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

• Main Authors: Kapcia, Konrad J, Reedyk, Maureen, Hajialamdari, Mojtaba, Ptok, Andrzej, Piekarz, Przemysław, Razavi, Fereidoon S, Oleś, Andrzej M, Kremer, Reinhard K
• Format: Journal Article
• Language: English
• Published: 25.11.2019
• Subjects: Physics - Materials Science; Physics - Superconductivity
• DOI: 10.48550/arxiv.1911.11057

Physical Review Research 2, 033108 (2020) 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$_7$.