28,770 research outputs found
Constraining neutrino physics with BBN and CMBR
We perform a likelihood analysis of the recent results on the anisotropy of
Cosmic Microwave Background Radiation from the BOOMERanG and DASI experiments
to show that they single out an effective number of neutrinos in good agreement
with standard Big Bang Nucleosynthesis. We also consider degenerate Big Bang
Nucleosynthesis to provide new bounds on effective relativistic degrees of
freedom and, in particular, on neutrino chemical potential
. When including Supernova Ia data we find, at , and , .Comment: 6 pages, 3 figures, some reference adde
Quantum Degenerate Mixture of Ytterbium and Lithium Atoms
We have produced a quantum degenerate mixture of fermionic alkali 6Li and
bosonic spin-singlet 174Yb gases. This was achieved using sympathetic cooling
of lithium atoms by evaporatively cooled ytterbium atoms in a far-off-resonant
optical dipole trap. We observe co-existence of Bose condensed (T/T_c~0.8)
174Yb with 2.3*10^4 atoms and Fermi degenerate (T/T_F~0.3) 6Li with 1.2*10^4
atoms. Quasipure Bose-Einstein condensates of up to 3*10^4 174Yb atoms can be
produced in single-species experiments. Our results mark a significant step
toward studies of few and many-body physics with mixtures of alkali and
alkaline-earth-like atoms, and for the production of paramagnetic polar
molecules in the quantum regime. Our methods also establish a convenient scheme
for producing quantum degenerate ytterbium atoms in a 1064nm optical dipole
trap.Comment: 4 pages, 3 figure
Suppression of hole-hole scattering in GaAs/AlGaAs heterostructures under uniaxial compression
Resistance, magnetoresistance and their temperature dependencies have been
investigated in the 2D hole gas at a [001] p-GaAs/AlGaAs
heterointerface under [110] uniaxial compression. Analysis performed in the
frame of hole-hole scattering between carriers in the two spin splitted
subbands of the ground heavy hole state indicates, that h-h scattering is
strongly suppressed by uniaxial compression. The decay time of the
relative momentum reveals 4.5 times increase at a uniaxial compression of 1.3
kbar.Comment: 5 pages, 3 figures. submitted to Phys.Rev.
A reduced coupled-mode description for the electron-ion energy relaxation in dense matter
We present a simplified model for the electron-ion energy relaxation in dense two-temperature systems that includes the effects of coupled collective modes. It also extends the standard Spitzer result to both degenerate and strongly coupled systems. Starting from the general coupled-mode description, we are able to solve analytically for the temperature relaxation time in warm dense matter and strongly coupled plasmas. This was achieved by decoupling the electron-ion dynamics and by representing the ion response in terms of the mode frequencies. The presented reduced model allows for a fast description of temperature equilibration within hydrodynamic simulations and an easy comparison for experimental investigations. For warm dense matter, both fluid and solid, the model gives a slower electron-ion equilibration than predicted by the classical Spitzer result
Sympathetic cooling in an optically trapped mixture of alkali and spin-singlet atoms
We report on the realization of a stable mixture of ultracold lithium and
ytterbium atoms confined in a far-off-resonance optical dipole trap. We observe
sympathetic cooling of 6Li by 174Yb and extract the s-wave scattering length
magnitude |a6Li-174Yb| = (13 \pm 3)a0 from the rate of inter-species
thermalization. Using forced evaporative cooling of 174Yb, we achieve reduction
of the 6Li temperature to below the Fermi temperature, purely through
inter-species sympathetic cooling.Comment: 4 pages, 3 figure
Some properties of two Nambu--Jona-Lasinio -type models with inputs from lattice QCD
We investigate the phase diagram of the so-called
Polyakov--Nambu--Jona-Lasinio (PNJL) model at finite temperature and nonzero
chemical potential. The calculations are performed in the light and strange
quark sectors (, , ), which includes the 't Hooft instanton induced
interaction term that breaks the axial symmetry, and the quarks are coupled to
the (spatially constant) temporal background gauge field. On one hand, a
special attention is payed to the critical end point (CEP). The strength of the
flavor-mixing interaction alters the CEP location, since when it becomes weaker
the CEP moves to low temperatures and can even disappear. On the other hand, we
also explore the connection between QCD, a nonlocal Nambu--Jona-Lasinio type
model and the Landau gauge gluon propagator. Possible links between the
quenched gluon propagator and low energy hadronic phenomenology are
investigated.Comment: Contribution to the International Meeting "Excited QCD", Peniche,
Portugal, 06 - 12 May 201
Connectivity and a Problem of Formal Geometry
Let be a product of weighted
projective spaces, and let be the diagonal of . We prove
an algebraization result for formal-rational functions on certain closed
subvarieties of along the intersection .Comment: 9 pages, to appear in the Proceedings volume "Experimental and
Theoretical Methods in Algebra, Geometry and Topology", series Springer
Proceedings in Mathematics & Statistic
Udryddelse af endemisk ektromeli i en kommerciel museavl ved hjaelp af hygiejniske og organisatoriske foranstaltninger
Friedmann cosmology with decaying vacuum density
Among the several proposals to solve the incompatibility between the observed
small value of the cosmological constant and the huge value obtained by quantum
field theories, we can find the idea of a decaying vacuum energy density,
leading from high values at early times of universe evolution to the small
value observed nowadays. In this paper we consider a variation law for the
vacuum density recently proposed by Schutzhold on the basis of quantum field
estimations in the curved, expanding background, characterized by a vacuum
density proportional to the Hubble parameter. We show that, in the context of
an isotropic and homogeneous, spatially flat model, the corresponding solutions
retain the well established features of the standard cosmology, and, in
addition, are in accordance with the observed cosmological parameters. Our
scenario presents an initial phase dominated by radiation, followed by a dust
era long enough to permit structure formation, and by an epoch dominated by the
cosmological term, which tends asymptotically to a de Sitter universe. Taking
the matter density equals to half of the vacuum energy density, as suggested by
observation, we obtain a universe age given by Ht = 1.1, and a decelerating
parameter equals to -1/2.Comment: Accepted for publication in General Relativity and Gravitatio
- …