Superconductivity mediated by polar modes in ferroelectric metals

C Enderlein,J Ferreira De Oliveira,S E Rowley,S S Saxena,E Baggio Saitovitch,D A Tompsett,G G Lonzarich

doi:10.1038/s41467-020-18438-0

Abstract

The occurrence of superconductivity in doped SrTiO3 at low carrier densities points to the presence of an unusually strong pairing interaction that has eluded understanding for several decades. We report experimental results showing the pressure dependence of the superconducting transition temperature, Tc, near to optimal doping that sheds light on the nature of this interaction. We find that Tc increases dramatically when the energy gap of the ferroelectric critical modes is suppressed, i.e., as the ferroelectric quantum critical point is approached in a way reminiscent to behaviour observed in magnetic counterparts. However, in contrast to the latter, the coupling of the carriers to the critical modes in ferroelectrics is predicted to be small. We present a quantitative model involving the dynamical screening of the Coulomb interaction and show that an enhancement of Tc near to a ferroelectric quantum critical point can arise due to the virtual exchange of longitudinal hybrid-polar-modes, even in the absence of a strong coupling to the transverse critical modes.

Highlights

The occurrence of superconductivity in doped SrTiO3 at low carrier densities points to the presence of an unusually strong pairing interaction that has eluded understanding for several decades
The phase diagram is characterized by (i) a ferroelectric transition temperature TCurie that terminates at a quantum critical point (QCP), (ii) a low-temperature crossover curve Tx separating a power law (1/ε0 ~ T2) and an exponential temperature dependence of 1/ε0 terminating at the QCP, and (iii) a hightemperature crossover curve separating classical (Curie–Weiss) and quantum behaviour of the temperature dependence of 1/ε0
The introduction of charge carriers leads to an additional axis (n) in the ferroelectric phase diagram of Fig. 1, which is available for exploration

Summary

Introduction

The occurrence of superconductivity in doped SrTiO3 at low carrier densities points to the presence of an unusually strong pairing interaction that has eluded understanding for several decades. We report experimental results showing the pressure dependence of the superconducting transition temperature, Tc, near to optimal doping that sheds light on the nature of this interaction. The temperature-quantum tuning parameter phase diagram of SrTiO3 and related materials has recently been discussed in terms of a phenomenological model involving the interaction of the local ferroelectric order-parameter field with itself, and with the strain field of the lattice3,S1–S13 (see Fig. 1). Umklapp processes may be neglected under our conditions as well as inter-valley scattering processes that would require the presence of multiple Fermi surface pockets well separated in the Brillouin zone These are not present in electron-doped SrTiO3 as confirmed by several quantum oscillation experiments and band structure calculations We present results showing the pressure dependence of the superconducting transition temperature and develop a superconductivity model appropriate for electron-doped SrTiO3 and related materials

Methods

Results

Conclusion