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arXiv:cond-mat/9409124 · International Journal of Modern Physics B

DYNAMICS OF T=0 BCS CONDENSATES

MICHAEL STONE

Published May 15, 1995• 30 citations
arXivHTMLPDF

Abstract

Fermi-surface bosonization is used to show that the long-wavelength, T=0, dynamics of a BCS superfluid or superconductor is described by a galilean invariant non-linear time-dependent Schrödinger equation. This equation is of same form as the Gross-Pitaevskii equation for a Bose superfluid, but the “wavefunction” is not the superfluid order parameter.

DOI
10.1142/s0217979295000586

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Fermi-surface bosonization is used to show that the long-wavelength, T=0, dynamics of a BCS superfluid or superconductor is described by a galilean invariant non-linear time-dependent Schrödinger equation. This equation is of same form as the Gross-Pitaevskii equation for a Bose superfluid, but the “wavefunction” is not the superfluid order parameter.
This chapter introduces the order parameter and the Gross--Pitaevskii equation. Although a Bose-Einstein C condensate (BEC) is a quantum many-body system which that can be described fully only using the many-body wavefunction, many of its simplest spatial dynamics can be understood using the order parameter and its dynamics. We first describe how the order parameter can be formally defined, and how this can be considered an effective spatial wavefunction of the BEC. We then derive the time-dependent evolution of the order parameter via the Gross--Pitaevskii equation, then and study various solutions of it. This includes plane-wave solutions, an infinite potential well solutions, and excited state solutions such as vortices and solitons. After introducing key concepts such as the Thomas--Fermi approximation and the healing length, we also discuss how the Gross--Pitaevskii equation can be cast in the form of hydrodynamic equations.
A standard perturbative expansion around the mean-field solution is used to derive the low-energy effective action for superconductors at T=0.T=0.T=0. Taking into account the density fluctuations at the outset we get the effective action where the density $\ensuremath{\rho}$ is the conjugated momentum to the phase $\ensuremath{\theta}$ of the order parameter. In the hydrodynamic regime, the dynamics of the superconductor is described by a time-dependent nonlinear Schr\"odinger (TDNLS) equation for the field $\ensuremath{\Psi}(x)=\sqrt{\ensuremath{\rho}/2}{e}^{i\ensuremath{\theta}}.$ 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.
Abstract Chapter 13 addresses Bose condensation in superfluids (and superconductors), which involves the field operator ψ having a c-number component (<ψ(x,t)>≠0), challenging number conservation. The nonlinear Gross-Pitaevskii equation is derived for this condensate wave function <ψ>=ψ−ψ˜, facilitating identification of the coherence length and the core region of vortex motion. The noncondensate Green’s function G˜1(1,1′)=−i<(ψ˜(1)ψ˜+(1′))+> and the nonvanishing anomalous correlation function F˜∗(2,1′)=−i<(ψ˜+(2)ψ˜+(1′))+> describe the dynamics and elementary excitations of the non-condensate states and are discussed in conjunction with Landau’s criterion for viscosity. Associated concepts of off-diagonal long-range order and the interpretation of <ψ> as a superfluid order parameter are also introduced. Anderson’s Bose-condensed state, as a phase-coherent wave packet superposition of number states, resolves issues of number conservation. Superconductivity involves bound Cooper pairs of electrons capable of Bose condensation and superfluid behavior. Correspondingly, the two-particle Green’s function has a term involving a product of anomalous bound-Cooper-pair condensate wave functions of the type F(1,2)=−i<(ψ(1)ψ(2))+>≠0, such that G2(1,2;1′,2′)=F(1,2)F+(1′,2′)+G˜2(1,2;1′,2′). Here, G˜2 describes the dynamics/excitations of the non-superfluid-condensate states, while nonvanishing F,F+ represent a phase-coherent wave packet superposition of Cooper-pair number states and off-diagonal long range order. Employing this form of G2 in the G1-equation couples the condensed state with the non-condensate excitations. Taken jointly with the dynamical equation for F(1,2), this leads to the Gorkov equations, encompassing the Bardeen–Cooper–Schrieffer (BCS) energy gap, critical temperature, and Bogoliubov-de Gennes eigenfunction Bogoliubons. Superconductor thermodynamics and critical magnetic field are discussed. For a weak magnetic field, the Gorkov-equations lead to Ginzburg–Landau theory and a nonlinear Schrödinger-like equation for the pair wave function and the associated supercurrent, along with identification of the Cooper pair density. Furthermore, Chapter 13 addresses the apparent lack of gauge invariance of London theory with an elegant variational analysis involving re-gauging the potentials, yielding a manifestly gauge invariant generalization of the London equation. Consistency with the equation of continuity implies the existence of Anderson’s acoustic normal mode, which is supplanted by the plasmon for Coulomb interaction. Type II superconductors and the penetration (and interaction) of quantized magnetic flux lines are also discussed. Finally, Chapter 13 addresses Josephson tunneling between superconductors.