190 research outputs found
Superconductivity and Rattling under High Pressure in the beta-Pyrochlore Oxide RbOs2O6
Rattling-induced superconductivity in the beta-pyrochlore oxide RbOs2O6 is
investigated under high pressures up to 6 GPa. Resistivity measurements in a
high-quality single crystal show that the superconducting transition
temperature Tc increases gradually from 6.3 K at ambient pressure to 8.8 K at
3.5 GPa, surprisingly remains almost constant at 8.8 \pm 0.1 K in a wide
pressure range between 3.5 (Po) and 4.8 GPa, and suddenly drops to 6.3 K at Ps
= 4.9 GPa, followed by a gradual decrease with further pressure increase. Two
anomalies in the temperature dependence of the normal-state resistivity are
observed at Po Ps, revealing the presence of two high-pressure
phases corresponding to the changes in Tc. The rattling of the Rb ion inside a
cage made of Os and O atoms may be slightly and seriously modified in these
high-pressure phases that probably have cages of reduced symmetry,
respectively, so that electron-rattler interactions that govern the
superconducting and transport properties of beta-RbOs2O6 are significantly
affected.Comment: arXiv admin note: text overlap with arXiv:1009.035
Spin fluctuations, magnetic long-range order and Fermi surface gapping in NaxCoO2
In this study an extended low energy phase diagram for NaxCoO2 is
experimentally established with emphasis on the high x range. It is based on
systematic heat capacity studies on both polycrystalline and single crystalline
samples and on uSR measurements. Main features are the existence of mass
enhancement, spin fluctuations without long-range order, and magnetic order
with associated Fermi surface gapping. The latter is seen in the electronic
density of states (DOS) and suppression of nuclear specific heat. While there
is agreement between the band structure and the low energy DOS in the low x
range, in the high x range (x > 0.6) the thermodynamically determined DOS is
approximately three times that deduced from the angle-resolved photoemission
spectroscopy (ARPES)-measured band dispersion or local-density approximation
(LDA) calculations.Comment: 9 pages, 5 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
Green's function of fully anharmonic lattice vibration
Motivated by the discovery of superconductivity in beta-pyrochlore oxides, we
study property of rattling motion coupled with conduction electrons. We derive
the general expression of the Green's function of fully anharmonic lattice
vibration within the accuracy of the second order perturbation of electron-ion
interaction by introducing self-energy, vertex-correction, and normalization
factor for each transition. Using the expression, we discuss the characteristic
properties of the spectral function in the entire range from weakly anharmonic
potential to double-well case, and calculate NMR relaxation rate due to the two
phonon Raman process
Intramolecular vibronic dynamics in molecular solids: C60
Vibronic coupling in solid C60 has been investigated with a combination of resonant photoemission spectroscopy (RPES) and resonant inelastic x-ray scattering (RIXS). Excitation as a function of energy within the lowest unoccupied molecular orbital resonance yielded strong oscillations in intensity and dispersion in RPES, and a strong inelastic component in RIXS. Reconciling these two observations establishes that vibronic coupling in this core hole excitation leads to predominantly inelastic scattering and localization of the excited vibrations on the molecule on a femtosecond time scale. The coupling extends throughout the widths of the frontier valence bands.
Superconducting Transition in the -Pyrochlore AOsO (A=Cs, Rb, K) under Pressure
Pressure dependence of superconducting transition temperature has
been determined through the DC magnetic measurements under pressure up to
=10 GPa for -pyrochlore oxides AOsO with A=Cs (=3.3 K), Rb (6.3 K) and K (9.6 K). Both for A=Rb and Cs,
increases with increasing and shows a saturation at 8.8
K, which is considered as the upper limit of inherent in
AOsO. In contrast, the curve for KOsO shows a
sharp maximum of 10 K at 0.5 GPa, and is higher than
for 01.5GPa, suggesting the enhanced
superconductivity due to the rattling of K ions.Comment: 4 pages, 4 figures, to be published in J. Phys. Soc. Jpn. Vol. 77,
No. 4 (2008
Phonon Dynamics and Multipolar Isomorphic Transition in beta-pyrochlore KOs2O6
We investigate with a microscopic model anharmonic K-cation oscillation
observed by neutron experiments in beta-pyrochlore superconductor KOs2O6, which
also shows a mysterious first-order structural transition at Tp=7.5 K. We have
identified a set of microscopic model parameters that successfully reproduce
the observed temperature dependence and the superconducting transition
temperature. Considering changes in the parameters at Tp, we can explain
puzzling experimental results about electron-phonon coupling and neutron data.
Our analysis demonstrates that the first-order transition is multipolar
transition driven by the octupolar component of K-cation oscillations. The
octupole moment does not change the symmetry and is characteristic to
noncentrosymmetric K-cation potential.Comment: 5 pages, 4 figures, submitted to J. Phys. Soc. Jp
FRC in Switzerland : research, applications and perspectives
This paper presents some applications of FRC in Switzerland and recent developments resulting from research activities carried out at Swiss university laboratories. The national recommendation SIA162/6 "Steel Fiber Reinforced Concrete" is also briefly presented in relation to the practical use of this material in terms of quality control and design. Finally, some perspectives of the use of FRC in the Swiss market are given
Screening, Coulomb pseudopotential, and superconductivity in alkali-doped Fullerenes
We study the static screening in a Hubbard-like model using quantum Monte
Carlo. We find that the random phase approximation is surprisingly accurate
almost up to the Mott transition. We argue that in alkali-doped Fullerenes the
Coulomb pseudopotential is not very much reduced by retardation
effects. Therefore efficient screening is important in reducing
sufficiently to allow for an electron-phonon driven superconductivity. In this
way the Fullerides differ from the conventional picture, where retardation
effects play a major role in reducing the electron-electron repulsion.Comment: 4 pages RevTeX with 2 eps figures, additional material available at
http://www.mpi-stuttgart.mpg.de/docs/ANDERSEN/fullerene
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