154 research outputs found
Hanle effect in coherent backscattering
We study the shape of the coherent backscattering (CBS) cone obtained when
resonant light illuminates a thick cloud of laser-cooled rubidium atoms in
presence of a homogenous magnetic field. We observe new magnetic
field-dependent anisotropies in the CBS signal. We show that the observed
behavior is due to the modification of the atomic radiation pattern by the
magnetic field (Hanle effect in the excited state).Comment: 4 pages, 3 figure
The Hanle Effect in 1D, 2D and 3D
This paper addresses the problem of scattering line polarization and the
Hanle effect in one-dimensional (1D), two-dimensional (2D) and
three-dimensional (3D) media for the case of a two-level model atom without
lower-level polarization and assuming complete frequency redistribution. The
theoretical framework chosen for its formulation is the QED theory of Landi
Degl'Innocenti (1983), which specifies the excitation state of the atoms in
terms of the irreducible tensor components of the atomic density matrix. The
self-consistent values of these density-matrix elements is to be determined by
solving jointly the kinetic and radiative transfer equations for the Stokes
parameters. We show how to achieve this by generalizing to Non-LTE polarization
transfer the Jacobi-based ALI method of Olson et al. (1986) and the iterative
schemes based on Gauss-Seidel iteration of Trujillo Bueno and Fabiani Bendicho
(1995). These methods essentially maintain the simplicity of the
Lambda-iteration method, but their convergence rate is extremely high. Finally,
some 1D and 2D model calculations are presented that illustrate the effect of
horizontal atmospheric inhomogeneities on magnetic and non-magnetic resonance
line polarization signals.Comment: 14 pages and 5 figure
A Substantial Amount of Hidden Magnetic Energy in the Quiet Sun
Deciphering and understanding the small-scale magnetic activity of the quiet
solar photosphere should help to solve many of the key problems of solar and
stellar physics, such as the magnetic coupling to the outer atmosphere and the
coronal heating. At present, we can see only of the complex
magnetism of the quiet Sun, which highlights the need to develop a reliable way
to investigate the remaining 99%. Here we report three-dimensional radiative
tranfer modelling of scattering polarization in atomic and molecular lines that
indicates the presence of hidden, mixed-polarity fields on subresolution
scales. Combining this modelling with recent observational data we find a
ubiquitous tangled magnetic field with an average strength of G,
which is much stronger in the intergranular regions of solar surface convection
than in the granular regions. So the average magnetic energy density in the
quiet solar photosphere is at least two orders of magnitude greater than that
derived from simplistic one-dimensional investigations, and sufficient to
balance radiative energy losses from the solar chromosphere.Comment: 21 pages and 2 figures (letter published in Nature on July 15, 2004
Spin relaxation in low-dimensional systems
We review some of the newest findings on the spin dynamics of carriers and
excitons in GaAs/GaAlAs quantum wells. In intrinsic wells, where the optical
properties are dominated by excitonic effects, we show that exciton-exciton
interaction produces a breaking of the spin degeneracy in two-dimensional
semiconductors. In doped wells, the two spin components of an optically created
two-dimensional electron gas are well described by Fermi-Dirac distributions
with a common temperature but different chemical potentials. The rate of the
spin depolarization of the electron gas is found to be independent of the mean
electron kinetic energy but accelerated by thermal spreading of the carriers.Comment: 1 PDF file, 13 eps figures, Proceedings of the 1998 International
Workshop on Nanophysics and Electronics (NPE-98)- Lecce (Italy
Einstein's quantum theory of the monatomic ideal gas: non-statistical arguments for a new statistics
In this article, we analyze the third of three papers, in which Einstein
presented his quantum theory of the ideal gas of 1924-1925. Although it failed
to attract the attention of Einstein's contemporaries and although also today
very few commentators refer to it, we argue for its significance in the context
of Einstein's quantum researches. It contains an attempt to extend and exhaust
the characterization of the monatomic ideal gas without appealing to
combinatorics. Its ambiguities illustrate Einstein's confusion with his initial
success in extending Bose's results and in realizing the consequences of what
later became to be called Bose-Einstein statistics. We discuss Einstein's
motivation for writing a non-combinatorial paper, partly in response to
criticism by his friend Ehrenfest, and we paraphrase its content. Its arguments
are based on Einstein's belief in the complete analogy between the
thermodynamics of light quanta and of material particles and invoke
considerations of adiabatic transformations as well as of dimensional analysis.
These techniques were well-known to Einstein from earlier work on Wien's
displacement law, Planck's radiation theory, and the specific heat of solids.
We also investigate the possible role of Ehrenfest in the gestation of the
theory.Comment: 57 pp
Resonant nonlinear magneto-optical effects in atoms
In this article, we review the history, current status, physical mechanisms,
experimental methods, and applications of nonlinear magneto-optical effects in
atomic vapors. We begin by describing the pioneering work of Macaluso and
Corbino over a century ago on linear magneto-optical effects (in which the
properties of the medium do not depend on the light power) in the vicinity of
atomic resonances, and contrast these effects with various nonlinear
magneto-optical phenomena that have been studied both theoretically and
experimentally since the late 1960s. In recent years, the field of nonlinear
magneto-optics has experienced a revival of interest that has led to a number
of developments, including the observation of ultra-narrow (1-Hz)
magneto-optical resonances, applications in sensitive magnetometry, nonlinear
magneto-optical tomography, and the possibility of a search for parity- and
time-reversal-invariance violation in atoms.Comment: 51 pages, 23 figures, to appear in Rev. Mod. Phys. in Oct. 2002,
Figure added, typos corrected, text edited for clarit
Zero-Field Dichroism in the Solar Chromosphere
We explain the linear polarization of the Ca II infrared triplet observed
close to the edge of the solar disk. In particular, we demonstrate that the
physical origin of the enigmatic polarizations of the 866.2 nm and 854.2 nm
lines lies in the existence of atomic polarization in their metastable lower
levels, which produces differential absorption of polarization components
(dichroism). To this end, we have solved the problem of the generation and
transfer of polarized radiation by taking fully into account all the relevant
optical pumping mechanisms in multilevel atomic models. We argue that
`zero-field' dichroism may be of great diagnostic value in astrophysics.Comment: 10 pages, 3 figure
Selective Absorption Processes as the Origin of Puzzling Spectral Line Polarization from the Sun
Magnetic fields play a key role in most astrophysical systems, from the Sun
to active galactic nuclei. They can be studied through their effects on atomic
energy levels, which produce polarized spectral lines. In particular,
anisotropic radiation pumping processes (which send electrons to higher atomic
levels) induce population imbalances that are modified by weak magnetic fields.
Here we report peculiarly polarized light in the He I 10830-\AA multiplet
observed in a coronal filament located at the centre of the solar disk. We show
that the polarized light arises from selective absorption from the ground level
of the triplet system of helium, and that it implies the presence of magnetic
fields of the order of a few gauss that are highly inclined with respect to the
solar radius vector. This disproves the common belief that population
imbalances in long-lived atomic levels are insignificant in the presence of
inclined fields with strengths in the gauss range, and demonstrates the
operation of the ground-level Hanle effect in an astrophysical plasma.Comment: 22 pages and 4 figure
Spintronics: Fundamentals and applications
Spintronics, or spin electronics, involves the study of active control and
manipulation of spin degrees of freedom in solid-state systems. This article
reviews the current status of this subject, including both recent advances and
well-established results. The primary focus is on the basic physical principles
underlying the generation of carrier spin polarization, spin dynamics, and
spin-polarized transport in semiconductors and metals. Spin transport differs
from charge transport in that spin is a nonconserved quantity in solids due to
spin-orbit and hyperfine coupling. The authors discuss in detail spin
decoherence mechanisms in metals and semiconductors. Various theories of spin
injection and spin-polarized transport are applied to hybrid structures
relevant to spin-based devices and fundamental studies of materials properties.
Experimental work is reviewed with the emphasis on projected applications, in
which external electric and magnetic fields and illumination by light will be
used to control spin and charge dynamics to create new functionalities not
feasible or ineffective with conventional electronics.Comment: invited review, 36 figures, 900+ references; minor stylistic changes
from the published versio
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