Resonant Phase Escape from the First Resistive State of Bi^sub 2^Sr^sub 2^CaCu^sub 2^O^sub y^ Intrinsic Josephson Junctions under Strong Microwave Irradiation

We report a study of the phase escape in Bi^sub 2^Sr^sub 2^CaCu^sub 2^O^sub y^ intrinsic Josephson junctions under the strong microwave irradiation, focusing on the switch from the first resistive state (2nd SW). The resonant double-peak structure is clearly observed in the switching current distributions below 10 K and is successfully explained by a quantum-mechanical model on the quantum phase escape under the strong microwave field. These results provide the first evidence for the formation of the energy level quantization for the 2nd SW, supporting that the macroscopic quantum tunneling for the 2nd SW survives up to ~10 K.