2,692 research outputs found
Acoustic and optical phonon dynamics from femtosecond time-resolved optical spectroscopy of superconducting iron pnictide Ca(Fe_0.944Co_0.056)_2As_2
We report temperature evolution of coherently excited acoustic and optical
phonon dynamics in superconducting iron pnictide single crystal
Ca(Fe_0.944Co_0.056)_2As_2 across the spin density wave transition at T_SDW ~
85 K and superconducting transition at T_SC ~20 K. Strain pulse propagation
model applied to the generation of the acoustic phonons yields the temperature
dependence of the optical constants, and longitudinal and transverse sound
velocities in the temperature range of 3.1 K to 300 K. The frequency and
dephasing times of the phonons show anomalous temperature dependence below T_SC
indicating a coupling of these low energy excitations with the Cooper-pair
quasiparticles. A maximum in the amplitude of the acoustic modes at T ~ 170 is
seen, attributed to spin fluctuations and strong spin-lattice coupling before
T_SDW.Comment: 6 pages, 4 figures (revised manuscript
Ultrafast quasiparticle dynamics in superconducting iron pnictide CaFe1.89Co0.11As2
Nonequilibrium quasiparticle relaxation dynamics is reported in
superconducting CaFe1.89Co0.11As2 single crystal using femtosecond
time-resolved pump-probe spectroscopy. The carrier dynamics reflects a
three-channel decay of laser deposited energy with characteristic time scales
varying from few hundreds of femtoseconds to order of few nanoseconds where the
amplitudes and time-constants of the individual electronic relaxation
components show significant changes in the vicinity of the spin density wave
(T_SDW ~ 85 K) and superconducting (T_SC ~ 20 K) phase transition temperatures.
The quasiparticles dynamics in the superconducting state reveals a charge gap
with reduced gap value of 2_0/k_BT_SC ~ 1.8. We have determined the
electron-phonon coupling constant \lemda to be ~ 0.14 from the temperature
dependent relaxation time in the normal state, a value close to those reported
for other types of pnictides. From the peculiar temperature-dependence of the
carrier dynamics in the intermediate temperature region between the
superconducting and spin density wave phase transitions, we infer a temperature
scale where the charge gap associated with the spin ordered phase is maximum
and closes on either side while approaching the two phase transition
temperatures.Comment: 6 pages, 4 figures (revised manuscript);
http://dx.doi.org/10.1016/j.ssc.2013.02.00
High contrast Mach-Zehnder lithium atom interferometer in the Bragg regime
We have constructed an atom interferometer of the Mach-Zehnder type,
operating with a supersonic beam of lithium. Atom diffraction uses Bragg
diffraction on laser standing waves. With first order diffraction, our
apparatus has given a large signal and a very good fringe contrast (74%), which
we believe to be the highest ever observed with atom interferometers. This
apparatus will be applied to high sensitivity measurementsComment: 6 pages, 3 figures, accepted by Appl. Phys.
X-ray spectral modelling of the AGN obscuring region in the CDFS: Bayesian model selection and catalogue
AGN are known to have complex X-ray spectra that depend on both the
properties of the accreting SMBH (e.g. mass, accretion rate) and the
distribution of obscuring material in its vicinity ("torus"). Often however,
simple and even unphysical models are adopted to represent the X-ray spectra of
AGN. In the case of blank field surveys in particular, this should have an
impact on e.g. the determination of the AGN luminosity function, the inferred
accretion history of the Universe and also on our understanding of the relation
between AGN and their host galaxies. We develop a Bayesian framework for model
comparison and parameter estimation of X-ray spectra. We take into account
uncertainties associated with X-ray data and photometric redshifts. We also
demonstrate how Bayesian model comparison can be used to select among ten
different physically motivated X-ray spectral models the one that provides a
better representation of the observations. Despite the use of low-count
spectra, our methodology is able to draw strong inferences on the geometry of
the torus. For a sample of 350 AGN in the 4 Ms Chandra Deep Field South field,
our analysis identifies four components needed to represent the diversity of
the observed X-ray spectra: (abridged). Simpler models are ruled out with
decisive evidence in favour of a geometrically extended structure with
significant Compton scattering. Regarding the geometry of the obscurer, there
is strong evidence against both a completely closed or entirely open toroidal
geometry, in favour of an intermediate case. The additional Compton reflection
required by data over that predicted by toroidal geometry models, may be a sign
of a density gradient in the torus or reflection off the accretion disk.
Finally, we release a catalogue with estimated parameters such as the accretion
luminosity in the 2-10 keV band and the column density, , of the
obscurer.Comment: 28 pages, 18 figures, catalogue available from
https://www.mpe.mpg.de/~jbuchner/agn_torus/analysis/cdfs4Ms_cat/, software
available from https://github.com/JohannesBuchner/BX
New Approaches to Data-Driven Civilian Oversight of Law Enforcement: An Introduction to the Second NACOLE/CJPR Special Issue
In April of 2016, National Association for Civilian Oversight of Law Enforcement (NACOLE) and John Jay College partnered to sponsor the Academic Symposium “Building Public Trust: Generating Evidence to Enhance Police Accountability and Legitimacy.” This essay introduces the Criminal Justice Policy Review Special Issue featuring peer-reviewed, empirical research papers first presented at the Symposium. We provide context for the Symposium in relation to contemporary national discourse on police accountability and legitimacy. In addition, we review each of the papers presented at the Symposium, and provide in-depth reviews of each of the manuscripts included in the Special Issue
The effect of induced forelimb lameness on thoracolumbar kinematics during treadmill locomotion
Reasons for performing study: Lameness has often been suggested to result in altered movement of the back, but there are no detailed studies describing such a relationship in quantitative terms. Objectives: To quantify the effect of induced subtle forelimb lameness on thoracolumbar kinematics in the horse. Methods: Kinematics of 6 riding horses was measured at walk and at trot on a treadmill before and after the induction of reversible forelimb lameness grade 2 (AAEP scale 1-5). Ground reaction forces (GRF) for individual limbs were calculated from kinematics. Results: The horses significantly unloaded the painful limb by 11.5% at trot, while unloading at walk was not significant. The overall flexion-extension range of back motion decreased on average by 0.2° at walk and increased by 3.3° at trot (P<0.05). Changes in angular motion patterns of vertebral joints were noted only at trot, with an increase in flexion of 0.9° at T10 (i.e. angle between T6, T10 and T13) during the stance phase of the sound diagonal and an increase in extension of the thoracolumbar area during stance of the lame diagonal (0.7° at T13, 0.8° at T17, 0.5° at L1, 0.4° at L3 and 0.3° at L5) (P<0.05). Lameness further caused a lateral bending of the cranial thoracic vertebral column towards the lame side (1.3° at T10 and 0.9° at T13) (P<0.05) during stance of the lame diagonal. Conclusions: Both range of motion and vertebral angular motion patterns are affected by subtle forelimb lameness. At walk, the effect is minimal, at trot the horses increased the vertebral range of motion and changed the pattern of thoracolumbar motion in the sagittal and horizontal planes, presumably in an attempt to move the centre of gravity away from the lame side and reduce the force on the affected limb. Potential relevance: Subtle forelimb lameness affects thoracolumbar kinematics. Future studies should aim at elucidating whether the altered movement patterns lead to back and/or neck dysfunction in the case of chronic lameness
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