Extremely strong-coupling superconductivity and anomalous lattice properties in the beta-pyrochlore oxide KOs2O6

Superconducting and normal-state properties of the beta-pyrochlore oxide KOs2O6 are studied by means of thermodynamic and transport measurements. It is shown that the superconductivity is of conventional s-wave type and lies in the extremely strong-coupling regime. Specific heat and resistivity measurements reveal that there are characteristic low-energy phonons that give rise to unusual scattering of carriers due to strong electron-phonon interactions. The entity of the low-energy phonons is ascribed to the heavy rattling of the K ion confined in an oversized cage made of OsO6 octahedra. It is suggested that this electron-rattler coupling mediates the Cooper pairing, resulting in the extremely strong-coupling superconductivity.Comment: 17 pages (only 4 pages included here. go to http://hiroi.issp.u-tokyo.ac.jp/Published%20papers/K-SC6.pdf for full pages), to be published in PR

Synthesis of SmFeAsO by an Easy and Versatile Route and its Physical Property Characterization

We report synthesis, structure, electrical transport and heat capacity of SmFeAsO. The title compound is synthesized by one-step encapsulation of stoichiometric FeAs, Sm, and Sm2O3 in an evacuated (10-5 Torr) quartz tube by prolong (72 hours) annealing at 1100oC. The as synthesized compound is crystallized in tetragonal structure with P4/nmm space group having lattice parameters a = 3.93726(33) A and c = 8.49802(07) A. The resistance (R-T) measurements on the compound exhibited ground state spin-density-wave (SDW)-like metallic steps below 140 K. Heat capacity CP(T) measurements on the title compound, showed an anomaly at around 140 K, which is reminiscent of the SDW ordering of the compound. At lower temperatures the CP(T) shows a clear peak at around 4.5 K. At lower temperature below 20 K, Cp(T) is also measured under an applied field of 7 Tesla. It is concluded that the CP(T) peak at 4.5 K is due to the anti-ferromagnetic(AFM) ordering of Sm3+ spins. These results are in confirmation with ordering of Sm in Sm2-xCexCuO4.Comment: 9 pages Text + Figs Contact Author ([email protected]

Investigation of the ferromagnetic transition in the correlated 4d perovskites SrRu1−x_{1-x}Rhx_xO3_3

The solid-solution SrRu$_{1-x}$Rh$_x$O$_3$ ($0\le x \le1$) is a variable-electron-configuration system forming in the nearly-cubic-perovskite basis, ranging from the ferromagnetic 4$d^4$ to the enhanced paramagnetic 4$d^5$. Polycrystalline single-phase samples were obtained over the whole composition range by a high-pressure-heating technique, followed by measurements of magnetic susceptibility, magnetization, specific heat, thermopower, and electrical resistivity. The ferromagnetic order in long range is gradually suppressed by the Rh substitution and vanishes at $x \sim 0.6$. The electronic term of specific-heat shows unusual behavior near the critical Rh concentration; the feature does not match even qualitatively with what was reported for the related perovskites (Sr,Ca)RuO$_3$. Furthermore, another anomaly in the specific heat was observed at $x \sim 0.9$.Comment: Accepted for publication in PR

Local spin and charge properties of beta-Ag0.33V2O5 studied by 51V NMR

Local spin and charge properties were studied on beta-Ag0.33V2O5, a pressure-induced superconductor, at ambient pressure using 51V-NMR and zero-field-resonance (ZFR) techniques. Three inequivalent Vi sites (i=1, 2, and 3) were identified from 51V-NMR spectra and the principal axes of the electric-field-gradient (EFG) tensor were determined in a metallic phase and the following charge-ordering phase. We found from the EFG analysis that the V1 sites are in a similar local environment to the V3 sites. This was also observed in ZFR spectra as pairs of signals closely located with each other. These results are well explained by a charge-sharing model where a 3d1 electron is shared within a rung in both V1-V3 and V2-V2 two-leg ladders.Comment: 12pages, 16figure