Experimental Evidence for Intra- and Inter-Unit-Cell Josephson Junctions in a YB${\mathrm{a}}_2$C${\mathrm{u}}_3$O ${}_{7-\delta }$ Single Crystal

Modulation of the dynamic resistance versus the magnetic field of ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-\delta }$ 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.