330 research outputs found
Mechanism of carrier-induced ferromagnetism in magnetic semiconductors
Taking into account both random impurity distribution and thermal
fluctuations of localized spins, we have performed a model calculation for the
carrier (hole) state in GaMnAs by using the coherent potential
approximation (CPA). The result reveals that a {\it p}-hole in the band tail of
GaMnAs is not like a free carrier but is rather virtually bounded
to impurity sites. The carrier spin strongly couples to the localized {\it d}
spins on Mn ions. The hopping of the carrier among Mn sites causes the
ferromagnetic ordering of the localized spins through the double-exchange
mechanism. The Curie temperature obtained by using conventional parameters
agrees well with the experimental result.Comment: 7 pages, 4 figure
The point of maximum curvature as a marker for physiological time series
We present a geometric analysis of the model of Stirling. In particular we analyze the curvature of a heart rate time series in response to a step like increment in the exercise intensity. We present solutions for the point of maximum curvature which can be used as a marker of physiological interest. This marker defines the point after which the heart rate no longer continues to rapidly rise and instead follows either a steady state or slow rise. These methods are then applied to find analytic solutions for a mono exponential model which is commonly used in the literature to model the response to a moderate exercise intensity. Numerical solutions are then found for the full model and parameter values presented in Stirling
The role of defects in fluorescent silicon carbide layers grown by sublimation epitaxy
Donor-acceptor co-doped SiC is a promising light converter for novel monolithic all-semiconductor white LEDs due to its broad-band donor-acceptor pair luminescence and potentially high internal quantum efficiency. Besides sufficiently high doping concentrations in an appropriate ratio yielding short radiative lifetimes, long nonradiative lifetimes are crucial for efficient light conversion. The impact of different types of defects is studied by characterizing fluorescent silicon carbide layers with regard to photoluminescence intensity, homogeneity and efficiency taking into account dislocation density and distribution. Different doping concentrations and variations in gas phase composition and pressure are investigated
Solubility limit and precipitate formation in Al-doped 4H-SiC epitaxial material
Heavily Al-doped 4H–SiC structures have been prepared by vapor phase epitaxy. Subsequent anneals have been carried out in an Ar atmosphere in a rf-heated furnace between 1500 °C and 2000 °C for 0.5 to 3 h. Secondary ion mass spectrometry has been utilized to obtain Al concentration versus depth as well as lateral distributions (ion images). Transmission electron microscopy(TEM) has been employed to study the crystallinity and determine phase composition after heat treatment. A solubility limit of ∼2×10²⁰ Al/cm³ (1900 °C) is extracted. Three-dimensional ion images show that the Al distribution does not remain homogeneous in layers heat treated at 1700 °C or above when the Al concentration exceeds 2×10²⁰ cm⁻³. Al-containing precipitates are identified by energy-filtered TEM.Financial support was partly received
from the Swedish Foundation for Strategic Research (SSF)
SiCEP program
Ferromagnetism in semiconductors and oxides: prospects from a ten years' perspective
Over the last decade the search for compounds combining the resources of
semiconductors and ferromagnets has evolved into an important field of
materials science. This endeavour has been fuelled by continual demonstrations
of remarkable low-temperature functionalities found for ferromagnetic
structures of (Ga,Mn)As, p-(Cd,Mn)Te, and related compounds as well as by ample
observations of ferromagnetic signatures at high temperatures in a number of
non-metallic systems. In this paper, recent experimental and theoretical
developments are reviewed emphasising that, from the one hand, they disentangle
many controversies and puzzles accumulated over the last decade and, on the
other, offer new research prospects.Comment: review, 13 pages, 8 figures, 109 reference
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