Probing the A-B Phase Interface in Superfluid 3He by Andreev Reflection of a Quasiparticle Beam.

Using a ballistic excitation beam generated by a quasiparticle blackbody radiator, we have probed the gap structure of a $B\ensuremath{-}A$ interface in superfluid ${}^{3}$He, stabilized magnetically. We see Andreev reflection both from the interface and from the field-distorted B-phase gap. As Andreev processes return excitations to the radiator enclosure, the fraction reflected governs the radiator temperature, from which we infer the maximum spin-dependent quasiparticle gap as a function of magnetic field. These measurements are the first to probe the superfluid ${}^{3}$He phase interface with quasiparticles.