Explaining the extra crystal field mode in ACeX2

TL;DR

This paper proposes that optical phonons can cause splitting of crystal electric field modes in Ce-based magnets, explaining previously unexplained high-energy modes observed in experiments.

Contribution

It introduces a phonon-induced mechanism for extra CEF modes, supported by DFT and point-charge calculations, applicable to various Ce$^{3+}$ compounds.

Findings

Phonon distortions can produce split CEF modes above phonon energies.

The mechanism explains observed extra modes in KCeO$_2$ and other Ce$^{3+}$ materials.

Density functional theory supports the phonon-CEF coupling explanation.

Abstract

A growing list of Ce-based magnets have shown an extra and heretofore unexplained crystal electric field (CEF) mode at high energies. We describe a process whereby an optical phonon can produce a split CEF mode well above the phonon energy. We use density functional theory and point-charge model calculations to estimate the phonon distortions and coupling to model this effect in KCeO$_2$, showing that it accounts for the extra CEF mode observed. This mechanism is generic, and may explain the extra modes observed on a variety of Ce$^{3+}$ compounds.