90 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
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
The neutron polaron as a constraint on nuclear density functionals
We study the energy of an impurity (polaron) that interacts strongly in a sea
of fermions when the effective range of the impurity-fermion interaction
becomes important, thereby mapping the Fermi polaron of condensed matter
physics and ultracold atoms to strongly interacting neutrons. We present
Quantum Monte Carlo results for this neutron polaron, and compare these with
effective field theory calculations that also include contributions beyond the
effective range. We show that state-of-the-art nuclear density functionals vary
substantially and generally underestimate the neutron polaron energy. Our
results thus provide constraints for adjusting the time-odd components of
nuclear density functionals to better characterize polarized systems.Comment: 5 pages, 3 figures; v2 corresponds to the published versio
Energy spectrum and effective mass using a non-local 3-body interaction
We recently proposed a nonlocal form for the 3-body induced interaction that
is consistent with the Fock space representation of interaction operators but
leads to a fractional power dependence on the density. Here we examine the
implications of the nonlocality for the excitation spectrum. In the
two-component weakly interacting Fermi gas, we find that it gives an effective
mass that is comparable to the one in many-body perturbation theory. Applying
the interaction to nuclear matter, it predicts a large enhancement to the
effective mass. Since the saturation of nuclear matter is partly due to the
induced 3-body interaction, fitted functionals should treat the effective mass
as a free parameter, unless the two- and three-body contributions are
determined from basic theory.Comment: 7 pages, 1 figure; V2 has a table showing the 3-body energies for two
phenomenological energy-density functional
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
Chiral Three-Nucleon Interactions in Light Nuclei, Neutron- Scattering, and Neutron Matter
We present quantum Monte Carlo calculations of light nuclei, neutron-
scattering, and neutron matter using local two- and three-nucleon ()
interactions derived from chiral effective field theory up to
next-to-next-to-leading order (NLO). The two undetermined low-energy
couplings are fit to the He binding energy and, for the first time, to the
spin-orbit splitting in the neutron- -wave phase shifts.
Furthermore, we investigate different choices of local -operator structures
and find that chiral interactions at NLO are able to simultaneously
reproduce the properties of systems and of neutron matter, in
contrast to commonly used phenomenological interactions.Comment: 5 pages, 3 figures, 1 table - updated version: small wording changes,
one reference chang
Quantum Monte Carlo calculations of light nuclei with local chiral two- and three-nucleon interactions
Local chiral effective field theory interactions have recently been developed
and used in the context of quantum Monte Carlo few- and many-body methods for
nuclear physics. In this work, we go over detailed features of local chiral
nucleon-nucleon interactions and examine their effect on properties of the
deuteron, paying special attention to the perturbativeness of the expansion. We
then turn to three-nucleon interactions, focusing on operator ambiguities and
their interplay with regulator effects. We then discuss the nuclear Green's
function Monte Carlo method, going over both wave-function correlations and
approximations for the two- and three-body propagators. Following this, we
present a range of results on light nuclei: Binding energies and distribution
functions are contrasted and compared, starting from several different
microscopic interactions.Comment: 21 pages, 14 figures, published version, Editor's Suggestio
- …