5,006 research outputs found
Sensitivities and correlations of nuclear structure observables emerging from chiral interactions
Starting from a set of different two- and three-nucleon interactions from
chiral effective field theory, we use the importance-truncated no-core shell
model for ab initio calculations of excitation energies as well as electric
quadrupole (E2) and magnetic dipole (M1) moments and transition strengths for
selected p-shell nuclei. We explore the sensitivity of the excitation energies
to the chiral interactions as a first step towards and systematic uncertainty
propagation from chiral inputs to nuclear structure observables. The
uncertainty band spanned by the different chiral interactions is typically in
agreement with experimental excitation energies, but we also identify
observables with notable discrepancies beyond the theoretical uncertainty that
reveal insufficiencies in the chiral interactions. For electromagnetic
observables we identify correlations among pairs of E2 or M1 observables based
on the ab initio calculations for the different interactions. We find extremely
robust correlations for E2 observables and illustrate how these correlations
can be used to predict one observable based on an experimental datum for the
second observable. In this way we circumvent convergence issues and arrive at
far more accurate results than any direct ab initio calculation. A prime
example for this approach is the quadrupole moment of the first 2^+ state in
C-12, which is predicted with an drastically improved accuracy.Comment: 11 pages, 8 figure
Nuclear Structure in the Framework of the Unitary Correlation Operator Method
Correlations play a crucial role in the nuclear many-body problem. We give an
overview of recent developments in nuclear structure theory aiming at the
description of these interaction-induced correlations by unitary
transformations. We focus on the Unitary Correlation Operator Method (UCOM),
which offers a very intuitive, universal and robust approach for the treatment
of short-range correlations. We discuss the UCOM formalism in detail and
highlight the connections to other methods for the description of short-range
correlations and the construction of effective interactions. In particular, we
juxtapose UCOM with the Similarity Renormalization Group (SRG) approach, which
implements the unitary transformation of the Hamiltonian through a very
flexible flow-equation formulation. The UCOM- and SRG-transformed interactions
are compared on the level of matrix elements and in many-body calculations
within the no-core shell model and with Hartree-Fock plus perturbation theory
for a variety of nuclei and observables. These calculations provide a detailed
picture of the similarities and differences as well as the advantages and
limitations of unitary transformation methods.Comment: 72 pages, 31 figure
Open-Shell Nuclei and Excited States from Multi-Reference Normal-Ordered Hamiltonians
We discuss the approximate inclusion of three-nucleon interactions into ab
initio nuclear structure calculations using a multi-reference formulation of
normal ordering and Wick's theorem. Following the successful application of
single-reference normal ordering for the study of ground states of closed-shell
nuclei, e.g., in coupled-cluster theory, multi-reference normal ordering opens
a path to open-shell nuclei and excited states. Based on different
multi-determinantal reference states we benchmark the truncation of the
normal-ordered Hamiltonian at the two-body level in no-core shell-model
calculations for p-shell nuclei, including 6-Li, 12-C, and 10-B. We find that
this multi-reference normal-ordered two-body approximation is able to capture
the effects of the 3N interaction with sufficient accuracy, both, for
ground-state and excitation energies, at the computational cost of a two-body
Hamiltonian. It is robust with respect to the choice of reference states and
has a multitude of applications in ab initio nuclear structure calculations of
open-shell nuclei and their excitations as well as in nuclear reaction studies.Comment: 6 pages, 4 figures, v2: update to published versio
Phase Diagram of Bosons in Two-Color Superlattices from Experimental Parameters
We study the zero-temperature phase diagram of a gas of bosonic 87-Rb atoms
in two-color superlattice potentials starting directly from the experimental
parameters, such as wavelengths and intensities of the two lasers generating
the superlattice. In a first step, we map the experimental setup to a
Bose-Hubbard Hamiltonian with site-dependent parameters through explicit
band-structure calculations. In the second step, we solve the many-body problem
using the density-matrix renormalization group (DMRG) approach and compute
observables such as energy gap, condensate fraction, maximum number
fluctuations and visibility of interference fringes. We study the phase diagram
as function of the laser intensities s_2 and s_1 as control parameters and show
that all relevant quantum phases, i.e. superfluid, Mott-insulator, and quasi
Bose-glass phase, and the transitions between them can be investigated through
a variation of these intensities alone.Comment: 4 pages, 3 figure
Nuclear Structure - "ab initio"
An ab-initio description of atomic nuclei that solves the nuclear many-body
problem for realistic nuclear forces is expected to possess a high degree of
predictive power. In this contribution we treat the main obstacle, namely the
short-ranged repulsive and tensor correlations induced by the realistic
nucleon-nucleon interaction, by means of a unitary correlation operator. This
correlator applied to uncorrelated many-body states imprints short-ranged
correlations that cannot be described by product states. When applied to an
observable it induces the correlations into the operator, creating for example
a correlated Hamiltonian suited for Slater determinants. Adding to the
correlated realistic interaction a correction for three-body effects,
consisting of a momentum-dependent central and spin-orbit two-body potential we
obtain an effective interaction that is successfully used for all nuclei up to
mass 60. Various results are shown.Comment: 9 pages, Invited talk and poster at the international symposium "A
New Era of Nuclear Structure Physics" (NENS03), Niigata, Japan, Nov. 19-22,
200
Spectra of Open-Shell Nuclei with Pad\'e-Resummed Degenerate Perturbation Theory
We apply degenerate many-body perturbation theory at high orders for the
ab-initio description of ground states and excitation spectra of open-shell
nuclei using soft realistic nucleon-nucleon interactions. We derive a recursive
formulation of standard degenerate many-body perturbation theory that enables
us to evaluate order-by-order perturbative energy and state corrections up to
the 30th order. We study 6,7-Li as test cases using a similarity
renormalization group (SRG) evolved nucleon-nucleon interaction from chiral
effective field theory. The simple perturbation series exhibits a strong, often
oscillatory divergence, as was observed previously for ground states of
closed-shell nuclei. Even for very soft interactions resulting from SRG
evolutions up to large flow parameter, i.e. low momentum scales, the
perturbation series still diverges. However, a resummation of the perturbation
series via Pad\'e approximants yields very stable and converged ground and
excited-state energies in very good agreement with exact no-core shell-model
calculations for the same model space.Comment: 8 pages, 4 figures; minor changes to match published versio
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