Number-Phase Fluctuations in Two-Band Superconductors

Abstract

It is shown that in a clean two-band (transition-metal) superconductor there appears a collective excitation corresponding to small fluctuations of the relative phase of the two “condensates”. In contrast to the transverse collective excitations in ordinary superconductors, the energy of this mode can be derived directly from existing experimental data; for Nb it lies about at the middle of the smaller single-particle gap. Direct experimental detection of this resonance is estimated to be rather beyond the limits of available techniques, but its effect on the ultrasound dispersion curve should be easily observable. The phenomenon is used to elucidate the physical nature of excitons in superconductors in general.