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.