Tip apex charged states in tunneling spectroscopy

The STM electron transport through a short wire on a surface is studied by means of Green's function method and the tight-binding Hamiltonian. It is shown that for a sharp tip, represented by a kind of bent wire, the apex atom is extra occupied. For the neutral tip the conductance is symmetrical versus the surface chemical potential whereas for charged apex atom the conductance symmetry obtained for positive and negative STM bias voltages is broken. In the last case, all differential conductance peaks are modified by the occupied tip state.