Thursday, December 16, 2010

Phys. Rev. Lett.

^{7} Li NMR Investigation of Li-Li Pair Ordering in the Paraelectric Phase of Weakly Substitutionally Disordered K_{1-x} Li_{x} TaO_{3}

Author(s): Boštjan Zalar, Andrija Lebar, David C. Ailion, R. O. Kuzian, I. V. Kondakova, and V. V. Laguta
Breaking of the average cubic symmetry in Li-doped potassium tantalate was observed with quadrupole-perturbed 7Li NMR at temperatures (150–400 K) far above the nominal glass transition temperature (≈50  K for Li concentration x=0.03). The observed spectrum consists of contributions from both isolated Li ions (i.e., with no nearest-neighbor Li) and from Li-Li pairs. The isolated Li ions move among six equivalent off-center sites in a potential having cubic symmetry. These have zero average electric field gradient and, hence, exhibit no quadrupole splitting. In addition, very low intensity, but well resolved, quadrupole satellites having a temperature-dependent splitting were observed. This splitting indicates that the various Li-Li pair configurations are not all equally probable. These are the first direct observations of biased Li ion ordering that persists in the paraelectric phase at temperatures high above the glass phase.

Anisotropic Spin Fluctuations and Superconductivity in “115” Heavy Fermion Compounds: ^{59} Co NMR Study in PuCoGa_{5}

Author(s): S.-H. Baek, H. Sakai, E. D. Bauer, J. N. Mitchell, J. A. Kennison, F. Ronning, and J. D. Thompson
We report results of 59Co nuclear magnetic resonance measurements on a single crystal of superconducting PuCoGa5 in its normal state. The nuclear spin-lattice relaxation rates and the Knight shifts as a function of temperature reveal an anisotropy of spin fluctuations with finite wave vector q. By comparison with the isostructural members, we conclude that antiferromagnetic XY-type anisotropy of spin fluctuations plays an important role in mediating superconductivity in these heavy fermion materials.

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