Behavior of a charged two-level fluctuator in an Al-AlO/sub x/-Al single-electron transistor in the normal and superconducting state

We have studied the behavior of a charged two-level fluctuator in an Al-AlO/sub x/-Al single-electron transistor (SET) in the normal state over a temperature range from 85 mK to 3 K. The fluctuator caused the SET's island charge to shift by /spl Delta/Q/sub o/=0.1/spl plusmn/0.025 e with an escape rate out of each state which was periodic in the gate voltage. We compare our results to a model which assumes the fluctuator resides in one of the tunnel junctions and discuss model predictions for when the device is in the superconducting state.