Fermi surface of the most dilute superconductor

TL;DR

This study reveals that the most dilute superconductor, SrTiO$_{3}$, maintains a tiny but well-defined Fermi surface and superconductivity at extremely low carrier concentrations, challenging existing theories of pairing mechanisms.

Contribution

It demonstrates the existence of a sharp Fermi surface and superconductivity in SrTiO$_{3}$ at unprecedentedly low carrier densities using Nernst effect measurements.

Findings

Superconductivity persists down to carrier concentrations of 5.5 x 10^{17} cm^{-3}.

The Fermi energy is as low as 1.1 meV, with a large band gap.

The Fermi surface is extremely small, single-component, and barely anisotropic.

Abstract

The origin of superconductivity in bulk SrTiO$_{3}$ is a mystery, since the non-monotonous variation of the critical transition with carrier concentration defies the expectations of the crudest version of the BCS theory. Here, employing the Nernst effect, an extremely sensitive probe of tiny bulk Fermi surfaces, we show that down to concentrations as low as $5.5 \times 10^{17}cm^{-3}$, the system has both a sharp Fermi surface and a superconducting ground state. The most dilute superconductor currently known has therefore a metallic normal state with a Fermi energy as little as 1.1 meV on top of a band gap as large as 3 eV. Occurrence of a superconducting instability in an extremely small, single-component and barely anisotropic Fermi surface implies strong constraints for the identification of the pairing mechanism.