Phonon echoes in an amorphous superconductor in the vortex state

Phonon echoes are observed in the amorphous superconductor CuZr₂ (T_c=2.4 K) in the vortex state at 10 mK. The phase-memory time T₂ of the tunnelling states and their relaxation time T₁ are measured at 10 mK. The latter is also obtained from a saturation-recovery experiment. T₁ and T₂ vary exponentially as a function of magnetic field between 0 and 5 kOe. This effect is explained by considering an interaction between the tunnelling states and the vortices that arise from the normal-electron density distribution near the vortex core.