918 research outputs found
"Cherenkov radiation" of a sound in a Bose-condensed gas
In terms of linearized Gross-Pitaevskii equation we have studied the process
of sound emission arises from a supersonic particle motion in a Bose-condensed
gas. By analogy with the method used for description of Vavilov-Cherenkov
phenomenon, we have found a friction work created by the particle generated
condensate polarization. For comparison we have found radiation intensity of
excitations. Both methods gives the same result
Physical Characterization of Warm Spitzer-observed Near-Earth Objects
Near-infrared spectroscopy of Near-Earth Objects (NEOs) connects diagnostic
spectral features to specific surface mineralogies. The combination of
spectroscopy with albedos and diameters derived from thermal infrared
observations can increase the scientific return beyond that of the individual
datasets. To that end, we have completed a spectroscopic observing campaign to
complement the ExploreNEOs Warm Spitzer program that obtained albedos and
diameters of nearly 600 NEOs (Trilling et al. 2010). Here we present the
results of observations using the low-resolution prism mode (~0.7-2.5 microns)
of the SpeX instrument on the NASA Infrared Telescope Facility (IRTF). We also
include near-infrared observations of ExploreNEOs targets from the MIT-UH-IRTF
Joint Campaign for Spectral Reconnaissance. Our dataset includes near-infrared
spectra of 187 ExploreNEOs targets (125 observations of 92 objects from our
survey and 213 observations of 154 objects from the MIT survey). We identify a
taxonomic class for each spectrum and use band parameter analysis to
investigate the mineralogies for the S-, Q-, and V-complex objects. Our
analysis suggests that for spectra that contain near-infrared data but lack the
visible wavelength region, the Bus-DeMeo system misidentifies some S-types as
Q-types. We find no correlation between spectral band parameters and
ExploreNEOs albedos and diameters. We find slightly negative Band Area Ratio
(BAR) correlations with phase angle for Eros and Ivar, but a positive BAR
correlation with phase angle for Ganymed. We find evidence for spectral phase
reddening for Eros, Ganymed, and Ivar. We identify the likely ordinary
chondrite type analog for a subset of our sample. Our resulting proportions of
H, L, and LL ordinary chondrites differ from those calculated for meteorite
falls and in previous studies of ordinary chondrite-like NEOs.Comment: 6 Tables, 9 Figure
Manifestation of superfluidity in an evolving Bose-condensed gas
We study the generation of excitations due to an ''impurity''(static
perturbation) placed into an oscillating Bose-condensed gas in the
time-dependent trapping field. It is shown that there are two regions for the
position of the local perturbation. In the first region the condensate flows
around the ''impurity'' without generation of excitations demonstrating
superfluid properties. In the second region the creation of excitations occurs,
at least within a limited time interval, revealing destruction of
superfluidity. The phenomenon can be studied by measuring the damping of
condensate oscillations at different positions of the ''impurity''
Spin wave dispersion softening in the ferromagnetic Kondo lattice model for manganites
Spin dynamics is calculated in the ferromagnetic (FM) state of the
generalized Kondo lattice model taking into account strong on-site correlations
between e_g electrons and antiferromagnetic (AFM) exchange among t_{2g} spins.
Our study suggests that competing FM double-exchange and AFM super-exchange
interaction lead to a rather nontrivial spin-wave spectrum. While spin
excitations have a conventional Dq^2 spectrum in the long-wavelength limit,
there is a strong deviation from the spin-wave spectrum of the isotropic
Heisenberg model close to the zone boundary. The relevance of our results to
the experimental data are discussed.Comment: 6 RevTex pages, 3 embedded PostScript figure
Doping dependence of the exchange energies in bilayer manganites: Role of orbital degrees of freedom
Recently, an intriguing doping dependence of the exchange energies in the
bilayer manganites has been observed in the neutron
scattering experiments. The intra-layer exchange only weakly changed with
doping while the inter-layer one drastically decreased. Here we propose a
theory which accounts for these experimental findings. We argue, that the
observed striking doping dependence of the exchange energies can be attributed
to the evaluation of the orbital level splitting with doping. The latter is
handled by the interplay between Jahn-Teller effect (supporting an axial
orbital) and the orbital anisotropy of the electronic band in the bilayer
structure (promoting an in-plane orbital), which is monitored by the Coulomb
repulsion. The presented theory, while being a mean-field type, describes well
the experimental data and also gives the estimates of the several interesting
energy scales involved in the problem.Comment: Added references, corrected typos. To appear in Phys. Rev.
Spin wave dispersion softening in the ferromagnetic Kondo lattice model for manganites
Spin dynamics is calculated in the ferromagnetic (FM) state of the
generalized Kondo lattice model taking into account strong on-site correlations
between e_g electrons and antiferromagnetic (AFM) exchange among t_{2g} spins.
Our study suggests that competing FM double-exchange and AFM super-exchange
interaction lead to a rather nontrivial spin-wave spectrum. While spin
excitations have a conventional Dq^2 spectrum in the long-wavelength limit,
there is a strong deviation from the spin-wave spectrum of the isotropic
Heisenberg model close to the zone boundary. The relevance of our results to
the experimental data are discussed.Comment: 6 RevTex pages, 3 embedded PostScript figure
- …