Local symmetry breaking and low-energy continuum in K$_2$ReCl$_6 $

TL;DR

This study uses polarization-resolved Raman scattering to explore structural phase transitions and local symmetry breaking in K$_2$ReCl$_6$, revealing persistent low-energy excitations and continuum linked to local octahedral fluctuations.

Contribution

It provides new insights into local symmetry breaking and low-energy excitations in K$_2$ReCl$_6$ through polarization-selective Raman measurements.

Findings

Violation of Raman selection rules in cubic phase

Persistent low-energy scattering continuum from 300K to 5K

Coupling of continuum to phonon mode above 76K

Abstract

Using polarization selective spontaneous Raman scattering, we have investigated the 5$d$ transition metal compound K$_2$ReCl$_6$ which displays a series of structural phase transitions. We observe a violation of the Raman selection rules in the cubic high temperature phase as well as a low-energy scattering continuum persistent throughout the investigated temperature range from 300 down to 5 K. The continuum couples to one of the phonon modes at temperatures above the lowest structural phase transition at 76 K. We propose a common origin of these observations caused by local fluctuations in the orientation of the ReCl$_6$ octahedra which locally break the long-range cubic symmetry. Consistent results from the related non-magnetic compound K$_2$SnCl$_6$ support this interpretation.