Variety of c-axis collective excitations in layered multigap   superconductors
  We present a dynamical theory for the phase differences along a stacked direction of intrinsic Josephson junctions (IJJ's) in layered multigap superconductors, motivated by the discovery of highly-anisotropic iron-based superconductors with thick perovskite-type blocking layers. The dynamical equations describing AC and DC intrinsic Josephson effects peculiar to multigap IJJ's are derived, and collective Leggett mode excitations in addition to the Josephson plasma established in single-gap IJJ's are predicted. The dispersion relations of their collective modes are explicitly displayed, and the remarkable peculiarity of the Leggett mode is demonstrated. 
Yukihiro Ota, Masahiko Machida, Tomio Koyama
cond-mat.supr-con cond-mat.mes-hall cond-mat.str-el quant-ph