13,630 research outputs found
ARPES view of orbitally resolved quasiparticle lifetimes in iron pnictides
We study with ARPES the renormalization and quasiparticle lifetimes of the
and / orbitals in two iron pnictides, LiFeAs and
Ba(FeCo)As (Co8). We find that both quantities depend
on orbital character rather than on the position on the Fermi Surface (for
example hole or electron pocket). In LiFeAs, the renormalizations are larger
for , while they are similar on both types of orbitals in Co8. The most
salient feature, which proved robust against all the ARPES caveats we could
think of, is that the lifetimes for exhibit a markedly different
behavior than those for /. They have smaller values near
and exhibit larger and temperature dependences. While the behavior of
is compatible with a Fermi liquid description, it is not the case for
/. This situation should have important consequences for the
physics of iron pnictides, which have not been considered up to now. More
generally, it raises interesting questions on how a Fermi liquid regime can be
established in a multiband system with small effective bandwidths
Brightness, distribution, and evolution of sunspot umbral dots
We present a 106-minute TiO (705.7nm) time series of high spatial and
temporal resolution that contains thousands of umbral dots (UDs) in a mature
sunspot in the active region NOAA 10667 at =0.95. The data were acquired
with the 1-m Swedish Solar Telescope on La Palma. With the help of a multilevel
tracking (MLT) algorithm the sizes, brightnesses, and trajectories of 12836
umbral dots were found and analyzed. The MLT allows UDs with very low contrast
to be reliably identified. Inside the umbra we determine a UD filling factor of
11%. The histogram of UD lifetimes is monotonic, i.e. a UD does not have a
typical lifetime. Three quarters of the UDs lived for less than 150s and showed
no or little motion. The histogram of the UD diameters exhibits a maximum at
225km, i.e. most of the UDs are spatially resolved. UDs display a typical
horizontal velocity of 420m/s and a typical peak intensity of 51% of the mean
intensity of the quiet photosphere, making them on average 20% brighter than
the local umbral background. Almost all mobile UDs (large birth-death distance)
were born close to the umbra-penumbra boundary, move towards the umbral center,
and are brighter than average. Notably bright and mobile UDs were also observed
along a prominent UD chain, both ends of which are located at the
umbra-penumbra boundary. Their motion started primarily at either of the ends
of the chain, continued along the chain, and ended near the chain's center. We
observed the splitting and merging of UDs and the temporal succession of both.
For the first time the evolution of brightness, size, and horizontal speed of a
typical UD could be determined in a statistically significant way. Considerable
differences between the evolution of central and peripheral UDs are found,
which point to a difference in origin
Correlations and Characterization of Emitting Sources
Dynamical and thermal characterizations of excited nuclear systems produced
during the collisions between two heavy ions at intermediate incident energies
are presented by means of a review of experimental and theoretical work
performed in the last two decades. Intensity interferometry, applied to both
charged particles (light particles and intermediate mass fragments) and to
uncharged radiation (gamma rays and neutrons) has provided relevant information
about the space-time properties of nuclear reactions. The volume, lifetime,
density and relative chronology of particle emission from decaying nuclear
sources has been extensively explored and has provided valuable information
about the dynamics of heavy-ion collisions. Similar correlation techniques
applied to coincidences between light particles and complex fragments are also
presented as a tool to determine the internal excitation energy of excited
primary fragments as it appears in secondary-decay phenomena.Comment: To appear on Euorpean Physics Journal A as part of the Topical Volume
"Dynamics and Thermodynamics with Nuclear Degrees of Freedom
Spin States Protected from Intrinsic Electron-Phonon-Coupling Reaching 100 ns Lifetime at Room Temperature in MoSe
We present time-resolved Kerr rotation measurements, showing spin lifetimes
of over 100 ns at room temperature in monolayer MoSe. These long lifetimes
are accompanied by an intriguing temperature dependence of the Kerr amplitude,
which increases with temperature up to 50 K and then abruptly switches sign.
Using ab initio simulations we explain the latter behavior in terms of the
intrinsic electron-phonon coupling and the activation of transitions to
secondary valleys. The phonon-assisted scattering of the photo-excited
electron-hole pairs prepares a valley spin polarization within the first few ps
after laser excitation. The sign of the total valley magnetization, and thus
the Kerr amplitude, switches as a function of temperature, as conduction and
valence band states exhibit different phonon-mediated inter-valley scattering
rates. However, the electron-phonon scattering on the ps time scale does not
provide an explanation for the long spin lifetimes. Hence, we deduce that the
initial spin polarization must be transferred into spin states which are
protected from the intrinsic electron-phonon coupling, and are most likely
resident charge carriers which are not part of the itinerant valence or
conduction band states.Comment: 18 pages, 17 figure
Black Holes in Galaxy Mergers: Evolution of Quasars
Based on numerical simulations of gas-rich galaxy mergers, we discuss a model
in which quasar activity is tied to the self-regulated growth of supermassive
black holes in galaxies. Nuclear inflow of gas attending a galaxy collision
triggers a starburst and feeds black hole growth, but for most of the duration
of the starburst, the black hole is heavily obscured by surrounding gas and
dust which limits the visibility of the quasar, especially at optical and UV
wavelengths. Eventually, feedback energy from accretion heats the gas and
expels it in a powerful wind, leaving a 'dead quasar'. Between buried and dead
phases there is a window during which the galaxy would be seen as a luminous
quasar. Because the black hole mass, radiative output, and distribution of
obscuring gas and dust all evolve strongly with time, the duration of this
phase of observable quasar activity depends on both the waveband and imposed
luminosity threshold. We determine the observed and intrinsic lifetimes as a
function of luminosity and frequency, and calculate observable lifetimes ~10
Myr for bright quasars in the optical B-band, in good agreement with empirical
estimates and much smaller than the black hole growth timescales ~100 Myr,
naturally producing a substantial population of 'buried' quasars. However,
observed and intrinsic energy outputs converge in the IR and hard X-ray bands
as attenuation becomes weaker and chances of observation greatly increase. We
obtain the distribution of column densities along sightlines in which the
quasar is seen above a given luminosity, and find that our result agrees
remarkably well with observed estimates of the column density distribution from
the SDSS for appropriate luminosity thresholds. (Abridged)Comment: 12 pages, 7 figures. Accepted for publication in ApJ (September
2005). Replacement with minor revisions from referee repor
Metastable level lifetimes from electron-shelving measurements with ion clouds and single ions
The lifetime of the 3d^2D_5/2-level in singly-ionized calcium has been
measured by the electron-shelving technique on different samples of rf trapped
ions. The metastable state has been directly populated by exciting the
dipole-forbidden 4S_1/2 - 3D_5/2 transition. In ion clouds, the natural
lifetime of this metastable level has been measured to be (1095+-27) ms. For
the single-ion case, we determined a lifetime of (1152+-20) ms. The
1sigma-error bars at the 2%-level have different origins for the two kinds of
experiments: data fitting methods for lifetime measurements in an ion cloud and
control of experimental parameters for a single ion. De-shelving effects are
extensively discussed. The influence of differing approaches for the processing
of the single-ion quantum jump data on the lifetime values is shown. Comparison
with recent measurements shows excellent agreement when evaluated from a given
method
Decorrelation Times of Photospheric Fields and Flows
We use autocorrelation to investigate evolution in flow fields inferred by
applying Fourier Local Correlation Tracking (FLCT) to a sequence of
high-resolution (0.3 \arcsec), high-cadence ( min) line-of-sight
magnetograms of NOAA active region (AR) 10930 recorded by the Narrowband Filter
Imager (NFI) of the Solar Optical Telescope (SOT) aboard the {\em Hinode}
satellite over 12--13 December 2006. To baseline the timescales of flow
evolution, we also autocorrelated the magnetograms, at several spatial
binnings, to characterize the lifetimes of active region magnetic structures
versus spatial scale. Autocorrelation of flow maps can be used to optimize
tracking parameters, to understand tracking algorithms' susceptibility to
noise, and to estimate flow lifetimes. Tracking parameters varied include: time
interval between magnetogram pairs tracked, spatial binning applied
to the magnetograms, and windowing parameter used in FLCT. Flow
structures vary over a range of spatial and temporal scales (including
unresolved scales), so tracked flows represent a local average of the flow over
a particular range of space and time. We define flow lifetime to be the flow
decorrelation time, . For , tracking results represent
the average velocity over one or more flow lifetimes. We analyze lifetimes of
flow components, divergences, and curls as functions of magnetic field strength
and spatial scale. We find a significant trend of increasing lifetimes of flow
components, divergences, and curls with field strength, consistent with Lorentz
forces partially governing flows in the active photosphere, as well as strong
trends of increasing flow lifetime and decreasing magnitudes with increases in
both spatial scale and .Comment: 48 pages, 20 figures, submitted to the Astrophysical Journal;
full-resolution images in manuscript (8MB) at
http://solarmuri.ssl.berkeley.edu/~welsch/public/manuscripts/flow_lifetimes_v2.pd
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