New instability toward inhomogeneous states of nonequilibrium superconductors

The new instability model of nonequilibrium superconductors recently proposed by Iguchi and Konno is explored to determine whether inhomogeneous superconducting states can be induced by high-energy quasiparticle injection. No superconducting states, whether homogeneous or not, can be found when the quasiparticle injection rate exceeds some critical value. For injection rates below the critical value, in addition to a stable homogeneous solution, a spatially periodic solution does exist but is, however, unstable against small fluctuations. Another model, which uses a more realistic approach to calculate the density of the relaxation phonons, is also presented and examined. This modified model does not predict a stable inhomogeneous solution either. Instead, it predicts a first-order-like transition between two homogeneous nonequilibrium states if the phonon escape time is sufficiently long.