2,517 research outputs found
Impurity Band Conduction in a High Temperature Ferromagnetic Semiconductor
The band structure of a prototypical dilute ferromagnetic semiconductor,
GaMnAs, is studied across the phase diagram via optical
spectroscopy. We prove that the Fermi energy () resides in a Mn induced
impurity band (IB). This conclusion is based upon careful analysis of the
frequency and temperature dependence of the optical conductivity
(). From our analysis of we infer
a large effective mass () of the carriers, supporting the view that
conduction occurs in an IB. Our results also provide useful insights into the
transport properties of Mn-doped GaAs.Comment: 4 pages, 4 figure
Quantum orbits of R-matrix type
Given a simple Lie algebra \gggg, we consider the orbits in \gggg^* which
are of R-matrix type, i.e., which possess a Poisson pencil generated by the
Kirillov-Kostant-Souriau bracket and the so-called R-matrix bracket. We call an
algebra quantizing the latter bracket a quantum orbit of R-matrix type. We
describe some orbits of this type explicitly and we construct a quantization of
the whole Poisson pencil on these orbits in a similar way. The notions of
q-deformed Lie brackets, braided coadjoint vector fields and tangent vector
fields are discussed as well.Comment: 18 pp., Late
Fundamentals of microcrack nucleation mechanics
A foundation for ultrasonic evaluation of microcrack nucleation mechanics is identified in order to establish a basis for correlations between plane strain fracture toughness and ultrasonic factors through the interaction of elastic waves with material microstructures. Since microcracking is the origin of (brittle) fracture, it is appropriate to consider the role of stress waves in the dynamics of microcracking. Therefore, the following topics are discussed: (1) microstress distributions with typical microstructural defects located in the stress field; (2) elastic wave scattering from various idealized defects; and (3) dynamic effective-properties of media with randomly distributed inhomogeneities
Immobilization of Polyethylene Oxide Surfactants for Non-Fouling Biomaterial Surfaces Using an Argon Glow Discharge Treatment
A non-fouling (protein-resistant) polymer surface is achieved by the covalent immobilization of polyethylene oxide (PEO) surfactants using an inert gas discharge treatment. Treated surfaces have been characterized using electron spectroscopy for chemical analysis (ESCA), static secondary ion mass spectrometry (SSIMS), water contact angle measurement, fibrinogen adsorption, and platelet adhesion. This paper is intended to review our recent work in using this simple surface modification process to obtain wettable polymer surfaces in general, and non-fouling biomaterial surfaces in particular
Antisite effect on ferromagnetism in (Ga,Mn)As
We study the Curie temperature and hole density of (Ga,Mn)As while
systematically varying the As-antisite density. Hole compensation by
As-antisites limits the Curie temperature and can completely quench long-range
ferromagnetic order in the low doping regime of 1-2% Mn. Samples are grown by
molecular beam epitaxy without substrate rotation in order to smoothly vary the
As to Ga flux ratio across a single wafer. This technique allows for a
systematic study of the effect of As stoichiometry on the structural,
electronic, and magnetic properties of (Ga,Mn)As. For concentrations less than
1.5% Mn, a strong deviation from Tc ~ p^0.33 is observed. Our results emphasize
that proper control of As-antisite compensation is critical for controlling the
Curie temperatures in (Ga,Mn)As at the low doping limit.Comment: 10 pages, 7 figure
The first products made in space: Monodisperse latex particles
The preparation of large particle size 3 to 30 micrometer monodisperse latexes in space confirmed that original rationale unequivocally. The flight polymerizations formed negligible amounts of coagulum as compared to increasing amounts for the ground-based polymerizations. The number of offsize large particles in the flight latexes was smaller than in the ground-based latexes. The particle size distribution broadened and more larger offsize particles were formed when the polymerizations of the partially converted STS-4 latexes were completed on Earth. Polymerization in space also showed other unanticipated advantages. The flight latexes had narrower particle size distributions than the ground-based latexes. The particles of the flight latexes were more perfect spheres than those of the ground-based latexes. The superior uniformity of the flight latexes was confirmed by the National Bureau of Standards acceptance of the 10 micrometer STS-6 latex and the 30 micrometer STS-11 latexes as Standard Reference Materials, the first products made in space for sale on Earth. The polymerization rates in space were the same as those on Earth within experimental error. Further development of the ground-based polymerization recipes gave monodisperse particles as large as 100 micrometer with tolerable levels of coagulum, but their uniformity was significantly poorer than the flight latexes. Careful control of the polymerization parameters gave uniform nonspherical particles: symmetrical and asymmetrical doublets, ellipsoids, egg-shaped, ice cream cone-shaped, and popcorn-shaped particles
Thermal transport in thin films measured by time-resolved, grazing incidence x-ray diffraction
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/98715/1/JApplPhys_110_102203.pd
Imaging nonequilibrium atomic vibrations with x-ray diffuse scattering
For over a century, x-ray scattering has been the most powerful tool for
determining the equilibrium structure of crystalline materials. Deviations from
perfect periodicity, for example due to thermal motion of the atoms, reduces
the intensity of the Bragg peaks as well as produces structure in the diffuse
scattering background. Analysis of the thermal diffuse scattering (TDS) had
been used to determine interatomic force constants and phonon dispersion in
relatively simple cases before inelastic neutron scattering became the
preferred technique to study lattice dynamics. With the advent of intense
synchrotron x-ray sources, there was a renewed interest in TDS for measuring
phonon dispersion. The relatively short x-ray pulses emanating from these
sources also enables the measurement of phonon dynamics in the time domain.
Prior experiments on nonequilibrium phonons were either limited by
time-resolution and/or to relatively long wavelength excitations. Here we
present the first images of nonequilibrium phonons throughout the Brillouin
zone in photoexcited III-V semiconductors, indium-phosphide and
indium-antimonide, using picosecond time-resolved diffuse scattering. In each
case, we find that the lattice remain out of equilibrium for several hundred
picoseconds up to nanoseconds after laser excitation. The non-equilibrium
population is dominated by transverse acoustic phonons which in InP are
directed along high-symmetry directions. The results have wide implications for
the detailed study of electron-phonon and phonon-phonon coupling in solids.Comment: 10 pages, 3 figure
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