3,773 research outputs found
Faraday rotation: effect of magnetic field reversals
The standard formula for the rotation measure, RM, which determines the
position angle, , due to Faraday rotation, includes
contributions only from the portions of the ray path where the natural modes of
the plasma are circularly polarized. In small regions of the ray path where the
projection of the magnetic field on the ray path reverses sign (called QT
regions) the modes are nearly linearly polarized. The neglect of QT regions in
estimating RM is not well justified at frequencies below a transition frequency
where mode coupling changes from strong to weak. By integrating the
polarization transfer equation across a QT region in the latter limit, I
estimate the additional contribution needed to correct this
omission. In contrast with a result proposed by \cite{BB10}, is
small and probably unobservable. I identify a new source of circular
polarization, due to mode coupling in an asymmetric QT region. I also identify
a new circular-polarization-dependent correction to the dispersion measure at
low frequencies.Comment: 25 pages 1 figure, accepted for publication in The Astrophysical
Journa
Signatures of integrability in charge and thermal transport in 1D quantum systems
Integrable and non-integrable systems have very different transport
properties. In this work, we highlight these differences for specific one
dimensional models of interacting lattice fermions using numerical exact
diagonalization. We calculate the finite temperature adiabatic stiffness (or
Drude weight) and isothermal stiffness (or ``Meissner'' stiffness) in
electrical and thermal transport and also compute the complete momentum and
frequency dependent dynamical conductivities and
. The Meissner stiffness goes to zero rapidly with system
size for both integrable and non-integrable systems. The Drude weight shows
signs of diffusion in the non-integrable system and ballistic behavior in the
integrable system. The dynamical conductivities are also consistent with
ballistic and diffusive behavior in the integrable and non-integrable systems
respectively.Comment: 4 pages, 4 figure
A Stellar Audit: The Computation of Encounter Rates for 47 Tucanae and omega Centauri
Using King-Mitchie Models, we compute encounter rates between the various
stellar species in the globular clusters Cen, and 47 Tuc. We also
compute event rates for encounters between single stars and a population of
primordial binaries. Using these rates, and what we have learnt from
hydrodynamical simulations of encounters performed earlier, we compute the
production rates of objects such as low-mass X-ray binaries (LMXBs), smothered
neutron stars and blue stragglers (massive main-sequence stars). If 10\% of the
stars are contained in primordial binaries, the production rate of interesting
objects from encounters involving these binaries is as large as that from
encounters between single stars. For example, encounters involving binaries
produce a significant number of blue stragglers in both globular cluster
models. The number of smothered neutron stars may exceed the number of low-mass
X-ray binaries (LMXBs) by a factor of 5-20, which may help explain why
millisecond pulsars are observed to outnumber LMXBs in globular clusters.Comment: uuencoded compressed postscript. The preprint is also available at
http://www.ast.cam.ac.uk/preprint/PrePrint.htm
Electrodynamic modeling of strong coupling between a metasurface and intersubband transitions in quantum wells
Strong light-matter coupling has recently been demonstrated in sub-wavelength
volumes by coupling engineered optical transitions in semiconductor
heterostructures (e.g., quantum wells) to metasurface resonances via near
fields. It has also been shown that different resonator shapes may lead to
different Rabi splittings, though this has not yet been well explained. In this
paper, our aim is to understand the correlation between resonator shape and
Rabi splitting, and in particular determine and quantify the physical
parameters that affect strong coupling by developing an equivalent circuit
network model whose elements describe energy and dissipation. Because of the
subwavelength dimension of each metasurface element, we resort to the
quasi-static (electrostatic) description of the near-field and hence define an
equivalent capacitance associated to each dipolar element of a flat
metasurface, and we show that this is also able to accurately model the
phenomenology involved in strong coupling between the metasurface and the
intersubband transitions in quantum wells. We show that the spectral properties
and stored energy of a metasurface/quantum-well system obtained using our model
are in good agreement with both full-wave simulation and experimental results.
We then analyze metasurfaces made of three different resonator geometries and
observe that the magnitude of the Rabi splitting increases with the resonator
capacitance in agreement with our theory, providing a phenomenological
explanation for the resonator shape dependence of the strong coupling process.Comment: 10 pages, 10 figure
Red giant collisions in the galactic centre
We simulate collisions involving red-giant stars in the centre of our galaxy.
Such encounters may explain the observed paucity of highly luminous red giants
within the central 0.2pc. The masses of the missing stars are likely to be in
the range 2 to 8 solar masses. Recent models of the galactic centre cluster's
density and velocity distributions are used to calculate two-body collision
rates. In particular we use stellar-evolution models to calculate the number of
collisions a star will have during different evolutionary phases. We find that
the number of two-body collisions per star is \lo 1 in the central 0.1 to 0.2
pc, depending strongly on the galactocentric radius. Using a 3D numerical
hydrodynamics code (SPH) we simulate encounters involving cluster stars of
various masses with 2 and 8 solar-mass red giants. The instantaneous mass loss
in such collisions is rarely enough to destroy either giant. A fraction of the
collisions do, however, lead to the formation of common envelope systems where
the impactor and giant's core are enshrouded by the envelope of the giant. Such
systems may evolve to expel the envelope, leaving a tight binary; the original
giant is destroyed. The fraction of collisions that produce common envelope
systems is sensitive to the local velocity dispersion and hence galactocentric
radius. Using our collision-rate calculations we compute the time-scales for a
giant star to suffer such a collision within the galactic centre. These
time-scales are >10^{9-10}years and so are longer than the lifetimes of stars
more-massive than 2 solar masses. Thus the observed paucity of luminous giants
is unlikely to be due to the formation of common envelope systems as a result
of two-body encounters involving giant stars.Comment: 10 pages, 11 figures, MNRAS (in press
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