230 research outputs found
The Influence of Magnetic Domain Walls on Longitudinal and Transverse Magnetoresistance in Tensile Strained (Ga,Mn)As Epilayers
We present a theoretical analysis of recent experimental measurements of
magnetoresistance in (Ga,Mn)As epilayers with perpendicular magnetic
anisotropy. The model reproduces the field-antisymmetric anomalies observed in
the longitudinal magnetoresistance in the planar geometry (magnetic field in
the epilayer plane and parallel to the current density), as well as the unusual
shape of the accompanying transverse magnetoresistance. The magnetoresistance
characteristics are attributed to circulating currents created by the presence
of magnetic domain walls
Exact entropy of dimer coverings for a class of lattices in three or more dimensions
We construct a class of lattices in three and higher dimensions for which the
number of dimer coverings can be determined exactly using elementary arguments.
These lattices are a generalization of the two-dimensional kagome lattice, and
the method also works for graphs without translational symmetry. The partition
function for dimer coverings on these lattices can be determined also for a
class of assignments of different activities to different edges.Comment: 4 pages, 2 figures; added results on partition function when
different edges have different weights; modified abstract; added reference
Dependence of ground state energy of classical n-vector spins on n
We study the ground state energy E_G(n) of N classical n-vector spins with
the hamiltonian H = - \sum_{i>j} J_ij S_i.S_j where S_i and S_j are n-vectors
and the coupling constants J_ij are arbitrary. We prove that E_G(n) is
independent of n for all n > n_{max}(N) = floor((sqrt(8N+1)-1) / 2) . We show
that this bound is the best possible. We also derive an upper bound for E_G(m)
in terms of E_G(n), for m<n. We obtain an upper bound on the frustration in the
system, as measured by F(n), which is defined to be (\sum_{i>j} |J_ij| +
E_G(n)) / (\sum_{i>j} |J_ij|). We describe a procedure for constructing a set
of J_ij's such that an arbitrary given state, {S_i}, is the ground state.Comment: 6 pages, 2 figures, submitted to Physical Review
Experimental exploration of compact convolutional neural network architectures for non-temporal real-time fire detection.
In this work we explore different Convolutional Neural Network (CNN) architectures and their variants for non-temporal binary fire detection and localization in video or still imagery. We consider the performance of experimentally defined, reduced complexity deep CNN architectures for this task and evaluate the effects of different optimization and normalization techniques applied to different CNN architectures (spanning the Inception, ResNet and EfficientNet architectural concepts). Contrary to contemporary trends in the field, our work illustrates a maximum overall accuracy of 0.96 for full frame binary fire detection and 0.94 for superpixel localization using an experimentally defined reduced CNN architecture based on the concept of InceptionV4. We notably achieve a lower false positive rate of 0.06 compared to prior work in the field presenting an efficient, robust and real-time solution for fire region detection
Current-Induced Polarization and the Spin Hall Effect at Room Temperature
Electrically-induced electron spin polarization is imaged in n-type ZnSe
epilayers using Kerr rotation spectroscopy. Despite no evidence for an
electrically-induced internal magnetic field, current-induced in-plane spin
polarization is observed with characteristic spin lifetimes that decrease with
doping density. The spin Hall effect is also observed, indicated by an
electrically-induced out-of-plane spin polarization with opposite sign for
spins accumulating on opposite edges of the sample. The spin Hall conductivity
is estimated as 3 +/- 1.5 Ohms**-1 m**-1/|e| at 20 K, which is consistent with
the extrinsic mechanism. Both the current-induced spin polarization and the
spin Hall effect are observed at temperatures from 10 K to 295 K.Comment: 5 pages, 4 figure
Internal magnetic fields in thin ZnSe epilayers
Strain induced spin-splitting is observed and characterized using pump-probe
Kerr rotation spectroscopy in n-ZnSe epilayers grown on GaAs substrates. The
spin-splitting energies are mapped out as a function of pump-probe separation,
applied voltage, and temperature in a series of samples of varying epilayer
thicknesses and compressive strain arising from epilayer-substrate lattice
mismatch. The strain is independently quantified using photoluminescence and
x-ray diffraction measurements. We observe that the magnitude of the spin
splitting increases with applied voltage and temperature, and is highly crystal
direction dependent, vanishing along [1 1-bar 0].Comment: 9 pages, 3 figure
Magnetoresistance Anomalies in (Ga,Mn)As Epilayers with Perpendicular Magnetic Anisotropy
We report the observation of anomalies in the longitudinal magnetoresistance
of tensile-strained (Ga,Mn)As epilayers with perpendicular magnetic anisotropy.
Magnetoresistance measurements carried out in the planar geometry (magnetic
field parallel to the current density) reveal "spikes" that are antisymmetric
with respect to the direction of the magnetic field. These anomalies always
occur during magnetization reversal, as indicated by a simultaneous change in
sign of the anomalous Hall effect. The data suggest that the antisymmetric
anomalies originate in anomalous Hall effect contributions to the longitudinal
resistance when domain walls are located between the voltage probes. This
interpretation is reinforced by carrying out angular sweeps of ,
revealing an antisymmetric dependence on the helicity of the field sweep.Comment: Submitted to Phys. Rev.
A Tunable Anomalous Hall Effect in a Non-Ferromagnetic System
We measure the low-field Hall resistivity of a magnetically-doped
two-dimensional electron gas as a function of temperature and
electrically-gated carrier density. Comparing these results with the carrier
density extracted from Shubnikov-de Haas oscillations reveals an excess Hall
resistivity that increases with decreasing temperature. This excess Hall
resistivity qualitatively tracks the paramagnetic polarization of the sample,
in analogy to the ferromagnetic anomalous Hall effect. The data are consistent
with skew-scattering of carriers by disorder near the crossover to
localization
- …