2,156 research outputs found
Space-division multiplexing for fiber-wireless communications
We envision the application of optical Space-division Multiplexing (SDM) to
the next generation fiber-wireless communications as a firm candidate to
increase the end user capacity and provide adaptive radiofrequency-photonic
interfaces. This approach relies on the concept of fiber-distributed signal
processing, where the SDM fiber provides not only radio access distribution but
also broadband microwave photonics signal processing. In particular, we present
two different SDM fiber technologies: dispersion-engineered heterogeneous
multicore fiber links and multicavity devices built upon the selective
inscription of gratings in homogeneous multicore fibers.Comment: 4 pages, 20th International Conference on Transparent Optical
Networks (ICTON), Girona (Spain), 2017. arXiv admin note: text overlap with
arXiv:1810.1213
Iron substitution in NdCoAsO: crystal structure and magnetic phase diagram
The effects of replacing small amounts of Co with Fe in NdCoAsO are reported.
Polycrystalline materials with compositions NdCo1-xFexAsO (x = 0.05, 0.10,
0.15, and 0.20) are studied and the results compared to previous reports for
NdCoAsO. Rietveld analysis of powder x-ray diffraction data shows that as Fe
replaces Co on the transition metal (T) site, the T-As distance increases, and
the As tetrahedra surrounding the T-site become more regular. Electrical
resistivity and magnetization measurements indicate that the three magnetic
phase transitions in NdCoAsO are suppressed as Co is replaced by Fe, and these
transitions are not observed above 1.8 K for x = 0.20. Based on these results,
the magnetic phase diagram for the Co-rich side of the NdCoAsO-NdFeAsO system
is constructed.Comment: Accepted for publication in Physical Review B, revised text and
figures, 5 pages, 5 figure
Anomalous Hall Effect in three ferromagnets: EuFe4Sb12, Yb14MnSb11, and Eu8Ga16Ge30
The Hall resistivity (Rho_xy), resistivity (Rho_xx), and magnetization of
three metallic ferromagnets are investigated as a function of magnetic field
and temperature. The three ferromagnets, EuFe4Sb12 (Tc = 84 K), Yb14MnSb11 (Tc
= 53 K), and Eu8Ga16Ge30 (Tc = 36 K) are Zintl compounds with carrier
concentrations between 1 x 10^21 cm^-3 and 3.5 x 10^21 cm^-3. The relative
decrease in Rho_xx below Tc [Rho_xx(Tc)/Rho_xx(2 K)] is 28, 6.5, and 1.3 for
EuFe4Sb12, Yb14MnSb11, and Eu8Ga16Ge30 respectively. The low carrier
concentrations coupled with low magnetic anisotropies allow a relatively clean
separation between the anomalous (Rho_'xy), and normal contributions to the
measured Hall resistivity. For each compound the anomalous contribution in the
zero field limit is fit to alpha Rho_xx + sigma_xy rho_xx^2 for temperatures T
< Tc. The anomalous Hall conductivity, sigma_xy, is -220 +- 5 (Ohm^-1 cm^-1),
-14.7 +- 1 (Ohm^-1 cm^-1), and 28 +- 3 (Ohm^-1 cm^-1) for EuFe4Sb12,
Yb14MnSb11, and Eu8Ga16Ge30 respectively and is independent of temperature for
T < Tc if the change in spontaneous magnetization (order parameter) with
temperature is taken into account. These data are consistent with recent
theories of the anomalous Hall effect that suggest that even for stochiometric
ferromagnetic crystals, such as those studied in this article, the intrinsic
Hall conductivity is finite at T = 0, and is a ground state property that can
be calculated from the electronic structure.Comment: 22 pages, 13 figures Submitted to PR
Superconductivity at 22 K in Co-doped BaFe2As2 Crystals
Here we report bulk superconductivity in BaFe1.8Co0.2As2 single crystals
below Tc = 22 K, as demonstrated by resistivity, magnetic susceptibility, and
specific heat data. Hall data indicate that the dominant carriers are
electrons, as expected from simple chemical reasoning. This is the first
example of superconductivity induced by electron doping in this family of
materials. In contrast to the cuprates, the BaFe2As2 system appears to tolerate
considerable disorder in the FeAs planes. First principles calculations for
BaFe1.8Co0.2As2 indicate the inter-band scattering due to Co is weak.Comment: 9 pages, 3 figure
Bismuth incorporation and the role of ordering in GaAsBi/GaAs structures
The structure and composition of single GaAsBi/GaAs epilayers grown by molecular beam epitaxy were investigated by optical and transmission electron microscopy techniques. Firstly, the GaAsBi layers exhibit two distinct regions and a varying Bi composition profile in the growth direction. In the lower (25 nm) region, the Bi content decays exponentially from an initial maximum value, while the upper region comprises an almost constant Bi content until the end of the layer. Secondly, despite the relatively low Bi content, CuPtB-type ordering was observed both in electron diffraction patterns and in fast Fourier transform reconstructions from high-resolution transmission electron microscopy images. The estimation of the long-range ordering parameter and the development of ordering maps by using geometrical phase algorithms indicate a direct connection between the solubility of Bi and the amount of ordering. The occurrence of both phase separation and atomic ordering has a significant effect on the optical properties of these layers
Counterrotating Stars in Simulated Galaxy Disks
Counterrotating stars in disk galaxies are a puzzling dynamical feature whose
origin has been ascribed to either satellite accretion events or to disk
instabilities triggered by deviations from axisymmetry. We use a cosmological
simulation of the formation of a disk galaxy to show that counterrotating
stellar disk components may arise naturally in hierarchically-clustering
scenarios even in the absence of merging. The simulated disk galaxy consists of
two coplanar, overlapping stellar components with opposite spins: an inner
counterrotating bar-like structure made up mostly of old stars surrounded by an
extended, rotationally-supported disk of younger stars. The opposite-spin
components originate from material accreted from two distinct filamentary
structures which at turn around, when their net spin is acquired, intersect
delineating a "V"-like structure. Each filament torques the other in opposite
directions; the filament that first drains into the galaxy forms the inner
counterrotating bar, while material accreted from the other filament forms the
outer disk. Mergers do not play a substantial role and most stars in the galaxy
are formed in situ; only 9% of all stars are contributed by accretion events.
The formation scenario we describe here implies a significant age difference
between the co- and counterrotating components, which may be used to
discriminate between competing scenarios for the origin of counterrotating
stars in disk galaxies.Comment: 7 pages, 7 figures. Accepted for publication in MNRA
Magnetic Phase Transitions in NdCoAsO
Magnetization measurements reveal that NdCoAsO undergoes three magnetic phase
transitions below room temperature. The crystal and magnetic structures of
NdCoAsO have been determined by powder neutron diffraction, and the effects of
the phase transitions on physical properties are reported. Near 69 K a
ferromagnetic state emerges with a small saturation moment of about 0.2 Bohr
magnetons, likely on Co atoms. At 14 K the material enters an antiferromagnetic
state with propagation vector (0 0 1/2) and small ordered moments (~0.4 Bohr
magnetons) on Co and Nd. Near 3.5 K a third transition is observed, and
corresponds to the antiferromagnetic ordering, with the same propagation
vector, of larger moments on Nd reaching 1.30(2) Bohr magnetons at 1.4 K. In
addition, transport properties and heat capacity results are presented, and
show anomalies at all three phase transitions.Comment: Some minor changes made, and lower temperature neutron diffraction
results are included. Accepted for publication in Physical Review
Curvature, twist and pose measurements using fiber Bragg gratings in multi-core fiber:A comparative study between helical and straight core fibers
This paper presents a technique to acquire measurements of curvature, twist and pose for two multi-core fibers; one with straight cores and the other with helical cores. Both the fibers have multiple fiber Bragg grating (FBG) sensors inscribed in the cores and the fibers are placed in known configurations in order to compare their measurement accuracy. For the curvature measurements both the fibers are placed in constant curvature slots; for the twist measurements, a set of twists are applied to each fiber and for the pose measurements the fibers are placed in molds of different shape. The mean curvature errors are 0.22 and 0.13 m(-1), in the helical and straight core fiber respectively. For the twist measurement the mean errors are 26.57 degrees 1m and 146.50 degrees 1m in the helical and straight core fiber, respectively. Lastly, the pose measurement consists of position and orientation where the orientation is represented in the axis-angle form. The mean position errors are 0.49 and 0.27 mm, the mean axis orientation errors are 0.12 degrees and 0.26 degrees degrees and the mean angle orientation errors are 1.10 degrees and 1.18 degrees, for the helical and straight core fiber, respectively. The results show that the twist measurement error is significantly low with the helical core fiber, thus helical core fiber is better suited than straight core fiber for applications where twist measurements are required. (C) 2020 Elsevier B.V. All rights reserved
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