38,539 research outputs found
The mass of the dark matter particle from theory and observations
We combine observed properties of galaxies as the core density and radius
with the theoretical linear evolution of density fluctuations computed from
first principles since the end of inflation till today. The halo radius r_0 is
computed in terms of cosmological parameters. The theoretical density profiles
rho(r)/rho(0) have an universal shape as a function of r/r_0 which reproduces
the observations. We show that the linear approximation to the Boltzmann-Vlasov
equation is valid for very large galaxies and correctly provides universal
quantities which are common to all galaxies, as the surface density and density
profile. By matching the theoretically computed surface density to its observed
value we obtain (i) the decreasing of the phase-space density during the MD era
(ii) the mass of the dark matter particle which turns to be between 1 and 2 keV
and the decoupling temperature T_d which turns to be above 100 GeV (iii) the
core vs. cusp discrimination: keV dark matter particles produce cored density
profiles while wimps (m \sim 100 GeV, T_d \sim 5 GeV) produce cusped profiles
at scales about 0.003 pc. These results are independent of the particle model
and vary very little with the statistics of the dark matter particle.
Non-universal galaxy quantities (which need to include non-linear effects as
mergers and baryons) are reproduced in the linear approximation up to a factor
of order one for the halo radius r_0, galaxy mass M_{gal}, halo central density
rho_{0} and velocity dispersion sqrt{{\bar {v^2}}_{halo}} in the limiting case
of large galaxies (both r_0 and M_{gal} large). This shows the power of the
linear approximation scheme: although it cannot capture the whole content of
the structure formation, it correctly provides universal quantities which as
well as the main non-universal galaxy properties.Comment: 17 pages, 15 figures, improved and expanded version to appear in New
Astronom
ESR study of the single-ion anisotropy in the pyrochlore antiferromagnet Gd2Sn2O7
Single-ion anisotropy is of importance for the magnetic ordering of the
frustrated pyrochlore antiferromagnets Gd2Ti2O7 and Gd2Sn2O7. The anisotropy
parameters for the Gd2Sn2O7 were measured using the electron spin resonance
(ESR) technique. The anisotropy was found to be of the easy plane type, with
the main constant D=140mK. This value is 35% smaller than the value of the
corresponding anisotropy constant in the related compound Gd2Ti2O7.Comment: 8 pages, 3 figure
Controlling the Spin Polarization of the Electron Current in a Semimagnetic Resonant-Tunneling Diode
The spin filtering effect of the electron current in a double-barrier
resonant-tunneling diode (RTD) consisting of ZnMnSe semimagnetic layers has
been studied theoretically. The influence of the distribution of the magnesium
ions on the coefficient of the spin polarization of the electron current has
been investigated. The dependence of the spin filtering degree of the electron
current on the external magnetic field and the bias voltage has been obtained.
The effect of the total spin polarization of the electron current has been
predicted. This effect is characterized by total suppression of the spin-up
component of electron current, that takes place when the Fermi level coincides
with the lowest Landau level for spin-up electrons in the RTD semimagnetic
emitter
Anderson Localization of Bogolyubov Quasiparticles in Interacting Bose-Einstein Condensates
We study the Anderson localization of Bogolyubov quasiparticles in an
interacting Bose-Einstein condensate (with healing length \xi) subjected to a
random potential (with finite correlation length \sigma_R). We derive
analytically the Lyapunov exponent as a function of the quasiparticle momentum
k and we study the localization maximum k_{max}. For 1D speckle potentials, we
find that k_{max} is proportional to 1/\xi when \xi is much larger than
\sigma_R while k_{max} is proportional to 1/\sigma_R when \xi is much smaller
than \sigma_R, and that the localization is strongest when \xi is of the order
of \sigma_R. Numerical calculations support our analysis and our estimates
indicate that the localization of the Bogolyubov quasiparticles is accessible
in current experiments with ultracold atoms.Comment: published version (no significant changes compared to last version
Localization of Bogoliubov quasiparticles in interacting Bose gases with correlated disorder
We study the Anderson localization of Bogoliubov quasiparticles (elementary
many-body excitations) in a weakly interacting Bose gas of chemical potential
subjected to a disordered potential . We introduce a general mapping
(valid for weak inhomogeneous potentials in any dimension) of the Bogoliubov-de
Gennes equations onto a single-particle Schr\"odinger-like equation with an
effective potential. For disordered potentials, the Schr\"odinger-like equation
accounts for the scattering and localization properties of the Bogoliubov
quasiparticles. We derive analytically the localization lengths for correlated
disordered potentials in the one-dimensional geometry. Our approach relies on a
perturbative expansion in , which we develop up to third order, and we
discuss the impact of the various perturbation orders. Our predictions are
shown to be in very good agreement with direct numerical calculations. We
identify different localization regimes: For low energy, the effective
disordered potential exhibits a strong screening by the quasicondensate density
background, and localization is suppressed. For high-energy excitations, the
effective disordered potential reduces to the bare disordered potential, and
the localization properties of quasiparticles are the same as for free
particles. The maximum of localization is found at intermediate energy when the
quasicondensate healing length is of the order of the disorder correlation
length. Possible extensions of our work to higher dimensions are also
discussed.Comment: Published versio
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