850 research outputs found
A Modified Self-Tuning Fuzzy-Neural Controller
This paper presents a modified self-tuning fuzzy-neural controller in the applications nonlinear model reference control system. In order to make the controller have the adaptive control capability, the immediate system error (e(k)) and error change ( e(k)) are used to be the inputs for fuzzy-neural tuning mechanism. For simplifying the construction of fuzzy system, nine rules are used in the rule table. To demonstrate the superiority of the controller we developed, several nonlinear model reference control systems are studied and simulated. The simulation results clearly show that the self-tuning fuzzy-neural controller has quite promising potential in the real control applications
Mesoscale magnetism at the grain boundaries in colossal magnetoresistive films
We report the discovery of mesoscale regions with distinctive magnetic
properties in epitaxial LaSrMnO films which exhibit
tunneling-like magnetoresistance across grain boundaries. By using
temperature-dependent magnetic force microscopy we observe that the mesoscale
regions are formed near the grain boundaries and have a different Curie
temperature (up to 20 K {\it higher}) than the grain interiors. Our images
provide direct evidence for previous speculations that the grain boundaries in
thin films are not magnetically and electronically sharp interfaces. The size
of the mesoscale regions varies with temperature and nature of the underlying
defect.Comment: 4 pages of text, 4 figure
Manipulating infrared photons using plasmons in transparent graphene superlattices
Superlattices are artificial periodic nanostructures which can control the
flow of electrons. Their operation typically relies on the periodic modulation
of the electric potential in the direction of electron wave propagation. Here
we demonstrate transparent graphene superlattices which can manipulate infrared
photons utilizing the collective oscillations of carriers, i.e., plasmons of
the ensemble of multiple graphene layers. The superlattice is formed by
depositing alternating wafer-scale graphene sheets and thin insulating layers,
followed by patterning them all together into 3-dimensional
photonic-crystal-like structures. We demonstrate experimentally that the
collective oscillation of Dirac fermions in such graphene superlattices is
unambiguously nonclassical: compared to doping single layer graphene,
distributing carriers into multiple graphene layers strongly enhances the
plasmonic resonance frequency and magnitude, which is fundamentally different
from that in a conventional semiconductor superlattice. This property allows us
to construct widely tunable far-infrared notch filters with 8.2 dB rejection
ratio and terahertz linear polarizers with 9.5 dB extinction ratio, using a
superlattice with merely five graphene atomic layers. Moreover, an unpatterned
superlattice shields up to 97.5% of the electromagnetic radiations below 1.2
terahertz. This demonstration also opens an avenue for the realization of other
transparent mid- and far-infrared photonic devices such as detectors,
modulators, and 3-dimensional meta-material systems.Comment: under revie
Impact of Charge Ordering on Magnetic Correlations in Perovskite (Bi,Ca)MnO_3
Single crystalline (Bi,Ca)MnO3 (74< %Ca <82) were studied with neutron
scattering, electron diffraction and bulk magnetic measurement. We discovered
dynamic ferromagnetic spin correlations at high temperatures, which are
replaced by antiferromagnetic spin fluctuations at a concomitant charge
ordering and structural transition. Our results indicate that thermal-activated
hopping of the Jahn-Teller active e_g electrons in these insulating materials,
nevertheless, induce ferromagnetic interaction through double-exchange
mechanism. It is the ordering of these charges competing with the
double-exchange ferromagnetic metallic state.Comment: 11 pages, 3 figures, Revte
Amplitude Damping for single-qubit System with single-qubit mixed-state Environment
We study a generalized amplitude damping channel when environment is
initially in the single-qubit mixed state. Representing the affine
transformation of the generalized amplitude damping by a three-dimensional
volume, we plot explicitly the volume occupied by the channels simulatable by a
single-qubit mixed-state environment. As expected, this volume is embedded in
the total volume by the channels which is simulated by two-qubit enviroment.
The volume ratio is approximately 0.08 which is much smaller than 3/8, the
volume ratio for generalized depolarizing channels.Comment: 13 pages, 2 figures incluided V2: homepage address is included in
reference V3: version to appear in J. Phys. A: Mathematical and Theoretica
Plasmon-phonon coupling in large-area graphene dot and antidot arrays
Nanostructured graphene on SiO2 substrates pave the way for enhanced
light-matter interactions and explorations of strong plasmon-phonon
hybridization in the mid-infrared regime. Unprecedented large-area graphene
nanodot and antidot optical arrays are fabricated by nanosphere lithography,
with structural control down to the sub-100 nanometer regime. The interaction
between graphene plasmon modes and the substrate phonons is experimentally
demonstrated and structural control is used to map out the hybridization of
plasmons and phonons, showing coupling energies of the order 20 meV. Our
findings are further supported by theoretical calculations and numerical
simulations.Comment: 7 pages including 6 figures. Supporting information is available upon
request to author
Mid-infrared plasmons in scaled graphene nanostructures
Plasmonics takes advantage of the collective response of electrons to
electromagnetic waves, enabling dramatic scaling of optical devices beyond the
diffraction limit. Here, we demonstrate the mid-infrared (4 to 15 microns)
plasmons in deeply scaled graphene nanostructures down to 50 nm, more than 100
times smaller than the on-resonance light wavelength in free space. We reveal,
for the first time, the crucial damping channels of graphene plasmons via its
intrinsic optical phonons and scattering from the edges. A plasmon lifetime of
20 femto-seconds and smaller is observed, when damping through the emission of
an optical phonon is allowed. Furthermore, the surface polar phonons in SiO2
substrate underneath the graphene nanostructures lead to a significantly
modified plasmon dispersion and damping, in contrast to a non-polar
diamond-like-carbon (DLC) substrate. Much reduced damping is realized when the
plasmon resonance frequencies are close to the polar phonon frequencies. Our
study paves the way for applications of graphene in plasmonic waveguides,
modulators and detectors in an unprecedentedly broad wavelength range from
sub-terahertz to mid-infrared.Comment: submitte
Grain boundary effects on magnetotransport in bi-epitaxial films of LaSrMnO
The low field magnetotransport of LaSrMnO (LSMO) films
grown on SrTiO substrates has been investigated. A high qualtity LSMO film
exhibits anisotropic magnetoresistance (AMR) and a peak in the
magnetoresistance close to the Curie temperature of LSMO. Bi-epitaxial films
prepared using a seed layer of MgO and a buffer layer of CeO display a
resistance dominated by grain boundaries. One film was prepared with seed and
buffer layers intact, while a second sample was prepared as a 2D square array
of grain boundaries. These films exhibit i) a low temperature tail in the low
field magnetoresistance; ii) a magnetoconductance with a constant high field
slope; and iii) a comparably large AMR effect. A model based on a two-step
tunneling process, including spin-flip tunneling, is discussed and shown to be
consistent with the experimental findings of the bi-epitaxial films.Comment: REVTeX style; 14 pages, 9 figures. Figure 1 included in jpeg format
(zdf1.jpg); the eps was huge. Accepted to Phys. Rev.
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