Experimental Evidence for Intra- and Inter-Unit-Cell Josephson Junctions in a YBa2Cu3O7- delta Single Crystal.

Abstract

Modulation of the dynamic resistance versus the magnetic field of ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}$ single crystal shows two groups of peak structures with nearly uniform magnetic field spacings. The spacings suggest two kinds of Josephson junctions: one formed by the Cu${\mathrm{O}}_{2}$-Y-Cu${\mathrm{O}}_{2}$ atomic planes within each unit cell and the other formed by the Cu${\mathrm{O}}_{2}$ bilayers between different unit cells. The intra-unit-cell junctions appear first as the temperature is lowered from ${T}_{c}$. The existence of Cu${\mathrm{O}}_{2}$-Y-Cu${\mathrm{O}}_{2}$ Josephson junctions implies two-dimensional superconductivity in the Cu${\mathrm{O}}_{2}$ atomic planes.