636 research outputs found
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
Etched distributed Bragg reflectors as three-dimensional photonic crystals: photonic bands and density of states
The photonic band dispersion and density of states (DOS) are calculated for
the three-dimensional (3D) hexagonal structure corresponding to a distributed
Bragg reflector patterned with a 2D triangular lattice of circular holes.
Results for the Si/SiO and GaAs/AlGaAs systems determine the optimal
parameters for which a gap in the 2D plane occurs and overlaps the 1D gap of
the multilayer. The DOS is considerably reduced in correspondence with the
overlap of 2D and 1D gaps. Also, the local density of states (i.e., the DOS
weighted with the squared electric field at a given point) has strong
variations depending on the position. Both results imply substantial changes of
spontaneous emission rates and patterns for a local emitter embedded in the
structure and make this system attractive for the fabrication of a 3D photonic
crystal with controlled radiative properties.Comment: 8 pages, 5 figures; to appear in Phys. Rev.
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
Spin-polarized transport and Andreev reflection in semiconductor/superconductor hybrid structures
We show that spin-polarized electron transmission across
semiconductor/superconductor (Sm/S) hybrid structures depends sensitively on
the degree of spin polarization as well as the strengths of potential and
spin-flip scattering at the interface. We demonstrate that increasing the Fermi
velocity mismatch in the Sm and S regions can lead to enhanced junction
transparency in the presence of spin polarization. We find that the Andreev
reflection amplitude at the superconducting gap energy is a robust measure of
the spin polarization magnitude, being independent of the strengths of
potential and spin-flip scattering and the Fermi velocity of the
superconductor.Comment: 4 pages, 2 figure
Coherent spin valve phenomena and electrical spin injection in ferromagnetic/semiconductor/ferromagnetic junctions
Coherent quantum transport in ferromagnetic/ semiconductor/ ferromagnetic
junctions is studied theoretically within the Landauer framework of ballistic
transport. We show that quantum coherence can have unexpected implications for
spin injection and that some intuitive spintronic concepts which are founded in
semi-classical physics no longer apply: A quantum spin-valve (QSV) effect
occurs even in the absence of a net spin polarized current flowing through the
device, unlike in the classical regime. The converse effect also arises, i.e. a
zero spin-valve signal for a non-vanishing spin-current. We introduce new
criteria useful for analyzing quantum and classical spin transport phenomena
and the relationships between them. The effects on QSV behavior of
spin-dependent electron transmission at the interfaces, interface Schottky
barriers, Rashba spin-orbit coupling and temperature, are systematically
investigated. While the signature of the QSV is found to be sensitive to
temperature, interestingly, that of its converse is not. We argue that the QSV
phenomenon can have important implications for the interpretation of
spin-injection in quantum spintronic experiments with spin-valve geometries.Comment: 15 pages including 11 figures. To appear in PR
Modeling water waves beyond perturbations
In this chapter, we illustrate the advantage of variational principles for
modeling water waves from an elementary practical viewpoint. The method is
based on a `relaxed' variational principle, i.e., on a Lagrangian involving as
many variables as possible, and imposing some suitable subordinate constraints.
This approach allows the construction of approximations without necessarily
relying on a small parameter. This is illustrated via simple examples, namely
the Serre equations in shallow water, a generalization of the Klein-Gordon
equation in deep water and how to unify these equations in arbitrary depth. The
chapter ends with a discussion and caution on how this approach should be used
in practice.Comment: 15 pages, 1 figure, 39 references. This document is a contributed
chapter to an upcoming volume to be published by Springer in Lecture Notes in
Physics Series. Other author's papers can be downloaded at
http://www.denys-dutykh.com
Radiation-induced oscillatory magnetoresistance as a sensitive probe of the zero-field spin splitting in high mobility GaAs/AlGaAs devices
We suggest an approach for characterizing the zero-field spin splitting of
high mobility two-dimensional electron systems, when beats are not readily
observable in the Shubnikov-de Haas effect. The zero-field spin splitting and
the effective magnetic field seen in the reference frame of the electron is
evaluated from a quantitative study of beats observed in radiation-induced
magnetoresistance oscillations.Comment: 4 pages, 4 color figure
Confining QCD Strings, Casimir Scaling, and a Euclidean Approach to High-Energy Scattering
We compute the chromo-field distributions of static color-dipoles in the
fundamental and adjoint representation of SU(Nc) in the loop-loop correlation
model and find Casimir scaling in agreement with recent lattice results. Our
model combines perturbative gluon exchange with the non-perturbative stochastic
vacuum model which leads to confinement of the color-charges in the dipole via
a string of color-fields. We compute the energy stored in the confining string
and use low-energy theorems to show consistency with the static quark-antiquark
potential. We generalize Meggiolaro's analytic continuation from parton-parton
to gauge-invariant dipole-dipole scattering and obtain a Euclidean approach to
high-energy scattering that allows us in principle to calculate S-matrix
elements directly in lattice simulations of QCD. We apply this approach and
compute the S-matrix element for high-energy dipole-dipole scattering with the
presented Euclidean loop-loop correlation model. The result confirms the
analytic continuation of the gluon field strength correlator used in all
earlier applications of the stochastic vacuum model to high-energy scattering.Comment: 65 pages, 13 figures, extended and revised version to be published in
Phys. Rev. D (results unchanged, 2 new figures, 1 new table, additional
discussions in Sec.2.3 and Sec.5, new appendix on the non-Abelian Stokes
theorem, old Appendix A -> Sec.3, several references added
Theory of spin-polarized bipolar transport in magnetic p-n junctions
The interplay between spin and charge transport in electrically and
magnetically inhomogeneous semiconductor systems is investigated theoretically.
In particular, the theory of spin-polarized bipolar transport in magnetic p-n
junctions is formulated, generalizing the classic Shockley model. The theory
assumes that in the depletion layer the nonequilibrium chemical potentials of
spin up and spin down carriers are constant and carrier recombination and spin
relaxation are inhibited. Under the general conditions of an applied bias and
externally injected (source) spin, the model formulates analytically carrier
and spin transport in magnetic p-n junctions at low bias. The evaluation of the
carrier and spin densities at the depletion layer establishes the necessary
boundary conditions for solving the diffusive transport equations in the bulk
regions separately, thus greatly simplifying the problem. The carrier and spin
density and current profiles in the bulk regions are calculated and the I-V
characteristics of the junction are obtained. It is demonstrated that spin
injection through the depletion layer of a magnetic p-n junction is not
possible unless nonequilibrium spin accumulates in the bulk regions--either by
external spin injection or by the application of a large bias. Implications of
the theory for majority spin injection across the depletion layer, minority
spin pumping and spin amplification, giant magnetoresistance, spin-voltaic
effect, biasing electrode spin injection, and magnetic drift in the bulk
regions are discussed in details, and illustrated using the example of a GaAs
based magnetic p-n junction.Comment: 36 pages, 11 figures, 2 table
Measurement of the photon-jet production differential cross section in collisions at \sqrt{s}=1.96~\TeV
We present measurements of the differential cross section dsigma/dpT_gamma
for the inclusive production of a photon in association with a b-quark jet for
photons with rapidities |y_gamma|< 1.0 and 30<pT_gamma <300 GeV, as well as for
photons with 1.5<|y_gamma|< 2.5 and 30< pT_gamma <200 GeV, where pT_gamma is
the photon transverse momentum. The b-quark jets are required to have pT>15 GeV
and rapidity |y_jet| < 1.5. The results are based on data corresponding to an
integrated luminosity of 8.7 fb^-1, recorded with the D0 detector at the
Fermilab Tevatron Collider at sqrt(s)=1.96 TeV. The measured cross
sections are compared with next-to-leading order perturbative QCD calculations
using different sets of parton distribution functions as well as to predictions
based on the kT-factorization QCD approach, and those from the Sherpa and
Pythia Monte Carlo event generators.Comment: 10 pages, 9 figures, submitted to Phys. Lett.
- …