Structural Order Parameter in the Pyrochlore Superconductor Cd2Re2O7

TL;DR

This paper investigates the structural phase transitions in the pyrochlore superconductor Cd2Re2O7, identifying the order parameter and proposing a thermodynamic phase diagram based on phonon mode instabilities.

Contribution

It introduces a detailed analysis of the structural order parameter and phase transitions in Cd2Re2O7, linking phonon modes to the observed transitions.

Findings

Both phase transitions are due to a common phonon mode instability.

The primary order parameter transforms according to E_u of O_h.

The transition at T_{s1} may be second order.

Abstract

It is shown that both structural phase transitions in Cd2Re2O7, which occur at T_{s1}=200 K and T_{s2}=120 K, are due to an instability of the Re tetrahedral network with respect to the same doubly degenerate long-wavelength phonon mode. The primary structural order parameter transforms according to the irreducible representation E_u of the point group O_h. We argue that the transition at T_{s1} may be of second order, in accordance with experimental data. We obtain the phase diagram in the space of phenomenological parameters and propose a thermodynamic path that Cd2Re2O7 follows upon cooling. Couplings of the itinerant electronic system and localized spin states in pyrochlores and spinels to atomic displacements are discussed.