3,464 research outputs found
Superfluid Pairing in Neutrons and Cold Atoms
Ultracold atomic gases and low-density neutron matter are unique in that they
exhibit pairing gaps comparable to the Fermi energy which in this sense are the
largest in the laboratory and in nature, respectively. This strong pairing
regime, or the crossover between BCS and BEC regimes, requires non-perturbative
treatments. We describe Quantum Monte Carlo results useful to understand the
properties of these systems, including infinite homogeneous matter and trapped
inhomogeneous gases.Comment: 14 pages, 4 figures; chapter in "50 Years of Nuclear BCS", edited by
R. A. Broglia and V. Zelevinsk
Quantum Monte Carlo calculations of neutron matter with chiral three-body forces
Chiral effective field theory (EFT) enables a systematic description of
low-energy hadronic interactions with controlled theoretical uncertainties. For
strongly interacting systems, quantum Monte Carlo (QMC) methods provide some of
the most accurate solutions, but they require as input local potentials. We
have recently constructed local chiral nucleon-nucleon (NN) interactions up to
next-to-next-to-leading order (NLO). Chiral EFT naturally predicts
consistent many-body forces. In this paper, we consider the leading chiral
three-nucleon (3N) interactions in local form. These are included in auxiliary
field diffusion Monte Carlo (AFDMC) simulations. We present results for the
equation of state of neutron matter and for the energies and radii of neutron
drops. In particular, we study the regulator dependence at the Hartree-Fock
level and in AFDMC and find that present local regulators lead to less
repulsion from 3N forces compared to the usual nonlocal regulators.Comment: 10 pages, 8 figures, 1 table, published versio
Zero-Temperature Dynamics of Plus/Minus J Spin Glasses and Related Models
We study zero-temperature, stochastic Ising models sigma(t) on a
d-dimensional cubic lattice with (disordered) nearest-neighbor couplings
independently chosen from a distribution mu on R and an initial spin
configuration chosen uniformly at random. Given d, call mu type I (resp., type
F) if, for every x in the lattice, sigma(x,t) flips infinitely (resp., only
finitely) many times as t goes to infinity (with probability one) --- or else
mixed type M. Models of type I and M exhibit a zero-temperature version of
``local non-equilibration''. For d=1, all types occur and the type of any mu is
easy to determine. The main result of this paper is a proof that for d=2,
plus/minus J models (where each coupling is independently chosen to be +J with
probability alpha and -J with probability 1-alpha) are type M, unlike
homogeneous models (type I) or continuous (finite mean) mu's (type F). We also
prove that all other noncontinuous disordered systems are type M for any d
greater than or equal to 2. The plus/minus J proof is noteworthy in that it is
much less ``local'' than the other (simpler) proof. Homogeneous and plus/minus
J models for d greater than or equal to 3 remain an open problem.Comment: 17 pages (RevTeX; 3 figures; to appear in Commun. Math. Phys.
Resonantly Interacting Fermions In a Box
We use two fundamental theoretical frameworks to study the finite-size
(shell) properties of the unitary gas in a periodic box: 1) an ab initio
Quantum Monte Carlo (QMC) calculation for boxes containing 4 to 130 particles
provides a precise and complete characterization of the finite-size behavior,
and 2) a new Density Functional Theory (DFT) fully encapsulates these effects.
The DFT predicts vanishing shell structure for systems comprising more than 50
particles, and allows us to extrapolate the QMC results to the thermodynamic
limit, providing the tightest bound to date on the ground-state energy of the
unitary gas: \xi_S <= 0.383(1). We also apply the new functional to
few-particle harmonically trapped systems, comparing with previous
calculations.Comment: Updated to correspond with published version: 4+ pages, 2 figures, 2
tables, Palatino and Euler font
Contact interaction in an unitary ultracold Fermi gas
An ultracold Fermi atomic gas at unitarity presents universal properties that
in the diluted limit can be well described by a contact interaction. By
employing a guide function with correct boundary conditions and making simple
modifications to the sampling procedure we are able to handle for the first
time a true contact interaction in a quantum Monte Carlo calculation. The
results are obtained with small variances. Our calculations for the Bertsch and
contact parameters are in excellent agreement with published experiments. The
possibility of using a more faithfully description of ultracold atomic gases
can help uncover features yet unknown of the ultracold atomic gases. In
addition, this work paves the way to perform quantum Monte Carlo calculations
for systems interacting with contact interactions, where in many cases the
description using potentials with finite effective range might not be accurate
Quantum Monte Carlo Calculations of Light Nuclei Using Chiral Potentials
We present the first Green's function Monte Carlo calculations of light
nuclei with nuclear interactions derived from chiral effective field theory up
to next-to-next-to-leading order. Up to this order, the interactions can be
constructed in a local form and are therefore amenable to quantum Monte Carlo
calculations. We demonstrate a systematic improvement with each order for the
binding energies of and systems. We also carry out the first
few-body tests to study perturbative expansions of chiral potentials at
different orders, finding that higher-order corrections are more perturbative
for softer interactions. Our results confirm the necessity of a three-body
force for correct reproduction of experimental binding energies and radii, and
pave the way for studying few- and many-nucleon systems using quantum Monte
Carlo methods with chiral interactions.Comment: 5 pages, 3 figures, 4 tables. Updated references. Cosmetic changes to
figures, tables, and equations; added a sentence clarifying the
correspondence between our real-space cutoffs and momentum-space cutoffs.
Other sentences were reworded for clarit
Microscopic calculation of the equation of state of nuclear matter and neutron star structure
We present results for neutron star models constructed with a new equation of
state for nuclear matter at zero temperature. The ground state is computed
using the Auxiliary Field Diffusion Monte Carlo (AFDMC) technique, with
nucleons interacting via a semi-phenomenological Hamiltonian including a
realistic two-body interaction. The effect of many-body forces is included by
means of additional density-dependent terms in the Hamiltonian. In this letter
we compare the properties of the resulting neutron-star models with those
obtained using other nuclear Hamiltonians, focusing on the relations between
mass and radius, and between the gravitational mass and the baryon number.Comment: modified version with a slightly different Hamiltonian and
parametrization of the EO
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