3,278 research outputs found
Thermodynamic properties in the normal and superconducting states of Na(x)CoO(2)*yH(2)O powder measured by heat capacity experiments
The heat capacity of superconducting Na(x)CoO(2)*yH(2)O was measured and the
data are discussed based on three different models: The thermodynamic
Ginzburg-Landau model, the BCS theory, and a model including the effects of
line nodes in the superconducting gap function. The electronic heat capacity is
separated from the lattice contribution in a thermodynamically consistent way
maintaining the entropy balance of superconducting and normal states at the
critical temperature. It is shown that for a fully gapped superconductor the
data can only be explained by a reduced (about 55 %) superconducting volume
fraction. The data are compatible with 100 % superconductivity in the case
where line nodes are present in the superconducting gap function.Comment: Revised, 19 pages, 3 figure
Possible singlet to triplet pairing transition in NaxCoO2 H2O
We present precise measurements of the upper critical field (Hc2) in the
recently discovered cobalt oxide superconductor. We have found that the
critical field has an unusual temperature dependence; namely, there is an
abrupt change of the slope of Hc2(T) in a weak field regime. In order to
explain this result we have derived and solved Gor'kov equations on a
triangular lattice. Our experimental results may be interpreted in terms of the
field-induced transition from singlet to triplet superconductivity.Comment: 6 pages, 5 figures, revte
Interleukin-18: a therapeutic target in rheumatoid arthritis?
Interleukin 18 (IL-18), a member of the IL-1 superfamily of cytokines has been demonstrated to be an important mediator of both innate and adaptive immune responses. Several reports have implicated its role in the pathogenesis of rheumatoid arthritis (RA). Although biologic therapy is firmly established in the treatment of a number of inflammatory diseases including RA, partial and non-responder patients constitute residual unmet clinical need. The aim of this article is to briefly review the biology of, and experimental approaches to IL-18 neutralisation, together with speculation as to the relative merits of IL-18 as an alternative to existing targets
A Novel Taxonomy of Opportunities and Risks in Massively Multiplayer Online Role Playing Games
The explosion and rapid embrace of Massively Multiplayer Online Role Playing Games (MMORPG) has provided players with unique, fully immersive three dimensional environments in which they can express themselves in a myriad of ways. Players can develop unique skill sets, share knowledge, explore and experiment with various identities and network with like-minded individuals. Evidence suggests however, that these opportunities are accompanied with a number of risks including addiction, desensitisation and threats to the privacy of personal information. This paper presents a novel Taxonomy of opportunities and risks in the specific context of Massively Multiplayer Online Role Playing Games
NaxCoO2: Enhanced low-energy excitations of electrons on a 2D triangular lattice
To elucidate the low-energy excitation spectrum of correlated electrons on a
2D triangular lattice, we have studied the electrical resistance and specific
heat down to 0.5 K and in magnetic fields up to 14 T, in NaxCoO2 samples with a
Na content ranging from x \approx 0.5 to 0.82. Two distinct regimes are
observed: for x from about 0.6 to x \approx 0.75 the specific heat is strongly
enhanced, with a pronounced upturn of C/T below about 10 K, reaching 47 mJ/(mol
K^2). This enhancement is suppressed in a magnetic field indicative of strong
low-energy spin fluctuations. At higher Na content the fluctuations are reduced
and mu-SR data confirm the SDW ground state below 22 K and the much reduced
heat capacity is field independent.Comment: Accepted in Physica
Coupling between electronic and structural degrees of freedom in the triangular lattice conductor NaxCoO2
The determination by powder neutron diffraction of the ambient temperature
crystal structures of compounds in the NaxCoO2 family, for 0.3 < x <= 1.0, is
reported. The structures consist of triangular CoO2 layers with Na ions
distributed in intervening charge reservoir layers. The shapes of the CoO6
octahedra that make up the CoO2 layers are found to be critically dependent on
the electron count and on the distribution of the Na ions in the intervening
layers, where two types of Na sites are available. Correlation of the shapes of
cobalt-oxygen octahedra, the Na ion positions, and the electronic phase diagram
in NaxCoO2 is made, showing how structural and electronic degrees of freedom
can be coupled in electrically conducting triangular lattice systems.Comment: 15 pages, 1 tables, 6 figures Submitted to Physical Review
Structure and Dynamics of Superconducting NaxCoO(2) Hydrate and Its Unhydrated Analog
Neutron scattering has been used to investigate the crystal structure and
lattice dynamics of superconducting Na0.3CoO2 1.4(H/D)2O, and the parent
Na0.3CoO2 material. The structure of Na0.3CoO2 consists of alternate layers of
CoO2 and Na and is the same as the structure at higher Na concentrations. For
the superconductor, the water forms two additional layers between the Na and
CoO2, increasing the c-axis lattice parameter of the hexagonal P63/mmc space
group from 11.16 A to 19.5 A. The Na ions are found to occupy a different
configuration from the parent compound, while the water forms a structure that
replicates the structure of ice. Both types of sites are only partially
occupied. The CoO2 layer in these structures is robust, on the other hand, and
we find a strong inverse correlation between the CoO2 layer thickness and the
superconducting transition temperature (TC increases with decreasing
thickness). The phonon density-of-states for Na0.3CoO2 exhibits distinct
acoustic and optic bands, with a high-energy cutoff of ~100 meV. The lattice
dynamical scattering for the superconductor is dominated by the hydrogen modes,
with librational and bending modes that are quite similar to ice, supporting
the structural model that the water intercalates and forms ice-like layers in
the superconductor.Comment: 14 pages, 7 figures, Phys. Rev. B (in press). Minor changes + two
figures removed as requested by refere
Superconductivity in a layered cobalt oxyhydrate NaCoO1.3HO
We report the electrical, magnetic and thermal measurements on a layered
cobalt oxyhydrate NaCoO1.3HO. Bulk superconductivity
at 4.3 K has been confirmed, however, the measured superconducting fraction is
relatively low probably due to the sample's intrinsic two-dimensional
characteristic. The compound exhibits weak-coupled and extreme type-II
superconductivity with the average energy gap and the
Ginzburg-Landau parameter of 0.50 meV and 140,
respectively. The normalized electronic specific heat data in the
superconducting state well fit the dependence, suggesting point nodes
for the superconducting gap structure.Comment: 4 pages, 3 figure
Evidence of many-body localization in 2D from quantum Monte Carlo simulation
We use the stochastic series expansion quantum Monte Carlo method, together
with the eigenstate-to-Hamiltonian mapping approach, to map the localized
ground states of the disordered two-dimensional Heisenberg model, to excited
states of a target Hamiltonian. The localized nature of the ground state is
established by studying the spin stiffness, local entanglement entropy, and
local magnetization. This construction allows us to define many body localized
states in an energy resolved phase diagram thereby providing concrete numerical
evidence for the existence of a many-body localized phase in two dimensions.Comment: 8 pages, 6 figure
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