Theory of macroscopic quantum tunneling in high-T_c c-axis Josephson junctions

摘要

We study macroscopic quantum tunneling (MQT) in oaxis twist Josephson junctions made of high-T_c superconductors in order to clarify the influence of the anisotropic order parameter symmetry (OPS) on MQT. The dependence of the MQT rate on the twist angle y about the c axis is calculated by using the functional integral and the bounce method. Due to the d-wave OPS, the y dependence of standard deviation of the switching current distribution and the crossover temperature from thermal activation to MQT are found to be given by cos 2γ and cos cos 2γ~(1/2) respectively. We also show that a dissipative effect resulting from the nodal quasiparticle excitation on MQT is negligibly small, which is consistent with recent MQT experiments using Bi_2Sr_2CaCu_2O_(8+δ) intrinsic junctions. These results indicate that MQT in c-axis twist junctions becomes a useful experimental tool for testing the OPS of high-T_c materials at low temperature, and suggest high potential of such junctions for qubit applications.