Phonon Echoes in Superconducting AmorphousPd30Zr70

Phonon echoes, the acoustic analog of spin or photon echoes, are observed in the superconducting ($T_c=2.6\mathrm{}$ K) metallic glass ${\mathrm{Pd}}_{30}$ ${\mathrm{Zr}}_{70}$ 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 $T_1^{}{}_{}{}^{-1\mathrm{}}$, which directly confirms the disappearance of an electronic relaxation channel for tunneling systems in the presence of a superconducting gap.