Discovery of a lowerature orthorhombic phase of the Cd2Re2 O7 superconductor

Abstract

The favorable combination of Raman spectroscopy of the lattice vibrations with ab initio phonon calculations enables us to pinpoint a lowerature orthorhombic phase which hosts the cryptic 1 K superconducting phase of the 5d pyrochlore-type superconductor Cd2Re2O7. Raman studies are performed on crystals with the natural isotope abundance and cadmium and oxygen isotope enriched samples. Characteristic splitting and the appearance of sharp phonon modes in the temperature-dependent Raman spectra prove a structural phase below ∼80 K, induced by a soft-mode phonon instability of the intermediateerature tetragonal crystal structure. The theory predicts that this lowerature orthorhombic crystal structure is described by the space group F222; it exhibits no phonon instabilities and has the lowest total energy. Future experiments on the F222 phase should establish whether spin-triplet components contribute to the superconducting phase.