Percolative Superconducting Pre-pairing in Cuprates

One of the important open questions in the physics of cuprate superconductors is the nature of superconducting pre-pairing above T_c. We present fundamental new insight into the pre-pairing regime, using a combination of techniques and several cuprate systems. We employ an unconventional probe – nonlinear conductivity – which is zero in the normal state, to find that the signals above T_c show an universal temperature dependence, irrespective of hole doping and cuprate family. Similar behavior is observed in nonlinear torque magnetization measurements, which unequivocally separate superconducting diamagnetism from normal-state paramagnetism, and in linear conductivity experiments. The results are incompatible with Ginzburg-Landau theory, but also rule out an extended temperature range of superconducting fluctuations. To explain our data, we employ a simple percolative model based on inhomogeneity of superconducting gaps. The calculated linear and nonlinear conductivity show excellent agreement with experiment, and the magnetization is explained qualitatively. The pre-pairing is thus controlled by intrinsic, universal nanoscale gap inhomogeneity.