Search results for " Monte Carlo"
showing 10 items of 400 documents
On the Lindemann criterion for quantum clusters at very low temperature.
2011
The Lindemann criterion to discern the solid-like or liquid-like nature of a quantum cluster at T = 0 is discussed. A critical analysis of current Lindemann parameters is presented and a new parameter is proposed that is appropriate to study quantum clusters made of identical particles. A simple model wave function is introduced to fix the range of variation of these parameters. The model presents two extreme limits that correspond to either a liquid-like or a solid-like system; besides, it fulfills the Bose symmetry and also permits evaluations without symmetrization. Variational and diffusion Monte Carlo calculations are also performed for clusters of spinless bosons interacting through L…
Wilsonʼs momentum shell renormalization group from Fourier Monte Carlo simulations
2011
Abstract Previous attempts to accurately compute critical exponents from Wilsonʼs momentum shell renormalization prescription suffered from the difficulties posed by the presence of an infinite number of irrelevant couplings. Taking the example of the 1d long-ranged Ising model , we calculate the momentum shell renormalization flow in the plane spanned by the coupling constants ( u 0 , r 0 ) for different values of the momentum shell thickness parameter b by simulation using our recently developed Fourier Monte Carlo algorithm. We report strong anomalies in the b-dependence of the fixed point couplings and the resulting exponents y τ and ω in the vicinity of a shell parameter b ⁎ 1 characte…
Sign problem of the fermionic shadow wave function
2014
We present a whole series of methods to alleviate the sign problem of the fermionic shadow wave function in the context of variational Monte Carlo. The effectiveness of our techniques is demonstrated on liquid ^{3}He. We found that although the variance is reduced, the gain in efficiency is restricted by the increased computational cost. Yet, this development not only extends the scope of the fermionic shadow wave function, but also facilitates highly accurate quantum Monte Carlo simulations previously thought not feasible.
Analysis of the incoherent intermediate scattering function in the framework of the idealized mode-coupling theory: A Monte Carlo study for polymer m…
1994
In this Monte Carlo simulation, we calculate the incoherent intermediate scattering function ${\mathrm{\ensuremath{\varphi}}}_{\mathit{q}}^{\mathit{s}}$(t) for a three-dimensional dense polymer melt after having made long relaxation runs in order to eliminate the history of the cooling procedure sufficiently. This function shows the signature of a two-step process in the temperature interval T\ensuremath{\in}[0.16,0.21] (the temperature is measured in units of an energy parameter introduced in the Hamiltonian of the model) whose time evolution was quantitatively analyzed in the framework of the idealized mode-coupling theory (MCT) within the \ensuremath{\beta}-relaxation regime. As a result…
Orbital-selective Mott transitions in the anisotropic two-band Hubbard model at finite temperatures
2005
The anisotropic degenerate two-orbital Hubbard model is studied within dynamical mean-field theory at low temperatures. High-precision calculations on the basis of a refined quantum Monte Carlo (QMC) method reveal that two distinct orbital-selective Mott transitions occur for a bandwidth ratio of 2 even in the absence of spin-flip contributions to the Hund exchange. The second transition -- not seen in earlier studies using QMC, iterative perturbation theory, and exact diagonalization -- is clearly exposed in a low-frequency analysis of the self-energy and in local spectra.
The fate of the resonating valence bond in graphene
2011
We apply a variational wave function capable of describing qualitatively and quantitatively the so called "resonating valence bond" in realistic materials, by improving standard ab initio calculations by means of quantum Monte Carlo methods. In this framework we clearly identify the Kekul\'e and Dewar contributions to the chemical bond of the benzene molecule, and we establish the corresponding resonating valence bond energy of these well known structures ($\simeq 0.01$eV/atom). We apply this method to unveil the nature of the chemical bond in undoped graphene and show that this picture remains only within a small "resonance length" of few atomic units.
Emery vs. Hubbard model for cuprate superconductors: A composite operator method study
2013
Within the Composite Operator Method (COM), we report the solution of the Emery model (also known as p-d or three band model), which is relevant for the cuprate high-Tc superconduc- tors. We also discuss the relevance of the often-neglected direct oxygen-oxygen hopping for a more accurate, sometimes unique, description of this class of materials. The benchmark of the solution is performed by comparing our results with the available quantum Monte Carlo ones. Both single- particle and thermodynamic properties of the model are studied in detail. Our solution features a metal-insulator transition at half filling. The resulting metal-insulator phase diagram agrees qual- itatively very well with …
Low-temperature anharmonic lattice deformations near rotator impurities: A quantum Monte Carlo approach.
1994
At zero temperature the equilibrium structures of a system consisting of a quantum rotator (${\mathrm{N}}_{2}$) embedded in a relaxing lattice (Ar) surrounding are studied with a variational approach. With symmetric wave functions (para-${\mathrm{N}}_{2}$), we obtain a cubic lattice deformation near the rotator, while with antisymmetric wave functions (ortho-${\mathrm{N}}_{2}$), we obtain a tetragonal lattice deformation forming a stable oriented ground state. At low temperatures, we investigate the properties of this system with a quantum Monte Carlo simulation. On top of the tetragonal deformation the width of the nearest-neighbor oscillations follows classical ``scaling'' laws according …
Drops of3Heatoms with good angular-momentum quantum numbers
2000
The stability of drops made of ${}^{3}\mathrm{He}$ atoms is studied by means of a Monte Carlo variational method using wave functions with good angular momentum quantum numbers. The number of constituents considered is in the range 34--40. It is found that the minimal bound drop requires 35 atoms (perhaps 34) and that the preferred wave function must have the maximum spin.
STABILITY AND SPECTRA OF SMALL 3He-4He CLUSTERS
2005
Diffusion Monte Carlo calculations have been systematically performed to analyze the stability of small mixed 3 He -4 He clusters, as well as their excitation spectra. The picture that emerges is that of systems with strong shell effects whose binding and excitation energies are essentially determined by the monopole properties of an effective Hamiltonian.