Josephson interferometry measurements of the superconducting pairing symmetry in the overdoped cuprate La_1.75Sr_0.25CuO₄ --- search for disorder-induced deviations from d-wave symmetry

Extensive measurements have shown that cuprate superconductors exhibit a robust d_x²_-y² global superconductor pairing symmetry. It has been proposed that in a disordered d-wave superconductor near criticality, the superconductivity will nucleate in spatially-separated puddles with orthogonal phase alignment of the d-wave order parameter. When these puddles are dilute, such as in highly overdoped samples, Josephson coupling between puddles can favor global s-wave superconductivity. To explore this regime, we perfromed Josephson interferometry measurements of LSCO-Au-Nb Josephson junctions formed on edges and corners of an overdoped single crystal of La_1.75Sr_0.25CuO₄ with a T_c of ~18K. In preliminary measurements of I_c vs. magnetic field, we observe deviations from Fraunhofer patterns expected for uniform edge junctions and from the characteristic corner junction pattern expected for d_x²_-y² symmetry. We see rapid modulations in the critical current indicating the presence of discrete order parameter domains with opposite sign. At temperatures near where the junction supercurrent vanishes, we observe diffraction patterns with a maximum centered on zero field in the corner junctions, as we would expect to see in an s-wave superconductor.