586 research outputs found
Qualitative understanding of the sign of t' asymmetry in the extended t-J Model and relevance for pairing properties
Numerical calculations illustrate the effect of the sign of the next
nearest-neighbor hopping term t' on the 2-hole properties of the t-t'-J model.
Working mainly on 2-leg ladders, in the -1.0 < t'/t < 1.0 regime, it is shown
that introducing t' in the t-J model is equivalent to effectively renormalizing
J, namely t' negative (positive) is equivalent to an effective t-J model with
smaller (bigger) J. This effect is present even at the level of a 2x2 plaquette
toy model, and was observed also in calculations on small square clusters.
Analyzing the transition probabilities of a hole-pair in the plaquette toy
model, it is argued that the coherent propagation of such hole-pair is enhanced
by a constructive interference between both t and t' for t'>0. This
interference is destructive for t'<0.Comment: 5 pages, 4 figures, to appear in PRB as a Rapid Communicatio
Spin-lattice Interaction For Ions In Low-symmetry Sites: The Case Of Mn2+:caco3
The spin-lattice interaction for Mn2+ ions as substitutional impurities in CaCO3 has been studied by electron-paramagnetic-resonance (EPR) experiments in crystals under externally applied uniaxial stresses. The shifts of the EPR lines of Mn2+ were measured as a function of the stress applied along the three different directions that accept uniaxial forces in this crystal. The values of six second-order spin-lattice coefficients for the Mn2+ site were obtained from the data. We also obtained two conditions relating linear combinations of the other four second-order coefficients. Contributions to the spin-lattice interaction of terms of fourth order in the effective spin have been detected. Our results are used to discuss the effect on the EPR spectrum of the different distortions of the carbonate ions surrounding the Mn2+. © 1994 The American Physical Society.49138583859
Quasiparticles and c-axis coherent hopping in high T_c superconductors
We study the problem of the low-energy quasiparticle spectrum of the extended
t-J model and analyze the coherent hopping between weakly coupled planes
described by this model. Starting with a two-band model describing the Cu-O
planes and the unoccupied bands associated to the metallic atoms located in
between the planes, we obtain effective hopping matrix elements describing the
c-axis charge transfer. A computational study of these processes shows an
anomalously large charge anisotropy for doping concentrations around and below
the optimal doping.Comment: 4 pages, 3 figure
Kinetic energy driven superconductivity in doped cuprates
Within the t-J model, the mechanism of superconductivity in doped cuprates is
studied based on the partial charge-spin separation fermion-spin theory. It is
shown that dressed holons interact occurring directly through the kinetic
energy by exchanging dressed spinon excitations, leading to a net attractive
force between dressed holons, then the electron Cooper pairs originating from
the dressed holon pairing state are due to the charge-spin recombination, and
their condensation reveals the superconducting ground-state. The electron
superconducting transition temperature is determined by the dressed holon pair
transition temperature, and is proportional to the concentration of doped holes
in the underdoped regime. With the common form of the electron Cooper pair, we
also show that there is a coexistence of the electron Cooper pair and
antiferromagnetic short-range correlation, and hence the antiferromagnetic
short-range fluctuation can persist into the superconducting state. Our results
are qualitatively consistent with experiments.Comment: 6 pages, Revtex, two figures are included, corrected typo
Suppression of static stripe formation by next-neighbor hopping
We show from real-space Hartree-Fock calculations within the extended Hubbard
model that next-nearest neighbor (t') hopping processes act to suppress the
formation of static charge stripes. This result is confirmed by investigating
the evolution of charge-inhomogeneous corral and stripe phases with increasing
t' of both signs. We propose that large t' values in YBCO prevent static stripe
formation, while anomalously small t' in LSCO provides an additional reason for
the appearance of static stripes only in these systems.Comment: 4 pages, 5 figure
Microwave-assisted and solvent-free peroxidative oxidation of 1-phenylethanol to acetophenone with a CuII–TEMPO catalytic system
Abstract The water-soluble copper(II) complex [Cu(H2R)(HL)]∙H2O (1) was prepared by reaction of copper(II) nitrate hydrate with (E)-2-(((1-hydroxynaphthalen-2-yl)methylene)amino) benzenesulfonic acid (H2L) and diethanolamine (H3R). It was characterized by IR and ESI-MS spectroscopies, elemental and X-ray crystal structural analyses. 1 shows a high catalytic activity for the solvent-free microwave (MW) assisted oxidation of 1-phenylethanol with tert-butylhydroperoxide, leading, in the presence of TEMPO, to yields up to 85% (TON = 850) in a remarkably short reaction time (15 min, with the corresponding TOF value of 3.40 × 103 h− 1) under low power (25 W) MW irradiation
Quasiparticles as composite objects in the RVB superconductor
We study the nature of the superconducting state, the origin of d-wave
pairing, and elementary excitations of a resonating valence bond (RVB)
superconductor. We show that the phase string formulation of the t-J model
leads to confinement of bare spinon and holon excitations in the
superconducting state, though the vacuum is described by the RVB state. Nodal
quasiparticles are obtained as composite excitations of spinon and holon
excitations. The d-wave pairing symmetry is shown to arise from short range
antiferromagnetic correlations
Impurity corrections to the thermodynamics in spin chains using a transfer-matrix DMRG method
We use the density matrix renormalization group (DMRG) for transfer matrices
to numerically calculate impurity corrections to thermodynamic properties. The
method is applied to two impurity models in the spin-1/2 chain, namely a weak
link in the chain and an external impurity spin. The numerical analysis
confirms the field theory calculations and gives new results for the crossover
behavior.Comment: 9 pages in revtex format including 5 embedded figures (using epsf).
To appear in PRB. The latest version in PDF format can be found at
http://fy.chalmers.se/~eggert/papers/DMRGimp.pd
Site-Dilution-Induced Antiferromagnetic Long-Range Order in Two-Dimensional Spin-Gapped Heisenberg Antiferromagnet
Effects of the site dilution on spin-gapped Heisenberg antiferromagnets with
and S=1 on a square lattice are investigated by means of the quantum
Monte Carlo method. It is found that effective magnetic moments induced around
the diluted sites exhibit the antiferromagnetic long-range order in the medium
of spin-singlet pairs. Their microscopic structure is examined in detail and
important roles of the higher dimensionality than one on the phenomenon are
discussed.Comment: RevTeX, 4 pages, 6 figure
Chitosan/TPP microparticles obtained by microemulsion method applied in controlled release of heparin
AbstractThis work deals with the preparation of chitosan/tripolyphosphate microparticles (CHT/TPP) using microemulsion system based on water/benzyl alcohol. The morphology of the microparticles was evaluated by scanning electron microscopy (SEM). The microparticles were also characterized through infrared spectroscopy (FTIR) and wide-angle X-ray scattering (WAXS). The morphology and crystallinity of microparticles depended mainly on CHT/TPP ratio. Studies of controlled release of HP were evaluated in distilled water and in simulated gastric fluid. Besides, the profile of HP releasing could be tailored by tuning the CHT/TPP molar ratio. Finally, these prospective results allow the particles to be employed as site-specific HP controlled release system
- …