1,637,483 research outputs found
A magnetic cycle of tau Bootis? The coronal and chromospheric view
Tau Bootis is a late F-type main sequence star orbited by a Hot Jupiter.
During the last years spectropolarimetric observations led to the hypothesis
that this star may host a global magnetic field that switches its polarity once
per year, indicating a very short activity cycle of only one year duration. In
our ongoing observational campaign, we have collected several X-ray
observations with XMM-Newton and optical spectra with TRES/FLWO in Arizona to
characterize tau Boo's corona and chromosphere over the course of the supposed
one-year cycle. Contrary to the spectropolarimetric reconstructions, our
observations do not show indications for a short activity cycle.Comment: 4 pages, 2 figures, appeared in Astronomical Notes 333, 1, 26-29
(2012
Keldysh field theory for nonequilibrium condensation in a parametrically pumped polariton system
We develop a quantum field theory for parametrically pumped polaritons using
Keldysh Green's function techniques. By considering the mean-field and Gaussian
fluctuations, we find that the low energy physics of the highly non-equilibrium
phase transition to the optical parametric oscillator regime is in many ways
similar to equilibrium condensation. In particular, we show that this phase
transition can be associated with an effective chemical potential, at which the
system's bosonic distribution function diverges, and an effective temperature.
As in equilibrium systems, the transition is achieved by tuning this effective
chemical potential to the energy of the lowest normal mode. Since the
occupations of the modes are available, we determine experimentally observable
properties, such as the luminescence and absorption spectra.Comment: 16 pages, 14 figure
Dispersive photoluminescence decay by geminate recombination in amorphous semiconductors
The photoluminescence decay in amorphous semiconductors is described by power
law at long times. The power-law decay of photoluminescence at
long times is commonly observed but recent experiments have revealed that the
exponent, , is smaller than the value 1.5 predicted from a
geminate recombination model assuming normal diffusion. Transient currents
observed in the time-of-flight experiments are highly dispersive characterized
by the disorder parameter smaller than 1. Geminate recombination rate
should be influenced by the dispersive transport of charge carriers. In this
paper we derive the simple relation, . Not only the
exponent but also the amplitude of the decay calculated in this study is
consistent with measured photoluminescence in a-Si:H.Comment: 18pages. Submitted for the publication in Phys. Rev.
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