Observation of a vortex-glass phase in polycrystalline ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathit{x}}$ in a magnetic field

We report the results of temperature- and field-dependent transport measurements on sintered, polycrystalline samples of ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{7\mathrm{-}\mathit{x}}$. A study of the E-J curves indicates a second-order phase transition, at low current density in the coupling of the grains, that exhibits the scaling behavior predicted for a vortex glass. The ohmic resistance at low current is observed to vanish at a nonzero temperature, and the E-J curves exhibit power-law behavior at that temperature. Below this temperature the E-J curves are consistent with a true critical current with zero linear resistance. The qualitative features of the data and the values of the critical exponents are consistent with the predictions of the vortex-glass theory. Our findings cannot be explained by the predictions of conventional flux-creep models.