4,044 research outputs found
Limits to compression with cascaded quadratic soliton compressors
We study cascaded quadratic soliton compressors and address the physical
mechanisms that limit the compression. A nonlocal model is derived, and the
nonlocal response is shown to have an additional oscillatory component in the
nonstationary regime when the group-velocity mismatch (GVM) is strong. This
inhibits efficient compression. Raman-like perturbations from the cascaded
nonlinearity, competing cubic nonlinearities, higher-order dispersion, and
soliton energy may also limit compression, and through realistic numerical
simulations we point out when each factor becomes important. We find that it is
theoretically possible to reach the single-cycle regime by compressing
high-energy fs pulses for wavelengths in a
-barium-borate crystal, and it requires that the system is in the
stationary regime, where the phase mismatch is large enough to overcome the
detrimental GVM effects. However, the simulations show that reaching
single-cycle duration is ultimately inhibited by competing cubic nonlinearities
as well as dispersive waves, that only show up when taking higher-order
dispersion into account.Comment: 16 pages, 5 figures, submitted to Optics Expres
Effective potential for composite operators and for an auxiliary scalar field in a Nambu-Jona-Lasinio model
We derive the effective potentials for composite operators in a
Nambu-Jona-Lasinio (NJL) model at zero and finite temperature and show that in
each case they are equivalent to the corresponding effective potentials based
on an auxiliary scalar field. The both effective potentials could lead to the
same possible spontaneous breaking and restoration of symmetries including
chiral symmetry if the momentum cutoff in the loop integrals is large enough,
and can be transformed to each other when the Schwinger-Dyson (SD) equation of
the dynamical fermion mass from the fermion-antifermion vacuum (or thermal)
condensates is used. The results also generally indicate that two effective
potentials with the same single order parameter but rather different
mathematical expressions can still be considered physically equivalent if the
SD equation corresponding to the extreme value conditions of the two potentials
have the same form.Comment: 7 pages, no figur
Nonlocal explanation of stationary and nonstationary regimes in cascaded soliton pulse compression
We study soliton pulse compression in materials with cascaded quadratic
nonlinearities, and show that the group-velocity mismatch creates two different
temporally nonlocal regimes. They correspond to what is known as the stationary
and nonstationary regimes. The theory accurately predicts the transition to the
stationary regime, where highly efficient pulse compression is possible.Comment: 3 pages, 2 figures, published verison in Optics Letters. Contains
revised equations, including an updated mode
Phase-field-crystal modeling of the (2x1)-(1x1) phase-transitions of Si(001) and Ge(001) surfaces
We propose a two-dimensional phase-field-crystal model for the
(21)-(11) phase transitions of Si(001) and Ge(001) surfaces.
The dimerization in the 21 phase is described with a
phase-field-crystal variable which is determined by solving an evolution
equation derived from the free energy. Simulated periodic arrays of
dimerization variable is consistent with scanning-tunnelling-microscopy images
of the two dimerized surfaces. Calculated temperature dependence of the
dimerization parameter indicates that normal dimers and broken ones coexist
between the temperatures describing the charactristic temperature width of the
phase-transition, and , and a first-order phase transition takes
place at a temperature between them. The dimerization over the whole
temperature is determined. These results are in agreement with experiment. This
phase-field-crystal approach is applicable to phase-transitions of other
reconstructed surface phases, especially semiconductor 1 reconstructed
surface phases.Comment: 10 pages with 4 figures include
Crystalline free energies of micelles of diblock copolymer solutions
We report a characterization of the relative stability and structural
behavior of various micellar crystals of an athermal model of AB-diblock
copolymers in solution. We adopt a previously devel- oped coarse-graining
representation of the chains which maps each copolymer on a soft dumbbell.
Thanks to this strong reduction of degrees of freedom, we are able to
investigate large aggregated systems, and for a specific length ratio of the
blocks f = MA/(MA + MB) = 0.6, to locate the order-disorder transition of the
system of micelles. Above the transition, mechanical and thermal properties are
found to depend on the number of particles per lattice site in the simulation
box, and the application of a recent methodology for multiple occupancy
crystals (B.M. Mladek et al., Phys. Rev. Lett. 99, 235702 (2007)) is necessary
to correctly define the equilibrium state. Within this scheme we have performed
free energy calculations at two reduced density {\rho}/{\rho}\ast = 4,5 and for
several cubic structures as FCC,BCC,A15. At both densities, the BCC symmetry is
found to correspond to the minimum of the unconstrained free energy, that is to
the stable symmetry among the few considered, while the A15 structure is almost
degenerate, indicating that the present sys- tem prefers to crystallize in less
packed structures. At {\rho}/{\rho}\ast = 4 close to melting, the Lindemann
ratio is fairly high (~ 0.29) and the concentration of vacancies is roughly 6%.
At {\rho}/{\rho}\ast = 5 the mechanical stability of the stable BCC structure
increases and the concentration of vacancies ac- cordingly decreases. The ratio
of the corona layer thickness to the core radius is found to be in good
agreement with experimental data for poly(styrene-b-isoprene)(22-12) in
isoprene selective solvent which is also reported to crystallize in the BCC
structure
Hyperfine Interactions in the Heavy Fermion CeMIn_5 Systems
The CeMIn_5 heavy fermion compounds have attracted enormous interest since
their discovery six years ago. These materials exhibit a rich spectrum of
unusual correlated electron behavior, and may be an ideal model for the high
temperature superconductors. As many of these systems are either
antiferromagnets, or lie close to an antiferromagnetic phase boundary, it is
crucial to understand the behavior of the dynamic and static magnetism. Since
neutron scattering is difficult in these materials, often the primary source of
information about the magnetic fluctuations is Nuclear Magnetic Resonance
(NMR). Therefore, it is crucial to have a detailed understanding of how the
nuclear moments interact with conduction electrons and the local moments
present in these systems. Here we present a detailed analysis of the hyperfine
coupling based on anisotropic hyperfine coupling tensors between nuclear
moments and local moments. Because the couplings are symmetric with respect to
bond axes rather than crystal lattice directions, the nuclear sites can
experience non-vanishing hyperfine fields even in high symmetry sites.Comment: 15 pages, 5 figure
Interpretation of hidden node methodology with network accuracy
Bayesian networks are constructed under a con-ditional independency assumption. This assump-tion however does not necessarily hold in prac-tice and may lead to loss of accuracy. We previ-ously proposed a hidden node methodology whereby Bayesian networks are adapted by the addition of hidden nodes to model the data de-pendencies more accurately. Empirical results in a computer vision application to classify and count the neural cell automatically showed that a modified network with two hidden nodes achieved significantly better performance with an average prediction accuracy of 83.9% com-pared to 59.31% achieved by the original net-work. In this paper we justify the improvement of performance by examining the changes in network accuracy using four network accuracy measurements; the Euclidean accuracy, the Co-sine accuracy, the Jensen-Shannon accuracy and the MDL score. Our results consistently show that the network accuracy improves by introduc-ing hidden nodes. Consequently, we were able to verify that the hidden node methodology helps to improve network accuracy and contribute to the improvement of prediction accuracy
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Optimization of the neutron yield in fusion plasmas produced by Coulomb explosions of deuterium clusters irradiated by a petawatt laser
The kinetic energy of hot (multi-keV) ions from the laser-driven Coulomb explosion of deuterium clusters and the resulting fusion yield in plasmas formed from these exploding clusters has been investigated under a variety of conditions using the Texas Petawatt laser. An optimum laser intensity was found for producing neutrons in these cluster fusion plasmas with corresponding average ion energies of 14 keV. The substantial volume (1-10 mm(3)) of the laser-cluster interaction produced by the petawatt peak power laser pulse led to a fusion yield of 1.6x10(7) neutrons in a single shot with a 120 J, 170 fs laser pulse. Possible effects of prepulses are discussed. DOI: 10.1103/PhysRevE.87.023106Glenn Focht Memorial FellowshipNNSA DE-FC52-08NA28512DOE Office of Basic Energy SciencesPhysic
Collapse arrest and soliton stabilization in nonlocal nonlinear media
We investigate the properties of localized waves in systems governed by
nonlocal nonlinear Schrodinger type equations. We prove rigorously by bounding
the Hamiltonian that nonlocality of the nonlinearity prevents collapse in,
e.g., Bose-Einstein condensates and optical Kerr media in all physical
dimensions. The nonlocal nonlinear response must be symmetric, but can be of
completely arbitrary shape. We use variational techniques to find the soliton
solutions and illustrate the stabilizing effect of nonlocality.Comment: 4 pages with 3 figure
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