Number-Phase Fluctuations in Two-Band Superconductors

It is shown that in a clean two-band (transition-metal) superconductor there appears a collective excitation corresponding to small fluctuations of th~ 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 th~ ultrasound dispersion curve should be easily observable. The phenomenon is used to elucidate the physical nature of excitons in superconductors in general.

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