273 research outputs found
Model of hopping dc conductivity via nearest neighbor boron atoms in moderately compensated diamond crystals
Expressions for dependences of the pre-exponential factor \sigma_3 and the
thermal activation energy \epsilon_3 of hopping electric conductivity of holes
via boron atoms on the boron atom concentration N and the compensation ratio K
are obtained in the quasiclassical approximation. It is assumed that the
acceptors (boron atoms) in charge states (0) and (-1) and the donors that
compensate them in the charge state (+1) form a nonstoichiometric simple cubic
lattice with translational period R_h = [(1 + K)N]^{-1/3} within the
crystalline matrix. A hopping event occurs only over the distance R_h at a
thermally activated accidental coincidence of the acceptor levels in charge
states (0) and (-1). Donors block the fraction K/(1 - K) of impurity lattice
sites. The hole hopping conductivity is averaged over all possible orientations
of the lattice with respect to the external electric field direction. It is
supposed that an acceptor band is formed by Gaussian fluctuations of the
potential energy of boron atoms in charge state (-1) due to Coulomb interaction
only between the ions at distance R_h. The shift of the acceptor band towards
the top of the valence band with increasing N due to screening (in the
Debye--H\"uckel approximation) of the impurity ions by holes hopping via
acceptor states was taken into account. The calculated values of \sigma_3(N)
and \epsilon_3(N) for K \approx 0.25 agree well with known experimental data at
the insulator side of the insulator--metal phase transition. The calculation is
carried out at a temperature two times lower than the transition temperature
from hole transport in v-band of diamond to hopping conductance via boron
atoms.Comment: 6 pages, 2 figure
Spin Precession and Oscillations in Mesoscopic Systems
We compare and contrast magneto-transport oscillations in the fully quantum
(single-electron coherent) and classical limits for a simple but illustrative
model. In particular, we study the induced magnetization and spin current in a
two-terminal double-barrier structure with an applied Zeeman field between the
barriers and spin disequilibrium in the contacts. Classically, the spin current
shows strong tunneling resonances due to spin precession in the region between
the two barriers. However, these oscillations are distinguishable from those in
the fully coherent case, for which a proper treatment of the electron phase is
required. We explain the differences in terms of the presence or absence of
coherent multiple wave reflections.Comment: 9 pages, 5 figure
Spin oscillations in transient diffusion of a spin pulse in n-type semiconductor quantum wells
By studying the time and spatial evolution of a pulse of the spin
polarization in -type semiconductor quantum wells, we highlight the
importance of the off-diagonal spin coherence in spin diffusion and transport.
Spin oscillations and spin polarization reverse along the the direction of spin
diffusion in the absence of the applied magnetic field are predicted from our
investigation.Comment: 5 pages, 4 figures, accepted for publication in PR
A jump-growth model for predator-prey dynamics: derivation and application to marine ecosystems
This paper investigates the dynamics of biomass in a marine ecosystem. A
stochastic process is defined in which organisms undergo jumps in body size as
they catch and eat smaller organisms. Using a systematic expansion of the
master equation, we derive a deterministic equation for the macroscopic
dynamics, which we call the deterministic jump-growth equation, and a linear
Fokker-Planck equation for the stochastic fluctuations. The McKendrick--von
Foerster equation, used in previous studies, is shown to be a first-order
approximation, appropriate in equilibrium systems where predators are much
larger than their prey. The model has a power-law steady state consistent with
the approximate constancy of mass density in logarithmic intervals of body mass
often observed in marine ecosystems. The behaviours of the stochastic process,
the deterministic jump-growth equation and the McKendrick--von Foerster
equation are compared using numerical methods. The numerical analysis shows two
classes of attractors: steady states and travelling waves.Comment: 27 pages, 4 figures. Final version as published. Only minor change
Spin battery operated by ferromagnetic resonance
Precessing ferromagnets are predicted to inject a spin current into adjacent
conductors via Ohmic contacts, irrespective of a conductance mismatch with, for
example, doped semiconductors. This opens the way to create a pure spin source
spin battery by the ferromagnetic resonance. We estimate the spin current and
spin bias for different material combinations.Comment: The estimate for the magnitude of the spin bias is improved. We find
that it is feasible to get a measurable signal of the order of the microwave
frequency already for moderate rf intensitie
B --> Phi K_S and Supersymmetry
The rare decay B --> Phi K_S is a well-known probe of physics beyond the
Standard Model because it arises only through loop effects yet has the same
time-dependent CP asymmetry as B --> Psi K_S. Motivated by recent data
suggesting new physics in B --> Phi K_S, we look to supersymmetry for possible
explanations, including contributions mediated by gluino loops and by Higgs
bosons. Chirality-preserving LL and RR gluino contributions are generically
small, unless gluinos and squarks masses are close to the current lower bounds.
Higgs contributions are also too small to explain a large asymmetry if we
impose the current upper limit on B(B_s --> mu mu). On the other hand,
chirality-flipping LR and RL gluino contributions can provide sizable effects
and while remaining consistent with related results in B --> Psi K_S, Delta
M_s, B --> X_s gamma and other processes. We discuss how the LR and RL
insertions can be distinguished using other observables, and we provide a
string-based model and other estimates to show that the needed sizes of mass
insertions are reasonable.Comment: 33 pages, 32 figures, Updated version for PRD. Includes discussions
of other recent works on this topic. Added discussions & plots for gluino
mass dependence and effects of theoretical uncertaintie
Multi-layered Ruthenium-modified Bond Coats for Thermal Barrier Coatings
Diffusional approaches for fabrication of multi-layered Ru-modified bond coats for thermal
barrier coatings have been developed via low activity chemical vapor deposition and high activity
pack aluminization. Both processes yield bond coats comprising two distinct B2 layers, based on
NiAl and RuAl, however, the position of these layers relative to the bond coat surface is reversed
when switching processes. The structural evolution of each coating at various stages of the
fabrication process has been and subsequent cyclic oxidation is presented, and the relevant
interdiffusion and phase equilibria issues in are discussed. Evaluation of the oxidation behavior of
these Ru-modified bond coat structures reveals that each B2 interlayer arrangement leads to the
formation of α-Al 2 O 3 TGO at 1100°C, but the durability of the TGO is somewhat different and in
need of further improvement in both cases
Spin relaxation: From 2D to 1D
In inversion asymmetric semiconductors, spin-orbit interactions give rise to
very effective relaxation mechanisms of the electron spin. Recent work, based
on the dimensionally constrained D'yakonov Perel' mechanism, describes
increasing electron-spin relaxation times for two-dimensional conducting layers
with decreasing channel width. The slow-down of the spin relaxation can be
understood as a precursor of the one-dimensional limit
Ideal Spin Filters: Theoretical Study of Electron Transmission Through Ordered and Disordered Interfaces Between Ferromagnetic Metals and Semiconductors
It is predicted that certain atomically ordered interfaces between some
ferromagnetic metals (F) and semiconductors (S) should act as ideal spin
filters that transmit electrons only from the majority spin bands or only from
the minority spin bands of the F to the S at the Fermi energy, even for F with
both majority and minority bands at the Fermi level. Criteria for determining
which combinations of F, S and interface should be ideal spin filters are
formulated. The criteria depend only on the bulk band structures of the S and F
and on the translational symmetries of the S, F and interface. Several examples
of systems that meet these criteria to a high degree of precision are
identified. Disordered interfaces between F and S are also studied and it is
found that intermixing between the S and F can result in interfaces with spin
anti-filtering properties, the transmitted electrons being much less spin
polarized than those in the ferromagnetic metal at the Fermi energy. A patent
application based on this work has been commenced by Simon Fraser University.Comment: RevTeX, 12 pages, 5 figure
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