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s-wave in iron-pnictide superconductorsOta, Yukihiro; Nakai, Noriyuki; Nakamura, Hiroki; Machida, Masahiko; Inotani, Daisuke*; Ohashi, Yoji*; Koyama, Tomio*; Matsumoto, Hideki*
no journal, ,
An enormous amount of studies has been devoted to the identification of the pairing symmetry in iron-based superconductors. In the paper, we focus on a heterotic (multiband) superconductor- insulator-(single-band)superconductor junction. First, we derive the Amgegaokar-Baratoff relation. We evaluate a theoretical lowerbound of 
for s-wave without sign change, which may correspond to a upper bound for s-wave. Next, we discuss how a fluctuation of relative phases between multiple gaps and the pairing symmetry correct the Shapiro step, e.g., the step height and the step position. On the basis of these results, we propose a conventional and direct method to identify the s-wave symmetry.
Ota, Yukihiro; Machida, Masahiko; Koyama, Tomio*
no journal, ,
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 
and iron-based superconductors. 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 drastically enhanced when the frequency of the Josephson-Leggett mode is less than the Josephson-plasma frequency.