The surface density of states (SDOS) in superfluid He at a diffusive wall was investigated by systematic measurements of the transverse acoustic impedance Z. The observed difference between the temperature dependence of Z in the A and B phases can be explained by a difference in the SDOS of each phase. In the B phase, an additional gap in the SDOS opened between the upper energy edge Δ* of the surface Andreev bound states band and the bulk energy gap Δ. The temperature dependence of Δ* was measured and was about 30% smaller than the theoretical value. In the A phase, a flat and gapless SDOS was confirmed experimentally.
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