Coupled Spin-Charge-Phonon Fluctuation in the All-In/All-Out Antiferromagnet Cd$_2$Os$_2$O$_7$

TL;DR

This paper uncovers a unique coupled spin-charge-phonon fluctuation in Cd$_2$Os$_2$O$_7$, driven by thermally excited carriers and associated with polaron hopping, linked to the material's metal-insulator transition.

Contribution

It reveals a novel fluctuation mechanism involving spin, charge, and phonons mediated by polaron dynamics in a 5d pyrochlore magnet.

Findings

Identification of high activation energy for spin-charge fluctuation

Observation of fluctuation frequency range from 10^6 to 10^{10} s^{-1}

Link between fluctuation and metal-insulator transition

Abstract

We report on a novel spin-charge fluctuation in the all-in-all-out pyrochlore magnet Cd$_2$Os$_2$O$_7$, where the spin fluctuation is driven by the conduction of thermally excited electrons/holes and associated fluctuation of Os valence. The fluctuation exhibits an activation energy significantly greater than the spin-charge excitation gap and a peculiar frequency range of $10^{6}$--$10^{10}$ s$^{-1}$. These features are attributed to the hopping motion of carriers as small polarons in the insulating phase, where the polaron state is presumably induced by the magnetoelastic coupling via the strong spin-orbit interaction. Such a coupled spin-charge-phonon fluctuation manifests as a part of the metal-insulator transition that is extended over a wide temperature range due to the modest electron correlation comparable with other interactions characteristic for 5$d$-subshell systems.