Phonon echoes in powders of high-$T_c$ superconducting ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$

Strong rf phonon echoes have been observed in the superconducting state of powders of high-$T_c$ ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$. We suggest that the coupling mechanism between rf fields and acoustic modes is different from that in the usual phonon echoes judging from the sudden disappearance of the echoes at $T_c$=90 K and from the angular dependence of the echo intensity. The temperature dependence of $T_2^{\mathrm{-}1}$ is tentatively ascribed to the ultrasonic attenuation due to electrons, and the energy gap parameter 2Δ/$k_B$ $T_c$ is estimated to be 3–7.