Collective oscillations in two-band superconducting systems with low carrier density

We study the collective oscillations in two-band superconducting systems with all possible intra-and interband interactions for arbitrary values of carrier density N ₀, including low carrier densities, when μ∼Δ. Allowance for processes of scattering of an interband Cooper pair into an intraband pair leads to the emergence of a new excitonic mode in addition to the Bogolyubov-Anderson acoustic mode and an excitonic mode of the Leggett type. The presence of a new order parameter Δ₂, in addition to Δ₁₁ and Δ₂₂, and the asymmetric cutoff of integrals lead to the mixing of fluctuations of the phases and amplitudes of the order parameters of different bands. This mixing and the dependence of the order parameters on N ₀ result in a strong dependence of these two excitonic modes on the carrier density N ₀. Zh. éksp. Teor. Fiz. 114, 1047–1066 (September 1998)