Coulomb staircase in STM current through granular films

The tunneling current through a junction array consisting of a scanning tunneling microscope tip and a granular film is studied both theoretically and experimentally. When the tunnel resistance between the tip and a granule on the surface is much larger than those between granules, a bottleneck of the tunneling current is created in the array and the Coulomb staircase (CS) appears in the $I-V$ curve at room temperature. It is shown that the period of the CS is given by the capacitance at the bottleneck even though a granular film consists of many tunnel junctions with different capacitances. Our study provides a possibility for single-electron-spin-electronic devices at room temperature.