ABSORPTION-EDGE SINGULARITIES FOR A NONEQUILIBRIUM FERMI SEA .1. 2ND-ORDER PERTURBATION-THEORY

The absorption threshold singularity induced by the conduction-electron- deep-hole final-state interaction is recovered from the Fermi golden rule using a simple second-order perturbation theory. This approach allows a direct physical understanding of the various contributions to the power-law critical exponent. We use this approach to get the absorption singularities for a nonequilibrium Fermi sea (μ1,μ2) instead of (0,μ). We find a diverging behavior at μ2 and a converging singularity at μ1. Unexpectedly, the critical exponents of these singularities disobey Hopfield's well accepted rule of thumb. © 1994 The American Physical Society.