8,433 research outputs found
Conformational Control of Exciton-Polariton Physics in Metal - Poly(9,9-dioctylfluorene) - Metal Cavities
Control is exerted over the exciton-polariton physics in metal -
Poly(9,9-dioctyl fluorene) - metal microcavities via conformational changes to
the polymer backbone. Using thin-film samples containing increasing fractions
of -phase chain segments, a systematic study is reported for the mode
characteristics and resulting light emission properties of cavities containing
two distinct exciton sub-populations within the same semiconductor. Ultrastrong
coupling for disordered glassy-phase excitons is observed from angle-resolved
reflectivity measurements, with Rabi splitting energies in excess of 1.05 eV
(more than 30% of the exciton transition energy) for both TE- and TM-polarized
light. A splitting of the lower polariton branch is then induced via
introduction of -phase excitons and increases with their growing
fraction. In all cases, the photoluminescence emanates from the lowermost
polariton branch, allowing conformational control to be exerted over the
emission energy and its angular variation. Dispersion-free cavities with highly
saturated blue-violet emission are thus enabled. Experimental results are
discussed in terms of the full Hopfield Hamiltonian generalized to the case of
two exciton oscillators. The importance of taking account of the molecular
characteristics of the semiconductor for an accurate description of its strong
coupling behaviour is directly considered, in specific relation to the role of
the vibronic structure
Breakdown patterns in Branly's coheror
We use thermal imaging of Joule heating to see for the first time electrical
conducting paths created by the so-called Branly effect in a two-dimensional
metallic granular medium (aluminium). Multiple breakdowns are shown to occur
when the medium is submitted to high voltage increases (more than 500 V) with
rise times close to one hundred of microseconds.Comment: 4 pages, 5 figures, related informations at
http://chemphys.weizmann.ac.il/~damien/index.htm
Light transport in cold atoms and thermal decoherence
By using the coherent backscattering interference effect, we investigate
experimentally and theoretically how coherent transport of light inside a cold
atomic vapour is affected by the residual motion of atomic scatterers. As the
temperature of the atomic cloud increases, the interference contrast
dramatically decreases emphazising the role of motion-induced decoherence for
resonant scatterers even in the sub-Doppler regime of temperature. We derive
analytical expressions for the corresponding coherence time.Comment: 4 pages - submitted to Physical Review Letter
Strain versus stress in a model granular material: a Devil's staircase
The series of equilibrium states reached by disordered packings of rigid,
frictionless discs in two dimensions, under gradually varying stress, are
studied by numerical simulations. Statistical properties of trajectories in
configuration space are found to be independent of specific assumptions ruling
granular dynamics, and determined by geometry only. A monotonic increase in
some macroscopic loading parameter causes a discrete sequence of
rearrangements. For a biaxial compression, we show that, due to the statistical
importance of such events of large magnitudes, the dependence of the resulting
strain on stress direction is a Levy flight in the thermodynamic limit.Comment: REVTeX, 4 pages, 5 included PostScript figures. New version altered
throughout text, very close to published pape
Diamagnetism of doped two-leg ladders and probing the nature of their commensurate phases
We study the magnetic orbital effect of a doped two-leg ladder in the
presence of a magnetic field component perpendicular to the ladder plane.
Combining both low-energy approach (bosonization) and numerical simulations
(density-matrix renormalization group) on the strong coupling limit (t-J
model), a rich phase diagram is established as a function of hole doping and
magnetic flux. Above a critical flux, the spin gap is destroyed and a Luttinger
liquid phase is stabilized. Above a second critical flux, a reentrance of the
spin gap at high magnetic flux is found. Interestingly, the phase transitions
are associated with a change of sign of the orbital susceptibility. Focusing on
the small magnetic field regime, the spin-gapped superconducting phase is
robust but immediately acquires algebraic transverse (i.e. along rungs) current
correlations which are commensurate with the 4k_F density correlations. In
addition, we have computed the zero-field orbital susceptibility for a large
range of doping and interactions ratio J/t : we found strong anomalies at low
J/t only in the vicinity of the commensurate fillings corresponding to delta =
1/4 and 1/2. Furthermore, the behavior of the orbital susceptibility reveals
that the nature of these insulating phases is different: while for delta = 1/4
a 4k_F charge density wave is confirmed, the delta = 1/2 phase is shown to be a
bond order wave.Comment: 15 pages, 17 figure
Internal states of model isotropic granular packings. III. Elastic properties
In this third and final paper of a series, elastic properties of numerically
simulated isotropic packings of spherical beads assembled by different
procedures and subjected to a varying confining pressure P are investigated. In
addition P, which determines the stiffness of contacts by Hertz's law, elastic
moduli are chiefly sensitive to the coordination number, the possible values of
which are not necessarily correlated with the density. Comparisons of numerical
and experimental results for glass beads in the 10kPa-10MPa range reveal
similar differences between dry samples compacted by vibrations and lubricated
packings. The greater stiffness of the latter, in spite of their lower density,
can hence be attributed to a larger coordination number. Voigt and Reuss bounds
bracket bulk modulus B accurately, but simple estimation schemes fail for shear
modulus G, especially in poorly coordinated configurations under low P.
Tenuous, fragile networks respond differently to changes in load direction, as
compared to load intensity. The shear modulus, in poorly coordinated packings,
tends to vary proportionally to the degree of force indeterminacy per unit
volume. The elastic range extends to small strain intervals, in agreement with
experimental observations. The origins of nonelastic response are discussed. We
conclude that elastic moduli provide access to mechanically important
information about coordination numbers, which escape direct measurement
techniques, and indicate further perspectives.Comment: Published in Physical Review E 25 page
A Study of Activated Processes in Soft Sphere Glass
On the basis of long simulations of a binary mixture of soft spheres just
below the glass transition, we make an exploratory study of the activated
processes that contribute to the dynamics. We concentrate on statistical
measures of the size of the activated processes.Comment: 17 pages, 9 postscript figures with epsf, uses harvmac.te
Forensic image analysis â CCTV distortion and artefacts
© 2018 Elsevier B.V. As a result of the worldwide deployment of surveillance cameras, authorities have gained a powerful tool that captures footage of activities of people in public areas. Surveillance cameras allow continuous monitoring of the area and allow footage to be obtained for later use, if a criminal or other act of interest occurs. Following this, a forensic practitioner, or expert witness can be required to analyse the footage of the Person of Interest. The examination ultimately aims at evaluating the strength of evidence at source and activity levels. In this paper, both source and activity levels are inferred from the trace, obtained in the form of CCTV footage. The source level alludes to features observed within the anatomy and gait of an individual, whilst the activity level relates to activity undertaken by the individual within the footage. The strength of evidence depends on the value of the information recorded, where the activity level is robust, yet source level requires further development. It is therefore suggested that the camera and the associated distortions should be assessed first and foremost and, where possible, quantified, to determine the level of each type of distortion present within the footage. A review of the âforensic image analysisâ review is presented here. It will outline the image distortion types and detail the limitations of differing surveillance camera systems. The aim is to highlight various types of distortion present particularly from surveillance footage, as well as address gaps in current literature in relation to assessment of CCTV distortions in tandem with gait analysis. Future work will consider the anatomical assessment from surveillance footage
Exciton-Polaritons in Uniaxially Aligned Organic Microcavities
Here we report the fabrication and optical characterization of organic
microcavities containing liquid-crystalline conjugated polymers (LCCPs):
poly(9,9-dioctylfluorene-co-benzothiadiazole) (F8BT), poly(9,9-dioctylfluorene)
(PFO) and poly(2,7-(9,9-dihexylfluorene)-co-bithiophene) (F6T2) aligned on top
of a thin transparent Sulfuric Dye 1 (SD1) photoalignment layer. We extract the
optical constants of the aligned films using variable angle spectroscopic
ellipsometry and fabricate metallic microcavities in which the ultrastrong
coupling regime is manifest both for the aligned and non-aligned LCCPs.
Transition dipole moment alignment enables a systematic increase in the
interaction strength, with unprecedented solid-state Rabi splitting energies up
to 1.80 eV for F6T2, the first to reach the visible spectrum; with an optical
gap of 2.79 eV this also gives the highest-to-date organic microcavity coupling
ratio, 65. We also demonstrate that the coupling strength is
polarization-dependent with bright polaritons photoluminescence for TE
polarization parallel to the transition dipoles and either no emission or
weakly coupled emission from the corresponding TM polarization. The use of
uniaxally aligned organic microcavities with switchable coupling strength
offers exciting prospects for direct observations of ultrastrong coupling
signatures, quantum simulation, polaritonics and condensation related
phenomena.Comment: Supplementary Information is available upon request (preferably via
e-mail
Forensic gait analysis â Morphometric assessment from surveillance footage
© 2019 Elsevier B.V. Following the technological rise of surveillance cameras and their subsequent proliferation in public places, the use of information gathered by such means for investigative and evaluative purposes sparked a large interest in the forensic community and within policing scenarios. In particular, it is suggested that analysis of the body, especially the assessment of gait characteristics, can provide useful information to aid the investigation. This paper discusses the influences upon gait to mitigate some of the limitations of surveillance footage, including those due to the varying anatomical differences between individuals. Furthermore, the differences between various techniques applied to assess gait are discussed, including biometric gait recognition, forensic gait analysis, tracking technology, and marker technology. This review article discusses the limitations of the current methods for assessment of gait; exposing gaps within the literature in regard to various influences impacting upon the gait cycle. Furthermore, it suggests a âmorphometricâ technique to enhance the available procedures to potentially facilitate the development of standardised protocols with supporting statistics and database. This in turn will provide meaningful information to forensic investigation, intelligence-gathering processes, and potentially as an additional method of forensic evaluation of evidence
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