492 research outputs found
The effects of a magnetic barrier and a nonmagnetic spacer in tunnel structures
The spin-polarized transport is investigated in a new type of magnetic tunnel
junction which consists of two ferromagnetic electrodes separated by a magnetic
barrier and a nonmagnetic metallic spacer. Based on the transfer matrix method
and the nearly-free-electron-approximation the dependence of the tunnel
magnetoresistance (TMR) and electron-spin polarization on the nonmagnetic layer
thickness and the applied bias voltage are studied theoretically. The TMR and
spin polarization show an oscillatory behavior as a function of the spacer
thickness and the bias voltage. The oscillations originate from the quantum
well states in the spacer, while the existence of the magnetic barrier gives
rise to a strong spin polarization and high values of the TMR. Our results may
be useful for the development of spin electronic devices based on coherent
transport.Comment: 15 pages, 5 figure
Thermodynamics of C incorporation on Si(100) from ab initio calculations
We study the thermodynamics of C incorporation on Si(100), a system where
strain and chemical effects are both important. Our analysis is based on
first-principles atomistic calculations to obtain the important lowest energy
structures, and a classical effective Hamiltonian which is employed to
represent the long-range strain effects and incorporate the thermodynamic
aspects. We determine the equilibrium phase diagram in temperature and C
chemical potential, which allows us to predict the mesoscopic structure of the
system that should be observed under experimentally relevant conditions.Comment: 5 pages, 3 figure
On composite likelihood in bivariate meta-analysis of diagnostic test accuracy studies
The composite likelihood (CL) is amongst the computational methods used for estimation of the generalized linear mixed model (GLMM) in the context of bivariate meta-analysis of diagnostic test accuracy studies. Its advantage is that the likelihood can be derived conveniently under the assumption of independence between the random effects, but there has not been a clear analysis of the merit or necessity of this method. For synthesis of diagnostic test accuracy studies, a copula mixed model has been proposed in the biostatistics literature. This general model includes the GLMM as a special case and can also allow for flexible dependence modelling, different from assuming simple linear correlation structures, normality and tail independence in the joint tails. A maximum likelihood (ML) method, which is based on evaluating the bi-dimensional integrals of the likelihood with quadrature methods has been proposed, and in fact it eases any computational difficulty that might be caused by the double integral in the likelihood function. Both methods are thoroughly examined with extensive simulations and illustrated with data of a published meta-analysis. It is shown that the ML method has non-convergence issues or computational difficulties and at the same time allows estimation of the dependence between study-specific sensitivity and specificity and thus prediction via summary receiver operating curves
Orthorectification of helicopter-borne high resolution experimental burn observation from infra red handheld imagers
To pursue the development and validation of coupled fire-atmosphere models, the wildland fire modeling community needs validation data sets with scenarios where fire-induced winds influence fire front behavior, and with high temporal and spatial resolution. Helicopter-borne infrared thermal cameras have the potential to monitor landscape-scale wildland fires at a high resolution during experimental burns. To extract valuable information from those observations, three-step image processing is required: (a) Orthorectification to warp raw images on a fixed coordinate system grid, (b) segmentation to delineate the fire front location out of the orthorectified images, and (c) computation of fire behavior metrics such as the rate of spread from the time-evolving fire front location. This work is dedicated to the first orthorectification step, and presents a series of algorithms that are designed to process handheld helicopter-borne thermal images collected during savannah experimental burns. The novelty in the approach lies on its recursive design, which does not require the presence of fixed ground control points, hence relaxing the constraint on field of view coverage and helping the acquisition of high-frequency observations. For four burns ranging from four to eight hectares, long-wave and mid infra red images were collected at 1 and 3 Hz, respectively, and orthorectified at a high spatial resolution (<1 m) with an absolute accuracy estimated to be lower than 4 m. Subsequent computation of fire radiative power is discussed with comparison to concurrent space-borne measurementsPeer ReviewedPostprint (published version
The Complexity of Flat Freeze LTL
We consider the model-checking problem for freeze LTL on one-counter automata (OCAs). Freeze LTL extends LTL with the freeze quantifier, which allows one to store different counter values of a run in registers so that they can be compared with one another. As the model-checking problem is undecidable in general, we focus on the flat fragment of freeze LTL, in which the usage of the freeze quantifier is restricted. Recently, Lechner et al. showed that model checking for flat freeze LTL on OCAs with binary encoding of counter updates is decidable and in 2NEXPTIME. In this paper, we prove that the problem is, in fact, NEXPTIME-complete no matter whether counter updates are encoded in unary or binary. Like Lechner et al., we rely on a reduction to the reachability problem in OCAs with parameterized tests (OCAPs). The new aspect is that we simulate OCAPs by alternating two-way automata over words. This implies an exponential upper bound on the parameter values that we exploit towards an NP algorithm for reachability in OCAPs with unary updates. We obtain our main result as a corollary
Si3AlP: A new promising material for solar cell absorber
First-principles calculations are performed to study the structural and
optoelectronic properties of the newly synthesized nonisovalent and
lattice-matched (Si2)0.6(AlP)0.4 alloy [T. Watkins et al., J. Am. Chem. Soc.
2011, 133, 16212.] We find that the ordered CC-Si3AlP with a basic unit of one
P atom surrounded by three Si atoms and one Al atom is the most stable one
within the experimentally observed unit cell.1 Si3AlP has a larger fundamental
band gap and a smaller direct band gap than Si, thus it has much higher
absorption in the visible light region. The calculated properties of Si3AlP
suggest that it is a promising candidate for improving the performance of the
existing Si-based solar cells. The understanding on the stability and band
structure engineering obtained in this study is general and can be applied for
future study of other nonisovalent and lattice-matched semiconductor alloys
A Placenta Derived C-Terminal Fragment of beta-Hemoglobin With Combined Antibacterial and Antiviral Activity
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