Ivan A. Sergienko

Metallic ferroelectricity in the pyrochlore Cd2Re2O7

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.

Superconducting states in the tetrahedral compound PrOs4Sb12

We find possible superconducting states for tetrahedral (Th) symmetry crystals with strong spin-orbit coupling using Landau theory. Additional symmetry breaking within the superconducting state is considered. We discuss nodes of the gap functions for the different states, secondary superconducting order parameters and coupling to the elastic strain. By comparing our results to experiments, we find that superconductivity in PrOs4Sb12 is best described by the three-dimensional representations of point group Th.

Goldstone-mode phonon dynamics in the pyrochlore Cd2Re2O7.

We have measured the polarized Raman scattering spectra of Cd2Re2O7, the first superconducting pyrochlore, as a function of temperature. For temperatures below the cubic-to-tetragonal structural phase transition (SPT) at 200 K, a peak with B1 symmetry develops at zero frequency with divergent intensity. We identify this peak as the first observation of the Goldstone phonon in a crystalline solid. The Goldstone phonon is a collective excitation that exists due to the breaking of the continuous symmetry with the SPT. Its emergence coincides with that of a Raman-active soft mode. The order parameter for both features derives from an unstable doubly degenerate vibration (with Eu symmetry) of the O1 atoms which drives the SPT.

Structural Order Parameter in the Pyrochlore Superconductor Cd2Re2O7

It is shown that both structural phase transitions in Cd 2 Re 2 O 7 , 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 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 Cd 2 Re 2 O 7 follows upon cooling. Couplings of the itinerant electronic system and localized spin states in pyrochlores and spinels to atomic displacements are discussed.

Landau theory of superconducting phases in Th crystals

Abstract We consider the Landau theory of phase transitions for multiple superconducting phases in T h crystals. All possible phase-transition sequences involving a single superconducting order parameter are found. These results may be applicable to PrOs 4 Sb 12 .

Are Th-doped Y2C3 and La2C3 two-band superconductors?

Abstract The superconducting sesquicarbides R 2 C 3 have noncentrosymmetric point group symmetry T d . Spin–orbit coupling lifts the spin degeneracy of electronic bands in the most of the Brillouin zone. Nevertheless, due to high symmetry, there are a few directions along which the Fermi surfaces must touch. This leads to two-band effects in the superconducting state.

Are Th-doped and two-band superconductors?

Abstract The superconducting sesquicarbides R 2 C 3 have noncentrosymmetric point group symmetry T d . Spin–orbit coupling lifts the spin degeneracy of electronic bands in the most of the Brillouin zone. Nevertheless, due to high symmetry, there are a few directions along which the Fermi surfaces must touch. This leads to two-band effects in the superconducting state.

Are Th-doped Y2C3Y2C3 and La2C3La2C3 two-band superconductors?

Abstract The superconducting sesquicarbides R 2 C 3 have noncentrosymmetric point group symmetry T d . Spin–orbit coupling lifts the spin degeneracy of electronic bands in the most of the Brillouin zone. Nevertheless, due to high symmetry, there are a few directions along which the Fermi surfaces must touch. This leads to two-band effects in the superconducting state.

Structural order parameter and itinerant electron magnetism in Cd2Re2O7

Abstract We describe the low-temperature behaviour of the magnetic susceptibility of Cd 2 Re 2 O 7 in terms of a Landau theory of structural phase transitions. We calculate the zero temperature Pauli susceptibility using a tight-binding approach to reveal the mechanism of coupling between the structural order parameter and itinerant magnetism.