1,263 research outputs found
Quantum transport properties of two-dimensional systems in disordered magnetic fields with a fixed sign
Quantum transport in disordered magnetic fields is investigated numerically
in two-dimensional systems. In particular, the case where the mean and the
fluctuation of disordered magnetic fields are of the same order is considered.
It is found that in the limit of weak disorder the conductivity exhibits a
qualitatively different behavior from that in the conventional random magnetic
fields with zero mean. The conductivity is estimated by the equation of motion
method and by the two-terminal Landauer formula. It is demonstrated that the
conductance stays on the order of even in the weak disorder limit. The
present behavior can be interpreted in terms of the Drude formula. The
Shubnikov-de Haas oscillation is also observed in the weak disorder regime.Comment: 6 pages, 7 figures, to appear in Phys. Rev.
Cosmology of hybrid metric-Palatini f(X)-gravity
A new class of modified theories of gravity, consisting of the superposition
of the metric Einstein-Hilbert Lagrangian with an term constructed
\`{a} la Palatini was proposed recently. The dynamically equivalent
scalar-tensor representation of the model was also formulated, and it was shown
that even if the scalar field is very light, the theory passes the Solar System
observational constraints. Therefore the model predicts the existence of a
long-range scalar field, modifying the cosmological and galactic dynamics. An
explicit model that passes the local tests and leads to cosmic acceleration was
also obtained. In the present work, it is shown that the theory can be also
formulated in terms of the quantity , where T and R are
the traces of the stress-energy and Ricci tensors, respectively. The variable X
represents the deviation with respect to the field equation trace of general
relativity. The cosmological applications of this hybrid metric-Palatini
gravitational theory are also explored, and cosmological solutions coming from
the scalar-tensor representation of f(X)-gravity are presented. Criteria to
obtain cosmic acceleration are discussed and the field equations are analyzed
as a dynamical system. Several classes of dynamical cosmological solutions,
depending on the functional form of the effective scalar field potential,
describing both accelerating and decelerating Universes are explicitly
obtained. Furthermore, the cosmological perturbation equations are derived and
applied to uncover the nature of the propagating scalar degree of freedom and
the signatures these models predict in the large-scale structure.Comment: 17 pages. V2: 18 pages; minor revision and references added; to
appear in JCA
Wormholes supported by hybrid metric-Palatini gravity
Recently, a modified theory of gravity was presented, which consists of the
superposition of the metric Einstein-Hilbert Lagrangian with an
term constructed \`{a} la Palatini. The theory possesses extremely interesting
features such as predicting the existence of a long-range scalar field, that
explains the late-time cosmic acceleration and passes the local tests, even in
the presence of a light scalar field. In this brief report, we consider the
possibility that wormholes are supported by this hybrid metric-Palatini
gravitational theory. We present here the general conditions for wormhole
solutions according to the null energy conditions at the throat and find
specific examples. In the first solution, we specify the redshift function, the
scalar field and choose the potential that simplifies the modified Klein-Gordon
equation. This solution is not asymptotically flat and needs to be matched to a
vacuum solution. In the second example, by adequately specifying the metric
functions and choosing the scalar field, we find an asymptotically flat
spacetime.Comment: 4 pages. V2: 5 pages, discussion added; matches published versio
Stereo vision based approach for extracting features from digital holograms
With digital holography one can record and reconstruct real world three-dimensional (3D) objects [1,2]. The recorded interference pattern includes information about both amplitude and phase of a wavefront reflected from or transmitted through the object. However, some of the hologram capture setups pose a problem for the reliable reconstruction of quantitative phase information. This can be because the twin image or noise corrupts the reconstructed phase. In such cases it is usual that only amplitude is reconstructed and used as the basis for metrology. A focus criterion is often applied to this reconstructed amplitude to extract depth information from the sensed 3D scene [3,4]. In this paper we present an alternative technique based on applying conventional computer stereo vision algorithms to amplitude reconstructions. We show the effectiveness of our technique using digital holograms of both macroscopic and microscopic real-world 3D objects. We discuss sensitivity to the depth of field of reconstructions, and which hologram capture setups are, and which are not, suitable for the technique
Shot noise and conductivity at high bias in bilayer graphene: Signatures of electron-optical phonon coupling
We have studied electronic conductivity and shot noise of bilayer graphene
(BLG) sheets at high bias voltages and low bath temperature K. As a
function of bias, we find initially an increase of the differential
conductivity, which we attribute to self-heating. At higher bias, the
conductivity saturates and even decreases due to backscattering from optical
phonons. The electron-phonon interactions are also responsible for the decay of
the Fano factor at bias voltages V. The high bias electronic
temperature has been calculated from shot noise measurements, and it goes up to
K at V. Using the theoretical temperature dependence of BLG
conductivity, we extract an effective electron-optical phonon scattering time
. In a 230 nm long BLG sample of mobility
cmVs, we find that decreases with increasing
voltage and is close to the charged impurity scattering time fs
at V.Comment: 7 pages, 7 figures. Extended version of the high bias part of version
1. The low bias part is discussed in arXiv:1102.065
Numerical reconstruction of digital holograms for conventional 3D display
True hologram video displays are currently under development, but are not yet available. Because of this restriction, conventional 3D displays can be used with digital holographic data. However when using conventional 3D displays, holographic data has to be processed correctly to meet the requirements of the display. A unique property of digital holograms, namely that a single hologram encodes multiple perspectives, can be used to achieve this goal. Reconstructions from digital holograms at different perspectives are processed further to meet the requirements of the conventional 3D display, which are typically based on stereoscopic images of the scene
Using disparity in digital holograms for three-dimensional object segmentation
Digital holography allows one to sense and reconstruct the amplitude and phase of a wavefront reflected from or
transmitted through a real-world three-dimensional (3D) object. However, some combinations of hologram capture setup
and 3D object pose problems for the reliable reconstruction of quantitative phase information. In particular, these are
cases where the twin image or noise corrupts the reconstructed phase. In such cases it is usual that only amplitude is
reconstructed and used as the basis for metrology. A focus criterion is often applied to this reconstructed amplitude to
extract depth information from the sensed 3D scene. In this paper we present an alternative technique based on applying
conventional stereo computer vision algorithms to amplitude reconstructions. In the technique, two perspectives are
reconstructed from a single hologram, and the stereo disparity between the pair is used to infer depth information for
different regions in the field of view. Such an approach has inherent simplifications in digital holography as the epipolar
geometry is known a priori. We show the effectiveness of the technique using digital holograms of real-world 3D
objects. We discuss extensions to multi-view algorithms, the effect of speckle, and sensitivity to the depth of field of
reconstruction
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