2,010 research outputs found
Forster energy transfer signatures in optically driven quantum dot molecules
The Forster resonant energy transfer mechanism (FRET) is investigated in
optically driven and electrically gated tunnel coupled quantum dot molecules.
Two novel FRET induced optical signatures are found in the dressed excitonic
spectrum. This is constructed from exciton level occupation as function of pump
laser energy and applied bias, resembling a level anticrossing spectroscopy
measurement. We observe a redistribution of spectral weight and splitting of
the exciton spectral lines. FRET among single excitons induces a splitting in
the spatially-direct exciton lines, away from the anticrossing due to charge
tunneling in the molecule. However, near the anticrossing, a novel signature
appears as a weak satellite line following an indirect exciton line. FRET
signatures may also occur among indirect excitons, appearing as split indirect
lines. In that case, the signatures appear also in the direct biexciton states,
as the indirect satellite mixes in near the tunneling anticrossing region
Discovery of Extremely Embedded X-ray Sources in the R Coronae Australis Star Forming Core
With the XMM-Newton and Chandra observatories, we detected two extremely
embedded X-ray sources in the R Corona Australis (R CrA) star forming core,
near IRS 7. These sources, designated as XB and XA, have X-ray absorption
columns of ~3e23 cm-2 equivalent to AV ~180 mag. They are associated with the
VLA centimeter radio sources 10E and 10W, respectively. XA is the counterpart
of the near-infrared source IRS 7, whereas XB has no K-band counterpart above
19.4 mag. This indicates that XB is younger than typical Class I protostars,
probably a Class 0 protostar or in an intermediate phase between Class 0 and
Class I. The X-ray luminosity of XB varied between 29<log LX <31.2 ergs s-1 on
timescales of 3-30 months. XB also showed a monotonic increase in X-ray
brightness by a factor of two in 30 ksec during an XMM-Newton observation. The
XMM-Newton spectra indicate emission from a hot plasma with kT ~3-4 keV and
also show fluorescent emission from cold iron. Though the X-ray spectrum from
XB is similar to flare spectra from Class I protostars in luminosity and
temperature, the light curve does not resemble the lightcurves of magnetically
generated X-ray flares because the variability timescale of XB is too long and
because variations in X-ray count rate were not accompanied by variations in
spectral hardness. The short-term variation of XB may be caused by the partial
blocking of the X-ray plasma, while the month-long flux enhancement may be
driven by mass accretion.Comment: 26 pages, 8 figures, To be published in ApJ in April 200
Constraints on Hidden Photon Models from Electron g-2 and Hydrogen Spectroscopy
The hidden photon model is one of the simplest models which can explain the
anomaly of the muon anomalous magnetic moment (g-2). The experimental
constraints are studied in detail, which come from the electron g-2 and the
hydrogen transition frequencies. The input parameters are set carefully in
order to take dark photon contributions into account and to prevent the
analysis from being self-inconsistent. It is shown that the new analysis
provides a constraint severer by more than one order of magnitude than the
previous result.Comment: 18 pages, 2 figures, 1 table. v2: minor correction
Phase diagram of Yukawa systems near the one‐component‐plasma limit revisited
Transition inverse temperatures (or Γ values) at the fluid–solid phase boundary of Yukawa systems near the one‐component‐plasma (OCP) limit have been evaluated by molecular dynamics simulations. These values are systematically smaller than those obtained in an earlier study by Farouki and Hamaguchi [J. Chem. Phys. 101, 9885 (1994)]. The discrepancy is attributed to the fact that, in the earlier study, the harmonic entropy constants were approximated by that of the OCP, whereas the new results are based on more accurate harmonic entropy constants obtained from lattice‐dynamics calculations. The new molecular dynamics simulations also confirm that the bcc–fcc phase transition curve is in good agreement with that of the quasiharmonic theory in the regime κ≤1.4, where κ is the ratio of the Wigner–Seitz radius to the Debye length. Examples of Yukawa systems include dusty plasmas and colloidal suspensions. © 1996 American Institute of Physics.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/69874/2/JCPSA6-105-17-7641-1.pd
One Dimensional Dynamical Models of the Carina Nebula Bubble
We have tested the two main theoretical models of bubbles around massive star
clusters, Castor et al. and Chevalier & Clegg, against observations of the well
studied Carina Nebula. The Castor et al. theory over-predicts the X-ray
luminosity in the Carina bubble by a factor of 60 and expands too rapidly, by a
factor of 4; if the correct radius and age are used, the predicted X-ray
luminosity is even larger. In contrast, the Chevalier & Clegg model
under-predicts the X-ray luminosity by a factor of 10. We modify the Castor et
al. theory to take into account lower stellar wind mass loss rates, radiation
pressure, gravity, and escape of or energy loss from the hot shocked gas. We
argue that energy is advected rather than radiated from the bubble. We
undertake a parameter study for reduced stellar mass loss rates and for various
leakage rates and are able to find viable models. The X-ray surface brightness
in Carina is highest close to the bubble wall, which is consistent with
conductive evaporation from cold clouds. The picture that emerges is one in
which the hot gas pressure is far below that found by dividing the
time-integrated wind luminosity by the bubble volume; rather, the pressure in
the hot gas is set by pressure equilibrium with the photoionized gas at T=10^4
K. It follows that the shocked stellar winds are not dynamically important in
forming the bubbles.Comment: Accepted to APJ. 47 pages, 13 figure
Vortex Creep Heating vs. Dark Matter Heating in Neutron Stars
Dark matter particles captured in neutron stars deposit their energy as heat.
This DM heating effect can be observed only if it dominates over other internal
heating effects in NSs. In this work, as an example of such an internal heating
source, we consider the frictional heating caused by the creep motion of
neutron superfluid vortex lines in the NS crust. The luminosity of this heating
effect is controlled by the strength of the interaction between the vortex
lines and nuclei in the crust, which can be estimated from the many-body
calculation of a high-density nuclear system as well as through the temperature
observation of old NSs. We show that both the temperature observation and
theoretical calculation suggest that the vortex creep heating dominates over
the DM heating. The vortex-nuclei interaction must be smaller than the
estimated values by several orders of magnitude to overturn this.Comment: 8 pages, 2 figure
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