37,662 research outputs found
An exterior for the G\"{o}del spacetime
We match the vacuum, stationary, cylindrically symmetric solution of
Einstein's field equations with , in a form recently given by Santos,
as an exterior to an infinite cylinder of dust cut out of a G\"{o}del universe.
There are three cases, depending on the radius of the cylinder. Closed timelike
curves are present in the exteriors of some of the solutions. There is a
considerable similarity between the spacetimes investigated here and those of
van Stockum referring to an infinite cylinder of rotating dust matched to
vacuum, with .Comment: 11 pages, LaTeX 2.09, no figures. Submitted to Classical and Quantum
Gravit
Detecting transit signatures of exoplanetary rings using SOAP3.0
CONTEXT. It is theoretically possible for rings to have formed around
extrasolar planets in a similar way to that in which they formed around the
giant planets in our solar system. However, no such rings have been detected to
date.
AIMS: We aim to test the possibility of detecting rings around exoplanets by
investigating the photometric and spectroscopic ring signatures in
high-precision transit signals.
METHODS: The photometric and spectroscopic transit signals of a ringed planet
is expected to show deviations from that of a spherical planet. We used these
deviations to quantify the detectability of rings. We present SOAP3.0 which is
a numerical tool to simulate ringed planet transits and measure ring
detectability based on amplitudes of the residuals between the ringed planet
signal and best fit ringless model.
RESULTS: We find that it is possible to detect the photometric and
spectroscopic signature of near edge-on rings especially around planets with
high impact parameter. Time resolution 7 mins is required for the
photometric detection, while 15 mins is sufficient for the spectroscopic
detection. We also show that future instruments like CHEOPS and ESPRESSO, with
precisions that allow ring signatures to be well above their noise-level,
present good prospects for detecting rings.Comment: 13 pages, 16 figures, 2 tables , accepted for publication in A&
Bulk-Edge correspondence of entanglement spectrum in 2D spin ground states
General local spin ground states, described by a Valence Bond Solid (VBS)
on a two dimensional lattice are studied. The norm of these ground states is
mapped to a classical O(3) model on the same lattice. Using this
quantum-to-classical mapping we obtain the partial density matrix
associated with a subsystem of the original ground state. We show that
the entanglement spectrum of in a translation invariant lattice
is given by the spectrum of a quantum spin chain at the boundary of region ,
with local Heisenberg type interactions between spin 1/2 particles.Comment: 8 pages, 4 figures, one section and references adde
Delocalization and wave-packet dynamics in one-dimensional diluted Anderson models
We study the nature of one-electron eigen-states in a one-dimensional diluted
Anderson model where every Anderson impurity is diluted by a periodic function
. Using renormalization group and transfer matrix techniques, we provide
accurate estimates of the extended states which appear in this model, whose
number depends on the symmetry of the diluting function . The density of
states (DOS) for this model is also numerically obtained and its main features
are related to the symmetries of the diluting function . Further, we show
that the emergence of extended states promotes a sub-diffusive spread of an
initially localized wave-packet.Comment: 6 pages, 6 figures, to appear in EPJ
Changes in postural sway behavior across the life span
The present study aimed to investigate human balance control by assessing postural sway on three groups representing three stages of life (6-12, 19-40 and 65-74 years old). There were 14 individuals in each group and they were tested during upright bipedal stance with either eyes open or closed. Focus was given to multiple sway indices representing multi-dimensional features of postural sway in quiet stance and included: the center of pressure area, amplitude, root mean square (RMS), velocity, jerkiness, and sample entropy. Results confirmed that children and seniors swayed more (p\u3c.004), faster (p\u3c.001) and their body sway was shakier (p\u3c.001) than young adults. Seniors also presented faster (p\u3c.006) and shakier (p\u3c.001) sway than children and a more unpredictable pattern of body sway in time (p\u3c.002) than children and young adults. In addition, children presented a more random anterior-posterior sway (p\u3c.034) and a more regular medio-lateral sway (p\u3c.043) than young adults, and a higher synchronization between anterior-posterior and medio-lateral body sway (p\u3c.012) than young adults and seniors. We also observed that postural control of children and young adults becomes relatively more challenged in experimental situations when eyes were closed for most postural indices. In conclusion, this study suggests that multi-dimension posturography is sensitive to detect subtle age-related changes in the postural behavior and each stage of life may have their own signature patterns of postural behavior. Therefore, we expect that quantifications of this nature may be used to assess not only postural instability and fall risk but also to aid the testing of the efficacy of balance interventional protocols
Disorder Induced Localized States in Graphene
We consider the electronic structure near vacancies in the half-filled
honeycomb lattice. It is shown that vacancies induce the formation of localized
states. When particle-hole symmetry is broken, localized states become
resonances close to the Fermi level. We also study the problem of a finite
density of vacancies, obtaining the electronic density of states, and
discussing the issue of electronic localization in these systems. Our results
also have relevance for the problem of disorder in d-wave superconductors.Comment: Replaced with published version. 4 pages, 4 figures. Fig. 1 was
revise
Phenomenological study of the electronic transport coefficients of graphene
Using a semi-classical approach and input from experiments on the
conductivity of graphene, we determine the electronic density dependence of the
electronic transport coefficients -- conductivity, thermal conductivity and
thermopower -- of doped graphene. Also the electronic density dependence of the
optical conductivity is obtained. Finally we show that the classical Hall
effect (low field) in graphene has the same form as for the independent
electron case, characterized by a parabolic dispersion, as long as the
relaxation time is proportional to the momentum.Comment: 4 pages, 1 figur
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