Phonon echoes in superconducting amorphous Pd sub (30) Zr sub (70).

Phonon echoes, the acoustic analog of spin or photon echoes, are observed in the superconducting (T sub c =2.6K) metallic glass Pd sub (30) Zr sub (70) below 50 mK. The echo generation is attributed to the coherent tunneling of two-level systems of the amorphous structure. Compared to a normal conducting amorphous metal we observe a very small decay rate T sub 1 sup (-1), which directly confirms the disappearance of an electronic relaxation channel for tunneling systems in the presence of a superconducting gap.