First-principles calculation of electron-phonon coupling in doped KTaO$_3$

TL;DR

This study uses first-principles calculations to analyze electron-phonon coupling in doped KTaO$_3$, revealing mode-specific interactions and suggesting ferroelectric fluctuations play a key role in its superconductivity.

Contribution

It provides a detailed mode-resolved analysis of electron-phonon coupling in doped KTaO$_3$ and highlights the importance of ferroelectric soft modes over traditional BCS mechanisms.

Findings

Electron-phonon coupling strongest in optical modes around $b3$ point.

Dome-shaped doping dependence of coupling strength in all directions.

Strong localization of coupling in soft optical modes suggests ferroelectric fluctuations influence superconductivity.

Abstract

Motivated by the recent experimental discovery of strongly surface-plane-dependent superconductivity at surfaces of KTaO$_3$ single crystals, we calculate the electron-phonon coupling strength, $\lambda$, of doped KTaO$_3$ along the reciprocal-space high-symmetry directions. Using the Wannier-function approach implemented in the EPW package, we calculate $\lambda$ across the experimentally covered doping range and compare its mode-resolved distribution along the [001], [110] and [111] reciprocal-space directions. We find that the electron-phonon coupling is strongest in the optical modes around the $\Gamma$ point, with some distribution to higher $k$ values in the [001] direction. The electron-phonon coupling strength as a function of doping has a dome-like shape in all three directions and its integrated total is largest in the [001] direction and smallest in the [111] direction, in contrast to the experimentally measured trends in critical temperatures. This disagreement points to a non-BCS character of the superconductivity. Instead, the strong localization of $\lambda$ in the soft optical modes around $\Gamma$ suggests an importance of ferroelectric soft-mode fluctuations, which is supported by our findings that the mode-resolved $\lambda$ values are strongly enhanced in polar structures. The inclusion of spin-orbit coupling has negligible influence on our calculated mode-resolved $\lambda$ values.