Low Temperature Vortex Liquid in La 2-x Sr x CuO 4

1. Low Temperature Vortex Liquid in La2-xSrxCuO4 (a) Ms (b) Md PI: N. P. Ong, Physics Department, Princeton University Supported by NSF through MRSEC grant DMR 0213706 What is the fate of superconductivity in the cuprates as the doping x0 at very low temperature T? To address this issue, we have used sensitive torque magnetometry (inset a) to measure the magnetization of La2-xSrxCuO4 with the DC field facilities at the NHMFL at Tallahassee. Seven crystals with 0.03 ≤x ≤ 0.09 were investigated. From the flexing of the cantilever in intense field H, we may infer the competing diamagnetic signal Md (of the vortex response) and the paramagnetic spin signal Ms (from local moments). As seen for sample x = 0.05 (panel b), the vortex signal Md peaks near 5 T but is strongly suppressed when H reaches the upper critical field Hc2 = 30 T. The local moments Ms are fully polarized at low T. The data from the 7 samples enable the phase diagram in the x-H plane to be drawn (Panel c). Superconductivity exists within the vortex-solid phase (red) which is surrounded by a vortex-liquid (pink). At T=0, the solid melts into a liquid of quantum vortices across the melting line H0. To answer the question posed, in zero H, vortex-antivortex pairs spontaneously appearto destroy long-range phase rigidity at the quantum critical point (x = 0.055), but the condensate survives as a quantum vortex liquid. This is intimately related to increased localization of the holes to form the local momentsas x0(blue shading). Caption (a) Crystal of magnetic moment m glued to torque magnetometer. (b) Curves of diamagnetic term Md and spin term Ms vs. H at selected T. (c) Vortex liquid (pink) and vortex solid (red) phases, and Hc2 and melting curve H0 in the x-H plane. (c) Li, Lu; Checkelsky, J.G.; Komiya, S.; Ando, Y. and Ong, N.P., Low-temperature vortex liquid in La2-xSrxCuO4, Nature Physics 3, 311 (2007)