747 research outputs found
The J1-J2 model: First order phase transition versus deconfinement of spinons
We revisit the phase transition from the N\'eel ordered to a valence bond
solid (VBS) state in the two-dimensional antiferromagnetic Heisenberg
model. In the first part we address the question whether or not this transition
could be an example of a second order phase transition due to a deconfinement
of spinons. We give arguments based on series expansion and spin-wave theory
that this is not the case and the transition is most likely first order. The
method proposed here to detect first order phase transitions seems to be very
sensitive and might be useful in other models as well. In the second part we
analyze possible VBS patterns in the magnetically disordered phase based on
numerical data for different susceptibilities, obtained in the ordered phase,
which test the breaking of lattice symmetries. We conclude that a columnar
dimerization pattern is the most likely candidate.Comment: 7 pages, 7 figures, small changes, references adde
Ab Initio Treatments of the Ising Model in a Transverse Field
In this article, new results are presented for the zero-temperature
ground-state properties of the spin-half transverse Ising model on various
lattices using three different approximate techniques. These are, respectively,
the coupled cluster method, the correlated basis function method, and the
variational quantum Monte Carlo method. The methods, at different levels of
approximation, are used to study the ground-state properties of these systems,
and the results are found to be in excellent agreement both with each other and
with results of exact calculations for the linear chain and results of exact
cumulant series expansions for lattices of higher spatial dimension. The
different techniques used are compared and contrasted in the light of these
results, and the constructions of the approximate ground-state wave functions
are especially discussed.Comment: 28 Pages, 4 Figures, 1 Tabl
Ab Initio Calculation of Spin Gap Behavior in CaV4O9
Second neighbor dominated exchange coupling in CaV4O9 has been obtained from
ab initio density functional (DF) calculations. A DF-based self-consistent
atomic deformation model reveals that the nearest neighbor coupling is small
due to strong cancellation among the various superexchange processes. Exact
diagonalization of the predicted Heisenberg model yields spin-gap behavior in
good agreement with experiment. The model is refined by fitting to the
experimental susceptibility. The resulting model agrees very well with the
experimental susceptibility and triplet dispersion.Comment: 4 pages; 3 ps figures included in text; Revte
Novel approach to description of spin liquid phases in low-dimensional quantum antiferromagnets
We consider quantum spin systems with dimerization, which at strong coupling
have singlet ground states. To account for strong correlations, the excitations
are described as dilute Bose gas of degenerate triplets with infinite on-site
repulsion. This approach is applied to the two-layer Heisenberg model at zero
temperature with general couplings. Our analytic results for the triplet gap,
the excitation spectrum and the location of the quantum critical point are in
excellent agreement with numerical results, obtained by dimer series
expansions.Comment: 4 pages, REVTex, 3 Postscript figure
Quantum and classical criticalities in the frustrated two-leg Heisenberg ladder
This talk was about the frustration-induced criticality in the
antiferromagnetic Heisenberg model on the two-leg ladder with exchange
interactions along the chains, rungs, and diagonals, and also about the effect
of thermal fluctuations on this criticlity. The method used is the bond
mean-field theory, which is based on the Jordan-Wigner transformation in
dimensions higher than one. In this paper, we will summarize the main results
presented in this talk, and report on new results about the couplings and
temperature dependences of the spin susceptibility.Comment: 6 pages, 4 figures, talk presented at the Theory Canada 3 conference
in 2007, submitted to the Canadian Journal of Physic
Phase Diagram of the BCC S=1/2 Heisenberg Antiferromagnet with First and Second Neighbor Exchange
We use linked-cluster series expansions, both at T=0 and high temperature, to
analyse the phase structure of the spin-\half Heisenberg antiferromagnet with
competing first and second-neighbor interactions on the 3-dimensional
body-centred-cubic lattice. At zero temperature we find a first-order quantum
phase transition at between AF (Ne\'el)
and AF ordered phases. The high temperature series yield quite accurate
estimates of the bounding critical line for the AF phase, and an apparent
critical line for the AF phase, with a bicritical point at , . The possibility that this latter transition is
first-order cannot be excluded.Comment: 10 pages, 4 figure
HGF/c-met/Stat3 signaling during skin tumor cell invasion: indications for a positive feedback loop
<p>Abstract</p> <p>Background</p> <p>Stat3 is a cytokine- and growth factor-inducible transcription factor that regulates cell motility, migration, and invasion under normal and pathological situations, making it a promising target for cancer therapeutics. The hepatocyte growth factor (HGF)/c-met receptor tyrosine kinase signaling pathway is responsible for stimulation of cell motility and invasion, and Stat3 is responsible for at least part of the c-met signal.</p> <p>Methods</p> <p>We have stably transfected a human squamous cell carcinoma (SCC) cell line (SRB12-p9) to force the expression of a dominant negative form of Stat3 (S3DN), which we have previously shown to suppress Stat3 activity. The <it>in vitro </it>and <it>in vivo </it>malignant behavior of the S3DN cells was compared to parental and vector transfected controls.</p> <p>Results</p> <p>Suppression of Stat3 activity impaired the ability of the S3DN cells to scatter upon stimulation with HGF (c-met ligand), enhanced their adhesion, and diminished their capacity to invade <it>in vitro </it>and <it>in vivo</it>. Surprisingly, S3DN cells also showed suppressed HGF-induced activation of c-met, and had nearly undetectable basal c-met activity, as revealed by a phospho-specific c-met antibody. In addition, we showed that there is a strong membrane specific localization of phospho-Stat3 in the wild type (WT) and vector transfected control (NEO4) SRB12-p9 cells, which is lost in the S3DN cells. Finally, co-immunoprecipitation experiments revealed that S3DN interfered with Stat3/c-met interaction.</p> <p>Conclusion</p> <p>These studies are the first confirm that interference with the HGF/c-met/Stat3 signaling pathway can block tumor cell invasion in an <it>in vivo </it>model. We also provide novel evidence for a possible positive feedback loop whereby Stat3 can activate c-met, and we correlate membrane localization of phospho-Stat3 with invasion <it>in vivo</it>.</p
Nonmagnetic Insulating States near the Mott Transitions on Lattices with Geometrical Frustration and Implications for -(ET)Cu
We study phase diagrams of the Hubbard model on anisotropic triangular
lattices, which also represents a model for -type BEDT-TTF compounds.
In contrast with mean-field predictions, path-integral renormalization group
calculations show a universal presence of nonmagnetic insulator sandwitched by
antiferromagnetic insulator and paramagnetic metals. The nonmagnetic phase does
not show a simple translational symmetry breakings such as flux phases,
implying a genuine Mott insulator. We discuss possible relevance on the
nonmagnetic insulating phase found in -(ET)Cu.Comment: 4pages including 7 figure
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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