Charging effects at the surfaces of HTSC films by CSTM

We have investigated the properties of ultrasmall capacitance junctions formed by the STM tip and conductive microstructures on the surfaces of evaporated YBaCuO films at 4.2K. We have found that most of the anomalous tunneling structures on the I-V curves that are in resemblance related to the superconductor and single junction Coulomb blockade behaviors can be explained by a voltage-biased double junction Coulomb blockade system. In one measurement, we have observed what appeared to be microwave induced current steps on the I-V curve.