1,053 research outputs found
NASA scientific and technical program: User survey
Results are presented of an intensive user requirements survey conducted by NASA's Scientific and Technical Information (STI) Program with the goal of improving the foundation for the user outreach program. The survey was carried out by interviewing 550 NASA scientists, engineers, and contractors and by analyzing 650 individual responses to a mailed out questionnaire. To analyze the user demographic data, a data base was built and used, and will be applied to ongoing analysis by the NASA STI Program
Review of the Natural History of the Handsome Fungus Beetles (Coleoptera: Cucujoidea: Endomychidae)
The literature pertaining to natural history of Endomychidae (Coleoptera: Cucujoidea) is reviewed. One hundred fungal host records are provided for 32 endomychid species. Twenty-three records of endomychid yeast endosymbionts are compiled. Summaries are also presented for feeding preferences, interactions with natural enemies, non-lethal symbiotic relationships, and pest activity within the family. Unusual endomychid behaviors and habitats are reviewed, with particular attention to gregariousness and defensive strategies within Endomychidae
End states, ladder compounds, and domain wall fermions
A magnetic field applied to a cross linked ladder compound can generate
isolated electronic states bound to the ends of the chain. After exploring the
interference phenomena responsible, I discuss a connection to the domain wall
approach to chiral fermions in lattice gauge theory. The robust nature of the
states under small variations of the bond strengths is tied to chiral symmetry
and the multiplicative renormalization of fermion masses.Comment: 10 pages, 4 figures; final version for Phys. Rev. Let
Spin transport theory in ferromagnet/semiconductor systems with non-collinear magnetization configurations
We present a comprehensive theory of spin transport in a non-degenerate
semiconductor that is in contact with multiple ferromagnetic terminals. The
spin dynamics in the semiconductor is studied during a perturbation of a
general, non-collinear magnetization configuration and a method is shown to
identify the various configurations from current signals. The conventional
Landauer-B\"{u}ttiker description for spin transport across Schottky contacts
is generalized by the use of a non-linearized I-V relation, and it is extended
by taking into account non-coherent transport mechanisms. The theory is used to
analyze a three terminal lateral structure where a significant difference in
the spin accumulation profile is found when comparing the results of this model
with the conventional model.Comment: 17 pages, 10 figure
Spin-filter effect of the europium chalcogenides: An exactly solved many-body model
A model Hamiltonian is introduced which considers the main features of the
experimental spin filter situation as s-f interaction, planar geometry and the
strong external electric field. The proposed many-body model can be solved
analytically and exactly using Green functions.
The spin polarization of the field-emitted electrons is expressed in terms of
spin-flip probabilities, which on their part are put down to the exactly known
dynamic quantities of the system.
The calculated electron spin polarization shows remarkable dependencies on
the electron velocity perpendicular to the emitting plane and the strength of
s-f coupling. Experimentally observed polarization values of about 90% are well
understood within the framework of the proposed model.Comment: accepted (Physical Review B); 10 pages, 11 figures;
http://orion.physik.hu-berlin.de
Thermodynamic basis of the concept of "recombination resistances"
The concept of "recombination resistance" introduced by Shockley and Read
(Phys. Rev. 87, 835 (1952)) is discussed within the framework of the
thermodynamics of irreversible processes ruled by the principle of the minimum
rate of entropy production. It is shown that the affinities of recombination
processes represent "voltages" in a thermodynamic Ohm-like law where the net
rates of recombinations represent the "currents". The quantities thus found
allow for the definition of the "dissipated power" which is to be related to
the rate of entropy production of the recombination processes dealt with.Comment: Submitted to Phys. Rev.
Surface metal-insulator transition in the Hubbard model
The correlation-driven metal-insulator (Mott) transition at a solid surface
is studied within the Hubbard model for a semi-infinite lattice by means of the
dynamical mean-field theory. The transition takes place at a unique critical
strength of the interaction. Depending on the surface geometry, the interaction
strength and the wave vector, we find one-electron excitations in the coherent
part of the surface-projected metallic spectrum which are confined to two
dimensions.Comment: LaTeX, 9 pages, 5 eps figures included, Phys. Rev. B (in press
Time-Dependent Current Partition in Mesoscopic Conductors
The currents at the terminals of a mesoscopic conductor are evaluated in the
presence of slowly oscillating potentials applied to the contacts of the
sample. The need to find a charge and current conserving solution to this
dynamic current partition problem is emphasized. We present results for the
electro-chemical admittance describing the long range Coulomb interaction in a
Hartree approach. For multiply connected samples we discuss the symmetry of the
admittance under reversal of an Aharonov-Bohm flux.Comment: 22 pages, 3 figures upon request, IBM RC 1971
Optical orientation in bipolar spintronic devices
Optical orientation is a highly efficient tool for the generation of
nonequilibrium spin polarization in semiconductors. Combined with
spin-polarized transport it offers new functionalities for conventional
electronic devices, such as pn junction bipolar diodes or transistors. In
nominally nonmagnetic junctions optical orientation can provide a source for
spin capacitance--the bias-dependent nonequilibrium spin accumulation--or for
spin-polarized current in bipolar spin-polarized solar cells. In magnetic
junctions, the nonequilibrium spin polarization generated by spin orientation
in a proximity of an equilibrium magnetization gives rise to the spin-voltaic
effect (a realization of the Silsbee-Johnson coupling), enabling efficient
control of electrical properties such as the I-V characteristics of the
junctions by magnetic and optical fields. This article reviews the main results
of investigations of spin-polarized and magnetic pn junctions, from spin
capacitance to the spin-voltaic effect.Comment: 9 pages, 10 figures; appeared in the special issue of Semicond. Sci.
Technol. on Optical Orientation, in honor of B. P. Zakharcheny
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