ in pure ${\mathrm{VSe}}_2),$ while it was never resolved in the NI areas.