5,395 research outputs found
Construction of Wannier functions from localized atomic-like orbitals
The problem of construction of the Wannier functions (WFs) in a restricted
Hilbert space of eigenstates of the one-electron Hamiltonian (forming
the so-called low-energy part of the spectrum) can be formulated in several
different ways. One possibility is to use the projector-operator techniques,
which pick up a set of trial atomic orbitals and project them onto the given
Hilbert space. Another possibility is to employ the downfolding method, which
eliminates the high-energy part of the spectrum and incorporates all related to
it properties into the energy-dependence of an effective Hamiltonian. We show
that by modifying the high-energy part of the spectrum of the original
Hamiltonian , which is rather irrelevant to the construction of WFs in
the low-energy part of the spectrum, these two methods can be formulated in an
absolutely exact and identical form, so that the main difference between them
is reduced to the choice of the trial orbitals. Concerning the latter part of
the problem, we argue that an optimal choice for trial orbitals can be based on
the maximization of the site-diagonal part of the density matrix. The main idea
is illustrated for a simple toy model, consisting of only two bands, as well as
for a more realistic example of bands in VO. An analogy with
the search of the ground state of a many-electron system is also discussed.Comment: 13 pages, 6 figure
Laser cooling of a trapped two-component Fermi gas
The collective Raman cooling of a trapped two-component Fermi gas is
analyzed. We develop the quantum master equation that describes the collisions
and the laser cooling, in the festina lente regime, where the heating due to
photon reabsorption can be neglected. The numerical results based on Monte
Carlo simulations show, that three-dimensional temperatures of the order of
0.008 T_F can be achieved. We analyze the heating related to the background
losses, and conclude that our laser-cooling scheme can maintain the temperature
of the gas without significant additional losses. Finally we derive an analytic
expression for the temperature of a trapped Fermi gas heated by background
collisions, that agrees very well with the data obtained from the numerical
simulation.Comment: 5 pages, 3 figure
On The Existence of Roton Excitations in Bose Einstein Condensates: Signature of Proximity to a Mott Insulating Phase
Within the last decade, artificially engineered Bose Einstein Condensation
has been achieved in atomic systems. Bose Einstein Condensates are superfluids
just like bosonic Helium is and all interacting bosonic fluids are expected to
be at low enough temperatures. One difference between the two systems is that
superfluid Helium exhibits roton excitations while Bose Einstein Condensates
have never been observed to have such excitations. The reason for the roton
minimum in Helium is its proximity to a solid phase. The roton minimum is a
consequence of enhanced density fluctuations at the reciprocal lattice vector
of the stillborn solid. Bose Einstein Condensates in atomic traps are not near
a solid phase and therefore do not exhibit roton minimum. We conclude that if
Bose Einstein Condensates in an optical lattice are tuned near a transition to
a Mott insulating phase, a roton minimum will develop at a reciprocal lattice
vector of the lattice. Equivalently, a peak in the structure factor will appear
at such a wavevector. The smallness of the roton gap or the largeness of the
structure factor peak are experimental signatures of the proximity to the Mott
transition.Comment: 4 pages, 5 figure
Rotating perfect fluid sources of the NUT metric
Locally rotationally symmetric perfect fluid solutions of Einstein's
gravitational equations are matched along the hypersurface of vanishing
pressure with the NUT metric. These rigidly rotating fluids are interpreted as
sources for the vacuum exterior which consists only of a stationary region of
the Taub-NUT space-time. The solution of the matching conditions leaves
generally three parameters in the global solution. Examples of perfect fluid
sources are discussed.Comment: 8 pages, late
The PLATO Dome A Site-Testing Observatory : instrumentation and first results
The PLATeau Observatory (PLATO) is an automated self-powered astrophysical observatory that was deployed to Dome A, the highest point on the Antarctic plateau, in 2008 January. PLATO consists of a suite of site-testing instruments designed to quantify the benefits of the Dome A site for astronomy, and science instruments designed to take advantage of the unique observing conditions. Instruments include CSTAR, an array of optical telescopes for transient astronomy; Gattini, an instrument to measure the optical sky brightness and cloud cover statistics; DASLE, an experiment to measure the statistics of the meteorological conditions within the near-surface layer; Pre-HEAT, a submillimeter tipping radiometer measuring the atmospheric transmission and water vapor content and performing spectral line imaging of the Galactic plane; and Snodar, an acoustic radar designed to measure turbulence within the near-surface layer. PLATO has run completely unattended and collected data throughout the winter 2008 season. Here we present a detailed description of the PLATO instrument suite and preliminary results obtained from the first season of operation
An Atom Laser with a cw Output Coupler
We demonstrate a continuous output coupler for magnetically trapped atoms.
Over a period of up to 100 ms a collimated and monoenergetic beam of atoms is
continuously extracted from a Bose- Einstein condensate. The intensity and
kinetic energy of the output beam of this atom laser are controlled by a weak
rf-field that induces spin flips between trapped and untrapped states.
Furthermore, the output coupler is used to perform a spectroscopic measurement
of the condensate, which reveals the spatial distribution of the magnetically
trapped condensate and allows manipulation of the condensate on a micrometer
scale.Comment: 4 pages, 4 figure
The origin of GEMS in IDPs as deduced from microstructural evolution of amorphous silicates with annealing
We present laboratory studies of the micro-structural evolution of an
amorphous ferro-magnesian silicate, of olivine composition, following thermal
annealing under vacuum. Annealing under vacuum was performed at temperatures
ranging from 870 to 1020 K. After annealing spheroidal metallic nano-particles
(2-50 nm) are found within the silicate films. We interpret this microstructure
in terms of a reduction of the initial amorphous silicate FeO component,
because of the carbon-rich partial pressure in the furnace due to pumping
mechanism. Annealing in a controlled oxygen-rich atmosphere confirms this
interpretation. The observed microstructures closely resemble those of the GEMS
(Glass with Embedded Metal and Sulphides) found in chondritic IDPs
(Interplanetary Dust Particles). Since IDPs contain abundant carbonaceous
matter, a solid-state reduction reaction may have occurred during heating in
the hot inner regions of the proto-solar disc. Related to this, the presence of
forsterite grains grown from the amorphous precursor material clearly
demonstrates that condensation from gaseous species is not required to explain
the occurrence of forsterite around young protostars and in comets. Forsterite
grains in these environments can be formed directly in the solid phase by
thermal annealing of amorphous ferro-magnesian silicates under reducing
conditions.Comment: 4 pages, 2 figures. Accepted for publication A&A Letter to the Edito
A method for collective excitation of Bose-Einstein condensate
It is shown that by an appropriate modification of the trapping potential one
may create collective excitation in cold atom Bose-Einstein condensate. The
proposed method is complementary to earlier suggestions. It seems to be
feasible experimentally --- it requires only a proper change in time of the
potential in atomic traps, as realized in laboratories already.Comment: 4 pages, 4 figures; major revision, several references added,
interacting particles case adde
Search for the Neutron Decay n X+ where X is a dark matter particle
In a recent paper submitted to Physical Review Letters, Fornal and Grinstein
have suggested that the discrepancy between two different methods of neutron
lifetime measurements, the beam and bottle methods can be explained by a
previously unobserved dark matter decay mode, n X+ where X
is a dark matter particle. We have performed a search for this decay mode over
the allowed range of energies of the monoenergetic gamma ray for X to be a dark
matter particle. We exclude the possibility of a sufficiently strong branch to
explain the lifetime discrepancy with greater than 4 sigma confidence.Comment: 6 pages 3 figure
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