Low-temperature structural change and magnetic anomaly in superconducting Cd2Re2O7

The structural phase transition at T*∼200 K in the superconducting pyrochlore oxide Cd2Re2O7 has been investigated using x-ray and electron diffraction experiments and 111Cd-NMR (nuclear-magnetic-resonance) measurements. The susceptibility for Cd2Re2O7 in the high-temperature region (T>>T*) obeys a Curie-Weiss law with a large and negative Weiss temperature, which indicates that this system possesses a localized-moment nature with antiferromagnetic interactions, leading to geometrical frustration. X-ray and electron-diffraction measurements suggest the tetramerization of Re below T*, which induces a nanosized stripe structure. From 111Cd-NMR experiments, a large orbital susceptibility is identified by means of a K-χ plot. Both the spin susceptibility and (T1T)-1 are found to decrease sharply below T*, signaling a decrease in density of states, which is apparently caused by a developing partial energy gap at the Fermi surface