Tetramer spin singlet instability in the fluorine-substituted pyrochlore superconducting system Cd2Re2O7−xFx

Abstract

We synthesized polycrystalline samples of the fluorine-substituted pyrochlore rhenates Cd2Re2O7−xFx, and investigated their magnetic, transport and structural properties. The transition temperature Ts1, where each Re4 tetrahedron in the Re pyrochlore network alternately expands and contracts, decreases with increasing x from 200 K at x = 0 to 100 K at x = 0.5. The strong x dependence of the magnetic and transport properties at the low-temperature phase indicates that the driving force of structural phase transition is fluctuations of the tetramer spin singlet formation in order to release the spin frustration in the pyrochlore lattice. Furthermore, we found unconventional superconducting properties in Cd2Re2O7−xFx. It was found that the superconducting phase transition temperature Tc markedly decreases with increasing x, suggesting that the addition of imperfection suppresses a condensation of Cooper-pair. In addition, the estimated upper critical field at zero temperature exceeds the Pauli paramagnetic limit and increases with increasing x in spite of the reduction of Tc. Hence, Cd2Re2O7−xFx is suggested to be an exotic superconductor realized in the itinerant electron systems on a spin frustrated lattice.