Spectroscopic signatures and origin of a hidden order in Ba$_2$MgReO$_6$

TL;DR

This study uncovers a unique ordering transition in Ba$_2$MgReO$_6$ involving both structural and electronic changes, revealing a complex interplay between electronic interactions and lattice effects.

Contribution

It demonstrates that in Ba$_2$MgReO$_6$, electronic interactions initiate the order, but the lattice determines the final ground state, challenging conventional classification of such transitions.

Findings

Simultaneous structural and charge quadrupole ordering at 33 K

Electronic interactions drive the ordering process

Lattice effects determine the ground state

Abstract

Clarifying the underlying mechanisms that govern ordering transitions in condensed matter systems is crucial for comprehending emergent properties and phenomena. While transitions are often classified as electronically driven or lattice-driven, we present a departure from this conventional paradigm in the case of the double perovskite Ba$_2$MgReO$_6$. Leveraging resonant and non-resonant elastic x-ray scattering techniques, we unveil the simultaneous ordering of structural distortions and charge quadrupoles at a critical temperature of $T_\mathrm{q}$$\sim$33 K. Using a variety of complementary first-principles-based computational techniques, we demonstrate that while electronic interactions drive the ordering at $T_\mathrm{q}$, it is ultimately the lattice that dictates the specific ground state that emerges. Our findings highlight the crucial interplay between electronic and lattice degrees of freedom, providing a unified framework to understand and predict unconventional emergent phenomena in quantum materials.