62 research outputs found
X-ray diffraction peak profiles from threading dislocations in GaN epitaxial films
We analyze the lineshape of x-ray diffraction profiles of GaN epitaxial
layers with large densities of randomly distributed threading dislocations. The
peaks are Gaussian only in the central, most intense part of the peak, while
the tails obey a power law. The decay typical for random dislocations
is observed in double-crystal rocking curves. The entire profile is well fitted
by a restricted random dislocation distribution. The densities of both edge and
screw threading dislocations and the ranges of dislocation correlations are
obtained
Electrically tunable GHz oscillations in doped GaAs-AlAs superlattices
Tunable oscillatory modes of electric-field domains in doped semiconductor
superlattices are reported. The experimental investigations demonstrate the
realization of tunable, GHz frequencies in GaAs-AlAs superlattices covering the
temperature region from 5 to 300 K. The orgin of the tunable oscillatory modes
is determined using an analytical and a numerical modeling of the dynamics of
domain formation. Three different oscillatory modes are found. Their presence
depends on the actual shape of the drift velocity curve, the doping density,
the boundary condition, and the length of the superlattice. For most bias
regions, the self-sustained oscillations are due to the formation, motion, and
recycling of the domain boundary inside the superlattice. For some biases, the
strengths of the low and high field domain change periodically in time with the
domain boundary being pinned within a few quantum wells. The dependency of the
frequency on the coupling leads to the prediction of a new type of tunable GHz
oscillator based on semiconductor superlattices.Comment: Tex file (20 pages) and 16 postscript figure
On the crystal lattice parameters of graphite-like phases of the B-C system
The structure of graphite-like BCx phases (x = 1, 1.5, 3, 4, 32) has been
studied using conventional X-ray diffraction. The results have been obtained,
which unambiguously point to turbostratic (one- dimensionally disordered)
structure of all phases under study. The crystal lattice parameters, sizes of
coherent scattering domains, and microstrain values have been defined, which
have allowed us to find a correlation between the structure and stoichiometry
of the phases synthesized at the same temperature
Collective coherence in planar semiconductor microcavities
Semiconductor microcavities, in which strong coupling of excitons to confined
photon modes leads to the formation of exciton-polariton modes, have
increasingly become a focus for the study of spontaneous coherence, lasing, and
condensation in solid state systems. This review discusses the significant
experimental progress to date, the phenomena associated with coherence which
have been observed, and also discusses in some detail the different theoretical
models that have been used to study such systems. We consider both the case of
non-resonant pumping, in which coherence may spontaneously arise, and the
related topics of resonant pumping, and the optical parametric oscillator.Comment: 46 pages, 12 figure
Superhard Phases of Simple Substances and Binary Compounds of the B-C-N-O System: from Diamond to the Latest Results (a Review)
The basic known and hypothetic one- and two-element phases of the B-C-N-O
system (both superhard phases having diamond and boron structures and
precursors to synthesize them) are described. The attention has been given to
the structure, basic mechanical properties, and methods to identify and
characterize the materials. For some phases that have been recently described
in the literature the synthesis conditions at high pressures and temperatures
are indicated.Comment: Review on superhard B-C-N-O phase
Discrete derived categories I: homomorphisms, autoequivalences and t-structures
Discrete derived categories were studied initially by Vossieck (J Algebra 243:168–176, 2001) and later by Bobiński et al. (Cent Eur J Math 2:19–49, 2004). In this article, we describe the homomorphism hammocks and autoequivalences on these categories. We classify silting objects and bounded t-structures
Aggression, anxiety and vocalizations in animals: GABA A and 5-HT anxiolytics
A continuing challenge for preclinical research on anxiolytic drugs is to capture the affective dimension that characterizes anxiety and aggression, either in their adaptive forms or when they become of clinical concern. Experimental protocols for the preclinical study of anxiolytic drugs typically involve the suppression of conditioned or unconditioned social and exploratory behavior (e.g., punished drinking or social interactions) and demonstrate the reversal of this behavioral suppression by drugs acting on the benzodiazepine-GABA A complex. Less frequently, aversive events engender increases in conditioned or unconditioned behavior that are reversed by anxiolytic drugs (e.g., fear-potentiated startle). More recently, putative anxiolytics which target 5-HT receptor subtypes produced effects in these traditional protocols that often are not systematic and robust. We propose ethological studies of vocal expressions in rodents and primates during social confrontations, separation from social companions, or exposure to aversive environmental events as promising sources of information on the affective features of behavior. This approach focusses on vocal and other display behavior with clear functional validity and homology. Drugs with anxiolytic effects that act on the benzodiazepine-GABA A receptor complex and on 5-HT 1A receptors systematically and potently alter specific vocalizations in rodents and primates in a pharmacologically reversible manner; the specificity of these effects on vocalizations is evident due to the effectiveness of low doses that do not compromise other physiological and behavioral processes. Antagonists at the benzodiazepine receptor reverse the effects of full agonists on vocalizations, particularly when these occur in threatening, startling and distressing contexts. With the development of antagonists at 5-HT receptor subtypes, it can be anticipated that similar receptor-specificity can be established for the effects of 5-HT anxiolytics.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/46351/1/213_2005_Article_BF02245590.pd
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