1,362 research outputs found
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
Numerical Jordan-Wigner approach for two dimensional spin systems
We present a numerical self consistent variational approach based on the
Jordan-Wigner transformation for two dimensional spin systems. We apply it to
the study of the well known quantum (S=1/2) antiferromagnetic XXZ system as a
function of the easy-axis anisotropy \Delta on a periodic square lattice. For
the SU(2) case the method converges to a N\'eel ordered ground state
irrespectively of the input density profile used and in accordance with other
studies. This shows the potential utility of the proposed method to investigate
more complicated situations like frustrated or disordered systems.Comment: Revtex, 8 pages, 4 figure
High-resolution tracking in a GEM-Emulsion detector
SHiP (Search for Hidden Particles) is a beam dump experiment proposed at the
CERN SPS aiming at the observation of long lived particles very weakly coupled
with ordinary matter mostly produced in the decay of charmed hadrons. The beam
dump facility of SHiP is also a copious factory of neutrinos of all three kinds
and therefore a dedicated neutrino detector is foreseen in the SHiP apparatus.
The neutrino detector exploits the Emulsion Cloud Chamber technique with a
modular structure, alternating walls of target units and planes of electronic
detectors providing the time stamp to the event. GEM detectors are one of the
possible choices for this task. This paper reports the results of the first
exposure to a muon beam at CERN of a new hybrid chamber, obtained by coupling a
GEM chamber and an emulsion detector. Thanks to the micrometric accuracy of the
emulsion detector, the position resolution of the GEM chamber as a function of
the particle inclination was evaluated in two configurations, with and without
the magnetic fiel
Charge fluctuations close to phase separation in the two dimensional t-J model
We have studied the t-J model using the Green Function Monte Carlo technique.
We have obtained accurate energies well converged in the thermodynamic limit,
by performing simulations up to 242 lattice sites. By studying the energy as a
function of hole doping we conclude that there is no phase separation in the
physical region, relevant for HTc superconductors. This finding is further
supported by the hole-hole correlation function calculation. Remarkably, by
approaching the phase separation instability, for ,this function
displays enhanced fluctuations at incommensurate wavevectors, scaling linearly
with the doping, in agreement with experimental findings.Comment: To appear on Phys. Rev. Let
Phase separation in the 2D Hubbard model: a fixed-node quantum Monte Carlo study
Fixed-node Green's function Monte Carlo calculations have been performed for
very large 16x6 2D Hubbard lattices, large interaction strengths U=10,20, and
40, and many (15-20) densities between empty and half filling. The nodes were
fixed by a simple Slater-Gutzwiller trial wavefunction. For each value of U we
obtained a sequence of ground-state energies which is consistent with the
possibility of a phase separation close to half-filling, with a hole density in
the hole-rich phase which is a decreasing function of U. The energies suffer,
however, from a fixed-node bias: more accurate nodes are needed to confirm this
picture. Our extensive numerical results and their test against size, shell,
shape and boundary condition effects also suggest that phase separation is
quite a delicate issue, on which simulations based on smaller lattices than
considered here are unlikely to give reliable predictions.Comment: 4 pages, 1 figure; revised version, more data point
From antiferromagnetism to d-wave superconductivity in the 2D t-J model
We have found that the two dimensional t-J model, for the physical parameter
range J/t = 0.4 reproduces the main experimental qualitative features of
High-Tc copper oxide superconductors: d-wave superconducting correlations are
strongly enhanced upon small doping and clear evidence of off diagonal long
range order is found at the optimal doping \delta ~ 0.15. On the other hand
antiferromagnetic long range order, clearly present at zero hole doping, is
suppressed at small hole density with clear absence of antiferromagnetism at
\delta >~ 0.1.Comment: 4 pages, 5 figure
Adaptive Optimization of Wave Functions for Fermion Lattice Models
We present a simulation algorithm for Hamiltonian fermion lattice models. A
guiding trial wave function is adaptively optimized during Monte Carlo
evolution. We apply the method to the two dimensional Gross-Neveu model and
analyze systematc errors in the study of ground state properties. We show that
accurate measurements can be achieved by a proper extrapolation in the
algorithm free parameters.Comment: 4 pages, 6 figures (Encapsulated PostScript
How resistant are levodopa-resistant axial symptoms? Response of freezing, posture, and voice to increasing levodopa intestinal infusion rates in Parkinson disease
Background and purpose: Treatment of freezing of gait (FoG) and other Parkinson disease (PD) axial symptoms is challenging. Systematic assessments of axial symptoms at progressively increasing levodopa doses are lacking. We sought to analyze the resistance to high levodopa doses of FoG, posture, speech, and altered gait features presenting in daily-ON therapeutic condition. Methods: We performed a pre-/postinterventional study including patients treated with levodopa/carbidopa intestinal gel infusion (LCIG) with disabling FoG in daily-ON condition. Patients were evaluated at their usual LCIG infusion rate (T1), and 1 h after 1.5× (T2) and 2× (T3) increase of the LCIG infusion rate by quantitative outcome measures. The number of FoG episodes (primary outcome), posture, speech, and gait features were objectively quantified during a standardized test by a blinded rater. Changes in motor symptoms, dyskinesia, and plasma levodopa concentrations were also analyzed. Results: We evaluated 16 patients with a mean age of 69 ± 9.4 years and treated with LCIG for a mean of 2.2 ± 2.1 years. FoG improved in 83.3% of patients by increasing the levodopa doses. The number of FoG episodes significantly decreased (mean = 2.3 at T1, 1.7 at T2, 1.2 at T3; p = 0.013). Posture and speech features did not show significant changes, whereas stride length (p = 0.049), turn duration (p = 0.001), and turn velocity (p = 0.024) significantly improved on doubling the levodopa infusion rate. Conclusions: In a short-term evaluation, the increase of LCIG dose can improve "dopa-resistant" FoG and gait issues in most advanced PD patients with overall good control of motor symptoms in the absence of clinically significant dyskinesia
Optimization of Gutzwiller Wavefunctions in Quantum Monte Carlo
Gutzwiller functions are popular variational wavefunctions for correlated
electrons in Hubbard models. Following the variational principle, we are
interested in the Gutzwiller parameters that minimize e.g. the expectation
value of the energy. Rewriting the expectation value as a rational function in
the Gutzwiller parameters, we find a very efficient way for performing that
minimization. The method can be used to optimize general Gutzwiller-type
wavefunctions both, in variational and in fixed-node diffusion Monte Carlo.Comment: 9 pages RevTeX with 10 eps figure
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