114 research outputs found
Constrained-Path Quantum Monte-Carlo Approach for Non-Yrast States Within the Shell Model
The present paper intends to present an extension of the constrained-path
quantum Monte-Carlo approach allowing to reconstruct non-yrast states in order
to reach the complete spectroscopy of nuclei within the interacting shell
model. As in the yrast case studied in a previous work, the formalism involves
a variational symmetry-restored wave function assuming two central roles.
First, it guides the underlying Brownian motion to improve the efficiency of
the sampling. Second, it constrains the stochastic paths according to the
phaseless approximation to control sign or phase problems that usually plague
fermionic QMC simulations. Proof-of-principle results in the valence space
are reported. They prove the ability of the scheme to offer remarkably accurate
binding energies for both even- and odd-mass nuclei irrespective of the
considered interaction.Comment: 11 pages, 4 figure
A constrained-path quantum Monte-Carlo approach for the nuclear shell model
International audienceA new QMC approach for the shell model yielding nearly exact spectroscopy of nuclei is presented. The originality of the formalism lies in the use of a variational symmetry-restored wave function to âsteerâ the Brownian motion, and to control the sign/phase problem that generally makes the traditional QMC samplings totally ineffective by causing a prohibitive growth of the statistical errors. Tests of convergence and proof-of-principle results are reported
Exotic spin, charge and pairing correlations of the two-dimensional doped Hubbard model: a symmetry entangled mean-field approach
Intertwining of spin, charge and pairing correlations in the repulsive
two-dimensional Hubbard model is shown through unrestricted variational
calculations, with projected wavefunctions free of symmetry breaking. A
crossover from incommensurate antiferromagnetism to stripe order naturally
emerges in the hole-doped region when increasing the on-site coupling. Although
effective pairing interactions are identified, they are strongly fragmented in
several modes including d-wave pairing and more exotic channels related to an
underlying stripe. We demonstrate that the entanglement of a mean-field
wavefunction by symmetry restoration can largely account for interaction
effects.Comment: Minor corrections, one reference adde
Intertwined orders from symmetry projected wavefunctions of repulsively interacting Fermi gases in optical lattices
Unconventional strongly correlated phases of the repulsive Fermi-Hubbard
model, which could be emulated by ultracold vapors loaded in optical lattices,
are investigated by means of energy minimizations with quantum number
projection before variation and without any assumed order parameter. In a
tube-like geometry of optical plaquettes to realize the four-leg ladder Hubbard
Hamiltonian, we highlight the intertwining of spin-, charge-, and pair-density
waves embedded in a uniform d-wave superfluid background. As the lattice
filling increases, this phase emerges from homogenous states exhibiting spiral
magnetism and evolves towards a doped antiferromagnet. A concomitant
enhancement of long-ranged d-wave pairing correlations is also found. Numerical
tests of the approach for two-dimensional clusters are carried out, too.Comment: 26 pages, 15 figures ; replaced with the published manuscript ;
substantial changes from previous versio
Symétries nucléaires à faible isospin
With the development of radioactive beams, an area of intense research in nuclear physics concerns the structure of exotic systems with roughly equal numbers of protons and neutrons. These nuclei might in fact develop a proton-neutron superfluidity whose importance compared to pairing correlations between like nucleons is currently investigated. The work presented in this thesis suggests to look at such a competition in an algebraic framework based on a Wigner SU(4) symmetry that combines the pseudo-spin and isospin degrees of freedom. After a detailed review of group theory in quantum mechanics, the validity of the pseudo-SU(4) classification is shown via a direct analysis of realistic shell model states. Its consequences on binding energies and ÎČ decay are also studied. Moreover, a simplified boson realisation with zero orbital angular momentum is used to find some physical features of N=Z nuclei such as the condensation of α-like structures or the destruction of isoscalar superfluid correlations by the spin-orbit potential. Finally, another bosonization scheme that includes quadrupole degrees of freedom (IBM-4 model) is tested for the first time by diagonalization of a full Hamiltonian deduced from a realistic shell model interaction. The quality of the results, especially for odd-odd nuclei, allows one to consider this boson approximation as an alternative to standard fermionic approaches for the collective structure of the exotic line NâŒZ=28-50.Avec le dĂ©veloppement des faisceaux radioactifs, un intĂ©rĂȘt particulier est actuellement portĂ© aux noyaux exotiques riches en protons. Ces structures offrent en effet la possibilitĂ© de dĂ©velopper une superfluiditĂ© proton-neutron dont l'importance vis Ă vis des corrĂ©lations d'appariement entre nuclĂ©ons identiques fait l'objet de nombreuses Ă©tudes thĂ©oriques. Le travail prĂ©sentĂ© propose prĂ©cisĂ©ment d'aborder ce problĂšme dans le cadre d'une approche algĂ©brique basĂ©e sur une symĂ©trie SU(4) de Wigner combinant les degrĂ©s de libertĂ© de pseudo-spin et d'isospin. AprĂšs avoir repris en dĂ©tail les implĂ©mentations de la thĂ©orie des groupes en mĂ©canique quantique, la pertinence de la classification pseudo-SU(4) est directement montrĂ©e au niveau des Ă©tats rĂ©alistes du modĂšle en couches. Ses consĂ©quences au niveau des masses et des transitions de dĂ©croissance ÎČ sont Ă©galement analysĂ©es. Sa rĂ©alisation partielle en bosons de moment orbital nul est de plus utilisĂ©e pour mettre en Ă©vidence un certain nombre de phĂ©nomĂšnes physiques spĂ©cifiques Ă la ligne N = Z comme la condensation en structures de type α ou la destruction par le potentiel spin-orbite des corrĂ©lations superfluides isoscalaires. Enfin, un autre schĂ©ma de bosonisation incluant des degrĂ©s de libertĂ© quadrupolaires (modĂšle IBM-4) est testĂ© pour la premiĂšre fois en diagonalisant un hamiltonien complet dĂ©duit d'une interaction rĂ©aliste du modĂšle en couches. La qualitĂ© des rĂ©sultats obtenus, plus particuliĂšrement pour les noyaux impair-impair, permet raisonnablement d'envisager l'utilisation de cette approximation en bosons comme alternative aux approches fermioniques standard pour Ă©lucider la structure collective de la ligne exotique N ⌠Z = 28-50
Information theory of open fragmenting systems
An information theory description of finite systems explicitly evolving in time is presented. We impose a MaxEnt variational principle on the Shannon entropy at a given time while the constraints are set at a former time. The resulting density matrix contains explicit time odd components in the form of collective flows. As a specific application we consider the dynamics of the expansion in connection with heavy ion experiments. Lattice gas and classical molecular dynamics simulations are shown. © 2007 American Institute of Physics.Fil:Ison, M.J. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales; Argentina.Fil:Dorso, C.O. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales; Argentina
Exact pairing correlations in one-dimensionally trapped fermions with stochastic mean-field wave-functions
Accepted for publication in Physical Review Letters.The canonical thermodynamic properties of a one-dimensional system of interacting spin-1/2 fermions with an attractive zero-range pseudo-potential are investigated within an exact approach. The density operator is evaluated as the statistical average of dyadics formed from a stochastic mean-field propagation of independent Slater determinants. For an harmonically trapped Fermi gas and for fermions confined in a 1D-like torus, we observe the transition to a quasi-BCS state with Cooper-like momentum correlations and an algebraic long-range order. For few trapped fermions in a rotating torus, a dominant superfluid component with quantized circulation can be isolated
Three fermions in a box at the unitary limit: universality in a lattice model
We consider three fermions with two spin components interacting on a lattice
model with an infinite scattering length. Low lying eigenenergies in a cubic
box with periodic boundary conditions, and for a zero total momentum, are
calculated numerically for decreasing values of the lattice period. The results
are compared to the predictions of the zero range Bethe-Peierls model in
continuous space, where the interaction is replaced by contact conditions. The
numerical computation, combined with analytical arguments, shows the absence of
negative energy solution, and a rapid convergence of the lattice model towards
the Bethe-Peierls model for a vanishing lattice period. This establishes for
this system the universality of the zero interaction range limit.Comment: 6 page
Sign-free stochastic mean-field approach to strongly correlated phases of ultracold fermions
We propose a new projector quantum Monte-Carlo method to investigate the
ground state of ultracold fermionic atoms modeled by a lattice Hamiltonian with
on-site interaction. The many-body state is reconstructed from Slater
determinants that randomly evolve in imaginary-time according to a stochastic
mean-field motion. The dynamics prohibits the crossing of the exact nodal
surface and no sign problem occurs in the Monte-Carlo estimate of observables.
The method is applied to calculate ground-state energies and correlation
functions of the repulsive two-dimensional Hubbard model. Numerical results for
the unitary Fermi gas validate simulations with nodal constraints.Comment: Accepted for publication in New Journal of Physic
Generalized Gibbs ensembles for time dependent processes
An information theory description of finite systems explicitly evolving in
time is presented for classical as well as quantum mechanics. We impose a
variational principle on the Shannon entropy at a given time while the
constraints are set at a former time. The resulting density matrix deviates
from the Boltzmann kernel and contains explicit time odd components which can
be interpreted as collective flows. Applications include quantum brownian
motion, linear response theory, out of equilibrium situations for which the
relevant information is collected within different time scales before entropy
saturation, and the dynamics of the expansion
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