Surface Andreev bound states of the superfluid 3He B phase

Surface Andreev bound states of superfluid 3He have been detected by transverse acoustic impedance measurement. In the diffusive limit of the quasiparticle scattering off a wall, a singularity in the temperature dependence of the acoustic impedance appears at a particular temperature. At this temperature, pair excitations of the surface Andreev bound state quasiparticle and the propagating Bogoliubov quasiparticle drastically change their nature due to a very sharp band edge of the surface Andreev bound states, resulting in the singularity of the acoustic impedance. This temperature dependence was well reproduced by a theory which properly takes into account the surface Andreev bound states. In a partially specular wall boundary condition achieved by coating the wall with thin 4He layers, the singularity shifted to a higher temperature since the bandwidth became broader as the specularity increased. The coating with 4He influences only the states in the vicinity of the wall and thus the shift is further evidence that the transverse acoustic impedance senses the surface Andreev bound states.