1,146 research outputs found
Prefect Klein tunneling in anisotropic graphene-like photonic lattices
We study the scattering of waves off a potential step in deformed honeycomb
lattices. For small deformations below a critical value, perfect Klein
tunneling is obtained. This means that a potential step in any direction
transmits waves at normal incidence with unit transmission probability,
irrespective of the details of the potential. Beyond the critical deformation,
a gap in the spectrum is formed, and a potential step in the deformation
direction reflects all normal-incidence waves, exhibiting a dramatic transition
form unit transmission to total reflection. These phenomena are generic to
honeycomb lattice systems, and apply to electromagnetic waves in photonic
lattices, quasi-particles in graphene, cold atoms in optical lattices
Suppression of geometrical barrier in crystals by Josephson vortex stacks
Differential magneto-optics are used to study the effect of dc in-plane
magnetic field on hysteretic behavior due to geometrical barriers in
crystals. In absence of in-plane field a vortex
dome is visualized in the sample center surrounded by barrier-dominated
flux-free regions. With in-plane field, stacks of Josephson vortices form
vortex chains which are surprisingly found to protrude out of the dome into the
vortex-free regions. The chains are imaged to extend up to the sample edges,
thus providing easy channels for vortex entry and for drain of the dome through
geometrical barrier, suppressing the magnetic hysteresis. Reduction of the
vortex energy due to crossing with Josephson vortices is evaluated to be about
two orders of magnitude too small to account for the formation of the
protruding chains. We present a model and numerical calculations that
qualitatively describe the observed phenomena by taking into account the
demagnetization effects in which flux expulsion from the pristine regions
results in vortex focusing and in the chain protrusion. Comparative
measurements on a sample with narrow etched grooves provide further support to
the proposed model.Comment: 12 figures (low res.) Higher resolution figures are available at the
Phys Rev B version. Typos correcte
Retinal metric: a stimulus distance measure derived from population neural responses
The ability of the organism to distinguish between various stimuli is limited
by the structure and noise in the population code of its sensory neurons. Here
we infer a distance measure on the stimulus space directly from the recorded
activity of 100 neurons in the salamander retina. In contrast to previously
used measures of stimulus similarity, this "neural metric" tells us how
distinguishable a pair of stimulus clips is to the retina, given the noise in
the neural population response. We show that the retinal distance strongly
deviates from Euclidean, or any static metric, yet has a simple structure: we
identify the stimulus features that the neural population is jointly sensitive
to, and show the SVM-like kernel function relating the stimulus and neural
response spaces. We show that the non-Euclidean nature of the retinal distance
has important consequences for neural decoding.Comment: 5 pages, 4 figures, to appear in Phys Rev Let
Incoherent white light solitons in logarithmically saturable noninstantaneous nonlinear media
We analytically demonstrate the existence of white light solitons in logarithmically saturable noninstantaneous nonlinear media. This incoherent soliton has elliptic Gaussian intensity profile, and elliptic Gaussian spatial correlation statistics. The existence curve of the soliton connects the strength of the nonlinearity, the spatial correlation distance as a function of frequency, and the characteristic width of the soliton. For this soliton to exist, the spatial correlation distance must be smaller for larger temporal frequency constituents of the beam
Incoherent white light solitons in logarithmically saturable noninstantaneous nonlinear media
We analytically demonstrate the existence of white light solitons in logarithmically saturable noninstantaneous nonlinear media. This incoherent soliton has elliptic Gaussian intensity profile, and elliptic Gaussian spatial correlation statistics. The existence curve of the soliton connects the strength of the nonlinearity, the spatial correlation distance as a function of frequency, and the characteristic width of the soliton. For this soliton to exist, the spatial correlation distance must be smaller for larger temporal frequency constituents of the beam
Quark Matter '99 --- Theoretical Summary: What Next?
I review the three broad areas where major progress has been reported: The
phase structure of strongly interacting matter, the properties of matter at the
instant when it freezes out into individual hadrons in the final stage of the
expansion of the hot fireball, and the status of the main signatures of the
formation of a quark-gluon plasma. In the final section I present some thoughts
about what should be done next, both in the experiemntal and the theoretical
arena.Comment: 10 pages, 1 figure, summary talk at Quark Matter '99, Torino, Italy,
somewhat modified, final versio
Coding âWhatâ and âWhenâ in the Archer Fish Retina
Traditionally, the information content of the neural response is quantified using statistics of the responses relative to stimulus onset time with the assumption that the brain uses onset time to infer stimulus identity. However, stimulus onset time must also be estimated by the brain, making the utility of such an approach questionable. How can stimulus onset be estimated from the neural responses with sufficient accuracy to ensure reliable stimulus identification? We address this question using the framework of colour coding by the archer fish retinal ganglion cell. We found that stimulus identity, âwhatâ, can be estimated from the responses of best single cells with an accuracy comparable to that of the animal's psychophysical estimation. However, to extract this information, an accurate estimation of stimulus onset is essential. We show that stimulus onset time, âwhenâ, can be estimated using a linear-nonlinear readout mechanism that requires the response of a population of 100 cells. Thus, stimulus onset time can be estimated using a relatively simple readout. However, large nerve cell populations are required to achieve sufficient accuracy
Random-Phase Solitons in Nonlinear Periodic Lattices
We predict the existence of random phase solitons in nonlinear periodic lattices. These solitons exist when the nonlinear response time is much longer than the characteristic time of random phase fluctuations. The intensity profiles, power spectra, and statistical (coherence) properties of these stationary waves conform to the periodicity of the lattice. The general phenomenon of such solitons is analyzed in the context of nonlinear photonic lattices
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