Metallic “Ferroelectricity” in the PyrochloreCd2Re2O7

Abstract

A class of materials known as "ferroelectric metals" was discussed theoretically by Anderson and Blount in 1965 [Phys. Rev. Lett. 14, 217 (1965)]], but to date no examples of this class of materials have been reported. Here we present measurements of the elastic moduli of Cd2Re2O7 through the 200 K cubic-to-tetragonal phase transition. A Landau analysis of the moduli reveals that the transition is consistent with Cd2Re2O7 being classified as a ferroelectric metal in the weaker sense described by Anderson and Blount (loss of a center of symmetry). First-principles calculations of the lattice instabilities indicate that the dominant lattice instability corresponds to a twofold degenerate mode with E(u) symmetry and that motions of the O ions forming the O octahedra dominate the energetics of the transition.

Electronic correlations in the ferroelectric metallic state ofLiOsO3

“…For instance LaSr 2 Cu 2 GaO 7 shares a very similar structure and the d 9 electronic configuration for Cu atoms with the high-temperature superconductor YBa 2 Cu 3 O 7 in which strong correlation effects are unambiguously recognized [7]. In the pyrochlore oxide Cd 2 Re 2 O 7 a piezoelectric transition with lifting of the inversion symmetry takes place [8] and a strong mass renormalization is measured [9].…”

Goldstone-Mode Phonon Dynamics in the PyrochloreCd2Re2O7

“…The corresponding low temperature space groups are I4m2 and I4 1 22, both tetragonal and non-centrosymmetric. The order parameter corresponds to a Brillouin zone centered (k = 0) doubly degenerate phonon of E u symmetry dominated by the displacements of oxygen atoms occupying position 48(f) of F d3m space group [12,13]. These atoms, which comprise 6 of the 7 oxygen per formula unit, are collectively referred to as the O1 atoms to distinguish them from the O2 atom at the 8(a) site.…”

“…The identical crystal was measured using resonance ultrasound spectroscopy [13], and Raman results were confirmed on a second crystal. Smooth surfaces reduced the scattered light and allowed for intrinsic intensity measurements very close to the laser line.…”

Successive spatial symmetry breaking under high pressure in the spin-orbit-coupled metal Cd2Re2O7Yamaura1, Takeda2, Ikeda3 et al. 2017Phys. Rev. B251270View full textAdd to dashboardCite

“…It undergoes a successive structural transition from high-temperature cubic Fd−3m (phase I) to tetragonal I−4m2 (phase II) at T s1 = 200 K and to tetragonal I4 1 22 (phase III) at T s2 = 120 K [13][14][15][16][17][18][19]. The T s1 transition of the second-order type is considered as a band Jahn-Teller transition lifting the highly degenerate bands, because there are large changes in the electronic properties in spite of the tiny tetragonal distortion at T s1 [23,30] On the other hand, the origin of the T s2 transition, which is of the first-order, is thus far unclear [15,31].…”