Interfacial effects and superconductivity in high-Tcmaterials

A series of quasihomogeneous superconducting powders of $\mathrm{Y}{\mathrm{Ba}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-x}$ have been prepared by sieving and characterized by electron microscopy and x-ray diffraction. Quantitative size-dependent and temperature-dependent magnetic levitation and susceptibility measurements of random powders and of field-oriented (at $T<\mathrm{}{T}_c=92\mathrm{}$ K) small grains (<20 μm) confirm previously observed symmetry-breaking effects associated with the twin boundary planes and suggest that the bulk anisotropic superconductivity is measurably perturbed by interfacial effects. Current models of interfacial superconductivity in high-$T_c$ materials are evaluated in light of these and other results and are found to be lacking.