665 research outputs found
Symmetry Breaking of Relativistic Multiconfiguration Methods in the Nonrelativistic Limit
The multiconfiguration Dirac-Fock method allows to calculate the state of
relativistic electrons in atoms or molecules. This method has been known for a
long time to provide certain wrong predictions in the nonrelativistic limit. We
study in full mathematical details the nonlinear model obtained in the
nonrelativistic limit for Be-like atoms. We show that the method with sp+pd
configurations in the J=1 sector leads to a symmetry breaking phenomenon in the
sense that the ground state is never an eigenvector of L^2 or S^2. We thereby
complement and clarify some previous studies.Comment: Final version, to appear in Nonlinearity. Nonlinearity (2010) in
pres
Trapped ion emulation of electric dipole moment of neutral relativistic particles
The electric dipole moments of various neutral elementary particles, such as
neutron, neutrinos, certain hypothetical dark matter particles and others, are
predicted to exist by the standard model of high energy physics and various
extensions of it. However, the predicted values are beyond the present
experimental capabilities. We propose to simulate and emulate the electric
dipole moment of neutral relativistic particles and the ensuing effects in the
presence of electrostatic field by emulation of an extended Dirac equation in
ion traps
Klein tunneling and Dirac potentials in trapped ions
We propose the quantum simulation of the Dirac equation with potentials,
allowing the study of relativistic scaterring and the Klein tunneling. This
quantum relativistic effect permits a positive-energy Dirac particle to
propagate through a repulsive potential via the population transfer to
negative-energy components. We show how to engineer scalar, pseudoscalar, and
other potentials in the 1+1 Dirac equation by manipulating two trapped ions.
The Dirac spinor is represented by the internal states of one ion, while its
position and momentum are described by those of a collective motional mode. The
second ion is used to build the desired potentials with high spatial
resolution.Comment: 4 pages, 3 figures, minor change
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
Electronic States of Graphene Grain Boundaries
We introduce a model for amorphous grain boundaries in graphene, and find
that stable structures can exist along the boundary that are responsible for
local density of states enhancements both at zero and finite (~0.5 eV)
energies. Such zero energy peaks in particular were identified in STS
measurements [J. \v{C}ervenka, M. I. Katsnelson, and C. F. J. Flipse, Nature
Physics 5, 840 (2009)], but are not present in the simplest pentagon-heptagon
dislocation array model [O. V. Yazyev and S. G. Louie, Physical Review B 81,
195420 (2010)]. We consider the low energy continuum theory of arrays of
dislocations in graphene and show that it predicts localized zero energy
states. Since the continuum theory is based on an idealized lattice scale
physics it is a priori not literally applicable. However, we identify stable
dislocation cores, different from the pentagon-heptagon pairs, that do carry
zero energy states. These might be responsible for the enhanced magnetism seen
experimentally at graphite grain boundaries.Comment: 10 pages, 4 figures, submitted to Physical Review
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
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
The molecular class C acid phosphatase of Chryseobacterium meningosepticum (OlpA) is a broad-spectrum nucleotidase with preferential activity on 5'-nucleotides
The olpA gene of Chryseobacterium meningosepticum, encoding a molecular class C phosphatase, was cloned and expressed in Escherichia coli. The gene encodes a 29-kDa polypeptide containing an amino-terminal signal peptide typical of bacterial membrane lipoproteins. Expression in E. coli results in a functional product that mostly partitions in the outer membrane. A secreted soluble OlpA derivative (sOlpA) lacking the N-terminal cysteine residue for lipid anchoring was produced in E. coli and purified by means of two steps of ion exchange chromatography. Analysis of the kinetic parameters of sOlpA with several organic phosphoesters revealed that the enzyme was able to efficiently hydrolyze nucleotide monophosphates, with a strong preference for 5'-nucleotides and for 3'-AMP. The enzyme was also able to hydrolyze sugar phosphates and beta-glycerol phosphate, although with a lower efficiency, whereas it was apparently inactive against nucleotide di- and triphosphates, diesters, and phytate. OlpA, therefore, can be considered a broad-spectrum nucleotidase with preference for 5'-nucleotides. Its functional behaviour exhibits differences from that of the Haemophilus influenzae OMP P4 lipoprotein, revealing functional heterogeneity among phosphatases of molecular class C
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
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
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