Leggett's modes in multi-band superconductors

Ota, Yukihiro; Machida, Masahiko  ; Koyama, Tomio*; Aoki, Hideo*

The discovery of iron-based superconductors has triggered the study for multi-band superconductors. Leggett's modes are expected to be important for characterizing multi-band superconductor. However, when considering the two-band model which is a minimal model for multi-band superconductors, it is characterized by only the magnitude of inter-band coupling. Thus, the relevance to s-wave superconductivity, which is a candidate for pairing symmetry in iron-based superconductors, is still unclear. Furthermore, a five-band model is necessary for a realistic theoretical description for iron-based superconductors. Hence, we examine Leggett's modes in a multi-band model. In particular, we focus on a three-band model, for simplicity. We reveal that two distinct classes of Leggett's modes exist under s-wave superconductivity. The difference between them is characterized via inter-band Josephson current addition or cancellation.