15,042 research outputs found
Resonance absolute quantum reflection at selected energies
The possibility of the resonance reflection (100 % at maximum) is revealed.
The corresponding exactly solvable models with the controllable numbers of
resonances, their positions and widths are presented.Comment: 5 pages, 2 figure
Quantum properties of the electron field in Kerr-Newman black hole manifolds
We study some spectral features of the one-particle electron Hamiltonian
obtained by separating the Dirac equation in a Kerr-Newman black hole
background. We find that the essential spectrum includes the whole real line.
As a consequence, there is no gap in the spectrum and discrete eigenvalues are
not allowed for any value of the black hole charge and angular momentum
. Our spectral analysis will be also related to the dissipation of the black
hole angular momentum and charge.Comment: 9 pages, uses revte
Suppressed dependence of polarization on epitaxial strain in highly polar ferroelectrics
A combined experimental and computational investigation of coupling between
polarization and epitaxial strain in highly polar ferroelectric
PbZr_0.2Ti_0.8O_3 (PZT) thin films is reported. A comparison of the properties
of relaxed (tetragonality c/a = 1.05) and highly-strained (c/a = 1.09)
epitaxial films shows that polarization, while being amongst the highest
reported for PZT or PbTiO_3 in either film or bulk forms (P_r = 82
microC/cm^2), is almost independent of the epitaxial strain. We attribute this
behavior to a suppressed sensitivity of the A-site cations to epitaxial strain
in these Pb-based perovskites, where the ferroelectric displacements are
already large, contrary to the case of less polar perovskites, such as BaTiO_3.
In the latter case, the A-site cation (Ba) and equatorial oxygen displacements
can lead to substantial polarization increases.Comment: 4 pages, 3 figure
Semiclassical time--dependent propagation in three dimensions: How accurate is it for a Coulomb potential?
A unified semiclassical time propagator is used to calculate the
semiclassical time-correlation function in three cartesian dimensions for a
particle moving in an attractive Coulomb potential. It is demonstrated that
under these conditions the singularity of the potential does not cause any
difficulties and the Coulomb interaction can be treated as any other
non-singular potential. Moreover, by virtue of our three-dimensional
calculation, we can explain the discrepancies between previous semiclassical
and quantum results obtained for the one-dimensional radial Coulomb problem.Comment: 8 pages, 4 figures (EPS
First-principles accurate total-energy surfaces for polar structural distortions of BaTiO3, PbTiO3, and SrTiO3: consequences to structural transition temperatures
Specific forms of the exchange correlation energy functionals in
first-principles density functional theory-based calculations, such as the
local density approximation (LDA) and generalized-gradient approximations
(GGA), give rise to structural lattice parameters with typical errors of -2%
and 2%. Due to a strong coupling between structure and polarization, the order
parameter of ferroelectric transitions, they result in large errors in
estimation of temperature dependent ferroelectric structural transition
properties. Here, we employ a recently developed GGA functional of Wu and Cohen
[Phys. Rev. B 73, 235116 (2006)] and determine total-energy surfaces for
zone-center distortions of BaTiO3, PbTiO3, and SrTiO3, and compare them with
the ones obtained with calculations based on standard LDA and GGA. Confirming
that the Wu and Cohen functional allows better estimation of structural
properties at 0 K, we determine a new set of parameters defining the effective
Hamiltonian for ferroelectric transition in BaTiO3. Using the new set of
parameters, we perform molecular-dynamics (MD) simulations under effective
pressures p=0.0 GPa, p=-2.0 GPa, and p=-0.005T GPa. The simulations under
p=-0.005T GPa, which is for simulating thermal expansion, show a clear
improvement in the cubic to tetragonal transition temperature and c/a parameter
of its ferroelectric tetragonal phase, while the description of transitions at
lower temperatures to orthorhombic and rhombohedral phases is marginally
improved. Our findings augur well for use of Wu-Cohen functional in studies of
ferroelectrics at nano-scale, particularly in the form of epitaxial films where
the properties depend crucially on the lattice mismatch.Comment: 10 pages, 7 figures, 3 tables, resubmitted to PR
The effect of radiative cooling on scaling laws of X-ray groups and clusters
We have performed cosmological simulations in a ÎCDM cosmology with and without radiative cooling in order to study the effect of cooling on the cluster scaling laws. Our simulations consist of 4.1 million particles each of gas and dark matter within a box size of 100 h-1 Mpc, and the run with cooling is the largest of its kind to have been evolved to z = 0. Our cluster catalogs both consist of over 400 objects and are complete in mass down to ~1013 h-1 Mâ. We contrast the emission-weighted temperature-mass (Tew-M) and bolometric luminosity-temperature (Lbol-Tew) relations for the simulations at z = 0. We find that radiative cooling increases the temperature of intracluster gas and decreases its total luminosity, in agreement with the results of Pearce et al. Furthermore, the temperature dependence of these effects flattens the slope of the Tew-M relation and steepens the slope of the Lbol-Tew relation. Inclusion of radiative cooling in the simulations is sufficient to reproduce the observed X-ray scaling relations without requiring excessive nongravitational energy injection
Generating quantum states through spin chain dynamics
Spin chains can realise perfect quantum state transfer between the two ends
via judicious choice of coupling strengths. In this paper, we study what other
states can be created by engineering a spin chain. We conclude that, up to
local phases, all single excitation quantum states with support on every site
of the chain can be created. We pay particular attention to the generation of
W-states that are superposed over every site of the chain.Comment: 9 pages, 1 figur
Improved detection of small atom numbers through image processing
We demonstrate improved detection of small trapped atomic ensembles through
advanced post-processing and optimal analysis of absorption images. A fringe
removal algorithm reduces imaging noise to the fundamental photon-shot-noise
level and proves beneficial even in the absence of fringes. A
maximum-likelihood estimator is then derived for optimal atom-number estimation
and is applied to real experimental data to measure the population differences
and intrinsic atom shot-noise between spatially separated ensembles each
comprising between 10 and 2000 atoms. The combined techniques improve our
signal-to-noise by a factor of 3, to a minimum resolvable population difference
of 17 atoms, close to our ultimate detection limit.Comment: 4 pages, 3 figure
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