404 research outputs found
Atomic Resolution Electron Holography
It has been demonstrated that electron holography is a very powerful tool to investigate an electromagnetic potential in medium resolution, since the phase of an electron wave is approximately proportional to the potential. Now, electron holography is at the second stage of development: to establish holography at atomic resolution and further to realize Gabor\u27s idea to improve the resolution restricted by the spherical aberration of the objective lens. We investigate the possibility of electron holography to get information at atomic resolution by computer simulations as well as by digital processing of electron holograms. We show that the phase distribution has more resemblance to the specimen structure than the amplitude distribution. We also compare electron holography with electron microscopy from an image processing point of view
Unusual superexchange pathways in a Ni triangular lattice of NiGaS with negative charge-transfer energy
We have studied the electronic structure of the Ni triangular lattice in
NiGaS using photoemission spectroscopy and subsequent model
calculations. The cluster-model analysis of the Ni 2 core-level spectrum
shows that the S 3 to Ni 3 charge-transfer energy is -1 eV and the
ground state is dominated by the configuration ( is a S 3 hole).
Cell perturbation analysis for the NiS triangular lattice indicates that
the strong S 3 hole character of the ground state provides the enhanced
superexchange interaction between the third nearest neighbor sites.Comment: 10 pages, 5 figures, accepted to PR
Deformation and Depinning of Superconducting Vortices from Artificial Defects: A Ginzburg-Landau Study
Using Ginzburg-Landau theory, we have performed detailed studies of vortices
in the presence of artificial defect arrays, for a thin film geometry. We show
that when a vortex approaches the vicinity of a defect, an abrupt transition
occurs in which the vortex core develops a ``string'' extending to the defect
boundary, while simultaneously the supercurrents and associated magnetic flux
spread out and engulf the defect. Current induced depinning of vortices is
shown to be dominated by the core string distortion in typical experimental
situations. Experimental consequences of this unusual depinning behavior are
discussed.Comment: 10 pages,9 figure
Domain walls in (Ga,Mn)As diluted magnetic semiconductor
We report experimental and theoretical studies of magnetic domain walls in an
in-plane magnetized (Ga,Mn)As dilute moment ferromagnetic semiconductor. Our
high-resolution electron holography technique provides direct images of domain
wall magnetization profiles. The experiments are interpreted based on
microscopic calculations of the micromagnetic parameters and
Landau-Lifshitz-Gilbert simulations. We find that the competition of uniaxial
and biaxial magnetocrystalline anisotropies in the film is directly reflected
in orientation dependent wall widths, ranging from approximately 40 nm to 120
nm. The domain walls are of the N\'eel type and evolve from near-
walls at low-temperatures to large angle [10]-oriented walls and small
angle [110]-oriented walls at higher temperatures.Comment: 5 pages, 4 figure
Darwin-Lagrangian Analysis for the Interaction of a Point Charge and a Magnet: Considerations Related to the Controversy Regarding the Aharonov-Bohm and Aharonov-Casher Phase Shifts
The classical electromagnetic interaction of a point charge and a magnet is
discussed by first calculating the interaction of point charge with a simple
model magnetic moment and then suggesting a multiparticle limit. The Darwin
Lagrangian is used to analyze the electromagnetic behavior of the model
magnetic moment (composed of two oppositely charged particles of different mass
in an initially circular orbit) interacting with a passing point charge. The
changing mangetic moment is found to put a force back on a passing charge; this
force is of order 1/c^2 and depends upon the magnitude of the magnetic moment.
It is suggested that in the limit of a multiparticle magnetic toroid, the
electric fields of the passing charge are screened out of the body of the
magnet while the magnetic fields penetrate into the magnet. This is consistent
with our understanding of the penetration of electromagnetic velocity fields
into ohmic conductors. Conservation laws are discussed. The work corresponds to
a classical electromagnetic analysis of the interaction which is basic to
understanding the controversy over the Aharonov-Bohm and Aharonov-Casher phase
shifts and represents a refutation of the suggestions of Aharonov, Pearle, and
Vaidman.Comment: 33 page
Interrelations Between the Neutron's Magnetic Interactions and the Magnetic Aharonov-Bohm Effect
It is proved that the phase shift of a polarized neutron interacting with a
spatially uniform time-dependent magnetic field, demonstrates the same physical
principles as the magnetic Aharonov-Bohm effect. The crucial role of inert
objects is explained, thereby proving the quantum mechanical nature of the
effect. It is also proved that the nonsimply connectedness of the field-free
region is not a profound property of the system and that it cannot be regarded
as a sufficient condition for a nonzero phase shift.Comment: 18 pages, 1 postscript figure, Late
Nonlocal Phases of Local Quantum Mechanical Wavefunctions in Static and Time-Dependent Aharonov-Bohm Experiments
We show that the standard Dirac phase factor is not the only solution of the
gauge transformation equations. The full form of a general gauge function (that
connects systems that move in different sets of scalar and vector potentials),
apart from Dirac phases also contains terms of classical fields that act
nonlocally (in spacetime) on the local solutions of the time-dependent
Schr\"odinger equation: the phases of wavefunctions in the Schr\"odinger
picture are affected nonlocally by spatially and temporally remote magnetic and
electric fields, in ways that are fully explored. These contributions go beyond
the usual Aharonov-Bohm effects (magnetic or electric). (i) Application to
cases of particles passing through static magnetic or electric fields leads to
cancellations of Aharonov-Bohm phases at the observation point; these are
linked to behaviors at the semiclassical level (to the old Werner & Brill
experimental observations, or their "electric analogs" - or to recent reports
of Batelaan & Tonomura) but are shown to be far more general (true not only for
narrow wavepackets but also for completely delocalized quantum states). By
using these cancellations, certain previously unnoticed sign-errors in the
literature are corrected. (ii) Application to time-dependent situations
provides a remedy for erroneous results in the literature (on improper uses of
Dirac phase factors) and leads to phases that contain an Aharonov-Bohm part and
a field-nonlocal part: their competition is shown to recover Relativistic
Causality in earlier "paradoxes" (such as the van Kampen thought-experiment),
while a more general consideration indicates that the temporal nonlocalities
found here demonstrate in part a causal propagation of phases of quantum
mechanical wavefunctions in the Schr\"odinger picture. This may open a direct
way to address time-dependent double-slit experiments and the associated causal
issuesComment: 49 pages, 1 figure, presented in Conferences "50 years of the
Aharonov-Bohm effect and 25 years of the Berry's phase" (Tel Aviv and
Bristol), published in Journ. Phys. A. Compared to the published paper, this
version has 17 additional lines after eqn.(14) for maximum clarity, and the
Abstract has been slightly modified and reduced from the published 2035
characters to the required 1920 character
Quantum Interference in Superconducting Wire Networks and Josephson Junction Arrays: Analytical Approach based on Multiple-Loop Aharonov-Bohm Feynman Path-Integrals
We investigate analytically and numerically the mean-field
superconducting-normal phase boundaries of two-dimensional superconducting wire
networks and Josephson junction arrays immersed in a transverse magnetic field.
The geometries we consider include square, honeycomb, triangular, and kagome'
lattices. Our approach is based on an analytical study of multiple-loop
Aharonov-Bohm effects: the quantum interference between different electron
closed paths where each one of them encloses a net magnetic flux. Specifically,
we compute exactly the sums of magnetic phase factors, i.e., the lattice path
integrals, on all closed lattice paths of different lengths. A very large
number, e.g., up to for the square lattice, exact lattice path
integrals are obtained. Analytic results of these lattice path integrals then
enable us to obtain the resistive transition temperature as a continuous
function of the field. In particular, we can analyze measurable effects on the
superconducting transition temperature, , as a function of the magnetic
filed , originating from electron trajectories over loops of various
lengths. In addition to systematically deriving previously observed features,
and understanding the physical origin of the dips in as a result of
multiple-loop quantum interference effects, we also find novel results. In
particular, we explicitly derive the self-similarity in the phase diagram of
square networks. Our approach allows us to analyze the complex structure
present in the phase boundaries from the viewpoint of quantum interference
effects due to the electron motion on the underlying lattices.Comment: 18 PRB-type pages, plus 8 large figure
Flicker Suppression in JPEG2000 using Segmentation-Based Adjustment of Block Truncation Lengths
Paradoxes of the Aharonov-Bohm and the Aharonov-Casher effects
For a believer in locality of Nature, the Aharonov-Bohm effect and the
Aharonov-Casher effect are paradoxes. I discuss these and other Aharonov's
paradoxes and propose a local explanation of these effects. If the solenoid in
the Aharonov-Bohm effect is treated quantum mechanically, the effect can be
explained via local interaction between the field of the electron and the
solenoid. I argue that the core of the Aharonov-Bohm and the Aharonov-Casher
effects is that of quantum entanglement: the quantum wave function describes
all systems together.Comment: To be published in Yakir Aharonov 80th birthday Festschrif
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