Pairing and the phase diagram of the normal coherence length ξ(N)(T, x) above T(c) of La(2−x)Sr(x)CuO(4) thin films probed by the Josephson effect

The long range proximity effect in high-T(c) c-axis Josephson junctions with a high-T(c) barrier of lower T(c) is still a puzzling phenomenon. It leads to supercurrents in junctions with much thicker barriers than would be allowed by the conventional proximity effect. Here we measured the T − x (Temperature-doping level) phase diagram of the barrier coherence length ξ(N)(T, x), and found an enhancement of ξ(N) at moderate under-doping and high temperatures. This indicates that a possible origin of the long range proximity effect in the cuprate barrier is the conjectured pre-formed pairs in the pseudogap regime, which increase the length scale over which superconducting correlations survive in the seemingly normal barrier. In more details, we measured the supercurrents I(c) of Superconducting - Normal - Superconducting SNS c-axis junctions, where S was optimally doped Y Ba(2)Cu(3)O(7−δ) below T(c) (90 K) and N was La(2−x)Sr(x)CuO(4) above its T(c) (<25 K) but in the pseudogap regime. From the exponential decay of I(c)(T) ∝ exp[−d/ξ(N)(T)], where d is the barrier thickness, the ξ(N)(T) values were extracted. By repeating these measurements for different barrier doping levels x, the whole phase diagram of ξ(N)(T, x) was obtained.