43,567 research outputs found
Color Dipole Moments for Edge Detection
Dipole and higher moments are physical quantities used to describe a charge
distribution. In analogy with electromagnetism, it is possible to define the
dipole moments for a gray-scale image, according to the single aspect of a
gray-tone map. In this paper we define the color dipole moments for color
images. For color maps in fact, we have three aspects, the three primary
colors, to consider. Associating three color charges to each pixel, color
dipole moments can be easily defined and used for edge detection.Comment: 8 page
Edge-Magnetoplasmon Wave-Packet Revivals in the Quantum Hall Effect
The quantum Hall effect is necessarily accompanied by low-energy excitations
localized at the edge of a two-dimensional electron system. For the case of
electrons interacting via the long-range Coulomb interaction, these excitations
are edge magnetoplasmons. We address the time evolution of localized
edge-magnetoplasmon wave packets. On short times the wave packets move along
the edge with classical E cross B drift. We show that on longer times the wave
packets can have properties similar to those of the Rydberg wave packets that
are produced in atoms using short-pulsed lasers. In particular, we show that
edge-magnetoplasmon wave packets can exhibit periodic revivals in which a
dispersed wave packet reassembles into a localized one. We propose the study of
edge-magnetoplasmon wave packets as a tool to investigate dynamical properties
of integer and fractional quantum-Hall edges. Various scenarios are discussed
for preparing the initial wave packet and for detecting it at a later time. We
comment on the importance of magnetoplasmon-phonon coupling and on quantum and
thermal fluctuations.Comment: 18 pages, RevTex, 7 figures and 2 tables included, Fig. 5 was
originally 3Mbyte and had to be bitmapped for submission to archive; in the
process it acquired distracting artifacts, to upload the better version, see
http://physics.indiana.edu/~uli/publ/projects.htm
Non-linear spin Seebeck effect due to spin-charge interaction in graphene
The abilities to inject and detect spin carriers are fundamental for research
on transport and manipulation of spin information. Pure electronic spin
currents have been recently studied in nanoscale electronic devices using a
non-local lateral geometry, both in metallic systems and in semiconductors. To
unlock the full potential of spintronics we must understand the interactions of
spin with other degrees of freedom, going beyond the prototypical electrical
spin injection and detection using magnetic contacts. Such interactions have
been explored recently, for example, by using spin Hall or spin thermoelectric
effects. Here we present the detection of non-local spin signals using
non-magnetic detectors, via an as yet unexplored non-linear interaction between
spin and charge. In analogy to the Seebeck effect, where a heat current
generates a charge potential, we demonstrate that a spin current in a
paramagnet leads to a charge potential, if the conductivity is energy
dependent. We use graphene as a model system to study this effect, as recently
proposed. The physical concept demonstrated here is generally valid, opening
new possibilities for spintronics
Fractional Spin Josephson Effect and Electrically Controlled Magnetization in Quantum Spin Hall Edges
We explore a spin Josephson effect in a system of two ferromagnets coupled by
a tunnel junction formed of 2D time-reversal invariant topological insulators.
In analogy with the more commonly studied instance of the Josephson effect for
charge in superconductors, we investigate properties of the phase-coherent {\it
spin} current resulting from the misalignment of the in-plane magnetization
angles of the two ferromagnets. We show that the topological insulating barrier
offers the exciting prospect of hosting a {\it fractional} spin Josephson
effect mediated by bound states at the ferromagnet-topological insulator
interface. We provide multiple perspectives to understand the periodic
nature of this effect. We discuss several measurable consequences, such as, the
generation of a transverse voltage signal which allows for purely electrical
measurements, an inverse of this effect where an applied voltage gives rise to
a transverse spin-current, and a fractional AC spin-Josephson effect.Comment: 5 pages, 2 figures (v2:fixed typo, added reference
Electrical writing, deleting, reading, and moving of magnetic skyrmioniums in a racetrack device
A magnetic skyrmionium (also called 2-skyrmion) can be understood as a
skyrmion - a topologically non-trivial magnetic whirl - which is situated in
the center of a second skyrmion with reversed magnetization. Here, we propose a
new optoelectrical writing and deleting mechanism for skyrmioniums in thin
films, as well as a reading mechanism based on the topological Hall voltage.
Furthermore, we point out advantages for utilizing skyrmioniums as carriers of
information in comparison to skyrmions with respect to the current-driven
motion. We simulate all four constituents of an operating skyrmionium-based
racetrack storage device: creation, motion, detection and deletion of bits. The
existence of a skyrmionium is thereby interpreted as a '1' and its absence as a
'0' bit.Comment: This is a post-peer-review, pre-copyedit version of an article
published in Scientific Reports. The final authenticated version is available
online at [DOI
Identifying single electron charge sensor events using wavelet edge detection
The operation of solid-state qubits often relies on single-shot readout using
a nanoelectronic charge sensor, and the detection of events in a noisy sensor
signal is crucial for high fidelity readout of such qubits. The most common
detection scheme, comparing the signal to a threshold value, is accurate at low
noise levels but is not robust to low-frequency noise and signal drift. We
describe an alternative method for identifying charge sensor events using
wavelet edge detection. The technique is convenient to use and we show that,
with realistic signals and a single tunable parameter, wavelet detection can
outperform thresholding and is significantly more tolerant to 1/f and
low-frequency noise.Comment: 11 pages, 4 figure
The chiral anomaly in real space
The chiral anomaly is based on a non-conserved chiral charge and can happen
in Dirac fermion systems under the influence of external electromagnetic
fields. In this case, the spectral flow leads to a transfer of right- to
left-moving excitations or vice versa. The corresponding transfer of chiral
particles happens in momentum space. We here describe an intriguing way to
introduce the chiral anomaly into real space. Our system consists of two
quantum dots that are formed at the helical edge of a quantum spin Hall
insulator on the basis of three magnetic impurities. Such a setup gives rise to
fractional charges which we show to be sharp quantum numbers for large barrier
strength. Interestingly, it is possible to map the system onto a quantum spin
Hall ring in the presence of a flux pierced through the ring where the relative
angle between the magnetization directions of the impurities takes the role of
the flux. The chiral anomaly in this system is then directly related to the
excess occupation of particles in the two quantum dots. This analogy allows us
to predict an observable consequence of the chiral anomaly in real space.Comment: 7 pages, 5 figure
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