1,682 research outputs found
Spin precession and inverted Hanle effect in a semiconductor near a finite-roughness ferromagnetic interface
Although the creation of spin polarization in various non-magnetic media via
electrical spin injection from a ferromagnetic tunnel contact has been
demonstrated, much of the basic behavior is heavily debated. It is reported
here for semiconductor/Al2O3/ferromagnet tunnel structures based on Si or GaAs
that local magnetostatic fields arising from interface roughness dramatically
alter and even dominate the accumulation and dynamics of spins in the
semiconductor. Spin precession in the inhomogeneous magnetic fields is shown to
reduce the spin accumulation up to tenfold, and causes it to be inhomogeneous
and non-collinear with the injector magnetization. The inverted Hanle effect
serves as experimental signature. This interaction needs to be taken into
account in the analysis of experimental data, particularly in extracting the
spin lifetime and its variation with different parameters (temperature, doping
concentration). It produces a broadening of the standard Hanle curve and
thereby an apparent reduction of the spin lifetime. For heavily doped n-type Si
at room temperature it is shown that the spin lifetime is larger than
previously determined, and a new lower bound of 0.29 ns is obtained. The
results are expected to be general and occur for spins near a magnetic
interface not only in semiconductors but also in metals, organic and
carbon-based materials including graphene, and in various spintronic device
structures.Comment: Final version, with text restructured and appendices added (25 pages,
9 figures). To appear in Phys. Rev.
Asymptotic Behaviour of Reduced-Order Filters
Reduced-order filters are proposed for linear and nonlinear systems and their long time behaviour is studied. Using the results of Ocone and Pardoux \cite{ocpa} on the asymptotic stability of the optimal filter with respect to its initial condition, the asymptotic efficiency of these filters is established in various cases
Asymptotic Optimality of Approximate Filters in Stochastic Systems with Colored Noises
In this report, a new interpretation of the main part of our study on asymptotic behaviour of reduced-order filters in [6] is given. Approximate filters are proposed for semi-linear and nonlinear stochastic systems with colored noises. Basically these filters are defined as those which are optimal when the noises are white. Their long time behavior is investigate- d and their asymptotic optimality is shown in two cases under some reasonable assumptions. At first the case of a system where the signal and observation dynamics are linear with respect to the state is considered; the approximate filter is a Kalman filter and the asymptotic analysis relies on a representati- on of the optimal filter which extends a formula known for a linear system with white noises and non-Gaussian initial condition. Then the case of a nonlinear system with limiting ergodic behavior is analyzed; here the approximate filter appears as the optimal filter corresponding to an incorrect prior distribution and the asymptotic study uses the results of Ocone and Pardoux on the asymptotic stability of optimal filters with respect to their initial condition
Kinematics and Convergent Tectonics of the Northwestern South American Plate During the Cenozoic
The interaction of the northern Nazca and southwestern Caribbean oceanic plates with northwestern South America (NWSA) and the collision of the Panama-Choco arc (PCA) have significant implications on the evolution of the northern Andes. Based on a quantitative kinematic reconstruction of the Caribbean and Farallon/Farallon-derived plates, we reconstructed the subducting geometries beneath NWSA and the PCA accretion to the continent. The persistent northeastward migration of the Caribbean plate relative to NWSA in Cenozoic time caused the continuous northward advance of the Farallon-Caribbean plate boundary, which in turn resulted in its progressive concave trench bending against NWSA. The increasing complexity during the Paleogene included the onset of Caribbean shallow subduction, the PCA approaching the continent, and the forced shallow Farallon subduction that ended in the fragmentation of the Farallon Plate into the Nazca and Cocos plates and the Coiba and Malpelo microplates by the late Oligocene. The convergence tectonics after late Oligocene comprised the accretional process of the PCA to NWSA, which evolved from subduction erosion of the forearc to collisional tectonics by the middle Miocene, as well as changes of convergence angle and slab dip of the Farallon-derived plates, and the attachment of the Coiba and Malpelo microplates to the Nazca plate around 9 Ma, resulting in a change of convergence directions. During the Pliocene, the Nazca slab broke at 5.5°N, shaping the modern configuration. Overall, the proposed reconstruction is supported by geophysical data and is well correlated with the magmatic and deformation history of the northern Andes
Electrical spin injection and detection in Germanium using three terminal geometry
In this letter, we report on successful electrical spin injection and
detection in \textit{n}-type germanium-on-insulator (GOI) using a
Co/Py/AlO spin injector and 3-terminal non-local measurements. We
observe an enhanced spin accumulation signal of the order of 1 meV consistent
with the sequential tunneling process via interface states in the vicinity of
the AlO/Ge interface. This spin signal is further observable up to
220 K. Moreover, the presence of a strong \textit{inverted} Hanle effect points
at the influence of random fields arising from interface roughness on the
injected spins.Comment: 4 pages, 3 figure
An Elementary Approach to Filtering in Systems with Fractional Brownian Observation Noise
The problem of optimal filtering is addressed for a signal observed through a possibly nonlinear channel driven by a fractional Brownian motion. An elementary and completely self-contained approach is developed. An appropriate Girsanov type result is proved and a process -- equivalent to the innovation process in the usual situation where the observation noise is a Brownian motion -- is introduced. Zakai's approach is partly extended to derive filtering equations when the signal process is a diffusion. The case of conditionally Gaussian linear systems is analyzed. Closed form equations are derived both for the mean of the optimal filter and the conditional variance of the filtering error. The results are explicit in various special cases
Electrical and thermal spin accumulation in germanium
In this letter, we first show electrical spin injection in the germanium
conduction band at room temperature and modulate the spin signal by applying a
gate voltage to the channel. The corresponding signal modulation agrees well
with the predictions of spin diffusion models. Then by setting a temperature
gradient between germanium and the ferromagnet, we create a thermal spin
accumulation in germanium without any tunnel charge current. We show that
temperature gradients yield larger spin accumulations than pure electrical spin
injection but, due to competing microscopic effects, the thermal spin
accumulation in germanium remains surprisingly almost unchanged under the
application of a gate voltage to the channel.Comment: 7 pages, 3 figure
Crossover from spin accumulation into interface states to spin injection in the germanium conduction band
Electrical spin injection into semiconductors paves the way for exploring new
phenomena in the area of spin physics and new generations of spintronic
devices. However the exact role of interface states in spin injection mechanism
from a magnetic tunnel junction into a semiconductor is still under debate. In
this letter, we demonstrate a clear transition from spin accumulation into
interface states to spin injection in the conduction band of -Ge. We observe
spin signal amplification at low temperature due to spin accumulation into
interface states followed by a clear transition towards spin injection in the
conduction band from 200 K up to room temperature. In this regime, the spin
signal is reduced down to a value compatible with spin diffusion model. More
interestingly, we demonstrate in this regime a significant modulation of the
spin signal by spin pumping generated by ferromagnetic resonance and also by
applying a back-gate voltage which are clear manifestations of spin current and
accumulation in the germanium conduction band.Comment: 5 pages, 4 figure
Srs2 removes deadly recombination intermediates independently of its interaction with SUMO-modified PCNA
Saccharomyces cerevisiae Srs2 helicase plays at least two distinct functions. One is to prevent recombinational repair through its recruitment by sumoylated Proliferating Cell Nuclear Antigen (PCNA), evidenced in postreplication-repair deficient cells, and a second one is to eliminate potentially lethal intermediates formed by recombination proteins. Both actions are believed to involve the capacity of Srs2 to displace Rad51 upon translocation on single-stranded DNA (ssDNA), though a role of its helicase activity may be important to remove some toxic recombination structures. Here, we described two new mutants, srs2R1 and srs2R3, that have lost the ability to hinder recombinational repair in postreplication-repair mutants, but are still able to remove toxic recombination structures. Although the mutants present very similar phenotypes, the mutated proteins are differently affected in their biochemical activities. Srs2R1 has lost its capacity to interact with sumoylated PCNA while the biochemical activities of Srs2R3 are attenuated (ATPase, helicase, DNA binding and ability to displace Rad51 from ssDNA). In addition, crossover (CO) frequencies are increased in both mutants. The different roles of Srs2, in relation to its eventual recruitment by sumoylated PCNA, are discussed
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