Electronic Raman scattering of two-band superconductors: a time-dependent Landau-Ginzburg theory approach

Massless Leggett mode in three-band superconductors with time-reversal-symmetry breaking

The Leggett mode associated with out-of-phase oscillations of the superconducting phase in multiband superconductors usually is heavy due to interband coupling, which makes its excitation and detection difficult. We report on the existence of a massless Leggett mode in three-band superconductors with time-reversal-symmetry breaking. The mass of this Leggett mode is small close to the time-reversal-symmetry-breaking transition and vanishes at the transition point, and thus locates within the smallest superconducting energy gap, which makes it stable and detectable, e.g., by means of the Raman spectroscopy. The thermodynamic consequences of this massless mode and possible realization in iron-based superconductors are also discussed.

Collective excitations and response functions of two-band band-singlet and spin-triplet superconductors

The two-band band-singlet and spin-triplet superconductors are studied in the framework of functional integrals with a focus on collective excitations and their influences on response functions. The long range Coulomb interaction has been included to preserve the electromagnetic U(1) gauge invariance. The U(1) phase fluctuation and the spin wave are identified as the collective excitations. The longitudinal spin response function turns out to be solely determined by quasiparticle excitation. The transverse spin response function is determined by both the quasiparticle excitation and the spin wave. The spin wave is found not to be renormalized by long range Coulomb interaction. The density response function is determined by both the quasiparticle excitation and the U(1) phase fluctuation, but in contrast to the case of spin response functions, both of them are strongly renormalized by the long range Coulomb interaction. The nondegeneracy of the band energy can affect the response functions significantly through the contribution by collective excitations.