Macroscopic quantum tunneling in multigap superconducting Josephson junctions: Escape rate enhancement via quantum fluctuations of Josephson-Leggett mode

We theoretically study the macroscopic quantum tunneling (MQT) in a hetero Josephson junction formed by a conventional single-gap superconductor and a multi-gap one such as {MgB}₂ and iron-based superconductors. In such a Josephson junction more than one phase differences are defined. We clarify the quantum dynamics of the phase differences and construct a theory for the MQT in the multi-gap Josephson junctions. The dynamics of the phase differences are strongly affected by the Josephson-Leggett mode, which is the out-of-phase oscillation mode of the phase differences. The escape rate is calculated in terms of the effective action renormalized by the Josephson-Leggett mode. It is shown that the escape rate is drastically enhanced when the frequency of the Josephson-Leggett mode is less than the Josephson-plasma frequency.