613 research outputs found
Electron spin relaxation in paramagnetic Ga(Mn)As quantum wells
Electron spin relaxation in paramagnetic Ga(Mn)As quantum wells is studied
via the fully microscopic kinetic spin Bloch equation approach where all the
scatterings, such as the electron-impurity, electron-phonon, electron-electron
Coulomb, electron-hole Coulomb, electron-hole exchange (the Bir-Aronov-Pikus
mechanism) and the - exchange scatterings, are explicitly included. The
Elliot-Yafet mechanism is also incorporated. From this approach, we study the
spin relaxation in both -type and -type Ga(Mn)As quantum wells. For
-type Ga(Mn)As quantum wells where most Mn ions take the interstitial
positions, we find that the spin relaxation is always dominated by the DP
mechanism in metallic region. Interestingly, the Mn concentration dependence of
the spin relaxation time is nonmonotonic and exhibits a peak. This behavior is
because that the momentum scattering and the inhomogeneous broadening have
different density dependences in the non-degenerate and degenerate regimes. For
-type Ga(Mn)As quantum wells, we find that Mn concentration dependence of
the spin relaxation time is also nonmonotonic and shows a peak. Differently,
this behavior is because that the - exchange scattering (or the
Bir-Aronov-Pikus) mechanism dominates the spin relaxation in the high Mn
concentration regime at low (or high) temperature, whereas the DP mechanism
determines the spin relaxation in the low Mn concentration regime. The
Elliot-Yafet mechanism also contributes the spin relaxation at intermediate
temperature. The spin relaxation time due to the DP mechanism increases with Mn
concentration due to motional narrowing, whereas those due to the spin-flip
mechanisms decrease with Mn concentration, which thus leads to the formation of
the peak.... (The remaining is omitted due to the space limit)Comment: 12 pages, 8 figures, Phys. Rev. B 79, 2009, in pres
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
Resonant Raman scattering by collective modes of the one-dimensional electron gas
We show that the low-energy peak in the polarized resonant Raman spectra of
quantum wires, which is commonly associated with ``single particle
excitations'', can be interpreted as signature of intra-band collective spin
excitations. A broad maximum in the resonant depolarized spectra is predicted
to exist above the frequency of the spin density excitation, due to
simultaneous but independent propagation of spin- and charge-density modes.Comment: 4 pages, accepted for publication in Phys. Rev. Let
Effects of tillage systems and mechanization on work time, fuel and energy consumption for cereal cropping in Austria
The machinery stock, fuel consumption and work time are crucial economic factors for the profit potential in the arable farming sector. The influence of five soil tillage systems (two conventional tillage systems and three conservation tillage systems) and two tractor sizes (92 kW-tractor and 59 kW-tractor) on work time, fuel and energy consumption was measured in the semi-arid region in Austria. The tractors were equipped with a high-performance flow meter and a radar sensor to measure the fuel consumption (L h-1) and working speed (km h-1). The conventional tillage with mouldboard plough has the highest working time and fuel consumption rate. The replacement of plough with a cultivator, reduces the work time and fuel consumption for soil tillage as well as the energy consumption per moved soil matter to more than 50% roughly. The highest saving effects (more than 85%) were achieved with the direct drilling without soil tillage system. A well loaded engine in a small tractor with small implements is more fuel efficient than a worse loaded engine in a “big tractor”. An adjusted tractor-implement combination, which is well implemented in the 59-kW mechanization, decreases the fuel consumption to up to 30% and 46%. Due to lower field capacity in the 59-kW mechanization, the work time is higher between 2.4% and 11.7%. Keywords: fuel consumption, mechanization, tillage system, work tim
Detection of large magneto-anisotropy of electron spin dephasing in a high-mobility two-dimensional electron system in a GaAs/AlGaAs quantum well
In time-resolved Faraday rotation experiments we have detected an inplane
anisotropy of the electron spin-dephasing time (SDT) in an
--modulation-doped GaAs/AlGaAs single quantum well. The SDT
was measured with magnetic fields of T, applied in the and
inplane crystal directions of the GaAs quantum well. For fields
along , we have found an up to a factor of about 2 larger SDT than
in the perpendicular direction. Fully microscopic calculations, by numerically
solving the kinetic spin Bloch equations considering the D'yakonov-Perel' and
the Bir-Aronov-Pikus mechanisms, reproduce the experimental findings
quantitatively. This quantitative analysis of the data allowed us to determine
the relative strengths of Rashba and Dresselhaus terms in our sample. Moreover,
we could estimate the SDT for spins aligned in the {\em inplane}
direction to be on the order of several nanoseconds, which is up to two orders
of magnitude larger than that in the perpendicular {\em inplane} direction.Comment: 4 pages, 4 figures, to be published in PR
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
Magnetization of noncircular quantum dots
We calculate the magnetization of quantum dots deviating from circular
symmetry for noninteracting electrons or electrons interacting according to the
Hartree approximation. For few electrons the magnetization is found to depend
on their number, and the shape of the dot. The magnetization is an ideal probe
into the many-electron state of a quantum dot.Comment: 11 RevTeX pages with 6 included Postscript figure
AC-conductance of a quantum wire with electron-electron interaction
The complex ac-response of a quasi-one dimensional electron system in the
one-band approximation with an interaction potential of finite range is
investigated. It is shown that linear response is exact for this model. The
influence of the screening of the electric field is discussed. The complex
absorptive conductance is analyzed in terms of resistive, capacitive and
inductive behaviors.Comment: 13 pages, REVTeX, 7 eps figures, to appear in Phys. Rev.
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