  The influence of charged STM tip on the electron transport through quantum
states on a surface is studied both theoretically and experimentally. The
current and the differential conductance calculations are carried out by means
of the Green's function technique and a tight-binding Hamiltonian. It is shown
that sharp STM tip is extra occupied and this additional charge breaks the
conductance symmetry for positive and negative STM voltages. The experiment on
Ag islands with two STM tips (blunt and sharp) confirms our theoretical
calculations.
