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