Scanning tunneling spectroscopy of Na on Cu(111)

Monolayer films of Na evaporated onto Cu(111) under UHV conditions are investigated with scanning tunneling microscopy and spectroscopy (STS) at low temperature. The STS data clearly reveal a two-dimensional overlayer state of Na on Cu(111). From STS and spectroscopic imaging, the binding energies and effective masses of this state are determined for various coverages. The state is demonstrated to be affected by spatial inhomogeneities such as Cu steps or local coverage variations. Drastic bias-dependent variations of apparent step heights and tunneling barrier heights are reported and interpreted in terms of the large contributions of the overlayer state to the tunneling current.