Pairing and the phase diagram of the normal coherence length $\xi_N(T,x)$ above $T_c$ of $La_{2-x}Sr_xCuO_4$ thin films probed by the Josephson effect

Abstract

Critical currents $I_c$ were measured in Superconducting - normal - superconducting SNS c-axis junctions, where S was optimally doped $YBa_2Cu_3O_{7-\delta}$ below $T_c$ (90 K) and N was $La_{2-x}Sr_xCuO_4$ above its $T_c$ ($<$25 K) but in the pseudogap regime. $I_c$ was measured versus temperature T in junctions with various N-barrier thicknesses $d$ and doping levels $x$. From the proximity effect relation $I_c(T)\propto exp[-d/\xi_N(T)]$, the normal coherence length $\xi_N(T)$ of N was extracted and plotted versus T. Besides finding a long range proximity effect as reported also by others, we found a crossing at about 55 K above which $\xi_N(x=0.1)>\xi_N(x=0.18)]$. This unexpected result where junctions with lower carrier density in the barrier have higher conductance, is attributed to the presence of pre-formed pairs in the pseudogap regime of the cuprate barrier. Our data yields a new phase diagram of $\xi_N(T,x)$ whose contours of constant $\xi_N$ follow the trend of the superconducting dome of $La_{2-x}Sr_xCuO_4$ above its $T_c$, with $\xi_N$ enhancement in the underdoped regime above 55 K.