175 research outputs found
Fermionization of two-component few-fermion systems in a one-dimensional harmonic trap
The nature of strongly interacting Fermi gases and magnetism is one of the
most important and studied topics in condensed-matter physics. Still, there are
many open questions. A central issue is under what circumstances strong
short-range repulsive interactions are enough to drive magnetic correlations.
Recent progress in the field of cold atomic gases allows to address this
question in very clean systems where both particle numbers, interactions and
dimensionality can be tuned. Here we study fermionic few-body systems in a one
dimensional harmonic trap using a new rapidly converging effective-interaction
technique, plus a novel analytical approach. This allows us to calculate the
properties of a single spin-down atom interacting with a number of spin-up
particles, a case of much recent experimental interest. Our findings indicate
that, in the strongly interacting limit, spin-up and spin-down particles want
to separate in the trap, which we interpret as a microscopic precursor of
one-dimensional ferromagnetism in imbalanced systems. Our predictions are
directly addressable in current experiments on ultracold atomic few-body
systems.Comment: 12 pages, 6 figures, published version including two appendices on
our new numerical and analytical approac
Tunneling Theory for Tunable Open Quantum Systems of Ultracold Atoms in One-Dimensional Traps
The creation of tunable open quantum systems is becoming feasible in current
experiments with ultracold atoms in low-dimensional traps. In particular, the
high degree of experimental control over these systems allows detailed studies
of tunneling dynamics, e.g., as a function of the trapping geometry and the
interparticle interaction strength. In order to address this exciting
opportunity we present a theoretical framework for two-body tunneling based on
the rigged Hilbert space formulation. In this approach, bound, resonant and
scattering states are included on an equal footing, and we argue that the
coupling of all these components is vital for a correct description of the
relevant threshold phenomena. In particular, we study the tunneling mechanism
for two-body systems in one-dimensional traps and different interaction
regimes. We find a strong dominance of sequential tunneling of single particles
for repulsive and weakly attractive systems, while there is a signature of
correlated pair tunneling in the calculated many-particle flux for strongly
attractive interparticle interaction.Comment: To be published in Phys. Rev. A (Rapid Communication
Power counting in chiral effective field theory and nuclear binding
Chiral effective field theory (EFT), as originally proposed by
Weinberg, promises a theoretical connection between low-energy nuclear
interactions and quantum chromodynamics (QCD). However, the important property
of renormalization-group (RG) invariance is not fulfilled in current
implementations and its consequences for predicting atomic nuclei beyond two-
and three-nucleon systems has remained unknown. In this work we present a first
and systematic study of recent RG-invariant formulations of EFT and their
predictions for the binding energies and other observables of selected nuclear
systems with mass-numbers up to . Specifically, we have carried out ab
initio no-core shell-model and coupled cluster calculations of the ground-state
energy of H, He, Li, and O using several recent
power-counting (PC) schemes at leading order (LO) and next-to-leading order
(NLO), where the subleading interactions are treated in perturbation theory.
Our calculations indicate that RG-invariant and realistic predictions can be
obtained for nuclei with mass number . We find, however, that
O is either unbound with respect to the four -particle
threshold, or deformed, or both. Similarly, we find that the Li
ground-state resides above the -deuteron separation threshold. These
results are in stark contrast with experimental data and point to either
necessary fine-tuning of all relevant counterterms, or that current
state-of-the-art RG-invariant PC schemes at LO in EFT lack necessary
diagrams -- such as three-nucleon forces -- to realistically describe nuclei
with mass number .Comment: 18 pages, 12 figure
Converging sequences in the ab initio no-core shell model
We demonstrate the existence of multiple converging sequences in the ab
initio no-core shell model. By examining the underlying theory of effective
operators, we expose the physical foundations for the alternative pathways to
convergence. This leads us to propose a revised strategy for evaluating
effective interactions for -body calculations in restricted model spaces. We
suggest that this strategy is particularly useful for applications to nuclear
processes in which states of both parities are used simultaneously, such as for
transition rates. We demonstrate the utility of our strategy with large-scale
calculations in light nuclei
An optimized chiral nucleon-nucleon interaction at next-to-next-to-leading order
We optimize the nucleon-nucleon interaction from chiral effective field
theory at next-to-next- to-leading order. The resulting new chiral force
NNLOopt yields \chi^2 \approx 1 per degree of freedom for laboratory energies
below approximately 125 MeV. In the A = 3, 4 nucleon systems, the contributions
of three-nucleon forces are smaller than for previous parametrizations of
chiral interactions. We use NNLOopt to study properties of key nuclei and
neutron matter, and demonstrate that many aspects of nuclear structure can be
understood in terms of this nucleon-nucleon interaction, without explicitly
invoking three-nucleon forces.Comment: 6 pages, 4 figure
The 14C(n,g) cross section between 10 keV and 1 MeV
The neutron capture cross section of 14C is of relevance for several
nucleosynthesis scenarios such as inhomogeneous Big Bang models, neutron
induced CNO cycles, and neutrino driven wind models for the r process. The
14C(n,g) reaction is also important for the validation of the Coulomb
dissociation method, where the (n,g) cross section can be indirectly obtained
via the time-reversed process. So far, the example of 14C is the only case with
neutrons where both, direct measurement and indirect Coulomb dissociation, have
been applied. Unfortunately, the interpretation is obscured by discrepancies
between several experiments and theory. Therefore, we report on new direct
measurements of the 14C(n,g) reaction with neutron energies ranging from 20 to
800 keV
Effective-interaction approach to the many-boson problem
We show that the convergence behavior of the many-body numerical
diagonalization scheme for strongly interacting bosons in a trap can be
significantly improved by the Lee-Suzuki method adapted from nuclear physics:
One can construct an effective interaction that acts in a space much smaller
than the original Hilbert space. In particular for short-ranged forces and
strong correlations, the method offers a good estimate of the energy and the
excitation spectrum, at a computational cost several orders of magnitude
smaller than that required by the standard method.Comment: 5 pages, 4 figure
Systematics of 2+ states in C isotopes from the ab initio no-core shell model
We study low-lying states of even carbon isotopes in the range A = 10 - 20
within the large- scale no-core shell model (NCSM). Using several accurate
nucleon-nucleon (NN) as well as NN plus three-nucleon (NNN) interactions, we
calculate excitation energies of the lowest 2+ state, the electromagnetic B(E2;
2+1 -> 0+1) transition rates, the 2+1 quadrupole moments as well as se- lected
electromagnetic transitions among other states. Recent experimental campaigns
to measure 2+-state lifetimes indicate an interesting evolution of nuclear
structure that pose a challenge to reproduce theoretically from first
principles. Our calculations do not include any effective charges or other
fitting parameters. However, calculated results extrapolated to infinite model
spaces are also presented. The model-dependence of those results is discussed.
Overall, we find a good agree- ment with the experimentally observed trends,
although our extrapolated B(E2; 2+1 -> 0+1) value for 16C is lower compared to
the most recent measurements. Relative transition strengths from higher excited
states are investigated and the influence of NNN forces is discussed. In
particular for 16C we find a remarkable sensitivity of the transition rates
from higher excited states to the details of the nuclear interactions.Comment: 22 pages, 8 figures, preprint version. Accepted for publication in
Journal of Physics G: Nuclear and Particle Physic
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