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 (Formula presented) 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.