Evidence for Long-Lived Quasiparticles Trapped in Superconducting Point Contacts

We have observed that the supercurrent across phase-biased, highly transmitting atomic size contacts is strongly reduced within a broad phase interval around pi. We attribute this effect to quasiparticle trapping in one of the discrete subgap Andreev bound states formed at the contact. Trapping occurs essentially when the Andreev energy is smaller than half the superconducting gap Delta, a situation in which the lifetime of trapped quasiparticles is found to exceed 100 mu s. The origin of this sharp energy threshold is presently not understood.