8,835 research outputs found
Radiative polarization of electrons in a strong laser wave
We reanalyze the problem of radiative polarization of electrons brought into
collision with a circularly polarized strong plane wave. We present an
independent analytical verification of formulae for the cross section given by
D.\,Yu. Ivanov et al [Eur.\ Phys.\ J. C \textbf{36}, 127 (2004)]. By choosing
the exact electron's helicity as the spin quantum number we show that the
self-polarization effect exists only for the moderately relativistic electrons
with energy and only for a non-head-on collision
geometry. In these conditions polarization degree may achieve the values up to
65%, but the effective polarization time is found to be larger than 1\,s even
for a high power optical or infrared laser with intensity parameter (). This
makes such a polarization practically unrealizable. We also compare these
results with the ones of some papers where the high degree of polarization was
predicted for ultrarelativistic case. We argue that this apparent contradiction
arises due to the different choice of the spin quantum numbers. In particular,
the quantum numbers which provide the high polarization degree represent
neither helicity nor transverse polarization, that makes the use of them
inconvenient in practice.Comment: minor changes compared to v3; to appear in PR
Kodaira-Spencer formality of products of complex manifolds
We shall say that a complex manifold is emph{Kodaira-Spencer formal} if its Kodaira-Spencer differential graded Lie algebra
is formal; if this happen, then the deformation theory of
is completely determined by the graded Lie algebra and the base space of the semiuniversal deformation is a quadratic singularity..
Determine when a complex manifold is Kodaira-Spencer formal is generally difficult and
we actually know only a limited class of cases where this happen. Among such examples we have
Riemann surfaces, projective spaces, holomorphic Poisson manifolds with surjective anchor map
and every compact K"{a}hler manifold with trivial or torsion canonical
bundle.
In this short note we investigate the behavior of this property under finite products. Let be compact complex manifolds; we prove that whenever and are
K"{a}hler, then is Kodaira-Spencer formal if and only if the same
holds for and . A revisit of a classical example by Douady shows that the above result fails if the K"{a}hler assumption is droppe
Absence of structural correlations of magnetic defects in heavy fermion LiV2O4
Magnetic defects have pronounced effects on the magnetic properties of the
face-centered cubic compound LiV2O4. The magnetic defects arise from crystal
defects present within the normal spinel structure. High-energy x-ray
diffraction studies were performed on LiV2O4 single crystals to search for
superstructure peaks or any other evidence of periodicity in the arrangement of
the crystal defects present in the lattice. Entire reciprocal lattice planes
are mapped out with help of synchrotron radiation. No noticeable differences in
the x-ray diffraction data between a crystal with high magnetic defect
concentration and a crystal with low magnetic defect concentration have been
found. This indicates the absence of any long-range periodicity or short-range
correlations in the arrangements of the crystal/magnetic defects.Comment: 6 pages, 4 figure
Engineering Time-Reversal Invariant Topological Insulators With Ultra-Cold Atoms
Topological insulators are a broad class of unconventional materials that are
insulating in the interior but conduct along the edges. This edge transport is
topologically protected and dissipationless. Until recently, all existing
topological insulators, known as quantum Hall states, violated time-reversal
symmetry. However, the discovery of the quantum spin Hall effect demonstrated
the existence of novel topological states not rooted in time-reversal
violations. Here, we lay out an experiment to realize time-reversal topological
insulators in ultra-cold atomic gases subjected to synthetic gauge fields in
the near-field of an atom-chip. In particular, we introduce a feasible scheme
to engineer sharp boundaries where the "edge states" are localized. Besides,
this multi-band system has a large parameter space exhibiting a variety of
quantum phase transitions between topological and normal insulating phases. Due
to their unprecedented controllability, cold-atom systems are ideally suited to
realize topological states of matter and drive the development of topological
quantum computing.Comment: 11 pages, 6 figure
Forcing function control of Faraday wave instabilities in viscous shallow fluids
We investigate the relationship between the linear surface wave instabilities
of a shallow viscous fluid layer and the shape of the periodic,
parametric-forcing function (describing the vertical acceleration of the fluid
container) that excites them. We find numerically that the envelope of the
resonance tongues can only develop multiple minima when the forcing function
has more than two local extrema per cycle. With this insight, we construct a
multi-frequency forcing function that generates at onset a non-trivial harmonic
instability which is distinct from a subharmonic response to any of its
frequency components. We measure the corresponding surface patterns
experimentally and verify that small changes in the forcing waveform cause a
transition, through a bicritical point, from the predicted harmonic
short-wavelength pattern to a much larger standard subharmonic pattern. Using a
formulation valid in the lubrication regime (thin viscous fluid layer) and a
WKB method to find its analytic solutions, we explore the origin of the
observed relation between the forcing function shape and the resonance tongue
structure. In particular, we show that for square and triangular forcing
functions the envelope of these tongues has only one minimum, as in the usual
sinusoidal case.Comment: 12 pages, 10 figure
Bottom-loading dilution refrigerator with ultra-high vacuum deposition capability
A Kelvinox 400 dilution refrigerator with the ability to load samples onto
the mixing chamber from the bottom of the cryostat has been combined with an
ultrahigh-vacuum (UHV) deposition chamber equipped with molecular beam sources.
The liquid helium cooled sample transfer mechanism is used in a manner that
allows films to be grown on substrates which are kept at temperatures of order
8K with chamber pressures in the 10^-9 to 10^-10 Torr range. This system
facilitates the growth of quench-condensed ultrathin films which must always be
kept below ~ 12K in a UHV environment during and after growth. Measurements can
be made on the films down to millikelvin temperatures and in magnetic fields up
to 15 T.Comment: 10 pages text, 1figur
Characterizing the Hofstadter butterfly's outline with Chern numbers
In this work, we report original properties inherent to independent particles
subjected to a magnetic field by emphasizing the existence of regular
structures in the energy spectrum's outline. We show that this fractal curve,
the well-known Hofstadter butterfly's outline, is associated to a specific
sequence of Chern numbers that correspond to the quantized transverse
conductivity. Indeed the topological invariant that characterizes the
fundamental energy band depicts successive stairways as the magnetic flux
varies. Moreover each stairway is shown to be labeled by another Chern number
which measures the charge transported under displacement of the periodic
potential. We put forward the universal character of these properties by
comparing the results obtained for the square and the honeycomb geometries.Comment: Accepted for publication in J. Phys. B (Jan 2009
Nature of the Magnetic Order in BaMn2As2
Neutron diffraction measurements have been performed on a powder sample of
BaMn2As2 over the temperature T range from 10 K to 675 K. These measurements
demonstrate that this compound exhibits collinear antiferromagnetic ordering
below the Neel temperature T_N = 625(1) K. The ordered moment mu = 3.88(4)
mu_B/Mn at T = 10 K is oriented along the c axis and the magnetic structure is
G-type, with all nearest-neighbor Mn moments antiferromagnetically aligned. The
value of the ordered moment indicates that the oxidation state of Mn is Mn^{2+}
with a high spin S = 5/2. The T dependence of mu suggests that the magnetic
transition is second-order in nature. In contrast to the closely related
AFe2As2 (A = Ca, Sr, Ba, Eu) compounds, no structural distortion is observed in
the magnetically ordered state of BaMn2As2.Comment: 4 pages, 3 figures, 1 table; v2: additional discussion of Mn-Mn
interactions; accepted for publication as a Rapid Communication in Phys. Rev.
- …