Locally modified charge-density waves in Na intercalated VSe2 studied by scanning tunneling microscopy and spectroscopy

We have observed local effects of the charge-density wave (CDW) in in situ Na-intercalated 1T-VSe2 using scanning tunneling microscopy and spectroscop y between 300 acid 60 K. Na intercalates nonuniformly, and divides the samp le into intercalated (I) and nonintercalated (NI) areas. Below the CDW tran sition temperature these areas displayed totally different CDW's. The NI ar eas seemed unaffected by Na, and showed a 4X4 CDW that did not differ from the CDW in pure VSe2. The intercalated areas showed a strong octahedral CDW (lambda(1)= 0.99 nm, lambda(2)= 0.65 nm, angle 70 degrees, rotated 20 degr ees with respect to the atomic lattice) occurring in three different orient ations. Additional spots in Fourier space at low temperatures were interpre ted to originate from Na ordering in a 1.3x 1.3 R45 degrees structure. Spec troscopy 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 compare d to -80 meV in pure VSe2) was observed, while the I areas showed a shift a way from E-F (-150 meV). The CDW energy gap was enlarged in the I areas (De lta approximate to 230 meV compared to Delta approximate to XO meV in pure VSe2), while it was never resolved in the NI areas. [S0163-1829(99)06511-X] .