752 research outputs found
Processes, Roles and Their Interactions
Taking an interaction network oriented perspective in informatics raises the
challenge to describe deterministic finite systems which take part in networks
of nondeterministic interactions. The traditional approach to describe
processes as stepwise executable activities which are not based on the
ordinarily nondeterministic interaction shows strong centralization tendencies.
As suggested in this article, viewing processes and their interactions as
complementary can circumvent these centralization tendencies.
The description of both, processes and their interactions is based on the
same building blocks, namely finite input output automata (or transducers).
Processes are viewed as finite systems that take part in multiple, ordinarily
nondeterministic interactions. The interactions between processes are described
as protocols.
The effects of communication between processes as well as the necessary
coordination of different interactions within a processes are both based on the
restriction of the transition relation of product automata. The channel based
outer coupling represents the causal relation between the output and the input
of different systems. The coordination condition based inner coupling
represents the causal relation between the input and output of a single system.
All steps are illustrated with the example of a network of resource
administration processes which is supposed to provide requesting user processes
exclusive access to a single resource.Comment: In Proceedings IWIGP 2012, arXiv:1202.422
Krotov: A Python implementation of Krotov's method for quantum optimal control
We present a new open-source Python package, krotov, implementing the quantum optimal control method of that name. It allows to determine time-dependent external fields for a wide range of quantum control problems, including state-to-state transfer, quantum gate implementation and optimization towards an arbitrary perfect entangler. Krotov's method compares to other gradient-based optimization methods such as gradient-ascent and guarantees monotonic convergence for approximately time-continuous control fields. The user-friendly interface allows for combination with other Python packages, and thus high-level customization
Correlation between microstructure and magnetotransport in organic semiconductor spin valve structures
We have studied magnetotransport in organic-inorganic hybrid multilayer
junctions. In these devices, the organic semiconductor (OSC) Alq
(tris(8-hydroxyquinoline) aluminum) formed a spacer layer between ferromagnetic
(FM) Co and Fe layers. The thickness of the Alq layer was in the range of
50-150 nm. Positive magnetoresistance (MR) was observed at 4.2 K in a current
perpendicular to plane geometry, and these effects persisted up to room
temperature. The devices' microstructure was studied by X-ray reflectometry,
Auger electron spectroscopy and polarized neutron reflectometry (PNR). The
films show well-defined layers with modest average chemical roughness (3-5 nm)
at the interface between the Alq and the surrounding FM layers.
Reflectometry shows that larger MR effects are associated with smaller
FM/Alq interface width (both chemical and magnetic) and a magnetically dead
layer at the Alq/Fe interface. The PNR data also show that the Co layer,
which was deposited on top of the Alq, adopts a multi-domain magnetic
structure at low field and a perfect anti-parallel state is not obtained. The
origins of the observed MR are discussed and attributed to spin coherent
transport. A lower bound for the spin diffusion length in Alq was estimated
as nm at 80 K. However, the subtle correlations between
microstructure and magnetotransport indicate the importance of interfacial
effects in these systems.Comment: 21 pages, 11 figures and 2 table
Assembly of multicellular constructs and microarrays of cells using magnetic nanowires
An approach is described for controlling the spatial organization of mammalian cells using ferromagnetic nanowires in conjunction with patterned micromagnet arrays. The nanowires are fabricated by electrodeposition in nanoporous templates, which allows for precise control of their size and magnetic properties. The high aspect ratio and large remanent magnetization of the nanowires enable suspensions of cells bound to Ni nanowires to be controlled with low magnetic fields. This was used to produce one- and two-dimensional field-tuned patterning of suspended 3T3 mouse fibroblasts. Self-assembled one-dimensional chains of cells were obtained through manipulation of the wires\u27 dipolar interactions. Ordered patterns of individual cells in two dimensions were formed through trapping onto magnetic microarrays of ellipsoidal permalloy micromagnets. Cell chains were formed on the arrays by varying the spacing between the micromagnets or the strength of fluid flow over the arrays. The positioning of cells on the array was further controlled by varying the direction of an external magnetic field. These results demonstrate the possibility of using magnetic nanowires to organize cells
Effects of carrier mobility and morphology in organic semiconductor spin valves
We studied spin transport in four organic semiconductors (OSCs) with different electronic properties. Magnetoresistance (MR) effects were observed up to room temperature in junctions based on an electron-carrying OSC, tris(8-hyroxyquinoline) aluminum (Alq3) and a hole-carrying OSC, copper phthalocyanine (CuPc). The MR shows similar temperature dependence for these two OSCs. We also investigated junctions based on two high lateral mobility electron-carrying OSCs, 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA) and N,N'-bis(4-trifluoromethylbenzyl)-1,4,5,8-naphthalenetetracarboxylic diimide (CF3-NTCDI). However, these junctions showed much weaker spin transport effects. Morphological studies suggest that these high mobility OSC films have much rougher surfaces than either Alq3 or CuPc, therefore the degradation of spin transport may originate from enhanced scattering due to the rougher FM/OSC interfaces. Our study shows that FM/OSC interfaces play an important role for spin transport in organic devices and need further exploration
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PH207x: Health in Numbers and PH278x: Human Health and Global Environmental Change: 2012-2013 Course Report
In the 2012-2013 academic year, the first two Harvard School of Public Health courses were offered through HarvardX on the edX platform: PH207x: Health in Numbers and PH278x: Human Health and Global Environmental Change. They were taught by Professors Earl Francis Cook and Marcello Pagano, and Aaron Bernstein and Jack Spengler, respectively. This report describes the structure of these two courses, the demographic characteristics of registrants, and the activity of students. This report was prepared by researchers external to the course teams and is based on examination of the courseware, analyses of the data collected by the edX platform, and interviews and consultations with the course faculty and team members
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