21,518 research outputs found
Dependence of spin dephasing on initial spin polarization in a high-mobility two-dimensional electron system
We have studied the spin dynamics of a high-mobility two-dimensional electron
system in a GaAs/Al_{0.3}Ga_{0.7}As single quantum well by time-resolved
Faraday rotation and time-resolved Kerr rotation in dependence on the initial
degree of spin polarization, P, of the electrons. By increasing the initial
spin polarization from the low-P regime to a significant P of several percent,
we find that the spin dephasing time, , increases from about 20 ps to
200 ps; Moreover, increases with temperature at small spin
polarization but decreases with temperature at large spin polarization. All
these features are in good agreement with theoretical predictions by Weng and
Wu [Phys. Rev. B {\bf 68}, 075312 (2003)]. Measurements as a function of spin
polarization at fixed electron density are performed to further confirm the
theory. A fully microscopic calculation is performed by setting up and
numerically solving the kinetic spin Bloch equations, including the
D'yakonov-Perel' and the Bir-Aronov-Pikus mechanisms, with {\em all} the
scattering explicitly included. We reproduce all principal features of the
experiments, i.e., a dramatic decrease of spin dephasing with increasing
and the temperature dependences at different spin polarizations.Comment: 8 pages, 8 figures, to be published in PR
Controlling ultrafast currents by the non-linear photogalvanic effect
We theoretically investigate the effect of broken inversion symmetry on the
generation and control of ultrafast currents in a transparent dielectric (SiO2)
by strong femto-second optical laser pulses. Ab-initio simulations based on
time-dependent density functional theory predict ultrafast DC currents that can
be viewed as a non-linear photogalvanic effect. Most surprisingly, the
direction of the current undergoes a sudden reversal above a critical threshold
value of laser intensity I_c ~ 3.8*10^13 W/cm2. We trace this switching to the
transition from non-linear polarization currents to the tunneling excitation
regime. We demonstrate control of the ultrafast currents by the time delay
between two laser pulses. We find the ultrafast current control by the
non-linear photogalvanic effect to be remarkably robust and insensitive to
laser-pulse shape and carrier-envelope phase
Effects of 3D Geometries on Cellular Gradient Sensing and Polarization
During cell migration, cells become polarized, change their shape, and move
in response to various internal and external cues. Cell polarization is defined
through the spatio-temporal organization of molecules such as PI3K or small
GTPases, and is determined by intracellular signaling networks. It results in
directional forces through actin polymerization and myosin contractions. Many
existing mathematical models of cell polarization are formulated in terms of
reaction-diffusion systems of interacting molecules, and are often defined in
one or two spatial dimensions. In this paper, we introduce a 3D
reaction-diffusion model of interacting molecules in a single cell, and find
that cell geometry has an important role affecting the capability of a cell to
polarize, or change polarization when an external signal changes direction. Our
results suggest a geometrical argument why more roundish cells can repolarize
more effectively than cells which are elongated along the direction of the
original stimulus, and thus enable roundish cells to turn faster, as has been
observed in experiments. On the other hand, elongated cells preferentially
polarize along their main axis even when a gradient stimulus appears from
another direction. Furthermore, our 3D model can accurately capture the effect
of binding and unbinding of important regulators of cell polarization to and
from the cell membrane. This spatial separation of membrane and cytosol, not
possible to capture in 1D or 2D models, leads to marked differences of our
model from comparable lower-dimensional models.Comment: 31 pages, 7 figure
Characterization and Application of Hard X-Ray Betatron Radiation Generated by Relativistic Electrons from a Laser-Wakefield Accelerator
The necessity for compact table-top x-ray sources with higher brightness,
shorter wavelength and shorter pulse duration has led to the development of
complementary sources based on laser-plasma accelerators, in contrast to
conventional accelerators. Relativistic interaction of short-pulse lasers with
underdense plasmas results in acceleration of electrons and in consequence in
the emission of spatially coherent radiation, which is known in the literature
as betatron radiation. In this article we report on our recent results in the
rapidly developing field of secondary x-ray radiation generated by high-energy
electron pulses. The betatron radiation is characterized with a novel setup
allowing to measure the energy, the spatial energy distribution in the
far-field of the beam and the source size in a single laser shot. Furthermore,
the polarization state is measured for each laser shot. In this way the emitted
betatron x-rays can be used as a non-invasive diagnostic tool to retrieve very
subtle information of the electron dynamics within the plasma wave. Parallel to
the experimental work, 3D particle-in-cell simulations were performed, proved
to be in good agreement with the experimental results.Comment: 38 pages, 19 figures, submitted to the Journal of Plasma Physic
- …