3,842 research outputs found
A Dimensional study of Disk Galaxies
We present a highly simplified model of the dynamical structure of a disk
galaxy where only two parameters fully determine the solution, mass and angular
momentum. We show through simple physical scalings that once the mass has been
fixed, the angular momentum parameter is expected to regulate such
critical galactic disk properties as colour, thickness of the disk and disk to
bulge ratio. It is hence expected to be the determinant physical ingredient
resulting in a given Hubble type. A simple analytic estimate of for
an observed system is provided. An explicit comparison of the distribution of
several galactic parameters against both Hubble type and is performed
using observed galaxies. Both such distributions exhibit highly similar
characteristics for all galactic properties studied, suggesting as a
physically motivated classification parameter for disk galaxies.Comment: 10 pages including 11 figures, Final version, MNRAS in pres
Ermakov Systems with Multiplicative Noise
Using the Euler-Maruyama numerical method, we present calculations of the
Ermakov-Lewis invariant and the dynamic, geometric, and total phases for
several cases of stochastic parametric oscillators, including the simplest case
of the stochastic harmonic oscillator. The results are compared with the
corresponding numerical noiseless cases to evaluate the effect of the noise.
Besides, the noiseless cases are analytic and their analytic solutions are
briefly presented. The Ermakov-Lewis invariant is not affected by the
multiplicative noise in the three particular examples presented in this work,
whereas there is a shift effect in the case of the phasesComment: 12 pages, 4 figures, 22 reference
Empirical distributions of galactic spin parameters from the SDSS
Using simple dimensional arguments for both spiral and elliptical galaxies,
we present formulas to derive an estimate of the halo spin parameter
for any real galaxy, in terms of common observational parameters. This allows a
rough estimate of , which we apply to a large volume limited sample of
galaxies taken from the SDSS data base. The large numbers involved (11,597)
allow the derivation of reliable distributions, as signal adds up
significantly in spite of the errors in the inferences for particular galaxies.
We find that if the observed distribution of is modeled with a
log-normal function, as often done for this distribution in dark matter halos
that appear in cosmological simulations, we obtain parameters and , interestingly consistent with
values derived from simulations. For spirals, we find a good correlation
between empirical values of and visually assigned Hubble types,
highlighting the potential of this physical parameter as an objective
classification tool.Comment: 8 pages, 6 figures, expanded final version, MNRAS (in press
Direct evidence for efficient ultrafast charge separation in epitaxial WS/graphene heterostructure
We use time- and angle-resolved photoemission spectroscopy (tr-ARPES) to
investigate ultrafast charge transfer in an epitaxial heterostructure made of
monolayer WS and graphene. This heterostructure combines the benefits of a
direct gap semiconductor with strong spin-orbit coupling and strong
light-matter interaction with those of a semimetal hosting massless carriers
with extremely high mobility and long spin lifetimes. We find that, after
photoexcitation at resonance to the A-exciton in WS, the photoexcited holes
rapidly transfer into the graphene layer while the photoexcited electrons
remain in the WS layer. The resulting charge transfer state is found to
have a lifetime of \,ps. We attribute our findings to differences in
scattering phase space caused by the relative alignment of WS and graphene
bands as revealed by high resolution ARPES. In combination with spin-selective
excitation using circularly polarized light the investigated WS/graphene
heterostructure might provide a new platform for efficient optical spin
injection into graphene.Comment: 28 pages, 14 figure
Remark on charge conjugation in the non relativistic limit
We study the non relativistic limit of the charge conjugation operation in the context of the Dirac equation coupled to an electromagnetic field.
The limit is well defined and, as in the relativistic case, ,
(parity) and (time reversal) are the generators of a matrix group
isomorphic to a semidirect sum of the dihedral group of eight elements and
. The existence of the limit is supported by an argument based in quantum
field theory. Also, and most important, the limit exists in the context of
galilean relativity. Finally, if one complexifies the Lorentz group and
therefore the galilean spacetime , then the explicit form of the matrix
for allows to interpret it, in this context, as the complex
conjugation of the spatial coordinates: . This result is
natural in a fiber bundle description.Comment: 8 page
Direct evidence for efficient ultrafast charge separation in epitaxial WS<sub>2</sub>/graphene heterostructures
We use time- and angle-resolved photoemission spectroscopy (tr-ARPES) to investigate ultrafast charge transfer in an epitaxial heterostructure made of monolayer WS2 and graphene. This heterostructure combines the benefits of a direct-gap semiconductor with strong spin-orbit coupling and strong light-matter interaction with those of a semimetal hosting massless carriers with extremely high mobility and long spin lifetimes. We find that, after photoexcitation at resonance to the A-exciton in WS2, the photoexcited holes rapidly transfer into the graphene layer while the photoexcited electrons remain in the WS2 layer. The resulting charge-separated transient state is found to have a lifetime of ∼1 ps. We attribute our findings to differences in scattering phase space caused by the relative alignment of WS2 and graphene bands as revealed by high-resolution ARPES. In combination with spin-selective optical excitation, the investigated WS2/graphene heterostructure might provide a platform for efficient optical spin injection into graphene
SPRINT INTERVAL TRAINING ON STATIONARY AIR BIKE SHOWS BENEFITS TO CARDIORESPIRATORY ADAPTATIONS WHILE BEING TIME-EFFICIENT
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