7,599 research outputs found
Magnetic phase transition in coherently coupled Bose gases in optical lattices
We describe the ground state of a gas of bosonic atoms with two coherently
coupled internal levels in a deep optical lattice in a one dimensional
geometry. In the single-band approximation this system is described by a
Bose-Hubbard Hamiltonian. The system has a superfluid and a Mott insulating
phase which can be either paramagnetic or ferromagnetic. We characterize the
quantum phase transitions at unit filling by means of a density matrix
renormalization group technique and compare it with a mean-field approach. The
presence of the ferromagnetic Ising-like transition modifies the Mott lobes. In
the Mott insulating region the system maps to the ferromagnetic spin-1/2 XXZ
model in a transverse field and the numerical results compare very well with
the analytical results obtained from the spin model. In the superfluid regime
quantum fluctuations strongly modify the phase transition with respect to the
well established mean-field three dimensional classical bifurcation.Comment: 6 pages, 3 figure
Instability of the superfluid flow as black-hole lasing effect
We show that the instability leading to the decay of the one-dimensional
superfluid flow through a penetrable barrier are due to the black-hole lasing
effect. This dynamical instability is triggered by modes resonating in an
effective cavity formed by two horizons enclosing the barrier. The location of
the horizons is set by , with being the local fluid
velocity and sound speed, respectively. We compute the critical velocity
analytically and show that it is univocally determined by the horizons
configuration. In the limit of broad barriers, the continuous spectrum at the
origin of the Hawking-like radiation and of the Landau energetic instability is
recovered.Comment: 18 pages, 3 figure
A New Technique for Screening Iron-Efficient Genotypes in Peach Rootstocks: Elicitation of Root Ferric Chelate Reductase by Manipulation of External Iron Concentrations
Non-Abelian dark matter and dark radiation
We propose a new class of dark matter models with unusual phenomenology. What
is ordinary about our models is that dark matter particles are WIMPs, they are
weakly coupled to the Standard Model and have weak scale masses. What is
unusual is that they come in multiplets of a new "dark" non-Abelian gauge group
with milli-weak coupling. The massless dark gluons of this dark gauge group
contribute to the energy density of the universe as a form of weakly
self-interacting dark radiation. In this paper we explore the consequences of
having i.) dark matter in multiplets ii.) self-interacting dark radiation and
iii.) dark matter which is weakly coupled to dark radiation. We find that i.)
dark matter cross sections are modified by multiplicity factors which have
significant consequences for collider searches and indirect detection, ii.)
dark gluons have thermal abundances which affect the CMB as dark radiation.
Unlike additional massless neutrino species the dark gluons are interacting and
have vanishing viscosity and iii.) the coupling of dark radiation to dark
matter represents a new mechanism for damping the large scale structure power
spectrum. A combination of additional radiation and slightly damped structure
is interesting because it can remove tensions between global CDM fits
from the CMB and direct measurements of the Hubble expansion rate () and
large scale structure ().Comment: 25 pages, 8 figures; v2: minor improvements, references added; v3:
added references and an acknowledgement note to J. Lesgourgues; accepted for
publication in PR
- …