# A possible mechanism for superconductivity in doped SrTiO3

## TL;DR

This paper proposes a mechanism for superconductivity in doped SrTiO3 involving strong electron-phonon coupling and bi-phonon exchange, explaining the observed critical temperature.

## Contribution

It introduces a novel pairing mechanism mediated by two transverse optical phonons, linked to the charged phonon effect in SrTiO3, supported by experimental spectral data.

## Key findings

- The bi-phonon mediated interaction matches the observed critical temperature.
- Charged phonon effects indicate strong electron-phonon coupling in SrTiO3.
- The proposed mechanism explains superconductivity at low electron densities.

## Abstract

The soft ferro-electric phonon in SrTiO3 observed with optical spectroscopy has an extraordinary strong spectral weight which is much stronger than expected in the limit of a perfectly ionic compound. The "charged phonon" in SrTiO3 is caused by the close-to-covalent character of the Ti-O ionic bond and implies a strong coupling between the soft ferro-electric phonon and the inter band transitions across the 3 eV gap of SrTiO3. We demonstrate that this coupling leads, in addition to the charged phonon effect, to a pairing interaction involving the exchange of two transverse optical phonons. This process owes its relevance to the strong electron-phonon coupling and to the fact that the interaction mediated by a single transverse optical phonon vanishes at low electron density. We use the experimental soft phonon spectral weight to calculate the strength of the bi-phonon mediated pairing interaction in the electron doped material and show that it is of the correct magnitude when compared to the experimental value of the superconducting critical temperature.