6,276 research outputs found
Superconductivity in Na_xCoO_2yH_2O by charge fluctuation
A new mechanism for superconductivity in the newly discovered Co-based oxide
is proposed by using charge fluctuation. A single-band extended Hubbard model
on the triangular lattice is studied within random phase approximation.
-wave triplet superconductivity is stabilized in the vicinity of
charge-density-wave instability, which is in sharp contrast with the
square-lattice case. The physical origin of the realization of the -wave
triplet state as well as the relevance to experiments are discussed
organic crystals: superconducting versus antiferromagnetic instabilities in an anisotropic triangular lattice Hubbard model
A Hubbard model at half-filling on an anisotropic triangular lattice has been
proposed as the minimal model to describe conducting layers of
organic materials. The model interpolates between the
square lattice and decoupled chains. The materials
present many similarities with cuprates, such as the presence of unconventional
metallic properties and the close proximity of superconducting and
antiferromagnetic phases. As in the cuprates, spin fluctuations are expected to
play a crucial role in the onset of superconductivity. We perform a
weak-coupling renormalization-group analysis to show that a superconducting
instability occurs. Frustration in the antiferromagnetic couplings, which
arises from the underlying geometrical arrangement of the lattice, breaks the
perfect nesting of the square lattice at half-filling. The spin-wave
instability is suppressed and a superconducting instability predominates. For
the isotropic triangular lattice, there are again signs of long-range magnetic
order, in agreement with studies at strong-coupling.Comment: 4 pages, 5 eps figs, to appear in Can. J. Phys. (proceedings of the
Highly Frustrated Magnetism (HFM-2000) conference, Waterloo, Canada, June
2000
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Tuning thermal properties and microphase separation in aliphatic polyester ABA copolymers
Four alkyl substituted ÎČ-lactones were investigated as monomers in ring opening polymerisation to produce a family of poly(3-hydroxyalkanoate)s. Homopolymers were synthesised using a robust aluminium salen catalyst, resulting in polymers with low dispersity (Ä < 1.1) and predictable molecular weights. ABA triblock copolymers were prepared using poly(L-lactic acid) as the A block and the afore- mentioned poly(3-hydroxyalkanoate) as the B block via a sequential addition method. Characterisation of these copolymers determined they were well controlled with low dispersities and predictable molecular weight. DSC analysis determined copolymers prepared from ÎČ-butyrolactone or ÎČ-valerolactone yielded polymers with tunable and predictable thermal properties. Copolymers prepared from ÎČ-heptanolactone yielded a microphase separated material as indicated by SAXS, with two distinct Tgs. The polymers could be readily cast into flexible films and their improved tensile properties were explored
Interpretations of suppression
We review the two main interpretations of suppression proposed in
the literature. The phase transition (or deconfining) scenario assumes that
below some critical value of the local energy density (or of some other
geometrical quantity which depends both on the colliding systems and on the
centrality of the collision), there is only nuclear absorption. Above this
critical value the absorptive cross-section is taken to be infinite, i.e. no
can survive in this hot region. In the hadronic scenario the
dissociates due both to nuclear absorption and to its interactions with
co-moving hadrons produced in the collision. No discontinuity exists in
physical observables. We show that an equally good description of the present
data is possible in either scenario.Comment: 12 pages, LaTeX, uses epsfig and ioplppt; review talk given by A.
Capella at the International Symposium on Strangness in Quark Matter,
Santorini (Greece), April 1997; Figs. 1 and 2 not available but can be found
in Refs. 13 and 6 respectivel
Correlation gap in the optical spectra of the two-dimensional organic metal (BEDT-TTF)_4[Ni(dto)_2]
Optical reflection measurements within the highly conducting (a,b)-plane of
the organic metal (BEDT-TTF)_4[Ni(dto)_2] reveal the gradual development of a
sharp feature at around 200 cm as the temperature is reduced below 150 K. Below
this frequency a narrow Drude-like response is observed which accounts for the
metallic behavior. Since de Haas-von Alphen oscillations at low temperatures
confirm band structure calculations of bands crossing the Fermi energy, we
assign the observed behavior to a two-dimensional metallic state in the
proximity of a correlation induced metal-insulator transition.Comment: 4 pages, 2 figure
Theoretical studies of the historical development of the accounting discipline: a review and evidence
Many existing studies of the development of accounting thought have either been atheoretical or have adopted Kuhn's model of scientific growth. The limitations of this 35-year-old model are discussed. Four different general neo-Kuhnian models of scholarly knowledge development are reviewed and compared with reference to an analytical matrix. The models are found to be mutually consistent, with each focusing on a different aspect of development. A composite model is proposed. Based on a hand-crafted database, author co-citation analysis is used to map empirically the entire literature structure of the accounting discipline during two consecutive time periods, 1972â81 and 1982â90. The changing structure of the accounting literature is interpreted using the proposed composite model of scholarly knowledge development
An Electrochemical Study of Frustrated Lewis Pairs: A Metal-free Route to Hydrogen Oxidation
[Image: see text] Frustrated Lewis pairs have found many applications in the heterolytic activation of H(2) and subsequent hydrogenation of small molecules through delivery of the resulting proton and hydride equivalents. Herein, we describe how H(2) can be preactivated using classical frustrated Lewis pair chemistry and combined with in situ nonaqueous electrochemical oxidation of the resulting borohydride. Our approach allows hydrogen to be cleanly converted into two protons and two electrons in situ, and reduces the potential (the required energetic driving force) for nonaqueous H(2) oxidation by 610 mV (117.7 kJ mol(â1)). This significant energy reduction opens routes to the development of nonaqueous hydrogen energy technology
Large-N solutions of the Heisenberg and Hubbard-Heisenberg models on the anisotropic triangular lattice: application to CsCuCl and to the layered organic superconductors -(BEDT-TTF)X
We solve the Sp(N) Heisenberg and SU(N) Hubbard-Heisenberg models on the
anisotropic triangular lattice in the large-N limit. These two models may
describe respectively the magnetic and electronic properties of the family of
layered organic materials -(BEDT-TTF)X. The Heisenberg model is
also relevant to the frustrated antiferromagnet, CsCuCl. We find rich
phase diagrams for each model. The Sp(N) antiferromagnet is shown to have five
different phases as a function of the size of the spin and the degree of
anisotropy of the triangular lattice. The effects of fluctuations at finite-N
are also discussed. For parameters relevant to CsCuCl the ground state
either exhibits incommensurate spin order, or is in a quantum disordered phase
with deconfined spin-1/2 excitations and topological order. The SU(N)
Hubbard-Heisenberg model exhibits an insulating dimer phase, an insulating box
phase, a semi-metallic staggered flux phase (SFP), and a metallic uniform
phase. The uniform and SFP phases exhibit a pseudogap. A metal-insulator
transition occurs at intermediate values of the interaction strength.Comment: Typos corrected, one reference added. 20 pages, 17 figures, RevTeX
3.
Finite-Temperature Properties across the Charge Ordering Transition -- Combined Bosonization, Renormalization Group, and Numerical Methods
We theoretically describe the charge ordering (CO) metal-insulator transition
based on a quasi-one-dimensional extended Hubbard model, and investigate the
finite temperature () properties across the transition temperature, . In order to calculate dependence of physical quantities such as the
spin susceptibility and the electrical resistivity, both above and below
, a theoretical scheme is developed which combines analytical
methods with numerical calculations. We take advantage of the renormalization
group equations derived from the effective bosonized Hamiltonian, where Lanczos
exact diagonalization data are chosen as initial parameters, while the CO order
parameter at finite- is determined by quantum Monte Carlo simulations. The
results show that the spin susceptibility does not show a steep singularity at
, and it slightly increases compared to the case without CO because
of the suppression of the spin velocity. In contrast, the resistivity exhibits
a sudden increase at , below which a characteristic dependence
is observed. We also compare our results with experiments on molecular
conductors as well as transition metal oxides showing CO.Comment: 9 pages, 8 figure
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