Structural Order Parameter in the Pyrochlore Superconductor Cd2Re2O7

  • Sergienko Ivan A.
    Department of Physics and Physical Oceanography, Memorial University of Newfoundland
  • Curnoe Stephanie H.
    Department of Physics and Physical Oceanography, Memorial University of Newfoundland

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  • Structural Order Parameter in the Pyrochlore Superconductor Cd<sub>2</sub>Re<sub>2</sub>O<sub>7</sub>

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Abstract

It is shown that both structural phase transitions in Cd2Re2O7, which occur at Ts1=200 K and Ts2=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 Eu of the point group Oh. We argue that the transition at Ts1 may be of the 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.

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