29,081 research outputs found
Two-Photon Polarization Interference for Pulsed SPDC in a PPKTP Waveguide
We study the spectral properties of Spontaneous Parametric Down Conversion in
a periodically poled waveguided structure of KTP crystal pumped by ultra-short
pulses. Our theoretical analysis reveals a strongly multimode and asymmetric
structure of the two-photon spectral amplitude for type-II SPDC. Experimental
evidence, based on Hong-Ou-Mandel polarization interference with narrowband
filtering, confirms this result.Comment: Submitted for publicatio
Evaluation of Critical Operating Conditions for a Semi-batch Reactor by Complementary Use of Sensitivity and Divergence Criteria
This paper presents a comparison of several effective methods of deriving the critical feeding conditions for the case of a semi-batch catalytic reactor used for the acetoacetylation of pyrrole with diketene in homogeneous liquid phase. The reaction is known to be of high risk due to the very exothermic (polymerisation) side-reactions involving reactive diketene. In order to perform the sensitivity analysis, both the Morbidelli-Varma sensitivity criterion and div-methods were used, the latter of which are based on the system’s Jacobian and Green’s function matrix analysis. The combined application of such criteria allows the optimal and safe setting of the reactor’s nominal operating conditions. Extended sensitivity analysis reveals possible QFS (quick onset, fair
conversion and smooth temperature profile) regions, under severe operating conditions characterized by fast enough main reaction that prevents the co-reactant accumulation, and leads to a quasi-insensitive semi-batch reactor behaviour
Magneto-mechanical interplay in spin-polarized point contacts
We investigate the interplay between magnetic and structural dynamics in
ferromagnetic atomic point contacts. In particular, we look at the effect of
the atomic relaxation on the energy barrier for magnetic domain wall migration
and, reversely, at the effect of the magnetic state on the mechanical forces
and structural relaxation. We observe changes of the barrier height due to the
atomic relaxation up to 200%, suggesting a very strong coupling between the
structural and the magnetic degrees of freedom. The reverse interplay is weak,
i.e. the magnetic state has little effect on the structural relaxation at
equilibrium or under non-equilibrium, current-carrying conditions.Comment: 5 pages, 4 figure
Breakdown of adiabatic transfer of light in waveguides in the presence of absorption
In atomic physics, adiabatic evolution is often used to achieve a robust and
efficient population transfer. Many adiabatic schemes have also been
implemented in optical waveguide structures. Recently there has been increasing
interests in the influence of decay and absorption, and their engineering
applications. Here it is shown that even a small decay can significantly
influence the dynamical behaviour of a system, above and beyond a mere change
of the overall norm. In particular, a small decay can lead to a breakdown of
adiabatic transfer schemes, even when both the spectrum and the eigenfunctions
are only sightly modified. This is demonstrated for the generalization of a
STIRAP scheme that has recently been implemented in optical waveguide
structures. Here the question how an additional absorption in either the
initial or the target waveguide influences the transfer property of the scheme
is addressed. It is found that the scheme breaks down for small values of the
absorption at a relatively sharp threshold, which can be estimated by simple
analytical arguments.Comment: 8 pages, 7 figures, revised and extende
Investigating Galactic supernova remnant candidates with LOFAR
We investigate six supernova remnant (SNR) candidates --- G51.21+0.11,
G52.37-0.70, G53.07+0.49, G53.41+0.03, G53.84-0.75, and the possible shell
around G54.1-0.3 --- in the Galactic Plane using newly acquired LOw-Frequency
ARray (LOFAR) High-Band Antenna (HBA) observations, as well as archival
Westerbork Synthesis Radio Telescope (WSRT) and Very Large Array Galactic Plane
Survey (VGPS) mosaics. We find that G52.37-0.70, G53.84-0.75, and the possible
shell around pulsar wind nebula G54.1+0.3 are unlikely to be SNRs, while
G53.07+0.49 remains a candidate SNR. G51.21+0.11 has a spectral index of
, but lacks X-ray observations and as such requires further
investigation to confirm its nature. We confirm one candidate, G53.41+0.03, as
a new SNR because it has a shell-like morphology, a radio spectral index of
and it has the X-ray spectral characteristics of a
1000-8000 year old SNR. The X-ray analysis was performed using archival
XMM-Newton observations, which show that G53.41+0.03 has strong emission lines
and is best characterized by a non-equilibrium ionization model, consistent
with an SNR interpretation. Deep Arecibo radio telescope searches for a pulsar
associated with G53.41+0.03 resulted in no detection, but place stringent upper
limits on the flux density of such a source if it is beamed towards Earth.Comment: 9 pages, 4 figures, 1 tabl
Spectral properties of orbital polarons in Mott insulators
We address the spectral properties of Mott insulators with orbital degrees of
freedom, and investigate cases where the orbital symmetry leads to Ising-like
superexchange in the orbital sector. The paradigm of a hole propagating by its
coupling to quantum fluctuations, known from the spin t-J model, then no longer
applies. We find instead that when one of the two orbital flavors is immobile,
as in the Falicov-Kimball model, trapped orbital polarons coexist with free
hole propagation emerging from the effective three-site hopping in the regime
of large on-site Coulomb interaction U. The spectral functions are found
analytically in this case within the retraceable path approximation in one and
two dimensions. On the contrary, when both of the orbitals are active, as in
the model for electrons in two dimensions, we find propagating
polarons with incoherent scattering dressing the moving hole and renormalizing
the quasiparticle dispersion. Here, the spectral functions, calculated using
the self-consistent Born approximation, are anisotropic and depend on the
orbital flavor. Unbiased conclusions concerning the spectral properties are
established by comparing the above results for the orbital t-J models with
those obtained using the variational cluster approximation or exact
diagonalization for the corresponding Hubbard models. The present work makes
predictions concerning the essential features of photoemission spectra of
certain fluorides and vanadates.Comment: 26 pages, 16 figures; to appear in Physical Review
Dark Energy as a Relic of the Vacuum-Energy Cancellation?
We analyze the dynamical implications of an exponential Lagrangian density
for the gravitational field, as referred to an isotropic FRW Universe. Then, we
discuss the features of the generalized deSitter phase, predicted by the new
Friedmann equation. The existence of a consistent deSitter solution arises only
if the ratio between the vacuum-energy density and that associated with the
fundamental length of the theory acquires a tantalizing negative character.
This choice allows us to explain the present universe dark energy as a relic of
the vacuum-energy cancellation due to the cosmological constant intrinsically
contained in our scheme. The corresponding scalar-tensor description of the
model is addressed too, and the behavior of the scalar field is analyzed for
both negative and positive values of the cosmological term. In the first case,
the Friedmann equation is studied both in vacuum and in presence of external
matter, while, in the second case, the quantum regime is approached in the
framework of ''repulsive'' properties of the gravitational interaction, as
described in recent issues in Loop Quantum Cosmology. In particular, in the
vacuum case, we find a pure non-Einsteinian effect, according to which a
negative cosmological constant provides an accelerating deSitter dynamics, in
the region where the series expansion of the exponential term does not hold.Comment: 24 pages, 2 figures, to appear on IJMP
First Passage and Cooperativity of Queuing Kinetics
We model the kinetics of ligand-receptor systems, where multiple ligands may
bind and unbind to the receptor, either randomly or in a specific order.
Equilibrium occupation and first occurrence of complete filling of the receptor
are determined and compared. At equilibrium, receptors that bind ligands
sequentially are more likely to be saturated than those that bind in random
order. Surprisingly however, for low cooperativity, the random process first
reaches full occupancy faster than the sequential one. This is true {\it
except} near a critical binding energy where a 'kinetic trap' arises and the
random process dramatically slows down when the number of binding sites . These results demonstrate the subtle interplay between cooperativity and
sequentiality for a wide class of kinetic phenomena, including chemical
binding, nucleation, and assembly line strategies.Comment: 5pp, 5 figure
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