Metallicity and superconductivity in doped strontium titanate

TL;DR

This paper reviews how doping strontium titanate leads to metallicity and superconductivity at very low carrier concentrations, highlighting its unique electronic properties and the connection to ferroelectricity.

Contribution

It provides a comprehensive overview of recent research on the electronic phases of doped strontium titanate, emphasizing its superconductivity and ferroelectricity interplay.

Findings

Superconductivity persists at extremely low doping levels.

Metallic behavior remains above room temperature despite low carrier density.

Superconductivity occurs beyond Migdal-Eliashberg approximation boundaries.

Abstract

Strontium titanate is a wide-gap semiconductor avoiding a ferroelectric instability thanks to quantum fluctuations. This proximity leads to strong screening of static Coulomb interaction and paves the way for the emergence of a very dilute metal with extremely mobile carriers at liquid-helium temperature. Upon warming, mobility decreases by several orders of magnitude. Yet, metallicity persists above room temperature even when the apparent mean-free-path falls below the electron wavelength. The superconducting instability survives at exceptionally low concentrations and beyond the boundaries of Migdal-Eliashberg approximation. An intimate connection between dilute superconductivity and aborted ferroelectricity is widely suspected. In this review, we will give a brief account of ongoing research on bulk strontium titanate as an insulator, a metal and a superconductor.