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We investigated macroscopic quantum tunneling (MQT) ofBi$_2$Sr$_2$CaCu$_2$O$_y$ intrinsic Josephson junctions (IJJs) with two devicestructures. One is a nanometer-thick small mesa structure with only two orthree IJJs and the other is a stack of a few hundreds of IJJs on a narrowbridge structure. Experimental results of switching current distribution forthe first switching events from zero-voltage state showed a good agreement withthe conventional theory for a single Josephson junction, indicating that acrossover temperature from thermal activation to MQT regime for the formerdevice structure was as high as that for the latter device structure. Togetherwith the observation of multiphoton transitions between quantized energy levelsin MQT regime, these results strongly suggest that the observed MQT behavior isintrinsic to a single IJJ in high-$T_c$ cuprates, independent of devicestructures. The switching current distribution for the second switching eventsfrom the first resistive state, which were carefully distinguished from thefirst switchings, was also compared between two device structures. In spite ofthe difference in the heat transfer environment, the second switching eventsfor both devices were found to show a similar temperature-independent behaviorup to a much higher temperature than the crossover temperature for the firstswitching. We argue that it cannot be explained in terms of the self-heatingowing to dissipative currents after the first switching. As possiblecandidates, the MQT process for the second switching and the effective increaseof electronic temperature due to quasiparticle injection are discussed.

Comparative study of macroscopic quantum tunneling in Bi_2Sr_2CaCu_2O_y intrinsic Josephson junctions with different device structures