Superconductivity in low carrier density metals challenges the conventional electron-phonon theory due to the absence of retardation required to overcome Coulomb repulsion. Here I will discuss how pairing mediated by energy fluctuations, ubiquitously present close to continuous phase transitions, occurs in dilute quantum critical polar metals and results in a dome-like dependence of the superconducting Tc on carrier density, characteristic of non-BCS superconductors. In quantum critical polar metals, the Coulomb repulsion is heavily screened, while the critical transverse optic phonons decouple from the electron charge. In the resulting vacuum, long-range attractive interactions emerge from the energy fluctuations of the critical phonons, resembling the gravitational interactions of a charge-neutral dark matter universe. Our estimates show that this mechanism may explain the critical temperatures observed in doped SrTiO3 in the appropriate density regime. Predictions for the enhancement of superconductivity near polar quantum criticality in two- and three-dimensional materials that can be used to test our theory are provided.
Work based on “Superconductivity from Energy Fluctuations in Dilute Quantum Critical Polar Metals,” P.A. Volkov, P. Chandra and P. Coleman, arXiv:2106.11295.
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