625 research outputs found
Frustration and sound attenuation in structural glasses
Three classes of harmonic disorder systems (Lennard-Jones like glasses,
percolators above threshold, and spring disordered lattices) have been
numerically investigated in order to clarify the effect of different types of
disorder on the mechanism of high frequency sound attenuation. We introduce the
concept of frustration in structural glasses as a measure of the internal
stress, and find a strong correlation between the degree of frustration and the
exponent alpha that characterizes the momentum dependence of the sound
attenuation . In particular, alpha decreases from
about d+1 in low-frustration systems (where d is the spectral dimension), to
about 2 for high frustration systems like the realistic glasses examined.Comment: Revtex, 4 pages including 4 figure
Broadband 200-nm second-harmonic generation in silicon in the telecom band.
Silicon is well known for its strong third-order optical nonlinearity, exhibiting efficient supercontinuum and four-wave mixing processes. A strong second-order effect that is naturally inhibited in silicon can also be observed, for example, by electrically breaking the inversion symmetry and quasi-phase matching the pump and the signal. To generate an efficient broadband second-harmonic signal, however, the most promising technique requires matching the group velocities of the pump and the signal. In this work, we utilize dispersion engineering of a silicon waveguide to achieve group velocity matching between the pump and the signal, along with an additional degree of freedom to broaden the second harmonic through the strong third-order nonlinearity. We demonstrate that the strong self-phase modulation and cross-phase modulation in silicon help broaden the second harmonic by 200 nm in the O-band. Furthermore, we show a waveguide design that can be used to generate a second-harmonic signal in the entire near-infrared region. Our work paves the way for various applications, such as efficient and broadband complementary-metal oxide semiconductor based on-chip frequency synthesizers, entangled photon pair generators, and optical parametric oscillators
Physical properties and radius variations in the HAT-P-5 planetary system from simultaneous four-colour photometry
The radii of giant planets, as measured from transit observations, may vary
with wavelength due to Rayleigh scattering or variations in opacity. Such an
effect is predicted to be large enough to detect using ground-based
observations at multiple wavelengths. We present defocussed photometry of a
transit in the HAT-P-5 system, obtained simultaneously through Stromgren u,
Gunn g and r, and Johnson I filters. Two more transit events were observed
through a Gunn r filter. We detect a substantially larger planetary radius in
u, but the effect is greater than predicted using theoretical model atmospheres
of gaseous planets. This phenomenon is most likely to be due to systematic
errors present in the u-band photometry, stemming from variations in the
transparency of Earth's atmosphere at these short wavelengths. We use our data
to calculate an improved orbital ephemeris and to refine the measured physical
properties of the system. The planet HAT-P-5b has a mass of 1.06 +/- 0.11 +/-
0.01 Mjup and a radius of 1.252 +/- 0.042 +/- 0.008 Rjup (statistical and
systematic errors respectively), making it slightly larger than expected
according to standard models of coreless gas-giant planets. Its equilibrium
temperature of 1517 +/- 29 K is within 60K of that of the extensively-studied
planet HD 209458b.Comment: Version 2 corrects the accidental omission of one author in the arXiv
metadata. Accepted for publication in MNRAS. 9 pages, 4 figures, 7 tables.
The properties of HAT-P-5 have been added to the Transiting Extrasolar Planet
Catalogue at http://www.astro.keele.ac.uk/~jkt/tepcat
Observation of Umklapp processes in non-crystalline materials
Umklapp processes are known to exist in cristalline materials, where they
control important properties such as thermal conductivity, heat capacity and
electrical conductivity. In this work we report the provocative observation of
Umklapp processes in a non-periodical system, namely liquid Lithium. The lack
of a well defined periodicity seems then not to prevent the existence of these
scattering processes mechanisms provided that the local order of the systems
i.e. the maxima of the static structure factor supply the equivalent of a
reciprocal lattice vector in the case of cristalline materials.Comment: 13 pages P
Phonon-like and single particle dynamics in liquid lithium
The dynamic structure factor, S(Q,E), of liquid lithium (T=475 K) has been
determined by inelastic x-ray scattering (IXS) in the momentum transfer region
(Q = 1.4-110 nm-1). These data allow to observe how, in a simple liquid, a
phonon-like collective mode evolves towards the single particle dynamics. As a
function of Q, one finds: i) at low Q's, a sound mode with a positive
dispersion of the sound velocity, ii) at intermediate Q's, excitations whose
energy oscillates similarly to phonons in the crystal Brillouin zones, and iii)
at high Q's, the S(Q,E) approaches a Gaussian shape, indicating that the single
particle dynamics has been reached.Comment: 3 pages and 5 figure
Electronic properties and orbital-filling mechanism in Rb-intercalated copper phthalocyanine
The evolution of the electronic properties of a thin film of copper phthalocyanine deposited on Al(100) and progressively intercalated with rubidium atoms was followed by photoemission and X-ray absorption spectroscopies. Electron donation from the Rb atoms to the C32H16N8Cu molecules results in the lifting of the degeneracy in the e(g) ligand-derived molecular orbital and the lowering of the molecular symmetry. For Rb similar to 2C32H16N8Cu, spectral evidence indicates that both donated electrons reside in the first split-off e(g)-derived level, thus creating an electronic inequivalence between the C atoms in the benzene rings. For higher Rb concentrations, a reduction of the Cu oxidation state is observed, together with a new Cu-derived state in valence-band photoemission spectra, testifying to the filling of the b(1g) orbital. Thus, even though b(1g) is the lowest unoccupied orbital of the neutral molecule, in the film, the Cu-derived b(1g)-derived states are occupied only after a partial filling of the e.-derived band has taken place. Despite the fact that the eg-derived spectral weight becomes larger as the rubidium content in the RbxC32H16N8Cu compound increases, no spectral density was observed at the Fermi level, showing that the film remains insulating for all of the investigated stoichiometries
Theories and analyses of functionally graded circular plates
This paper presents the governing equations and analytical solutions of the classical and shear deformation theories of functionally graded axisymmetric circular plates. The classical, first-order, and third-order shear deformation theories are presented, accounting for through-thickness variation of two-constituent functionally graded material, modified couple stress effect, and the von Kármán nonlinearity. Analytical solutions for bending of the linear theories, some of which are not readily available in the literature, are included to show the influence of the material variation, boundary conditions, and loads
Aging of the Nonlinear Optical Susceptibility of colloidal solutions
Using Z-scan and dynamic light scattering measurements we investigate the
nonlinear optics response of a colloidal solution undergoing dynamics slowing
down with age. We study the high optical nonlinearity of an organic dye
(Rhodamine B) dispersed in a water-clay (Laponite) solution, at different clay
concentrations (2.0 wt% - 2.6 wt%), experiencing the gelation process. We
determine the clay platelets self diffusion coefficient and, by its comparison
with the structural relaxation time, we conclude that the gelation process
proceeds through the structuring of interconnecting clay platelets network
rather than through clusters growth and aggregation.Comment: 4 figures, 4 page
The nature of the short wavelength excitations in vitreous silica: X-Rays Brillouin scattering study
The dynamical structure factor (S(Q,E)) of vitreous silica has been measured
by Inelastic X-ray Scattering varying the exchanged wavevector (Q) at fixed
exchanged energy (E) - an experimental procedure that, contrary to the usual
one at constant Q, provides spectra with much better identified inelastic
features. This allows the first direct evidence of Brillouin peaks in the
S(Q,E) of SiO_2 at energies above the Boson Peak (BP) energy, a finding that
excludes the possibility that the BP marks the transition from propagating to
localised dynamics in glasses.Comment: 4 pages, 3 Postscript figures. To appear in Physical Review Letter
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