41,670 research outputs found
Quantum Hall fractions in ultracold atomic vapors
Atomic vapors can be prepared and manipulated at very low densities and
temperatures. When they are rotating, they can reach a quantum Hall regime in
which there should be manifestations of the fractional quantum Hall effect. We
discuss the appearance of the principal sequence of fractions nu =p/(p+- 1) for
bosonic atoms. The termination point of this series is the paired Moore-Read
Pfaffian state. Exotic states fill the gap between the paired state and the
vortex lattice expected at high filling of the lowest Landau level. In
fermionic vapors, the p-wave scattering typical of ultralow energy collisions
leads to the hard-core model when restricted to the lowest Landau level.Comment: 7 pages, 2 figs, brief review submitted to Modern Physics Letters
Parafermionic states in rotating Bose-Einstein condensates
We investigate possible parafermionic states in rapidly rotating ultracold
bosonic atomic gases at lowest Landau level filling factor nu=k/2. We study how
the system size and interactions act upon the overlap between the true ground
state and a candidate Read-Rezayi state. We also consider the quasihole states
which are expected to display non-Abelian statistics. We numerically evaluate
the degeneracy of these states and show agreement with a formula given by E.
Ardonne. We compute the overlaps between low-lying exact eigenstates and
quasihole candidate wavefunctions. We discuss the validity of the parafermion
description as a function of the filling factor.Comment: 23 pages, 10 figure
From interstellar abundances to grain composition: the major dust constituents Mg, Si and Fe
We analyse observational correlations for three elements entering into the
composition of interstellar silicate and oxide grains. Using current solar
abundances (Asplund et al. 2009), we convert the gas-phase abundances into
dust-phase abundances for 196 sightlines. We deduce a sharp difference in
abundances for sightlines located at low (|b|<30\degr) and high
(|b|>30\degr) galactic latitudes. For high-latitude stars the ratios Mg/Si
and Fe/Si in dust are close to 1.5. For disk stars they are reduced to and . The derived numbers indicate that
1) the dust grains cannot be the mixture of silicates with olivine and pyroxene
composition only and some amount of magnesium or iron (or both) should be in
another population and 2) the destruction of Mg-rich grains in the warm medium
is more effective than of Fe-rich grains. We reveal a decrease of dust-phase
abundances and correspondingly an increase of gas-phase abundances with
distance for stars with D\ga 400\,pc. We attribute this fact to an
observational selection effect: a systematic trend toward smaller observed
hydrogen column density for distant stars. We find differences in abundances
for disk stars with low (E({\rm B-V}) \la 0.2) and high (E({\rm B-V}) \ga
0.2) reddenings which reflect the distinction between the sightlines passing
through diffuse and translucent interstellar clouds. For Scorpius-Ophiuchus we
detect an uniform increase of dust-phase abundances of Mg and Si with an
increase of the ratio of total to selective extinction and a
decrease of the strength of the far-UV extinction. This is the first evidence
for a growth of Mg-Si grains due to accretion in the interstellar medium.Comment: 16 pages, 16 figures, accepted for publication in Astronomy and
Astrophysic
Field-Induced Disorder Point in Non-Collinear Ising Spin Chains
We perform a theoretical study of a non-collinear Ising ferrimagnetic spin
chain inspired by the compound Co(hfac)2NITPhOMe. The basic building block of
its structure contains one Cobalt ion and one organic radical each with a spin
1/2. The exchange interaction is strongly anisotropic and the corresponding
axes of anisotropy have a period three helical structure. We introduce and
solve a model Hamiltonian for this spin chain. We show that the present
compound is very close to a so-called disorder point at which there is a
massive ground state degeneracy. We predict the equilibrium magnetization
process and discuss the impact of the degeneracy on the dynamical properties by
using arguments based on the Glauber dynamics.Comment: revtex 4, 10 pages, 7 figure
Pairing in ultracold Fermi gases in the lowest Landau level
We study a rapidly rotating gas of unpolarized spin-1/2 ultracold fermions in
the two-dimensional regime when all atoms reside in the lowest Landau level.
Due to the presence of the spin degree of freedom both s-wave and p-wave
interactions are allowed at ultralow temperatures. We investigate the phase
diagram of this system as a function of the filling factor in the lowest Landau
level and in terms of the ratio between s- and p-wave interaction strengths. We
show that the presence of attractive interactions induces a wide regime of
phase separation with formation of maximally compact droplets that are either
fully polarized or composed of spin-singlets. In the regime with no phase
separation, we give evidence for fractional quantum Hall states. Most notably,
we find two distinct singlet states at the filling nu =2/3 for different
interactions. One of these states is accounted for by the composite fermion
theory while the other one is a paired state for which we identify two
competing descriptions with different topological structure. This paired state
may be an Abelian liquid of composite spin-singlet Bose molecules with Laughlin
correlations. Alternatively, it may be a known non-Abelian paired state,
indicated by good overlaps with the corresponding trial wavefunction. By fine
tuning of the scattering lengths it is possible to create the non-Abelian
critical Haldane-Rezayi state for nu =1/2 and the permanent state of Moore and
Read for nu =1. For purely repulsive interactions, we also find evidence for a
gapped Halperin state at nu=2/5.Comment: 12 pages, 9 figs (best viewed in color), published version with
additional evidence for a non-Abelian spin singlet state at filling nu=2/
Improving the dynamical overlap algorithm
We present algorithmic improvements to the overlap Hybrid Monte Carlo
algorithm, including preconditioning techniques and improvements to the
correction step, used when one of the eigenvalues of the Kernel operator
changes sign, which is now O(\Delta t^2) exact.Comment: 6 pages, 3 figures; poster contribution at Lattice 2005(Algorithms
and machines
Effective medium theories for irregular fluffy structures: aggregation of small particles
We study the extinction efficiencies as well as scattering properties of
particles of different porosity. Calculations are performed for porous
pseudospheres with small size (Rayleigh) inclusions using the discrete dipole
approximation. Five refractive indices of materials covering the range from
to were selected. They correspond to biological
particles, dirty ice, silicate, amorphous carbon and soot in the visual part of
spectrum. We attempt to describe the optical properties of such particles using
Lorenz-Mie theory and a refractive index found from some effective medium
theory (EMT) assuming the particle is homogeneous. We refer to this as the
effective model.
It is found that the deviations are minimal when utilizing the EMT based on
the Bruggeman mixing rule. Usually the deviations in extinction factor do not
exceed for particle porosity and size parameters
x_{\rm porous} = 2 \pi r_{\rm s, porous}/\lambda \la 25. The deviations are
larger for scattering and absorption efficiencies and smaller for particle
albedo and asymmetry parameter. Our calculations made for spheroids confirm
these conclusions. Preliminary consideration shows that the effective model
represents the intensity and polarization of radiation scattered by fluffy
aggregates quite well. Thus, the effective models of spherical and
non-spherical particles can be used to significantly simplify computations of
the optical properties of aggregates containing only Rayleigh inclusions.Comment: 24 pages, 9 figures, accepted for publication in Applied Optic
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