3,492 research outputs found
Femtosecond study of the interplay between excitons, trions, and carriers in (Cd,Mn)Te quantum wells
We present an absorption study of the neutral and positively charged exciton
(trion) under the influence of a femtosecond, circularly polarized, resonant
pump pulse. Three populations are involved: free holes, excitons, and trions,
all exhibiting transient spin polarization. In particular, a polarization of
the hole gas is created by the formation of trions. The evolution of these
populations is studied, including the spin flip and trion formation processes.
The contributions of several mechanisms to intensity changes are evaluated,
including phase space filling and spin-dependent screening. We propose a new
explanation of the oscillator strength stealing phenomena observed in p-doped
quantum wells, based on the screening of neutral excitons by charge carriers.
We have also found that binding heavy holes into charged excitons excludes them
from the interaction with the rest of the system, so that oscillator strength
stealing is partially blockedComment: 4 pages, 4 figure
Excitonic - vibronic coupled dimers: A dynamic approach
The dynamical properties of exciton transfer coupled to polarization
vibrations in a two site system are investigated in detail. A fixed point
analysis of the full system of Bloch - oscillator equations representing the
coupled excitonic - vibronic flow is performed. For overcritical polarization a
bifurcation converting the stable bonding ground state to a hyperbolic unstable
state which is basic to the dynamical properties of the model is obtained. The
phase space of the system is generally of a mixed type: Above bifurcation chaos
develops starting from the region of the hyperbolic state and spreading with
increasing energy over the Bloch sphere leaving only islands of regular
dynamics. The behaviour of the polarization oscillator accordingly changes from
regular to chaotic.Comment: uuencoded compressed Postscript file containing text and figures. In
case of questions, please, write to [email protected]
PP-waves with torsion and metric-affine gravity
A classical pp-wave is a 4-dimensional Lorentzian spacetime which admits a
nonvanishing parallel spinor field; here the connection is assumed to be
Levi-Civita. We generalise this definition to metric compatible spacetimes with
torsion and describe basic properties of such spacetimes. We use our
generalised pp-waves for constructing new explicit vacuum solutions of
quadratic metric-affine gravity.Comment: 17 pages, LaTeX2
Transition from the Couette-Taylor system to the plane Couette system
We discuss the flow between concentric rotating cylinders in the limit of
large radii where the system approaches plane Couette flow. We discuss how in
this limit the linear instability that leads to the formation of Taylor
vortices is lost and how the character of the transition approaches that of
planar shear flows. In particular, a parameter regime is identified where
fractal distributions of life times and spatiotemporal intermittency occur.
Experiments in this regime should allow to study the characteristics of shear
flow turbulence in a closed flow geometry.Comment: 5 pages, 5 figure
Fuzzy cellular model for on-line traffic simulation
This paper introduces a fuzzy cellular model of road traffic that was
intended for on-line applications in traffic control. The presented model uses
fuzzy sets theory to deal with uncertainty of both input data and simulation
results. Vehicles are modelled individually, thus various classes of them can
be taken into consideration. In the proposed approach, all parameters of
vehicles are described by means of fuzzy numbers. The model was implemented in
a simulation of vehicles queue discharge process. Changes of the queue length
were analysed in this experiment and compared to the results of NaSch cellular
automata model.Comment: The original publication is available at http://www.springerlink.co
Beam-Normal Single Spin Asymmetry in Elastic Electron Scattering off Si and Zr
We report on a new measurement of the beam-normal single spin asymmetry
in the elastic scattering of 570 MeV transversely polarized
electrons off Si and Zr at . The
studied kinematics allow for a comprehensive comparison with former results on
C. No significant mass dependence of the beam-normal single spin
asymmetry is observed in the mass regime from C to Zr.Comment: Submitted for publication to Physics Letters
Gallium scintigraphy in the diagnosis and total lymphoid irradiation of Takayasu's arteritis
Takayasu's arteritis (TA) in children causes appreciable morbidity and mortality, predominantly as a result of the complication of renovascular hypertension (RVH). Ten children with TA, complicated by RVH, were treated at our centre over the past decade. An initial raised erythrocyte sedimentation rate (ESR) and a purified protein derivative greater than 15 mm were present in every case. More recently, gallium scintigraphy has been used to demonstrate sites of active inflammation in affected vessels (3/4 patients) which became negative after total lymphoid irradiation (TU). The latter was used in the last 6 children, and appeared to be effective in controlling disease activity as evinced in the normalisation of their ESRs and negative findings on gallium scintigraphy (in all 3 patients with prior active inflammation). Because of vascular damage caused by the vasculitic process, surgical intervention is often required to improve organ perfusion, particularly of the kidney/so Renal autografting (or allografting) seems preferable (6/11 kidneys functional) to renal bypass grafting (5/5 kidneys clotted). Patient survival improved when TU was used in addition to standard surgical and medical therapy; this included steroids and antituberculous therapy with TU, and steroids and cyclophosphamide in the two relapses. Five of 6 patients treated with TU were alive after 32 - 54 months' follow-up, while 4 patients who received standard medical and surgical therapy but not TU all died within 18 months of diagnosis. Gallium scintigraphy is a helpful diagnostic tool in assessing vasculitic activity in TA; TU is an important mode of immunosuppression, but still needs to be compared with cyclophosphamide as the major immunosuppressive
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