Vanishing magnetization relaxation in the high field quantum limit in ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$

We have investigated the magnetic response of untwinned single crystals of ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ at millikelvin temperatures using a Bi thin film magnetometer of micron dimensions. Below T=0.8 K, the magnetization relaxation rate S crosses over from thermally activiated to quantum behavior. Above a sharply defined and strongly temperature-dependent threshold field, S disappears altogether. In concert with the vanishing magnetization relaxation, discrete steps appear in the magnetic hysteresis B(H), each of which corresponds to the ‘‘stick-slip’’ motion of ${10}^3$ vortices under the magnetometer.