Resonant Phase Escape from the First Resistive State of Bi₂Sr₂CaCu₂O_y Intrinsic Josephson Junctions under Strong Microwave Irradiation

Comparative Study of First and Second Switches with Nearly Identical Switching Current in Bi2Sr2Ca0.85Y0.15Cu2Oy Intrinsic Josephson Junctions

“…This result sharply contrasts to the previous results for IJJs with I 1st SW ≪ I 2nd SW , strongly supporting that the anomalous enhancement of T 2nd cr is attributed to the influence of the phase-running state after the 1st SW. We also observe that the microwave-induced resonant escapes for both SWs manifest almost the same behaviors up to ∼15 K. The detailed analyses indicate that the decrease of the Josephson plasma frequency ω p with increasing the microwave power is stronger than that of ∆U, in contrast to the previous studies. 12,13) Furthermore, the energy level numbers within the potential well for both SWs was found to remain more than 20 under the strong microwaves. These results suggest that the microwave irradiation is not used for a multi-photon absorption process from the ground state to the first excited state, but induces the strong oscillation such as a nonlinear bifurcation in the potential well.…”

“…11) In addition, the microwave-induced resonant peak structure was observed in the switching current distribution below T 2nd cr , suggesting the occurrence of the quantum phase escapes and the formation of energy level quantization (ELQ) in a potential well. 12,13) Thus, many experimental results 7,[9][10][11][12][13][14][15] support that the MQT state is realized even in the 2nd SWs of IJJs, although the value of T cr (typically ∼ 10 K for the optimal doped Bi2212 IJJs) is much higher than the value predicted in the conventional theory. 16) As a qualitative explanation for this anomaly, we proposed that the ac Josephson current occurring after the 1st SW possibly passes through other junctions near the phase-switched junction, inducing the rapid oscillation in a tilted-washboard potential and effectively reducing the barrier height ∆U(I) for the 2nd SW. 11) Similar effects are expected for the quantum phase escapes under the strong microwave irradiation.…”

“…[2][3][4][5] The most striking feature is that a crossover temperature, T cr , to the MQT state in the phase switch from the first to the second resistive branch (2nd SW) is largely enhanced compared to that from the zero voltage to the first resistive branch (1st SW) in the multi-branched current-voltage (I-V) characteristics of IJJs. [6][7][8][9][10][11][12][13][14][15] An early study 6) suggested that such an enhancement was due to the self-heating effects in the first resistive state, based on the consideration that a series array of uniform Josephson junctions should show the same value of T cr as long as the current flowing in each junction agrees with the externally-applied bias current.However, the anomalous increase of T 2nd cr was found to be independent of the device structure of IJJs with different heat-transfer environment 7) and the order of phase switches, 10) throwing doubt on the self-heating effects after the 1st SW.…”

“…17) These effects are also confirmed for Bi2212-IJJs under the strong microwaves. 12,13,15,18) Thus, the alternating current generating after the 1st SW is also expected to increase the MQT rate due to the effective reduction of ∆U(I). This scenario qualitatively explains the anomalous enhancement of T cr for the 2nd and higher order SWs.…”

“…An important fact leading to the above proposal is that the switching current I SW for the 1st SW often becomes smaller than that for the 2nd SW (I 1st SW < I 2nd SW ), because of the proximity effect of the normal-metal electrode in the mesa-type IJJs, 9,14,18,19) or the amorphous damage due to focused ion beam (FIB) microfabrication in the bridge-type IJJs. 10,12,15,20) The reduced switching current brings the phase-running state after the 1st SW, generating a temporal oscillation in a tilted washboard potential to describe the 2nd SW. This suggests that a study on IJJs with almost identical I 1st SW and I 2nd SW will provide novel information.…”