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

Magnetic, thermodynamic, and electrical transport properties of the noncentrosymmetric B20 germanides MnGe and CoGe

We present magnetization, specific heat, resistivity, and Hall effect measurements on the cubic B20 phase of MnGe and CoGe and compare to measurements of isostructural FeGe and electronic structure calculations. In MnGe, we observe a transition to a magnetic state at $T_c=275$ K as identified by a sharp peak in the ac magnetic susceptibility, as well as second phase transition at lower temperature that becomes apparent only at finite magnetic field. We discover two phase transitions in the specific heat at temperatures much below the Curie temperature one of which we associate with changes to the magnetic structure. A magnetic field reduces the temperature of this transition which corresponds closely to the sharp peak observed in the ac susceptibility at fields above 5 kOe. The second of these transitions is not affected by the application of field and has no signature in the magnetic properties or our crystal structure parameters. Transport measurements indicate that MnGe is metal with a negative magnetoresistance similar to that seen in isostructural FeGe and MnSi. Hall effect measurements reveal a carrier concentration of about 0.5 carriers per formula unit also similar to that found in FeGe and MnSi. CoGe is shown to be a low carrier density metal with a very small, nearly temperature independent diamagnetic susceptibility.Comment: 16 pages 23 figure

A Possible Phase Transition in beta-pyrochlore Compounds

We investigate a lattice of interacting anharmonic oscillators by using a mean field theory and exact diagonalization. We construct an effective five-state hopping model with intersite repulsions as a model for beta-pyrochlore AOs_2O_6(A=K, Rb or Cs). We obtain the first order phase transition line from large to small oscillation amplitude phases as temperature decreases. We also discuss the possibility of a phase with local electric polarizations. Our theory can explain the origin of the mysterious first order transition in KOs_2O_6.Comment: 4 pages, 4 figures, submitted to J. Phys. Soc. Jp

Crystal Structure and Magnetism of the Linear-Chain Copper Oxides Sr5Pb3-xBixCuO12

The title quasi-1D copper oxides (0=< x =<0.4) were investigated by neutron diffraction and magnetic susceptibility studies. Polyhedral CuO4 units in the compounds were found to comprise linear-chains at inter-chain distance of approximately 10 A. The parent chain compound (x = 0), however, shows less anisotropic magnetic behavior above 2 K, although it is of substantially antiferromagnetic (mu_{eff}= 1.85 mu_{B} and Theta_{W} = -46.4 K) spin-chain system. A magnetic cusp gradually appears at about 100 K in T vs chi with the Bi substitution. The cusp (x = 0.4) is fairly characterized by and therefore suggests the spin gap nature at Delta/k_{B} ~ 80 K. The chain compounds hold electrically insulating in the composition range.Comment: To be published in PR

Structural Order Parameter in the Pyrochlore Superconductor Cd2Re2O7

It is shown that both structural phase transitions in Cd2Re2O7, which occur at T_{s1}=200 K and T_{s2}=120 K, are due to an instability of the Re tetrahedral network with respect to the same doubly degenerate long-wavelength phonon mode. The primary structural order parameter transforms according to the irreducible representation E_u of the point group O_h. We argue that the transition at T_{s1} may be of second order, in accordance with experimental data. We obtain the phase diagram in the space of phenomenological parameters and propose a thermodynamic path that Cd2Re2O7 follows upon cooling. Couplings of the itinerant electronic system and localized spin states in pyrochlores and spinels to atomic displacements are discussed.Comment: 5 pages. Submitted to J. Phys. Soc. Jpn. Best quality figures are available at http://www.physics.mun.ca/~isergien/pubs.htm

Coexistence of metallic and nonmetallic properties in the pyrochlore Lu2Rh2O7

Transition metal oxides of the $4d$ and $5d$ block have recently become the targets of materials discovery, largely due to their strong spin-orbit coupling that can generate exotic magnetic and electronic states. Here we report the high pressure synthesis of Lu$_2$Rh$_2$O$_7$, a new cubic pyrochlore oxide based on $4d^5$ Rh$^{4+}$ and characterizations via thermodynamic, electrical transport, and muon spin relaxation measurements. Magnetic susceptibility measurements reveal a large temperature-independent Pauli paramagnetic contribution, while heat capacity shows an enhanced Sommerfeld coefficient, $\gamma$ = 21.8(1) mJ/mol-Rh K$^2$. Muon spin relaxation measurements confirm that Lu$_2$Rh$_2$O$_7$ remains paramagnetic down to 2 K. Taken in combination, these three measurements suggest that Lu$_2$Rh$_2$O$_7$ is a correlated paramagnetic metal with a Wilson ratio of $R_W = 2.5$. However, electric transport measurements present a striking contradiction as the resistivity of Lu$_2$Rh$_2$O$_7$ is observed to monotonically increase with decreasing temperature, indicative of a nonmetallic state. Furthermore, although the magnitude of the resistivity is that of a semiconductor, the temperature dependence does not obey any conventional form. Thus, we propose that Lu$_2$Rh$_2$O$_7$ may belong to the same novel class of non-Fermi liquids as the nonmetallic metal FeCrAs.Comment: 11 pages, 5 figure

Plaquette Ordering in SU(4) Antiferromagnets

We use fermion mean field theory to study possible plaquette ordering in the antiferromagnetic SU(4) Heisenberg model. We find the ground state for both the square and triangular lattices to be the disconnected plaquette state. Our mean field theory gives a first order transition for plaquette ordering for the triangular lattice. Our results suggest a large number of low lying states.Comment: 16 pages, 5 figure

Direct observation of electron density reconstruction at the metal-insulator transition in NaOsO3

5d transition metal oxides offer new opportunities to test our understanding of the interplay of correlation effects and spin-orbit interactions in materials in the absence of a single dominant interaction. The subtle balance between solid-state interactions can result in new mechanisms that minimize the interaction energy, and in material properties of potential use for applications. We focus here on the 5d transition metal oxide NaOsO3, a strong candidate for the realization of a magnetically driven transition from a metallic to an insulating state exploiting the so-called Slater mechanism. Experimental results are derived from non-resonant and resonant x-ray single crystal diffraction at the Os L-edges. A change in the crystallographic symmetry does not accompany the metal-insulator transition in the Slater mechanism and, indeed, we find no evidence of such a change in NaOsO3. An equally important experimental observation is the emergence of the (300) Bragg peak in the resonant condition with the onset of magnetic order. The intensity of this space-group forbidden Bragg peak continuously increases with decreasing temperature in line with the square of intensity observed for an allowed magnetic Bragg peak. Our main experimental results, the absence of crystal symmetry breaking and the emergence of a space-group forbidden Bragg peak with developing magnetic order, support the use of the Slater mechanism to interpret the metal-insulator transition in NaOsO3. We successfully describe our experimental results with simulations of the electronic structure and, also, with an atomic model based on the established symmetry of the crystal and magnetic structure.Comment: 6 figure