Spin-dependent anisotropic electron-phonon coupling in KTaO$_3$

TL;DR

This study investigates the spin-dependent electron-phonon interactions in KTaO$_3$, revealing dominant coupling to soft TO modes with significant spin-non-conserving effects, and introduces a versatile computational approach for such analyses.

Contribution

It provides a detailed ab initio analysis of electron-phonon coupling in KTaO$_3$, highlighting spin-dependent effects and developing a general DFPT-based method applicable to other materials.

Findings

Strong coupling to soft TO mode with intraband and interband contributions

Spin-non-conserving matrix elements are particularly relevant for the soft mode

Developed a DFPT protocol for calculating electron-phonon interactions in any mode

Abstract

KTaO$_3$ (KTO) is an incipient ferroelectric, characterized by a softening of the lowest transverse optical (TO) mode with decreasing temperature. Cooper pairing in the recently discovered KTO-based heterostructures has been proposed to be mediated by the soft TO mode. Here we study the electron coupling to the zone-center odd-parity modes of bulk KTO by means of relativistic Density Functional Perturbation Theory (DFPT). The coupling to the soft TO mode is by far the largest, with comparable contributions from both intraband and interband processes. Remarkably, we find that for this mode, spin-non-conserving matrix elements are particularly relevant. We develop a three-band microscopic model with spin-orbit coupled $t_{2g}$ orbitals that reproduces the main features of the ab initio results. For the highest energy band, the coupling can be understood as a "dynamical" isotropic Rashba effect. In contrast, for the two lowest bands, the Rashba-like coupling becomes strongly anisotropic. The DFPT protocol implemented here enables the calculation of the full electron-phonon coupling matrix projected onto any mode of interest, and it is easily applicable to other systems.