Superconductivity at low density near a ferroelectric quantum critical point: doped SrTiO

TL;DR

This paper presents a theoretical model explaining the unconventional low-density superconductivity in doped SrTiO3 near a ferroelectric quantum critical point, emphasizing dynamical screening, phonon-mediated pairing, and the role of ferroelectric fluctuations.

Contribution

The study introduces a comprehensive model incorporating dynamical Coulomb screening and phonon interactions to explain superconductivity in doped SrTiO3 near ferroelectric criticality, aligning well with experimental observations.

Findings

Transition temperature depends on doping and dielectric properties.

Transverse optical phonons are crucial for pairing.

The model explains the anomalous isotope effect.

Abstract

Recent experiments on electron- or hole-doped SrTiO$_{3}$ have revealed a hitherto unknown form of superconductivity, where the Fermi energy of the paired electrons is much lower than the energies of the bosonic excitations thought to be responsible for the attractive interaction. We show that this situation requires a fresh look at the problem calling for (i) a systematic modeling of the dynamical screening of the Coulomb interaction by ionic and electronic charges, (ii) a transverse optical phonon mediated pair interaction and (iii) a determination of the energy range over which the pairing takes place. We argue that the latter is essentially given by the limiting energy beyond which quasiparticles cease to be well defined. The model allows to find the transition temperature as a function of both, the doping concentration and the dielectric properties of the host system, in good agreement with experimental data. The additional interaction mediated by the transverse optical soft phonon is shown to be essential in explaining the observed anomalous isotope effect. The model allows to capture the effect of the incipient (or real) ferroelectric phase in pure, or oxygen isotope substituted SrTiO$_{3}$ .