Direct observation of the critical state field profile in a ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathit{y}}$ single crystal

We report direct imaging of the critical state in a ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathit{y}}$ single crystal. The self-field profiles are obtained by mechanically scanning a high-spatial-resolution Hall-probe array above the crystal surface. The measured field distributions confirm Bean’s assumption of homogeneity, setting the relevant length scale for the evaluation of the critical currents as the sample size, and are a proof against the weak-link behavior in such crystals.