1,545 research outputs found
Stochastic homogenization of the laser intensity to improve the irradiation uniformity of capsules directly driven by thousands laser beams
Illumination uniformity of a spherical capsule directly driven by laser beams has been assessed numerically. Laser facilities characterized by ND = 12, 20, 24, 32, 48 and 60 directions of irradiation with associated a single laser beam or a bundle of NB laser beams have been considered. The laser beam intensity profile is assumed super-Gaussian and the calculations take into account beam imperfections as power imbalance and pointing errors. The optimum laser intensity profile, which minimizes the root-mean-square deviation of the capsule illumination, depends on the values of the beam imperfections. Assuming that the NB beams are statistically independents is found that they provide a stochastic homogenization of the laser intensity associated to the whole bundle, reducing the errors associated to the whole bundle by the factor  , which in turn improves the illumination uniformity of the capsule. Moreover, it is found that the uniformity of the irradiation is almost the same for all facilities and only depends on the total number of laser beams Ntot = ND × NB
Absorption spectrum of a weakly n-doped semiconductor quantum well
We calculate, as a function of temperature and conduction band electron
density, the optical absorption of a weakly n-doped, idealized semiconductor
quantum well. In particular, we focus on the absorption band due to the
formation of a charged exciton. We conceptualize the charged exciton as an
itinerant excitation intimately linked to the dynamical response of itinerant
conduction band electrons to the appearance of the photo-generated valence band
hole. Numerical results for the absorption in the vicinity of the exciton line
are presented and the spectral weights associated with, respectively, the
charged exciton band and the exciton line are analyzed in detail. We find, in
qualitative agreement with experimental data, that the spectral weight of the
charged exciton grows with increasing conduction band electron density and/or
decreasing temperature at the expense of the exciton.Comment: 5 pages, 4 figure
Magnetic Phase Diagram of GdNi2B2C: Two-ion Magnetoelasticity and Anisotropic Exchange Couplings
Extensive magnetization and magnetostriction measurements were carried out on
a single crystal of GdNi2B2C along the main tetragonal axes. Within the
paramagnetic phase, the magnetic and strain susceptibilities revealed a weak
anisotropy in the exchange couplings and two-ion tetragonal-preserving
alpha-strain modes. Within the ordered phase, magnetization and
magnetostriction revealed a relatively strong orthorhombic distortion mode and
rich field-temperature phase diagrams. For H//(100) phase diagram, three
field-induced transformations were observed, namely, at: Hd(T), related to the
domain alignment; Hr(T), associated with reorientation of the moment towards
the c-axis; and Hs(T), defining the saturation process wherein the exchange
field is completely counterbalanced. On the other hand, For H//(001) phase
diagram, only two field-induced transformations were observed, namely at: Hr(T)
and Hs(T). For both phase diagrams, Hs(T) follows the relation
Hs[1-(T/Tn)^2]^(1/2)kOe with Hs(T-->0)=128.5(5) kOe and Tn(H=0)=19.5 K. In
contrast, the thermal evolution of Hr(T) along the c-axis (much simpler than
along the a-axis) follows the relation Hr[1-T/Tr]^(1/3) kOe where
Hr(T-->0)=33.5(5) kOe and Tr(H=0)=13.5 K. It is emphasized that the
magnetoelastic interaction and the anisotropic exchange coupling are important
perturbations and therefore should be explicitly considered if a complete
analysis of the magnetic properties of the borocarbides is desired
Magnetic and lattice polaron in Holstein-t-J model
We investigate the interplay between the formation of lattice and magnetic
polaron in the case of a single hole in the antiferromagnetic background. We
present an exact analytical solution of the Holstein-t-J model in infinite
dimensions. Ground state energy, electron-lattice correlation function, spin
bag dimension as well as spectral properties are calculated. The magnetic and
hole-lattice correlations sustain each other, i.e. the presence of
antiferromagnetic correlations favors the formation of the lattice polaron at
lower value of the electron-phonon coupling while the polaronic effect
contributes to reduce the number of spin defects in the antiferromagnetic
background. The crossover towards a spin-lattice small polaron region of the
phase diagram becomes a discontinuous transition in the adiabatic limit.Comment: revtex, 8 eps figures included NEW version. Appendix with a full
proof include
Surface effects in multiband superconductors. Application to MgB
Metals with many bands at the Fermi level can have different band dependent
gaps in the superconducting state. The absence of translational symmetry at an
interface can induce interband scattering and modify the superconducting
properties. We dicuss the relevance of these effects to recent experiments in
MgB
Magnetic polarons in weakly doped high-Tc superconductors
We consider a spin Hamiltonian describing - exchange interactions
between localized spins of a finite antiferromagnet as well as -
interactions between a conducting hole () and localized spins. The spin
Hamiltonian is solved numerically with use of Lanczos method of
diagonalization. We conclude that - exchange interaction leads to
localization of magnetic polarons. Quantum fluctuations of the antiferromagnet
strengthen this effect and make the formation of polarons localized in one site
possible even for weak - coupling. Total energy calculations, including
the kinetic energy, do not change essentially the phase diagram of magnetic
polarons formation. For parameters reasonable for high- superconductors
either a polaron localized on one lattice cell or a small ferron can form. For
reasonable values of the dielectric function and - coupling, the
contributions of magnetic and phonon terms in the formation of a polaron in
weakly doped high- materials are comparable.Comment: revised, revtex-4, 12 pages 8 eps figure
Superconductivity in the SU(N) Anderson Lattice at U=\infty
We present a mean-field study of superconductivity in a generalized N-channel
cubic Anderson lattice at U=\infty taking into account the effect of a
nearest-neighbor attraction J. The condition U=\infty is implemented within the
slave-boson formalism considering the slave bosons to be condensed. We consider
the -level occupancy ranging from the mixed valence regime to the Kondo
limit and study the dependence of the critical temperature on the various model
parameters for each of three possible Cooper pairing symmetries (extended s,
d-wave and p-wave pairing) and find interesting crossovers. It is found that
the d- and p- wave order parameters have, in general, very similar critical
temperatures. The extended s-wave pairing seems to be relatively more stable
for electronic densities per channel close to one and for large values of the
superconducting interaction J.Comment: Seven Figures; one appendix. Accepted for publication in Phys. Rev.
Phase Separation of Rigid-Rod Suspensions in Shear Flow
We analyze the behavior of a suspension of rigid rod-like particles in shear
flow using a modified version of the Doi model, and construct diagrams for
phase coexistence under conditions of constant imposed stress and constant
imposed strain rate, among paranematic, flow-aligning nematic, and log-rolling
nematic states. We calculate the effective constitutive relations that would be
measured through the regime of phase separation into shear bands. We calculate
phase coexistence by examining the stability of interfacial steady states and
find a wide range of possible ``phase'' behaviors.Comment: 23 pages 19 figures, revised version to be published in Physical
Review
Real-time and non-invasive measurements of cell mechanical behaviour with optical coherence phase microscopy
There is an unmet need in tissue engineering for non-invasive, label-free monitoring of cell mechanical behaviour in their physiological environment. Here, we describe a novel optical coherence phase microscopy (OCPM) set-up which can map relative cell mechanical behaviour in monolayers and 3D systems non-invasively, and in real-time. 3T3 and MCF-7 cells were investigated, with MCF-7 demonstrating an increased response to hydrostatic stimulus indicating MCF-7 being softer than 3T3, demonstrating the ability to provide qualitative data on cell mechanical behaviour. Quantitative measurements of 6% agarose beads have been taken with commercial Cell Scale Microsquisher® system demonstrating that their mechanical properties are in the same order of magnitude of cells, indicating that this is an appropriate test sample for the novel method desctibed
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