570 research outputs found
Zero-energy states and fragmentation of spin in the easy-plane antiferromagnet on a honeycomb lattice
The core of the vortex in the Neel order parameter for an easy-plane
antiferromagnet on honeycomb lattice is demonstrated to bind two zero-energy
states. Remarkably, a single electron occupying this mid-gap band has its spin
fragmented between the two sublattices: Whereas it yields a vanishing total
magnetization it shows a finite Neel order, orthogonal to the one of the
assumed background. The requisite easy-plane anisotropy may be introduced by a
magnetic field parallel to the graphene layer, for example. The results are
relevant for spin-1/2 fermions on graphene's or optical honeycomb lattice, in
the strongly interacting regime.Comment: 4 pages; cosmetic changes; published versio
Velocity in Lorentz-Violating Fermion Theories
We consider the role of the velocity in Lorentz-violating fermionic quantum
theory, especially emphasizing the nonrelativistic regime. Information about
the velocity will be important for the kinematical analysis of scattering and
other problems. Working within the minimal standard model extension, we derive
new expressions for the velocity. We find that generic momentum and spin
eigenstates may not have well-defined velocities. We also demonstrate how
several different techniques may be used to shed light on different aspects of
the problem. A relativistic operator analysis allows us to study the behavior
of the Lorentz-violating Zitterbewegung. Alternatively, by studying the time
evolution of Gaussian wave packets, we find that there are Lorentz-violating
modifications to the wave packet spreading and the spin structure of the wave
function.Comment: 24 page
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
Slowly decaying classical fields, unitarity, and gauge invariance
In classical external gauge fields that fall off less fast than the inverse
of the evolution parameter (time) of the system the implementability of a
unitary perturbative scattering operator (-matrix) is not guaranteed,
although the field goes to zero. The importance of this point is exposed for
the counter-example of low-dimensionally expanding systems. The issues of gauge
invariance and of the interpretation of the evolution at intermediate times are
also intricately linked to that point.Comment: 8 pages, no figure
Some new results concerning the vacuum in Dirac Hole Theory
In Dirac's hole theory the vacuum state is generally believed to be the state
of minimum energy. It will be shown that this is not, in fact, the case and
that there must exist states in hole theory with less energy than the vacuum
state. It will be shown that energy can be extracted from the hole theory
vacuum state through the application of an electric field.Comment: Accepted by Physica Scripta, 19 page
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
T-MOD PATHWAY, A REDUCED SEQUENCE FOR IDENTIFICATION OF GRAM-NEGATIVE URINARY-TRACT PATHOGENS
In this paper, we describe a reduced sequence of identification that includes T-mod medium, a selective and
differential isolation medium which allows accurate presumptive identification of the most common gramnegative
bacteria encountered in urine samples. The present study, performed on bacteria isolated from 1,762
independent urine samples, has shown that a few selected tests (lysine and ornithine decarboxylase, urease and
trehalose fermentation tests) improve the identification accuracy of T-mod, making it possible both to identify
the less frequent species and to prevent some misidentifications of Klebsiella pneumoniae and Proteus mirabilis.
The proposed work flow agreed with conventional identification protocols to a 99.3% extent and allowed
identification of 87.4% of the isolates directly from the primary plate, 11.4% after 1 to 3 additional tests, and
1.2% after an identification gallery
Zitterbewegung of relativistic electrons in a magnetic field and its simulation by trapped ions
One-electron 3+1 and 2+1 Dirac equations are used to calculate the motion of
a relativistic electron in a vacuum in the presence of an external magnetic
field. First, calculations are carried on an operator level and exact
analytical results are obtained for the electron trajectories which contain
both intraband frequency components, identified as the cyclotron motion, as
well as interband frequency components, identified as the trembling motion
(Zitterbewegung, ZB). Next, time-dependent Heisenberg operators are used for
the same problem to compute average values of electron position and velocity
employing Gaussian wave packets. It is shown that the presence of a magnetic
field and the resulting quantization of the energy spectrum has pronounced
effects on the electron Zitterbewegung: it introduces intraband frequency
components into the motion, influences all the frequencies and makes the motion
stationary (not decaying in time) in case of the 2+1 Dirac equation. Finally,
simulations of the 2+1 Dirac equation and the resulting electron ZB in the
presence of a magnetic field are proposed and described employing trapped ions
and laser excitations. Using simulation parameters achieved in recent
experiments of Gerritsma and coworkers we show that the effects of the
simulated magnetic field on ZB are considerable and can certainly be observed.Comment: 19 pages, 9 figures, published versio
Causality, particle localization and positivity of the energy
Positivity of the Hamiltonian alone is used to show that particles, if
initially localized in a finite region, immediately develop infinite tails.Comment: To appear in: Irreversibility and Causality in Quantum Theory --
Semigroups and Rigged Hilbert Spaces, edited by A. Bohm, H.-D. Doebner and P.
Kielanowski, Springer Lecture Notes in Physics, Vol. 504 (1998
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