Synthesis, Crystal Structure and Magnetic Properties of the Linear-Chain Cobalt Oxide Sr5Pb3CoO12

The novel spin-chain cobalt oxide Sr5Pb3CoO12 [P-62m, a = 10.1093(2) A and c = 3.562 51(9) A at 295 K] is reported. Polycrystalline sample of the compound was studied by neutron diffraction (at 6 and 295 K) and magnetic susceptibility measurements (5 to 390 K). The cobalt oxide was found to be analogous to the copper oxide Sr5Pb3CoO12, which is comprised of magnetic-linear chains at inter-chain distance of 10 A. Although the cobalt oxide chains (mu_eff of 3.64 mu_B per Co) are substantially antiferromagnetic (theta_W = -38.8 K), neither low-dimensional magnetism nor long-range ordering has been found; a local-structure disorder in the chains might impact on the magnetism. This compound is highly electrically insulating.Comment: To be published in J. Solid State Che

Metallic conductivity and a Ca substitution study of NaRh2O4 comprising a double chain system

The metallic compound NaRh2O4 forms a full range solid solution to the insulating phase CaRh2O4. At a Na concentration of 0.25 moles per formula unit, we found an unexpected contribution to the specific heat at low temperature [K. Yamaura et al. Chem. Mater. 17 (2005) 359]. To address this issue, specific heat and ac and dc magnetic susceptibilities were additionally measured under a variety of conditions for the Na0.25 sample. A new set of data clearly indicate the additional specific heat is magnetic in origin; however, the magnetic entropy is fairly small (~1 % of Schottky term for a simple splitting doublet), and there is no other evidence to suggest that a magnetic phase transition is responsible for the anomalous specific heat.Comment: Accepted for publication in a special issue of Physica B (the proceedings of SCES05

Magnetic studies of the lightly Ru doped perovskite rhodates Sr(Ru,Rh)O3_3

The solid solution between the ferromagnetic metal SrRuO$_3$ and the enhanced paramagnetic metal SrRhO$_3$ was recently reported [K. Yamaura et al., Phys. Rev. B 69 (2004) 024410], and an unexpected feature was found in the specific heat data at $x$=0.9 of SrRu$_{1-x}$Rh$_x$O$_3$. The feature was reinvestigated further by characterizing additional samples with various Ru concentrations in the vicinity of $x$=0.9. Specific heat and magnetic susceptibility data indicate that the feature reflects a peculiar magnetism of the doped perovskite, which appears only in the very narrow composition range 0.85$<$$x$$\le$0.95.Comment: Accepted for publication in a special issue of Physica B (the proceedings of SCES04

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

Electronic properties of the novel 4d metallic oxide SrRhO3

The novel 4d perovskite compound SrRhO3 was investigated by isovalent doping studies. The solubility limits of Ca and Ba onto Sr-site were below 80% and 20%, respectively. Although SrRhO3 was chemically compressed, approximately 5.7% by the Ca doping, no significant influence was observed on the magnetic and electrical properties.Comment: To be published in a special issue of Physica B (the proceedings of LT23

Non-magnetic pair-breaking effect on La(Fe_{1-x}Zn_{x})AsO_{0.85} studied by NMR and NQR

$^{75}$As and $^{139}$La NMR and nuclear quadrupole resonance (NQR) studies on Zn-substituted LaFeAsO$_{0.85}$ have been performed to investigate the Zn-impurity effects microscopically. Although superconductivity in LaFeAsO$_{0.85}$ disappears by 3% Zn substitution, we found that NMR/NQR spectra and NMR physical quantities in the normal state are hardly changed, indicating that the crystal structure and electronic states are not modified by Zn substitution. Our results suggest that the suppression of superconductivity by Zn substitution is not due to the change of the normal-state properties, but due to strong non-magnetic pair-breaking effect to superconductivity.Comment: 5 pages, 4 figures, This paper was chosen as "Paper of Editors' Suggestion

Magnetic phase transitions in SmCoAsO

Magnetization, x-ray diffraction and specific-heat measurements reveal that SmCoAsO undergoes three magnetic phase transitions. A ferromagnetic transition attributed to the Co ions, emerges at TC=57 K with a small saturation moment of 0.15muB/Co. Reorientation of the Co moment to an antiferromagnetic state is obtained at TN2=45 K. The relative high paramagnetic effective moment Peff=1.57 MuB/Co indicates an itinerant ferromagnetic state of the Co sublattice. The third magnetic transition at TN1=5 K is observed clearly in the specific-heat study only. Both magnetic and 57Fe Mossbauer studies show that substitution of small quantities of Fe for Co was unsuccessful.Comment: 10pages text+Figures: comments welcome ([email protected]

Effects of Rattling Phonons on the Quasiparticle Excitation and Dynamics in the Superconducting β\beta-Pyrochlore KOs2_2O6_6

Microwave penetration depth $\lambda$ and surface resistance at 27 GHz are measured in high quality crystals of KOs$_2$O$_6$. Firm evidence for fully-gapped superconductivity is provided from $\lambda(T)$. Below the second transition at $T_{\rm p}\sim 8$ K, the superfluid density shows a step-like change with a suppression of effective critical temperature $T_{\rm c}$. Concurrently, the extracted quasiparticle scattering time shows a steep enhancement, indicating a strong coupling between the anomalous rattling motion of K ions and quasiparticles. The results imply that the rattling phonons help to enhance superconductivity, and that K sites freeze to an ordered state with long quasiparticle mean free path below $T_{\rm p}$.Comment: 5 pages, 5 figures, to be published in Phys. Rev. Let

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]