Exploring exotic superconducting order in La-based cuprate materials using Josephson interferometry

Abstract

Josephson junctions are especially useful devices for studying exotic superconducting materials, because the transmission of superconductivity through a junction depends on the difference in phase of the superconducting order parameter across the junction. We present the results of two measurements which involve the fabrication of Josephson junctions on unconventional high-temperature superconductors to study the physical processes which govern novel superconducting states within these materials. The first experiment involves measuring the current-phase relation (CPR) of La$_{2-x}$Ba$_x$CuO$_4$ (LBCO)-Au-Nb Josephson junctions. Using two independent measurement methods, we observe a clear sin(2$\phi$) component of the CPR, a signature of pair-density wave (PDW) order. This component is strongest at x=0.125 doping, where PDW order is believed to be strongest, and increases in magnitude with temperature as conventional superconductivity is suppressed. Likewise, at x=0.155 doping, where PDW order is not expected to dominate, the CPR primarily contains the conventional sin($\phi$) term. Together, these results provide strong support to the prediction that charge, spin and superconducting order intertwine to form a pair-density wave state in LBCO near x=1/8 doping. In the second experiment, we perform Josephson interferometry measurements of the superconducting order parameter of La$_{2−x}$Sr$_x$CuO$_4$ (LSCO) in the heavily overdoped (x$>$0.25) regime. Combining experimental measurements and simulations of I$_c(\Phi)$, we find evidence for granular superconductivity and circulating currents in LSCO near criticality.