Andreev Scattering at Rough Surfaces
In an Andreev reflection, a quasiparticle excitation turns from a particle into a hole or the other way around, reverses its direction of propagation and, to a good approximation, conserves its momentum. Such a process is peculiar to superconductors and superfluids where a quasiparticle can combine into a Cooper pair with another quasiparticle of nearly opposite momentum and substitute in its own place the hole or the particle left behind by the creation of the newly acquired partner. Andreev scattering is usually brought about by spatial changes in the order parameter of a superfluid. The prototype case is a quasiparticle meeting an increasing and eventually overwhelmingly large gap. Any rapid change of the order parameter, however, can call forth Andreev scattering. There is an enormous wealth of circumstances in which Andreev scattering may take place. The present article is about almost pure so called quantum Andreev scattering, i.e. Andreev scattering by a rapid change into a smaller gap than the energy of the quasiparticle as seen at surfaces or walls of p-wave superfluids. Some quasiparticles impinging on a wall of a superfluid container simply never reach the wall but turn back without exchanging momentum with the wall no matter how rough the wall may be. Not only the fate of the manifestly Andreev scattered particle will be at the focus of our interest here.
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