Low-temperature vortex liquid in La2-xSrxCuO4

In copper oxide superconductors, the lightly doped ( small dopant concentration, x) region is of major interest(1,2) because superconductivity, antiferromagnetism and the pseudogap state come together near a critical doping value, x(c). But the way in which superconductivity is destroyed as x is decreased at very low temperatures, T, is not clear(3-7). Does the pair condensate vanish abruptly at a critical value, x(c)? Or is phase coherence of the condensate destroyed by spontaneous vortices - as is the case at elevated T (refs 8 - 10)? So far, magnetization data at low T are very sparse in this region of the phase diagram. Here, we report torque magnetometry measurements on La2-xSrxCuO4, which show that, in zero magnetic field, quantum phase fluctuations destroy superconductivity at x(c) approximate to 0.055. The phase-disordered condensate survives to x = 0.03. In finite field H, the vortex solid-to-liquid transition occurs at H lower than the depairing field, H-c2. The resulting phase diagram reveals the large fraction of the x - H plane occupied by the quantum vortex liquid.