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Effective action for superconductors and BCS-Bose crossover

S. De Palo, C. Castellani, C. Di Castro, B. K. Chakraverty

DOI 10.1103/PhysRevB.60.564 · Physical Review B

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Abstract

A standard perturbative expansion around the mean-field solution is used to derive the low-energy effective action for superconductors at T=0. Taking into account the density fluctuations at the outset we get the effective action where the density ρ is the conjugated momentum to the phase θ of the order parameter. In the hydrodynamic regime, the dynamics of the superconductor is described by a time-dependent nonlinear Schrödinger (TDNLS) equation for the field Ψ(x)=ρ/2eiθ. The evolution of the density fluctuations in the crossover from weak-coupling (BCS) to strong-coupling (Bose condensation of localized pairs) superconductivity is discussed for the attractive Hubbard model. In the bosonic limit, the TDNLS equation reduces to the Gross-Pitaevskii equation for the order parameter, as in the standard description of superfluidity. The conditions under which a phase-only action can be derived in the presence of a long-range interaction to describe the physics of the superconductivity of “bad metals” are discussed.

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