Isotope effect in superconducting n-doped SrTiO$_3$

TL;DR

Replacing oxygen isotopes in doped SrTiO$_3$ significantly enhances its superconducting critical temperature and magnetic field, indicating strong electron-phonon coupling and supporting models involving ferroelectric soft modes.

Contribution

This study demonstrates a large isotope effect on superconductivity in SrTiO$_3$, revealing the importance of lattice vibrations and ferroelectric modes in its pairing mechanism.

Findings

$T_c$ increases by ~50% with $^{18}$O substitution

$H_{c2}$ doubles with isotope substitution

Supports strong coupling to ferroelectric soft modes

Abstract

We report the influence on the superconducting critical temperature $T_c$ in doped SrTiO$_3$ of the substitution of the natural $^{16}$O atoms by the heavier isotope $^{18}$O. We observe that for a wide range of doping this substitution causes a strong ($\sim 50 \%$) enhancement of $T_c$. Also the magnetic critical field $H_{c2}$ is increased by a factor $\sim 2$. Such a strong impact on $T_c$ and $H_{c2}$, with a sign opposite to conventional superconductors, is unprecedented. The observed effect could be the consequence of strong coupling of the doped electrons to lattice vibrations (phonons), a notion which finds support in numerous optical and photo-emission studies. The unusually large size of the observed isotope effect supports a recent model for superconductivity in these materials based on strong coupling to the ferroelectric soft modes of SrTiO$_{3}$.