246 research outputs found
Accurate nonrelativistic ground-state energies of 3d transition metal atoms
We present accurate nonrelativistic ground-state energies of the transition
metal atoms of the 3d series calculated with Fixed-Node Diffusion Monte Carlo
(FN-DMC). Selected multi-determinantal expansions obtained with the CIPSI
method (Configuration Interaction using a Perturbative Selection made
Iteratively) and including the most prominent determinants of the full CI
expansion are used as trial wavefunctions. Using a maximum of a few tens of
thousands determinants, fixed-node errors on total DMC energies are found to be
greatly reduced for some atoms with respect to those obtained with Hartree-Fock
nodes. The FN-DMC/(CIPSI nodes) ground-state energies presented here are, to
the best of our knowledge, the most accurate values reported so far. Thanks to
the variational property of FN-DMC total energies, the results also provide
lower bounds for the absolute value of all-electron correlation energies,
.Comment: 5 pages, 3 table
An efficient sampling algorithm for Variational Monte Carlo
We propose a new algorithm for sampling the -body density in the Variational Monte Carlo (VMC)
framework. This algorithm is based upon a modified Ricci-Ciccotti
discretization of the Langevin dynamics in the phase space
improved by a Metropolis acceptation/rejection step. We show through some
representative numerical examples (Lithium, Fluorine and Copper atoms, and
phenol molecule), that this algorithm is superior to the standard sampling
algorithm based on the biased random walk (importance sampling).Comment: 23 page
Dynamical Symmetry Enlargement Versus Spin-Charge Decoupling in the One-Dimensional SU(4) Hubbard Model
We investigate dynamical symmetry enlargement in the half-filled SU(4)
Hubbard chain using non-perturbative renormalization group and Quantum Monte
Carlo techniques. A spectral gap is shown to open for arbitrary Coulombic
repulsion . At weak coupling, , a SO(8) symmetry between
charge and spin-orbital excitations is found to be dynamically enlarged at low
energy. At strong coupling, , the charge degrees of freedom
dynamically decouple and the resulting effective theory in the spin-orbital
sector is that of the SO(6) antiferromagnetic Heisenberg model. Both regimes
exhibit spin-Peierls order. However, although spin-orbital excitations are
in the SO(6) regime they are in the SO(8) one. The
cross-over between these regimes is discussed.Comment: 4 pages, 2 figure
Monte Carlo Calculation of the Spin-Stiffness of the Two-Dimensional Heisenberg Model
Using a collective-mode Monte Carlo method (the Wolff-Swendsen-Wang
algorithm), we compute the spin-stiffness of the two-dimensional classical
Heisenberg model. We show that it is the relevant physical quantity to
investigate the behaviour of the model in the very low temperature range
inaccessible to previous studies based on correlation length and susceptibility
calculations.Comment: 6 pages, latex, 3 postscript figures appended, DIM preprint 93-3
Coexistence of solutions in dynamical mean-field theory of the Mott transition
In this paper, I discuss the finite-temperature metal-insulator transition of
the paramagnetic Hubbard model within dynamical mean-field theory. I show that
coexisting solutions, the hallmark of such a transition, can be obtained in a
consistent way both from Quantum Monte Carlo (QMC) simulations and from the
Exact Diagonalization method. I pay special attention to discretization errors
within QMC. These errors explain why it is difficult to obtain the solutions by
QMC close to the boundaries of the coexistence region.Comment: 3 pages, 2 figures, RevTe
The Fermion Monte Carlo revisited
In this work we present a detailed study of the Fermion Monte Carlo algorithm
(FMC), a recently proposed stochastic method for calculating fermionic
ground-state energies [M.H. Kalos and F. Pederiva, Phys. Rev. Lett. vol. 85,
3547 (2000)]. A proof that the FMC method is an exact method is given. In this
work the stability of the method is related to the difference between the
lowest (bosonic-type) eigenvalue of the FMC diffusion operator and the exact
fermi energy. It is shown that within a FMC framework the lowest eigenvalue of
the new diffusion operator is no longer the bosonic ground-state eigenvalue as
in standard exact Diffusion Monte Carlo (DMC) schemes but a modified value
which is strictly greater. Accordingly, FMC can be viewed as an exact DMC
method built from a correlated diffusion process having a reduced Bose-Fermi
gap. As a consequence, the FMC method is more stable than any transient method
(or nodal release-type approaches). We illustrate the various ideas presented
in this work with calculations performed on a very simple model having only
nine states but a full sign problem. Already for this toy model it is clearly
seen that FMC calculations are inherently uncontrolled.Comment: 49 pages with 4 postscript figure
Optimization of ground and excited state wavefunctions and van der Waals clusters
A quantum Monte Carlo method is introduced to optimize excited state trial
wavefunctions. The method is applied in a correlation function Monte Carlo
calculation to compute ground and excited state energies of bosonic van der
Waals clusters of upto seven particles. The calculations are performed using
trial wavefunctions with general three-body correlations
Pseudogap and high-temperature superconductivity from weak to strong coupling. Towards quantitative theory
This is a short review of the theoretical work on the two-dimensional Hubbard
model performed in Sherbrooke in the last few years. It is written on the
occasion of the twentieth anniversary of the discovery of high-temperature
superconductivity. We discuss several approaches, how they were benchmarked and
how they agree sufficiently with each other that we can trust that the results
are accurate solutions of the Hubbard model. Then comparisons are made with
experiment. We show that the Hubbard model does exhibit d-wave
superconductivity and antiferromagnetism essentially where they are observed
for both hole and electron-doped cuprates. We also show that the pseudogap
phenomenon comes out of these calculations. In the case of electron-doped high
temperature superconductors, comparisons with angle-resolved photoemission
experiments are nearly quantitative. The value of the pseudogap temperature
observed for these compounds in recent photoemission experiments has been
predicted by theory before it was observed experimentally. Additional
experimental confirmation would be useful. The theoretical methods that are
surveyed include mostly the Two-Particle Self-Consistent Approach, Variational
Cluster Perturbation Theory (or variational cluster approximation), and
Cellular Dynamical Mean-Field Theory.Comment: 32 pages, 51 figures. Slight modifications to text, figures and
references. A PDF file with higher-resolution figures is available at
http://www.physique.usherbrooke.ca/senechal/LTP-toc.pd
Dynamical Mean Field Theory of the Antiferromagnetic Metal to Antiferromagnetic Insulator Transition
We study the antiferromagnetic metal to antiferromagnetic insulator using
dynamical mean field theory and exact diagonalization methods. We find two
qualitatively different behaviors depending on the degree of magnetic
correlations. For strong correlations combined with magnetic frustration, the
transition can be described in terms of a renormalized slater theory, with a
continuous gap closure driven by the magnetism but strongly renormalized by
correlations. For weak magnetic correlations, the transition is weakly first
order.Comment: 4 pages, uses epsfig,4 figures,notational errors rectifie
Tissue transglutaminase mediates the pro-malignant effects of oncostatin M receptor over-expression in cervical squamous cell carcinoma.
Oncostatin M receptor (OSMR) is commonly over-expressed in advanced cervical squamous cell carcinoma (SCC), producing a significantly worse clinical outcome. Cervical SCC cells that over-express OSMR show enhanced responsiveness to the major ligand OSM, which induces multiple pro-malignant effects, including increased cell migration and invasiveness. Here, we show that tissue transglutaminase (TGM2) is an important mediator of the ligand-dependent phenotypic effects of OSMR over-expression in SCC cells. TGM2 expression correlated with disease progression and with OSMR levels in clinical samples of cervical and oral SCC. TGM2 depletion in cervical SCC cells abrogated OSM-induced migration on fibronectin-coated surfaces and invasiveness through extracellular matrix, while ectopic expression of TGM2 increased cell motility and invasiveness. Confocal microscopy and co-immunoprecipitation assays showed that TGM2 interacted with integrin-α5ÎČ1 in the presence of fibronectin in cervical SCC cells, with OSM treatment strengthening the interaction. Importantly, integrin-α5ÎČ1 and fibronectin were also over-expressed in cervical and oral SCC, where levels correlated with those of OSMR and TGM2. This combined tissue and in vitro study demonstrates for the first time that stimulation of over-expressed OSMR in cervical SCC cells activates TGM2/integrin-α5ÎČ1 interactions and induces pro-malignant changes. We conclude that an OSMR/TGM2/integrin-α5ÎČ1/fibronectin pathway is of biological significance in cervical SCC and a candidate for therapeutic targeting
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