Low-Temperature Phase of the Cd$_2$Re$_2$O$_7$ Superconductor: Ab initio Phonon Calculations and Raman Scattering

TL;DR

This study combines ab initio phonon calculations and Raman scattering experiments to identify a new low-temperature phase transition in the superconductor Cd$_2$Re$_2$O$_7$, revealing a novel orthorhombic structure below 80 K.

Contribution

It provides the first combined theoretical and experimental evidence for a new low-temperature phase transition in Cd$_2$Re$_2$O$_7$, including identification of a lower-energy orthorhombic structure.

Findings

Identification of a phonon soft mode inducing a structural phase transition.

Observation of a new phase transition around 80 K with Raman spectroscopy.

Confirmation of a lower-energy orthorhombic crystal structure.

Abstract

Using an {\it ab initio} approach, we report a phonon soft mode in the tetragonal structure described by the space group $I4_{1}22$ of the $1$ K $5d$ superconductor Cd$_2$Re$_2$O$_7$. It induces an orthorhombic distortion to a crystal structure described by the space group $F222$ which hosts the superconducting state. This new phase has a lower total energy than the other known crystal structures of Cd$_2$Re$_2$O$_7$. Comprehensive temperature dependent Raman scattering experiments on isotope enriched samples, $^{116}$Cd$_2$Re$_2{^{18}}$O$_7$, not only confirm the already known structural phase transitions but also allow us to identify a new characteristic temperature regime around $\sim 80$ K, below which the Raman spectra undergo remarkable changes with the development of several sharp modes and mode splitting. Together with the results of the \textit{ab initio} phonon calculations we take these observations as strong evidence for another phase transition to a novel low-temperature crystal structure of Cd$_2$Re$_2$O$_7$.