803 research outputs found
Circularly Polarized Resonant Rayleigh Scattering and Skyrmions in the = 1 Quantum Hall Ferromagnet
We use the circularly polarized resonant Rayleigh scattering (RRS) to study
the quantum Hall ferromagnet at = 1. At this filling factor we observe a
right handed copolarized RRS which probes the Skyrmion spin texture of the
electrons in the photoexcited grounds state. The resonant scattering is not
present in the left handed copolarization, and this can be related to the
correlation between Skymionic effects, screening and spin wave excitations.
These results evidence that RRS is a valid method for the study of the spin
texture of the quantum Hall states
Statics and Dynamics of the Wormlike Bundle Model
Bundles of filamentous polymers are primary structural components of a broad
range of cytoskeletal structures, and their mechanical properties play key
roles in cellular functions ranging from locomotion to mechanotransduction and
fertilization. We give a detailed derivation of a wormlike bundle model as a
generic description for the statics and dynamics of polymer bundles consisting
of semiflexible polymers interconnected by crosslinking agents. The elastic
degrees of freedom include bending as well as twist deformations of the
filaments and shear deformation of the crosslinks. We show that a competition
between the elastic properties of the filaments and those of the crosslinks
leads to renormalized effective bend and twist rigidities that become
mode-number dependent. The strength and character of this dependence is found
to vary with bundle architecture, such as the arrangement of filaments in the
cross section and pretwist. We discuss two paradigmatic cases of bundle
architecture, a uniform arrangement of filaments as found in F-actin bundles
and a shell-like architecture as characteristic for microtubules. Each
architecture is found to have its own universal ratio of maximal to minimal
bending rigidity, independent of the specific type of crosslink induced
filament coupling; our predictions are in reasonable agreement with available
experimental data for microtubules. Moreover, we analyze the predictions of the
wormlike bundle model for experimental observables such as the tangent-tangent
correlation function and dynamic response and correlation functions. Finally,
we analyze the effect of pretwist (helicity) on the mechanical properties of
bundles. We predict that microtubules with different number of protofilaments
should have distinct variations in their effective bending rigidity
Orbital current mode in elliptical quantum dots
An orbital current mode peculiar to deformed quantum dots is theoretically
investigated; first by using a simple model that allows to interpret
analytically its main characteristics, and second, by numerically solving the
microscopic equations of time evolution after an initial perturbation within
the time-dependent local-spin-density approximation. Results for different
deformations and sizes are shown.Comment: 4 REVTEX pages, 4 PDF figures, accepted in PRB:R
Wurtzite quantum wires with strong spatial confinement: polarization anisotropies in single wire spectroscopy
We report GaAs/AlGaAs nanowires in the one-dimensional (1D) quantum limit.
The ultrathin wurtzite GaAs cores between 20-40\,nm induce large confinement
energies of several tens of meV, allowing us to experimentally resolve up to
four well separated subband excitations in microphotoluminescence spectroscopy.
Our detailed experimental and theoretical polarization-resolved study reveals a
strong diameter-dependent anisotropy of these transitions: We demonstrate that
the polarization of the detected photoluminescence is governed by the symmetry
of the wurtzite 1D quantum wire subbands on the one hand, but also by the
dielectric mismatch of the wires with the surrounding material on the other
hand. The latter effect leads to a strong attenuation of perpendicularly
polarized light in thin dielectric wires, making the thickness of the AlGaAs
shell an important factor in the observed polarization behavior. Including the
dielectric mismatch to our k.p-based simulated polarization-resolved spectra of
purely wurtzite GaAs quantum wires, we find an excellent agreement between
experiment and theory
Theory of Resonant Raman Scattering in One Dimensional Electronic systems
A theory of resonant Raman scattering spectroscopy of one dimensional
electronic systems is developed on the assumptions that (i) the excitations of
the one dimensional electronic system are described by the Luttinger Liquid
model, (ii) Raman processes involve virtual excitations from a filled valence
band to an empty state of the one dimensional electronic system and (iii)
excitonic interactions between the valence and conduction bands may be
neglected. Closed form analytic expressions are obtained for the Raman
scattering cross sections, and are evaluated analytically and numerically for
scattering in the polarized channel, revealing a "double-peak" structure with
the lower peak involving multispinon excitations with total spin S=0 and the
higher peak being the conventional plasmon. A key feature of our results is a
nontrivial power law dependence, involving the Luttinger Liquid exponents, of
the dependence of the Raman cross sections on the difference of the laser
frequency from resonance. We find that near resonance the calculated ratio of
intensity in the lower energy feature to the intensity in the higher energy
feature saturates at a value of the order of unity (times a factor of the ratio
of the velocities of the two modes). We explicate the differences between the
'Luttinger liquid' and 'Fermi liquid' calculations of RRS spectra and argue
that excitonic effects, neglected in all treatments so far, are essential for
explaining the intensity ratios observed in quantum wires. We also discuss
other Luttinger liquid features which may be observed in future RRS
experiments
Oscillation modes of two-dimensional nanostructures within the time-dependent local-spin-density approximation
We apply the time-dependent local-spin-density approximation as general
theory to describe ground states and spin-density oscillations in the linear
response regime of two-dimensional nanostructures of arbitrary shape. For this
purpose, a frequency analysis of the simulated real-time evolution is
performed. The effect on the response of the recently proposed spin-density
waves in the ground state of certain parabolic quantum dots is considered. They
lead to the prediction of a new class of excitations, soft spin-twist modes,
with energies well below that of the spin dipole oscillation.Comment: 4 RevTex pages and 4 GIF figures, accepted in PR
Cyclotron effect on coherent spin precession of two-dimensional electrons
We investigate the spin dynamics of high-mobility two-dimensional electrons
in GaAs/AlGaAs quantum wells grown along the and directions by
time-resolved Faraday rotation at low temperatures. In measurements on the
-grown structures without external magnetic fields, we observe coherent
oscillations of the electron spin polarization about the effective spin-orbit
field. In non-quantizing magnetic fields applied normal to the sample plane,
the cyclotron motion of the electrons rotates the effective spin-orbit field.
This rotation leads to fast oscillations in the spin polarization about a
non-zero value and a strong increase in the spin dephasing time in our
experiments. These two effects are absent in the -grown structure due to
the different symmetry of its effective spin-orbit field. The measurements are
in excellent agreement with our theoretical model.Comment: 4 pages, 3 figure
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