What Makes Cuprate Superconductors so Exceptional?*

A comprehensive experiment will be decsribed in which over 2,000 single-crystal LSCO films were grown by molecular beam epitaxy and studied for 12 years. Resistivity, R_H, magnetoresistance, T_c, penetration depth, and coherence length have been measured precisely as a function of temperature T (down to 300 mK), magnetic field B (up to 90 T), doping, and in-plane azimuth angle. [1-3]
The key findings are as follows. (i) The superconducting phase stiffness is extremely low, comparable to T_c. (ii) The superfluid density N_s(T) decreases linearly with T, up to T_c. (iii) T_c scales with N_s0 linearly but with an offset, except very close to the dome edges where it scales as √N_s0. (iv) The superconducting state develops from an electronic nematic state that breaks the C₄ symmetry of the underlying crystal lattice. (v) The electron fluid behaves as if it were comprised of two components, one Fermi-liquid (FL) like and the other showing resistivity linear in T and B, diminishing with increased doping, and tracking the nematicity, N_s0, and T_c.
The related results of other groups show that the above appears to be typical of high-T_c cuprates and independent on the details of the Fermi surface, the number of CuO₂ planes in the unit cell, the presence or absence of CuO chains, the density and the nature of dopants, the superconducting gap size, etc.
We conclude that high-T_c superconductivity in cuprates involves some new physics that entails strong pairing, strong electron correlations, strong thermal phase fluctuations, and strong pair-breaking, intrinsic but T- and doping-dependent.
References
[1] I. Bozovic, X. He, J. Wu and A. T. Bollinger, Nature 536, 309 (2016).
[2] J. Wu, A. T. Bollinger, X. He and I. Bozovic, Nature 547, 432 (2017).
[3] P. Girardo-Gallo et al., Science 361, 479 (2018).