title: Collective excitations and response functions of two-band band-singlet and spin-triplet superconductors.
abstract: 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.
authors.affiliation: Department of Physics and Basic Science Research Institute, Sogang University, Seoul, 121-742, Korea. hyunlee@sogang.ac.kr
authors.forename: H C
authors.initials: HC
authors.lastname: Lee