Electron quasi-itinerancy intertwined with quantum order by disorder in pyrochlore iridate magnetism

TL;DR

This paper explores how electron quasi-itinerancy and quantum order by disorder jointly influence magnetism in pyrochlore iridates, revealing a non-coplanar magnetic order driven by quantum fluctuations and electron mobility.

Contribution

It introduces a comprehensive theoretical framework linking electron quasi-itinerancy with quantum order by disorder in pyrochlore iridates, highlighting their combined effect on magnetic ground states.

Findings

Quantum order by disorder selects a non-coplanar magnetic ground state.

Electron quasi-itinerancy extends the non-coplanar order to intermediate coupling regimes.

The study provides insights into magnetic behavior relevant to iridates and related materials.

Abstract

We point out the emergence of magnetism from the interplay of electron quasi-itinerancy and quantum order by disorder in pyrochlore iridates. Like other Mott insulating iridates, the Ir$^{4+}$ ion in pyrochlore iridates develops an effective ${J=1/2}$ moment from the on-site spin-orbit coupling. We consider the generic symmetry-allowed exchange between these local moments on a pyrochlore lattice and obtain the mean-field phase diagram. Assuming the superexchange is mediated by direct and/or indirect electron hopping via intermediate oxygens, we derive the exchange interactions in the strong coupling regime from the Hubbard model. This exchange has a degenerate classical ground state manifold and quantum fluctuation selects a non-coplanar ground state, known as quantum order by disorder. Extending to the intermediate coupling regime, the same non-coplanar order is selected from the degenerate manifold by the kinetic energy, which is dubbed "electron quasi-itinerancy". We discuss the experimental relevance of our results and electron quasi-itinerancy among other iridates and $4d/5d$ magnets.