1,076 research outputs found
Exploring small energy scales with x-ray absorption and dichroism
Soft x-ray linear and circular dichroism (XLD, XMCD) experiments at the Ce
M edges are being used to determine the energy scales characterizing
the Ce degrees of freedom in the ultrathin ordered surface intermetallic
CeAg/Ag(111). We find that all relevant interactions, i. e. Kondo
scattering, crystal field splitting and magnetic exchange coupling occur on
small scales. Our study demonstrates the usefulness of combining x-ray
absorption experiments probing linear and circular dichroism owing to their
strong sensitivity for anisotropies in both charge distribution and
paramagnetic response, respectively.Comment: 5 pages, 4 figure
Shell-Model Monte Carlo Simulations of BCS-BEC Crossover in Few-Fermion Systems
We study a trapped system of fermions with a zero-range two-body interaction
using the shell-model Monte Carlo method, providing {\em ab initio} results for
the low particle number limit where mean-field theory is not applicable. We
present results for the -body energies as function of interaction strength,
particle number, and temperature. The subtle question of renormalization in a
finite model space is addressed and the convergence of our method and its
applicability across the BCS-BEC crossover is discussed. Our findings indicate
that very good quantitative results can be obtained on the BCS side, whereas at
unitarity and in the BEC regime the convergence is less clear. Comparison to
N=2 analytics at zero and finite temperature, and to other calculations in the
literature for show very good agreement.Comment: 6 pages, 5 figures, Revtex4, final versio
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
Reconstruction of cloud geometry using a scanning cloud radar
Clouds are one of the main reasons of uncertainties in the forecasts of weather and climate. In part, this is due to limitations of remote sensing of cloud microphysics. Present approaches often use passive spectral measurements for the remote sensing of cloud microphysical parameters. Large uncertainties are introduced by three-dimensional (3-D) radiative transfer effects and cloud inhomogeneities. Such effects are largely caused by unknown orientation of cloud sides or by shadowed areas on the cloud. Passive ground-based remote sensing of cloud properties at high spatial resolution could be crucially improved with this kind of additional knowledge of cloud geometry. To this end, a method for the accurate reconstruction of 3-D cloud geometry from cloud radar measurements is developed in this work. Using a radar simulator and simulated passive measurements of model clouds based on a large eddy simulation (LES),the effects of different radar scan resolutions and varying interpolation methods are evaluated. In reality, a trade-off between scan resolution and scan duration has to be found as clouds change quickly. A reasonable choice is a scan resolution of 1 to 2\degree. The most suitable interpolation procedure identified is the barycentric interpolation method. The 3-D reconstruction method is demonstrated using radar scans of convective cloud cases with the Munich miraMACS, a 35 GHz scanning cloud radar. As a successful proof of concept, camera imagery collected at the radar location is reproduced for the observed cloud cases via 3-D volume reconstruction and 3-D radiative transfer simulation. Data sets provided by the presented reconstruction method will aid passive spectral ground-based measurements of cloud sides to retrieve microphysical parameters
Reconstruction of cloud geometry using a scanning cloud radar
Clouds are one of the main reasons of uncertainties in the forecasts of weather and climate. In part, this is due to limitations of remote sensing of cloud microphysics. Present approaches often use passive spectral measurements for the remote sensing of cloud microphysical parameters. Large uncertainties are introduced by three-dimensional (3-D) radiative transfer effects and cloud inhomogeneities. Such effects are largely caused by unknown orientation of cloud sides or by shadowed areas on the cloud. Passive ground-based remote sensing of cloud properties at high spatial resolution could be crucially improved with this kind of additional knowledge of cloud geometry. To this end, a method for the accurate reconstruction of 3-D cloud geometry from cloud radar measurements is developed in this work. Using a radar simulator and simulated passive measurements of model clouds based on a large eddy simulation (LES),the effects of different radar scan resolutions and varying interpolation methods are evaluated. In reality, a trade-off between scan resolution and scan duration has to be found as clouds change quickly. A reasonable choice is a scan resolution of 1 to 2\degree. The most suitable interpolation procedure identified is the barycentric interpolation method. The 3-D reconstruction method is demonstrated using radar scans of convective cloud cases with the Munich miraMACS, a 35 GHz scanning cloud radar. As a successful proof of concept, camera imagery collected at the radar location is reproduced for the observed cloud cases via 3-D volume reconstruction and 3-D radiative transfer simulation. Data sets provided by the presented reconstruction method will aid passive spectral ground-based measurements of cloud sides to retrieve microphysical parameters
Thomas-Fermi Approximation for a Condensate with Higher-order Interactions
We consider the ground state of a harmonically trapped Bose-Einstein
condensate within the Gross-Pitaevskii theory including the effective-range
corrections for a two-body zero-range potential. The resulting non-linear
Schr\"odinger equation is solved analytically in the Thomas-Fermi approximation
neglecting the kinetic energy term. We present results for the chemical
potential and the condensate profiles, discuss boundary conditions, and compare
to the usual Thomas-Fermi approach. We discuss several ways to increase the
influence of effective-range corrections in experiment with magnetically
tunable interactions. The level of tuning required could be inside experimental
reach in the near future.Comment: 8 pages, RevTex4 format, 5 figure
Carbon-rich presolar grains from massive stars : subsolar ¹²C/¹³C and ¹⁴N/¹⁵N ratios and the mystery of ¹⁵N
Carbon-rich grains with isotopic anomalies compared to the Sun are found in primitive meteorites. They were made by stars, and carry the original stellar nucleosynthesis signature. Silicon carbide grains of Type X and C and low-density (LD) graphites condensed in the ejecta of core-collapse supernovae. We present a new set of models for the explosive He shell and compare them with the grains showing ¹²C/¹³C and ¹⁴N/¹⁵N ratios lower than solar. In the stellar progenitor H was ingested into the He shell and not fully destroyed before the explosion. Different explosion energies and H concentrations are considered. If the supernova shock hits the He-shell region with some H still present, the models can reproduce the C and N isotopic signatures in C-rich grains. Hot-CNO cycle isotopic signatures are obtained, including a large production of ¹³C and ¹⁵N. The short-lived radionuclides ²²Na and ²⁶Al are increased by orders of magnitude. The production of radiogenic ²²Ne from the decay of ²²Na in the He shell might solve the puzzle of the Ne-E(L) component in LD graphite grains. This scenario is attractive for the SiC grains of type AB with ¹⁴N/¹⁵N ratios lower than solar, and provides an alternative solution for SiC grains originally classified as nova grains. Finally, this process may contribute to the production of ¹⁴N and ¹⁵N in the Galaxy, helping to produce the ¹⁴N/¹⁵N ratio in the solar system
Few-Body Bound Complexes in One-dimensional Dipolar Gases and Non-Destructive Optical Detection
We consider dipolar interactions between heteronuclear molecules in
low-dimensional geometries. The setup consists of two one-dimensional tubes. We
study the stability of possible few-body complexes in the regime of repulsive
intratube interaction, where the binding arises from intertube attraction. The
stable dimers, trimers, and tetramers are found and we discuss their properties
for both bosonic and fermionic molecules. To observe these complexes we propose
an optical non-destructive detection scheme that enables in-situ observation of
the creation and dissociation of the few-body complexes. A detailed description
of the expected signal of such measurements is given using the numerically
calculated wave functions of the bound states. We also discuss implications on
the many-body physics of dipolar systems in tubular geometries, as well as
experimental issues related to the external harmonic confinement along the tube
and the prospect of applying an in-tube optical lattice to increase the
effective dipole strength.Comment: 16 pages, 15 figures, published versio
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