2 An Intrinsic Tendency of Electronic Phase Separation into Two Superconducting States

The effect of hydrostatic pressure up to 2 GPa on the superconductiong transitions in La 2−x Sr x CuO 4+δ is investigated. The ambient and high pressure properties of two series of samples with x=0 and x=0.015 and 0< δ <0.1 are characterized and compared by ac-susceptibility measurements. At ambient pressure both sets of samples fit into the same phase diagram as a function of the total hole concentration, n h. For n h < 0.085 there is a single supercon-ducting transition (T c ≈ 30K) with an unusually large pressure coefficient, dT (30) c /dp ≈ 10 K/GP a. At higher hole density (n h > 0.085) a second su-perconducting transition (T c ≈ 15K) follows the first transition upon cooling and the pressure shift of this transition is negative, dT (15) c /dp ≈ −4 K/GP a. At the boundary as the hole density is close to 0.085 the phase separation can be induced by pressure. The results are explained in terms of a strong correlation of the interstitial oxygen with the hole system in the CuO-planes. Pressure, applied at ambient temperature, causes a redistribution of holes.The mobile oxygen dopants follow and enhance T c as well as the tendency to phase 1 separation. If pressure is changed at low temperature (< 100K) the effects on T c and phase separations are greatly diminished because the interstitial oxygen becomes immobile at low T. Our results indicate that the dopant effects are important. Dopants and holes should be treated as a single globally correlated state. When thermodynamic euqilibrium is approached in the oxygen-doped samples, we find that there is an intrinsic tendency of electronic phase separation of doped holes into two distinct superconducting states.