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$_7$.
Highlights
Superconductivity in materials with noncentrosymmetric crystal structures currently attracts broad attention [1,2]
The purpose of this paper is to highlight the discovery of a different noncentrosymmetric orthorhombic LT phase for which we find strong evidence both from detailed temperature-dependent Raman scattering and from density-functional theory (DFT) calculations of the phonon dispersions
The structural phase transitions (SPTs) at ∼200 K, Kendziora et al [17] observed a peak at zero frequency as well as a low-frequency phonon of A1 symmetry which they attributed to a soft mode associated with the cubic to tetragonal SPT
Summary
Superconductivity in materials with noncentrosymmetric crystal structures currently attracts broad attention [1,2]. Materials containing heavy 4d (5d) transition-metal elements with strong spin-orbit coupling are especially promising candidates to search for such states [5]. Among a number of unusual properties reported for CRO, a series of structural phase transitions (SPTs) is of particular interest since it essentially determines the low-temperature (LT) structural and electronic properties [11]. It is common belief that superconductivity occurs in this latter phase Both currently known LT tetragonal space groups (phase II and phase III) are subgroups of the RT cubic space group (phase I) (see Fig. 1). The purpose of this paper is to highlight the discovery of a different noncentrosymmetric orthorhombic LT phase (phase IV, space group F 222, No 22) for which we find strong evidence both from detailed temperature-dependent Raman scattering and from density-functional theory (DFT) calculations of the phonon dispersions (discussed below).
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