508 research outputs found
Spin-orbit coupled Bose-Einstein condensate in a tilted optical lattice
Bloch oscillations appear for a particle in a weakly tilted periodic
potential. The intrinsic spin Hall effect is an outcome of a spin-orbit
coupling. We demonstrate that both these phenomena can be realized
simultaneously in a gas of weakly interacting ultracold atoms exposed to a
tilted optical lattice and to a set of spatially dependent light fields
inducing an effective spin-orbit coupling. It is found that both the spin Hall
as well as the Bloch oscillation effects may coexist, showing, however, a
strong correlation between the two. These correlations are manifested as a
transverse spin current oscillating in-phase with the Bloch oscillations.Comment: 12 pages, 7 figure
First quantized approaches to neutrino oscillations and second quantization
Neutrino oscillations are treated from the point of view of relativistic
first quantized theories and compared to second quantized treatments. Within
first quantized theories, general oscillation probabilities can be found for
Dirac fermions and charged spin 0 bosons. A clear modification in the
oscillation formulas can be obtained and its origin is elucidated and confirmed
to be inevitable from completeness and causality requirements. The left-handed
nature of created and detected neutrinos can also be implemented in the first
quantized Dirac theory in presence of mixing; the probability loss due to the
changing of initially left-handed neutrinos to the undetected right-handed
neutrinos can be obtained in analytic form. Concerning second quantized
approaches, it is shown in a calculation using virtual neutrino propagation
that both neutrinos and antineutrinos may also contribute as intermediate
particles. The sign of the contributing neutrino energy may have to be chosen
explicitly without being automatic in the formalism. At last, a simple second
quantized description of the flavor oscillation phenomenon is devised. In this
description there is no interference terms between positive and negative
components, but it still gives simple normalized oscillation probabilities. A
new effect appearing in this context is an inevitable but tiny violation of the
initial flavor of neutrinos. The probability loss due to the conversion of
left-handed neutrinos to right-handed neutrinos is also presented.Comment: version accepted for publicatio
Spin dynamics in the Kapitza-Dirac effect
Electron spin dynamics in Kapitza-Dirac scattering from a standing laser wave
of high frequency and high intensity is studied. We develop a fully
relativistic quantum theory of the electron motion based on the time-dependent
Dirac equation. Distinct spin dynamics, with Rabi oscillations and complete
spin-flip transitions, is demonstrated for Kapitza-Dirac scattering involving
three photons in a parameter regime accessible to future high-power X-ray laser
sources. The Rabi frequency and, thus, the diffraction pattern is shown to
depend crucially on the spin degree of freedom
Exact solution of the Schrodinger equation with the spin-boson Hamiltonian
We address the problem of obtaining the exact reduced dynamics of the
spin-half (qubit) immersed within the bosonic bath (enviroment). An exact
solution of the Schrodinger equation with the paradigmatic spin-boson
Hamiltonian is obtained. We believe that this result is a major step ahead and
may ultimately contribute to the complete resolution of the problem in
question. We also construct the constant of motion for the spin-boson system.
In contrast to the standard techniques available within the framework of the
open quantum systems theory, our analysis is based on the theory of block
operator matrices.Comment: 9 pages, LaTeX, to appear in Journal of Physics A: Mathematical and
Theoretica
A structure-based proposal for the catalytic mechanism of the bacterial acid phosphatase AphA belonging to the DDDD superfamily of phosphohydrolases
The Escherichia coli gene aphA codes for a periplasmic acid phosphatase called AphA, belonging to class B bacterial phosphatases, which is part of the DDDD superfamily of phosphohydrolases. After our first report about its crystal structure, we have started a series of crystallographic studies aimed at understanding of the catalytic mechanism of the enzyme. Here, we report three crystal structures of the AphA enzyme in complex with the hydrolysis products of nucleoside monophosphate substrates and a fourth with a proposed intermediate analogue that appears to be covalently bound to the enzyme. Comparison with the native enzyme structure and with the available X-ray structures of different phosphatases provides clues about the enzyme chemistry and allows us to propose a catalytic mechanism for AphA, and to discuss it with respect to the mechanism of other bacterial and human phosphatases. (c) 2005 Elsevier Ltd. All rights reserved
Relating pseudospin and spin symmetries through charge conjugation and chiral transformations: the case of the relativistic harmonic oscillator
We solve the generalized relativistic harmonic oscillator in 1+1 dimensions,
i.e., including a linear pseudoscalar potential and quadratic scalar and vector
potentials which have equal or opposite signs. We consider positive and
negative quadratic potentials and discuss in detail their bound-state solutions
for fermions and antifermions. The main features of these bound states are the
same as the ones of the generalized three-dimensional relativistic harmonic
oscillator bound states. The solutions found for zero pseudoscalar potential
are related to the spin and pseudospin symmetry of the Dirac equation in 3+1
dimensions. We show how the charge conjugation and chiral
transformations relate the several spectra obtained and find that for massless
particles the spin and pseudospin symmetry related problems have the same
spectrum, but different spinor solutions. Finally, we establish a relation of
the solutions found with single-particle states of nuclei described by
relativistic mean-field theories with scalar, vector and isoscalar tensor
interactions and discuss the conditions in which one may have both nucleon and
antinucleon bound states.Comment: 33 pages, 10 figures, uses revtex macro
Dynamics of chiral oscillations - A comparative analysis with spin-flipping
Chiral oscillation as well as spin flipping effects correspond to quantum
phenomena of fundamental importance in the context of particle physics and, in
particular, of neutrino physics. From the point of view of first quantized
theories, we are specifically interested in appointing the differences between
chirality and helicity by obtaining their dynamic equations for a fermionic
Dirac-type particle (neutrino). We also identify both effects when the
non-minimal coupling with an external (electro)magnetic field in the neutrino
interacting Lagrangian is taken into account. We demonstrate that, however,
there is no constraint between chiral oscillations, when it takes place in
vacuum, and the process of spin flipping related to the helicity quantum
number, which does not take place in vacuum. To conclude, we show that the
origin of chiral oscillations (in vacuum) can be interpreted as position very
rapid oscillation projections onto the longitudinal direction of momentum.Comment: 14 pages, no figure
Exact results for `bouncing' Gaussian wave packets
We consider time-dependent Gaussian wave packet solutions of the Schrodinger
equation (with arbitrary initial central position, x_0, and momentum, p_0, for
an otherwise free-particle, but with an infinite wall at x=0, so-called
bouncing wave packets. We show how difference or mirror solutions of the form
psi(x,t)-psi(-x,t) can, in this case, be normalized exactly, allowing for the
evaluation of a number of time-dependent expectation values and other
quantities in closed form. For example, we calculate _t explicitly which
illustrates how the free-particle kinetic (and hence total) energy is affected
by the presence of the distant boundary. We also discuss the time dependence of
the expectation values of position, _t, and momentum, _t, and their
relation to the impulsive force during the `collision' with the wall. Finally,
the x_0,p_0 --> 0 limit is shown to reduce to a special case of a non-standard
free-particle Gaussian solution. The addition of this example to the literature
then expands on the relatively small number of Gaussian solutions to quantum
mechanical problems with familiar classical analogs (free particle, uniform
acceleration, harmonic oscillator, unstable oscillator, and uniform magnetic
field) available in closed form.Comment: 14 pages, 1 embedded .eps figur
Relativistic Spin-Flavor States in Light Front Dynamics
Orthonormal spin-flavor wave functions of Lorentz covariant quark models of
the Bakamjian-Thomas type are constructed for nucleon resonances. Three
different bases are presented. The manifestly Lorentz covariant Dirac-Melosh
basis is related to the Pauli-Melosh basis and the symmetrized Bargmann-Wigner
basis that are manifestly orthogonal.Comment: 30 pages, 8 tables, no figs; submitted to Ann.Phys.(NY
Complementary Domains of Retinoic Acid Production and Degradation in the Early Chick Embryo
AbstractExcess retinoids as well as retinoid deprivation cause abnormal development, suggesting that retinoid homeostasis is critical for proper morphogenesis. RALDH-2 and CYP26, two key enzymes that carry out retinoic acid (RA) synthesis and degradation, respectively, were cloned from the chick and show significant homology with their orthologs in other vertebrates. Expression patterns of RALDH-2 and CYP26 genes were determined in the early chick embryo by in situ hybridization. During gastrulation and neurulation RALDH-2 and CYP26 were expressed in nonoverlapping regions, with RALDH-2 transcripts localized to the presumptive presomitic and lateral plate mesoderm and CYP26 mRNA to the presumptive mid- and forebrain. The two domains of expression were separated by an approximately 300-μm-wide gap, encompassing the presumptive hindbrain. In the limb region, a similar spatial segregation of RALDH-2 and CYP26 expression was found at stages 14 and 15. Limb region mesoderm expressed RALDH-2, whereas the overlying limb ectoderm expressed CYP26. RA-synthesizing and -degrading enzymatic activities were measured biochemically in regions expressing RALDH-2 or CYP26. Regions expressing RALDH-2 generated RA efficiently from precursor retinal but degraded RA only inefficiently. Conversely, tissue expressing CYP26 efficiently degraded but did not synthesize RA. Localized regions of RA synthesis and degradation mediated by these two enzymes may therefore provide a mechanism to regulate RA homeostasis spatially in vertebrate embryos
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