317 research outputs found
Capture of carriers to screened charged centres and low temperature shallow impurity electric field break down in semiconductors
Free carrier capture by a screened Coulomb potential in semiconductors are
considered. It is established that with decreasing screening radius the capture
cross section decreases drastically, and it goes to zero when .
On the basis of this result a new mechanism of shallow impurity electric field
break down in semiconductors is suggested.Comment: 8 pages, latex, 1 figure in gif format, to be submitted to "Journal
of Condensed Matter
Generation of spin currents via Raman scattering
We show theoretically that stimulated spin flip Raman scattering can be used
to inject spin currents in doped semiconductors with spin split bands. A pure
spin current, where oppositely oriented spins move in opposite directions, can
be injected in zincblende crystals and structures. The calculated spin current
should be detectable by pump-probe optical spectroscopy and anomalous Hall
effect measurement
Correlation effects in sequential energy branching: an exact model of the Fano statistics
Correlation effects in in the fluctuation of the number of particles in the
process of energy branching by sequential impact ionizations are studied using
an exactly soluble model of random parking on a line. The Fano factor F
calculated in an uncorrelated final-state "shot-glass" model does not give an
accurate answer even with the exact gap-distribution statistics. Allowing for
the nearest-neighbor correlation effects gives a correction to F that brings F
very close to its exact value. We discuss the implications of our results for
energy resolution of semiconductor gamma detectors, where the value of F is of
the essence. We argue that F is controlled by correlations in the cascade
energy branching process and hence the widely used final-state model estimates
are not reliable -- especially in the practically relevant cases when the
energy branching is terminated by competition between impact ionization and
phonon emission.Comment: 11 pages, 4 figures. Submitted to Physical Review
Dynamic avalanche breakdown of a p-n junction: deterministic triggering of a plane streamer front
We discuss the dynamic impact ionization breakdown of high voltage p-n
junction which occurs when the electric field is increased above the threshold
of avalanche impact ionization on a time scale smaller than the inverse
thermogeneration rate. The avalanche-to-streamer transition characterized by
generation of dense electron-hole plasma capable to screen the applied external
electric field occurs in such regimes. We argue that the experimentally
observed deterministic triggering of the plane streamer front at the electric
field strength above the threshold of avalanche impact ionization but yet below
the threshold of band-to-band tunneling is generally caused by field-enhanced
ionization of deep-level centers. We suggest that the process-induced sulfur
centers and native defects such as EL2, HB2, HB5 centers initiate the front in
Si and GaAs structures, respectively. In deep-level free structures the plane
streamer front is triggered by Zener band-to-band tunneling.Comment: 4 pages, 2 figure
Tunneling spin-galvanic effect
It has been shown that tunneling of spin-polarized electrons through a
semiconductor barrier is accompanied by generation of an electric current in
the plane of the interfaces. The direction of this interface current is
determined by the spin orientation of the electrons, in particular the current
changes its direction if the spin orientation changes the sign. Microscopic
origin of such a 'tunneling spin-galvanic' effect is the spin-orbit
coupling-induced dependence of the barrier transparency on the spin orientation
and the wavevector of electrons.Comment: 3 pages, 2 figure
Characterization of deep impurities in semiconductors by terahertz tunneling ionization
Tunneling ionization in high frequency fields as well as in static fields is suggested as a method for the characterization of deep impurities in semiconductors. It is shown that an analysis of the field and temperature dependences of the ionization probability allows to obtain defect parameters like the charge of the impurity, tunneling times, the Huang–Rhys parameter, the difference between optical and thermal binding energy, and the basic structure of the defect adiabatic potentials. Compared to static fields, high frequency electric fields in the terahertz-range offer various advantages, as they can be applied contactlessly and homogeneously even to bulk samples using the intense radiation of a high power pulsed far-infrared laser. Furthermore, impurity ionization with terahertz radiation can be detected as photoconductive signal with a very high sensitivity in a wide range of electric field strengths
Temperature influence on the properties of thin Si₃N₄ films
Applying Raman spectroscopy, small-angle x-ray scattering, and atomic force microscopy it were studied phase composition and surface morphology of nanoscale films Si₃N₄ (obtained by RF magnetron sputtering
Hydrodynamic Simulations of Counterrotating Accretion Disks
Hydrodynamic simulations have been used to study accretion disks consisting
of counterrotating components with an intervening shear layer(s).
Configurations of this type can arise from the accretion of newly supplied
counterrotating matter onto an existing corotating disk. The grid-dependent
numerical viscosity of our hydro code is used to simulate the influence of a
turbulent viscosity of the disk. Firstly, we consider the case where the gas
well above the disk midplane rotates with angular rate +\Omega(r) and that well
below has the same properties but rotates with rate -\Omega(r). We find that
there is angular momentum annihilation in a narrow equatorial boundary layer in
which matter accretes supersonically with a velocity which approaches the
free-fall velocity and the average accretion speed of the disk can be
enormously larger than that for a conventional \alpha-disk rotating in one
direction. Secondly, we consider the case of a corotating accretion disk for
rr_t. In this case we observed, that
matter from the annihilation layer lost its stability and propagated inward
pushing matter of inner regions of the disk to accrete. Thirdly, we
investigated the case where counterrotating matter inflowing from large radial
distances encounters an existing corotating disk. Friction between the
inflowing matter and the existing disk is found to lead to fast boundary layer
accretion along the disk surfaces and to enhanced accretion in the main disk.
These models are pertinent to the formation of counterrotating disks in
galaxies and possibly in Active Galactic Nuclei and in X-ray pulsars in binary
systems.Comment: LaTeX, 18 pages, to appear in Ap
A new numerical approach to Anderson (de)localization
We develop a new approach for the Anderson localization problem. The
implementation of this method yields strong numerical evidence leading to a
(surprising to many) conjecture: The two dimensional discrete random
Schroedinger operator with small disorder allows states that are dynamically
delocalized with positive probability. This approach is based on a recent
result by Abakumov-Liaw-Poltoratski which is rooted in the study of spectral
behavior under rank-one perturbations, and states that every non-zero vector is
almost surely cyclic for the singular part of the operator.
The numerical work presented is rather simplistic compared to other numerical
approaches in the field. Further, this method eliminates effects due to
boundary conditions.
While we carried out the numerical experiment almost exclusively in the case
of the two dimensional discrete random Schroedinger operator, we include the
setup for the general class of Anderson models called Anderson-type
Hamiltonians.
We track the location of the energy when a wave packet initially located at
the origin is evolved according to the discrete random Schroedinger operator.
This method does not provide new insight on the energy regimes for which
diffusion occurs.Comment: 15 pages, 8 figure
Humification and Humic Acid Composition of Suspended Soil in Oligotrophous Environments in South Vietnam
Humification is considered to be a global process that is implemented in soils and organic sediments and also in natural water and air. The term “suspended soils” has become increasingly common in recent years. Suspended soils are defined as the part of the organic matter that has not undergone the full decomposition process and has not turned into the humus of terrestrial soils. Suspended soils were shown to contain higher total nitrogen, phosphorus, and potassium contents than the forest soil, but the moisture content in suspended soils was significantly lower. Our study of the structural composition of humic substances in suspended soils was conducted with an aim of evaluating the humification rates and structural composition of humic acids in the suspended soil in tropical forests of South Vietnam. Soil samples from three selected areas were investigated: the soil under phorophytes (mineral soil presented by samples of topsoil of the typical dry savanna landscape) and two soils from epiphytous formations. Samples were collected from savanna-type sparse communities, located on oligotrophous plains in Phú Quốc Island (South Vietnam) in 2015. General properties of the soil and the elemental composition of suspended soils were determined, and the humic substance chemical composition was evaluated using solid state 13C-NMR. Data obtained showed that the pH of the soils under phorophytes was higher than in the suspended soils; basal respiration did not tend to change indices between soils under phorophytes and suspended soils, but the suspended soil was less enriched by nitrogen than the soil under phorophytes. This can be related to the total amount of organic matter exposed to humification in various soils and to the presence of an essential portion of mineral particles in the soil under phorophytes. Data on elemental composition of the humic acids (HAs) indicated that one method of humification is implemented in all three soils that were investigated. The humic acids of the phorophyte soil showed the same content of aromatic fraction as the suspended soil. The most comparable soil type in terms of humic substance composition is Cambisols from humid forests of subboreal and subtropical zones. The humification process implemented in suspended soils showed the absence of mineral compounds or mineral fine earth, which indicated that humification in conditions of pure organic substrates can result in formation of deep humified organic matter, as shown by humic acids with an essential aromatic fraction content
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