5,212 research outputs found
Rotating frames and gauge invariance in two-dimensional many-body quantum systems
We study the quantization of many-body systems in two dimensions in rotating
coordinate frames using a gauge invariant formulation of the dynamics. We
consider reference frames defined by linear and quadratic gauge conditions. In
both cases we discuss their Gribov ambiguities and commutator algebra. We
construct the momentum operators, inner-product and Hamiltonian in both types
of gauges, for systems with and without translation invariance. The analogy
with the quantization of QED in non-covariant gauges is emphasized. Our results
are applied to quasi-rigid systems in the Eckart frame.Comment: 18 pages, LaTeX2e, AmsLaTe
Planetary nebulae and stellar kinematics in the flattened elliptical galaxy NGC 1344
We present photometric and kinematic information obtained by measuring 197
planetary nebulae (PNs) discovered in the flattened Fornax elliptical galaxy
NGC 1344 (also known as NGC 1340) with an on-band, off-band, grism + on-band
filter technique. We build the PN luminosity function (PNLF) and use it to
derive a distance modulus m-M=31.4, slightly smaller than, but in good
agreement with, the surface brightness fluctuation distance. The PNLF also
provides an estimate of the specific PN formation rate: 6x10^-12 PNs per year
per solar luminosity. Combining the positional information from the on-band
image with PN positions measured on the grism + on-band image, we can measure
the radial velocities of 195 PNs, some of them distant more than 3 effective
radii from the center of NGC 1344. We complement this data set with stellar
kinematics derived from integrated spectra along the major and minor axes, and
parallel to the major axis of NGC 1344. The line-of-sight velocity dispersion
profile indicates the presence of a dark matter halo around this galaxy.Comment: 45 pages, 18 figures, accepted for publication in Ap
Electrical conductivity measured in atomic carbon chains
The first electrical conductivity measurements of monoatomic carbon chains
are reported in this study. The chains were obtained by unraveling carbon atoms
from graphene ribbons while an electrical current flowed through the ribbon
and, successively, through the chain. The formation of the chains was
accompanied by a characteristic drop in the electrical conductivity. The
conductivity of carbon chains was much lower than previously predicted for
ideal chains. First-principles calculations using both density functional and
many-body perturbation theory show that strain in the chains determines the
conductivity in a decisive way. Indeed, carbon chains are always under varying
non-zero strain that transforms its atomic structure from cumulene to polyyne
configuration, thus inducing a tunable band gap. The modified electronic
structure and the characteristics of the contact to the graphitic periphery
explain the low conductivity of the locally constrained carbon chain.Comment: 21 pages, 9 figure
Delocalization of Wannier-Stark ladders by phonons: tunneling and stretched polarons
We study the coherent dynamics of a Holstein polaron in strong electric
fields. A detailed analytical and numerical analysis shows that even for small
hopping constant and weak electron-phonon interaction, polaron states can
become delocalized if a resonance condition develops between the original
Wannier-Stark states and the phonon modes, yielding both tunneling and
`stretched' polarons. The unusual stretched polarons are characterized by a
phonon cloud that {\em trails} the electron, instead of accompanying it. In
general, our novel approach allows us to show that the polaron spectrum has a
complex nearly-fractal structure, due to the coherent coupling between states
in the Cayley tree which describes the relevant Hilbert space. The eigenstates
of a finite ladder are analyzed in terms of the observable tunneling and
optical properties of the system.Comment: 7 pages, 4 figure
Study of Heart Rate Variability in Bipolar Disorder: Linear and Non-Linear Parameters during Sleep.
The aim of the study is to define physiological parameters and vital signs that may be related to the mood and mental status in patients affected by bipolar disorder. In particular we explored the autonomic nervous system through the analysis of the heart rate variability. Many different parameters, in the time and in the frequency domain, linear and non-linear were evaluated during the sleep in a group of normal subject and in one patient in four different conditions. The recording of the signals was performed through a wearable sensorized T-shirt. Heart rate variability (HRV) signal and movement analysis allowed also obtaining sleep staging and the estimation of REM sleep percentage over the total sleep time. A group of eight normal females constituted the control group, on which normality ranges were estimated. The pathologic subject was recorded during four different nights, at time intervals of at least 1âweek, and during different phases of the disturbance. Some of the examined parameters (MEANNN, SDNN, RMSSD) confirmed reduced HRV in depression and bipolar disorder. REM sleep percentage was found to be increased. LempelâZiv complexity and sample entropy, on the other hand, seem to correlate with the depression level. Even if the number of examined subjects is still small, and the results need further validation, the proposed methodology and the calculated parameters seem promising tools for the monitoring of mood changes in psychiatric disorders
Suppression of hole-hole scattering in GaAs/AlGaAs heterostructures under uniaxial compression
Resistance, magnetoresistance and their temperature dependencies have been
investigated in the 2D hole gas at a [001] p-GaAs/AlGaAs
heterointerface under [110] uniaxial compression. Analysis performed in the
frame of hole-hole scattering between carriers in the two spin splitted
subbands of the ground heavy hole state indicates, that h-h scattering is
strongly suppressed by uniaxial compression. The decay time of the
relative momentum reveals 4.5 times increase at a uniaxial compression of 1.3
kbar.Comment: 5 pages, 3 figures. submitted to Phys.Rev.
Detection, photometry and slitless radial velocities of 535 planetary nebulae in the flattened elliptical galaxy NGC 4697
We have detected 535 planetary nebulae (PNs) in NGC 4697, using the classic
on-band, off-band filter technique with the Focal Reducer and Spectrograph
(FORS) at the Cassegrain focus of the first 8-meter telescope unit of the ESO
Very Large Telescope. From our photometry we have built the [O III] 5007
planetary nebula luminosity function (PNLF) of NGC 4697. It indicates a
distance of 10.5 Mpc, substantially smaller than a previous estimate of 24 Mpc
used in earlier dynamical studies. The PNLF also provides an estimate of the
specific PN formation rate. Combining the information from on-band images with
PN positions on dispersed, slitless grism images, we have obtained radial
velocities for 531 of the 535 PNs. They provide kinematic information up to a
distance of almost three effective radii from the nucleus. Some rotation is
detected in the outer regions, but the rotation curve of this galaxy appears to
drop beyond one effective radius. Assuming an isotropic velocity distribution,
the velocity dispersion profile is consistent with no dark matter within three
effective radii of the nucleus (however, some dark matter can be present if the
velocity distribution is anisotropic). We obtain a blue mass-to-light ratio of
11. Earlier M/L ratios for NGC 4697 were too small, because of the too large
distance used for their derivation.Comment: 52 pages, 24 Postscript figures, ApJ 2001, in pres
SeDAR: Reading Floorplans Like a HumanâUsing Deep Learning to Enable Human-Inspired Localisation
This is the final version. Available from Springer Verlag via the DOI in this record.âŻThe use of human-level semantic information to aid robotic tasks has recently become an important area for both Computer
Vision and Robotics. This has been enabled by advances in Deep Learning that allow consistent and robust semantic understanding. Leveraging this semantic vision of the world has allowed human-level understanding to naturally emerge from many
different approaches. Particularly, the use of semantic information to aid in localisation and reconstruction has been at the
forefront of both fields. Like robots, humans also require the ability to localise within a structure. To aid this, humans have
designed high-level semantic maps of our structures called floorplans. We are extremely good at localising in them, even with
limited access to the depth information used by robots. This is because we focus on the distribution of semantic elements,
rather than geometric ones. Evidence of this is that humans are normally able to localise in a floorplan that has not been
scaled properly. In order to grant this ability to robots, it is necessary to use localisation approaches that leverage the same
semantic information humans use. In this paper, we present a novel method for semantically enabled global localisation. Our
approach relies on the semantic labels present in the floorplan. Deep Learning is leveraged to extract semantic labels from
RGB images, which are compared to the floorplan for localisation. While our approach is able to use range measurements if
available, we demonstrate that they are unnecessary as we can achieve results comparable to state-of-the-art without them.EPSRCInnovate UKNVIDIA Corporatio
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