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. $f$-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 $f$-wave triplet state as well as the relevance to experiments are discussed

κ−(BEDT−TTF)2X\kappa-(BEDT-TTF)_2X 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 $\kappa-(BEDT-TTF)_2X$ organic materials. The model interpolates between the square lattice and decoupled chains. The $\kappa-(BEDT-TTF)_2X$ 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

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

Tuning thermal properties and microphase separation in aliphatic polyester ABA copolymers

Four alkyl substituted β-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 (Đ < 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 β-butyrolactone or β-valerolactone yielded polymers with tunable and predictable thermal properties. Copolymers prepared from β-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

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 ($T$) properties across the transition temperature, $T_{\rm CO}$. In order to calculate $T$ dependence of physical quantities such as the spin susceptibility and the electrical resistivity, both above and below $T_{\rm CO}$, 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-$T$ is determined by quantum Monte Carlo simulations. The results show that the spin susceptibility does not show a steep singularity at $T_{\rm CO}$, 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 $T_{\rm CO}$, below which a characteristic $T$ 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

Switching the stereochemical outcome of 6-endo-trig cyclizations; Synthesis of 2,6-Cis-6-substituted 4-oxopipecolic acids

A base-mediated 6-endo-trig cyclization of readily accessible enone-derived α-amino acids has been developed for the direct synthesis of novel 2,6-cis-6- substituted-4-oxo-L-pipecolic acids. A range of aliphatic and aryl side chains were tolerated by this mild procedure to give the target compounds in good overall yields. Molecular modeling of the 6-endo-trig cyclization allowed some insight as to how these compounds were formed, with the enolate intermediate generated via an equilibrium process, followed by irreversible tautomerization/neutralization providing the driving force for product formation. Stereoselective reduction and deprotection of the resulting 2,6-cis-6-substituted 4-oxo-L-pipecolic acids to the corresponding 4-hydroxy-L-pipecolic acids was also performed

Interpretations of J/ψJ/\psi suppression

We review the two main interpretations of $J/\psi$ 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 $J/\psi$ can survive in this hot region. In the hadronic scenario the $J/\psi$ 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

Charge order and superconductivity in a two-dimensional triangular lattice at n=2/3

To investigate the possibility of charge order and superconductivity in a doped two-dimensional triangular lattice, we study the extended Hubbard model with variational Monte Carlo method. At n=2/3, a commensurate filling for a triangular lattice, it is shown that the nearest-neighbor Coulomb interaction V induces honeycomb-type charge order and antiferromagnetic spin order at U>10t. We also discuss the possibility of superconductivity induced by charge fluctuation and the relation to the superconductivity in Na_{0.35}CoO_{2}1.3H_{2}O and theta-type organic condoctors.Comment: 4 pages, 5 figure

Spin wave analysis to the spatially-anisotropic Heisenberg antiferromagnet on triangular lattice

We study the phase diagram at T=0 of the antiferromagnetic Heisenberg model on the triangular lattice with spatially-anisotropic interactions. For values of the anisotropy very close to J_alpha/J_beta=0.50, conventional spin wave theory predicts that quantum fluctuations melt the classical structures, for S=1/2. For the regime J_beta<J_alpha, it is shown that the incommensurate spiral phases survive until J_beta/J_alpha=0.27, leaving a wide region where the ground state is disordered. The existence of such nonmagnetic states suggests the possibility of spin liquid behavior for intermediate values of the anisotropy.Comment: Revised version, 4 pages, Latex (twocolumn), 4 figures as eps files. To appear in PR