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D Pelc, M Vuckovic, M Grbic, M Pozek, G. Yu, T Sasagawa, M. Greven, N. Barisic:
"Emergence of superconductivity in the cuprates via a universal percolation process";
Nature Communications, 9 (2018), 4327; S. 1 - 10.

A pivotal step toward understanding unconventional superconductors would be to decipherhow superconductivity emerges from the unusual normal state. In the cuprates, traces ofsuperconducting pairing appear above the macroscopic transition temperatureTc, yetextensive investigation has led to disparate conclusions. The main difficulty has been toseparate superconducting contributions from complex normal-state behaviour. Here weavoid this problem by measuring nonlinear conductivity, an observable that is zero in thenormal state. We uncover for several representative cuprates that the nonlinear conductivityvanishes exponentially aboveTc, both with temperature and magneticfield, and exhibitstemperature-scaling characterized by a universal scaleΞ0. Attempts to model the responsewith standard Ginzburg-Landau theory are systematically unsuccessful. Instead, ourfindingsare captured by a simple percolation model that also explains other properties of the cup-rates. We thus resolve a long-standing conundrum by showing that the superconductingprecursor in the cuprates is strongly affected by intrinsic inhomogeneity.

http://dx.doi.org/10.1038/s41467-018-06707-y