Charge multipoles correlations and order in Cs$_2$TaCl$_6$

TL;DR

This study investigates the complex phase transition in Cs$_2$TaCl$_6$, revealing charge quadrupole ordering driven by charge, structural, and spin interactions, using combined experimental and computational methods.

Contribution

It demonstrates the emergence of long-range charge quadrupole order in Cs$_2$TaCl$_6$ and highlights the effectiveness of integrated techniques in studying hidden orders.

Findings

Heat capacity anomaly at $T_Q$ in single crystals

Local charge quadrupoles develop into long-range order

Resonant x-ray scattering supports computational results

Abstract

We examine the role of charge, structural, and spin degrees of freedom in the previously poorly understood phase transition in the 5$d^1$ transition-metal double perovskite Cs$_2$TaCl$_6$, using a combination of computational and experimental techniques. Our heat capacity measurements of single-crystalline Cs$_2$TaCl$_6$, reveal a clear anomaly at the transition temperature, $T_\mathrm{Q}$, which was not previously observed in polycrystalline samples. Density functional calculations indicate the emergence of local charge quadrupoles in the cubic phase, mediated by the paramagnetic spins or local structural distortions which then develop into long-range ordered charge quadrupoles in the tetragonal phase. Our resonant elastic x-ray scattering on Cs$_2$TaCl$_6$, single crystals lend support to our calculations. Our work provides new insight into the phase transition in Cs$_2$TaCl$_6$, at $T_\mathrm{Q}$, and demonstrates the utility of this combination of techniques in understanding the complex physics of hidden orders in paramagnetic spin-orbit-entangled compounds.