Measurements of electrical resistivity ͑͒, dc magnetization ͑M͒, and specific heat ͑C͒ have been performed on layered oxide Li x CoO 2 ͑0.25Յ x Յ 0.99͒ using single-crystal specimens. The versus temperature ͑T͒ curve for x = 0.90 and 0.99 is found to be insulating but a metallic behavior is observed for 0.25Յ x Յ 0.71. At T S ϳ 155 K, a sharp anomaly is observed in the -T, M-T, and C / T-T curves for x = 0.66 with thermal hysteresis, indicating the first-order character of the transition. The transition at T S ϳ 155 K is observed for the wide range of x = 0.46-0.71. It is found that the M-T curve measured after rapid cool becomes different from that after slow cool below T F , which is ϳ130 K for x = 0.46-0.71. T F is found to agree with the temperature at which the motional narrowing in the 7 Li NMR linewidth is observed, indicating that the Li ions stop diffusing and order at the regular site below T F . The ordering of Li ions below T F ϳ 130 K is likely to be triggered and stabilized by the charge ordering in CoO 2 layers below T S .

Pressure dependence of superconducting transition temperature T c has been determined through the DC magnetic measurements under pressure up to P ¼ 10 GPa for -pyrochlore oxides AOs 2 O 6 with A = Cs (T c ¼ 3:3 K), Rb (6.3 K), and K (9.6 K). Both for A = Rb and Cs, T c increases with increasing P and shows a saturation at T cm $ 8:8 K, which is considered as the upper limit of T c inherent in AOs 2 O 6 . In contrast, the T c -P curve for KOs 2 O 6 shows a sharp maximum of $10 K at P $ 0:5 GPa, and T c is higher than T cm for 0 P 1:5 GPa, suggesting the enhanced superconductivity due to the rattling of K ions.In recent years, there has been a great deal of interest in pyrochlore or spinel oxides, in which a variety of remarkable low-temperature properties, e.g., metal-insulator 1-4) or superconducting transition, 5-7) the exotic magnetic ground state such as spin ice 8,9) and heavy mass Fermi-liquid behavior, 10) have been found, exploring new types of electronic behavior on the geometrically frustrated lattice. Among these, the recent discovery of -pyrochlore superconductor AOs 2 O 6 (A = Cs, 11) Rb 12,13) and K 14) ) may stimulate renewed interest in superconductivity on geometrically frustrated structures, as in LiTi 2 O 4 7) and Cd 2 Re 2 O 7 . 5,6) Indeed, the superconductivity in AOs 2 O 6 has been the subject of intensive research and several experiments suggest the conventional s-wave superconductivity. [15][16][17][18][19][20] One of the characteristic features of AOs 2 O 6 is the anharmonic ''rattling'' motion of A ions in an oversized cage of Os-O network, 21) which has been experimentally inferred from the specific heat data showing the existence of low frequency Einstein mode contribution, 22,23) and also demonstrated by the recent NMR experiments. 24) Especially for KOs 2 O 6 , the superconducting transition at T c ¼ 9:6 K is followed by a second transition concerning to the rattling freedom of K ion at T p $ 7:5 K, and a field-independent specific heat anomaly of first order transition has been observed at T p . 25,26) The measurements of electrical resistivity as a function of temperature T in high magnetic fields have revealed that ðTÞ changes the curvature from a concave-downward one at high temperatures to a Fermiliquid behavior ðTÞ / AT 2 below T p , suggesting that the electron-rattling phonon scattering disappears below T p . 27,28) In RbOs 2 O 6 and CsOs 2 O 6 , such a crossover of ðTÞ has been observed at T Ã $ 15 and $20 K, respectively, suggesting that the rattling motion is frozen below T * . 28) Moreover, a strong coupling between the rattling motion of K ions and quasiparticles has been evidenced through the recent microwave penetration depth study, where it is suggested that the rattling phonons help to enhance superconductivity and K sites freeze to an ordered state below T p . 19) Also, the ordering of K ions at T p has been suggested from the theoretical side. 29) Thus, there exists an intriguing possibility that novel superconductivity enhanced by the rattling phonons is realized ...

No abstract

A realistic simulation of a magnetic recording medium necessitates the use of a realistic grain microstructure. However, the calculation of the demagnetizing field for irregular structures is difficult. In this article an approach to the calculation of demagnetizing tensors is described which allows for the use of arbitrary microstructures and also allows nonmagnetic grain boundaries to be included in the model. Simulations of media with various grain size distributions are described. A narrower distribution of grain sizes results in higher signal-to-noise ratios for recorded tracks and increased resistance to stray fields.

No abstract