25,577 research outputs found
Large-signal dynamic behavior of distributed-feedback lasers including lateral effects
The large-signal behavior of DFB lasers is analyzed, including lateral as well as longitudinal variations in carrier density, photon density, and refractive index. The effective index method and other approximations are used to reduce the complex three-dimensional problem to one dimension. The coupled wave and carrier rate equations are then solved in a self-consistent manner. Lateral spatial carrier hole burning and lateral diffusion are found to affect the relaxation oscillation frequency and damping rate of DFB lasers, depending on their detailed structure. The effective time-averaged linewidth enhancement factor is also affected. In symmetric AR-coated λ/4 phase-shifted lasers the side mode suppression ratio can be deteriorated significantly by lateral spatial hole burning when kL is large.published_or_final_versio
Inferring Population Preferences via Mixtures of Spatial Voting Models
Understanding political phenomena requires measuring the political
preferences of society. We introduce a model based on mixtures of spatial
voting models that infers the underlying distribution of political preferences
of voters with only voting records of the population and political positions of
candidates in an election. Beyond offering a cost-effective alternative to
surveys, this method projects the political preferences of voters and
candidates into a shared latent preference space. This projection allows us to
directly compare the preferences of the two groups, which is desirable for
political science but difficult with traditional survey methods. After
validating the aggregated-level inferences of this model against results of
related work and on simple prediction tasks, we apply the model to better
understand the phenomenon of political polarization in the Texas, New York, and
Ohio electorates. Taken at face value, inferences drawn from our model indicate
that the electorates in these states may be less bimodal than the distribution
of candidates, but that the electorates are comparatively more extreme in their
variance. We conclude with a discussion of limitations of our method and
potential future directions for research.Comment: To be published in the 8th International Conference on Social
Informatics (SocInfo) 201
A standing-wave thermoacoustic engine driven by liquid nitrogen
Thermoacoustic oscillation at cryogenic temperatures, such as Taconis oscillation, has been typically suppressed in the former studies, and few efforts have been made to enhance it. We proposed a standing-wave thermoacoustic engine (TE) driven by liquid cryogens instead of the conventional heat to enhance the thermoacoustic effects and utilize the cold energy. Experimental and theoretical work has been performed on a self-made standingwave TE to demonstrate the feasibility and the operating characteristics of the engine driven by the liquid nitrogen. Experiments show that with nitrogen at 0.5 MPa as a working gas, a pressure ratio of 1.21 is obtained on the TE driven by liquid nitrogen with a much lower temperature difference along the stack compared to that of the conventional TE. The onset temperature difference decreases by 28.9% with helium at 0.63 MPa as a working gas, compared to that of the conventional TE. This study verifies the feasibility of enhancing the thermoacoustic oscillation at cryogenic temperatures. The TEs driven by liquid cryogens such as liquid nitrogen and liquefied nature gas (LNG), may be an alternative for recovering the cold energy
The particle carriers of field-aligned currents in the Earth's magnetotail during a substorm
Although the particle carriers of field-aligned currents (FACs) in the Earth's magnetotail play an important role in the transfer of momentum and energy between the solar wind, magnetosphere, and ionosphere, the characteristics of the FAC carriers have been poorly understood. Taking advantage of multiinstrument magnetic field and plasma data collected by the four spacecraft of the Cluster constellation as they traversed the northern plasma sheet boundary layer in the magnetotail on 14 September 2004, we identified the species type and energy range of the FAC carriers for the first time. The results indicate that part of tailward FACs is carried by energetic keV ions, which are probably originated from the ionosphere through outflow, and they are not too small (~2 nA/m2) to be ignored. The earthward (tailward) FACs are mainly carried by the dominant tailward (earthward) motion of electrons, and higher-energy electrons (from ~0.5 to 26 keV) are the main carriers
Fourier Magnetic Imaging with Nanoscale Resolution and Compressed Sensing Speed-up using Electronic Spins in Diamond
Optically-detected magnetic resonance using Nitrogen Vacancy (NV) color
centres in diamond is a leading modality for nanoscale magnetic field imaging,
as it provides single electron spin sensitivity, three-dimensional resolution
better than 1 nm, and applicability to a wide range of physical and biological
samples under ambient conditions. To date, however, NV-diamond magnetic imaging
has been performed using real space techniques, which are either limited by
optical diffraction to 250 nm resolution or require slow, point-by-point
scanning for nanoscale resolution, e.g., using an atomic force microscope,
magnetic tip, or super-resolution optical imaging. Here we introduce an
alternative technique of Fourier magnetic imaging using NV-diamond. In analogy
with conventional magnetic resonance imaging (MRI), we employ pulsed magnetic
field gradients to phase-encode spatial information on NV electronic spins in
wavenumber or k-space followed by a fast Fourier transform to yield real-space
images with nanoscale resolution, wide field-of-view (FOV), and compressed
sensing speed-up.Comment: 31 pages, 10 figure
Novel SNPs polymorphism of bovine CACNA2D1 gene and their association with somatic cell score
Mastitis is a major cause of economic loss in dairy cattle. In this study, the bovine CACNA2D1 gene was taken as a candidate gene for mastitis resistance. The objective of this study was to identify single nucleotide polymorphisms (SNPs) in the bovine CACNA2D1 gene and evaluate the association of these SNPs with mastitis in cattle. Through DNA sequencing and PCR-RFLP analysis, three mutations C367400T, A496561G and G519663A were detected in the cattle CACNA2D1 gene. Altogether 240 dairy cattle of three breeds (Holstein, Simmental, and Sanhe cattle) were genotyped and allele frequencies were determined. The effects of CACNA2D1 polymorphisms on somatic cell score (SCS) were analyzed and a significant association was found between G519663A and SCS. The mean of genotype GG was significantly lower than those of genotypes AG and AA. The results of this research will be useful in further studies to determine the role of the CACNA2D1 gene in mastitis resistance and further work will be necessary to investigate whether the CACNA2D1 gene play a role in defending the host from mastitis.Key words: Association analysis, CACNA2D1 gene, dairy breeds, mastitis, somatic cell score
Quantum interference and Klein tunneling in graphene heterojunctions
The observation of quantum conductance oscillations in mesoscopic systems has
traditionally required the confinement of the carriers to a phase space of
reduced dimensionality. While electron optics such as lensing and focusing have
been demonstrated experimentally, building a collimated electron interferometer
in two unconfined dimensions has remained a challenge due to the difficulty of
creating electrostatic barriers that are sharp on the order of the electron
wavelength. Here, we report the observation of conductance oscillations in
extremely narrow graphene heterostructures where a resonant cavity is formed
between two electrostatically created bipolar junctions. Analysis of the
oscillations confirms that p-n junctions have a collimating effect on
ballistically transmitted carriers. The phase shift observed in the conductance
fringes at low magnetic fields is a signature of the perfect transmission of
carriers normally incident on the junctions and thus constitutes a direct
experimental observation of ``Klein Tunneling.''Comment: 13 pages and 6 figures including supplementary information. The paper
has been modified in light of new theoretical results available at
arXiv:0808.048
Mid-infrared frequency comb spanning an octave based on an Er fiber laser and difference-frequency generation
We describe a coherent mid-infrared continuum source with 700 cm-1 usable
bandwidth, readily tuned within 600 - 2500 cm-1 (4 - 17 \mum) and thus covering
much of the infrared "fingerprint" molecular vibration region. It is based on
nonlinear frequency conversion in GaSe using a compact commercial 100-fs-pulsed
Er fiber laser system providing two amplified near-infrared beams, one of them
broadened by a nonlinear optical fiber. The resulting collimated mid-infrared
continuum beam of 1 mW quasi-cw power represents a coherent infrared frequency
comb with zero carrier-envelope phase, containing about 500,000 modes that are
exact multiples of the pulse repetition rate of 40 MHz. The beam's
diffraction-limited performance enables long-distance spectroscopic probing as
well as maximal focusability for classical and ultraresolving near-field
microscopies. Applications are foreseen also in studies of transient chemical
phenomena even at ultrafast pump-probe scale, and in high-resolution gas
spectroscopy for e.g. breath analysis.Comment: 8 pages, 2 figures revised version, added reference
Microscopic Polarization in Bilayer Graphene
Bilayer graphene has drawn significant attention due to the opening of a band
gap in its low energy electronic spectrum, which offers a promising route to
electronic applications. The gap can be either tunable through an external
electric field or spontaneously formed through an interaction-induced symmetry
breaking. Our scanning tunneling measurements reveal the microscopic nature of
the bilayer gap to be very different from what is observed in previous
macroscopic measurements or expected from current theoretical models. The
potential difference between the layers, which is proportional to charge
imbalance and determines the gap value, shows strong dependence on the disorder
potential, varying spatially in both magnitude and sign on a microscopic level.
Furthermore, the gap does not vanish at small charge densities. Additional
interaction-induced effects are observed in a magnetic field with the opening
of a subgap when the zero orbital Landau level is placed at the Fermi energy
Observation of An Evolving Magnetic Flux Rope Prior To and During A Solar Eruption
Explosive energy release is a common phenomenon occurring in magnetized
plasma systems ranging from laboratories, Earth's magnetosphere, the solar
corona and astrophysical environments. Its physical explanation is usually
attributed to magnetic reconnection in a thin current sheet. Here we report the
important role of magnetic flux rope structure, a volumetric current channel,
in producing explosive events. The flux rope is observed as a hot channel prior
to and during a solar eruption from the Atmospheric Imaging Assembly (AIA)
telescope on board the Solar Dynamic Observatory (SDO). It initially appears as
a twisted and writhed sigmoidal structure with a temperature as high as 10 MK
and then transforms toward a semi-circular shape during a slow rise phase,
which is followed by fast acceleration and onset of a flare. The observations
suggest that the instability of the magnetic flux rope trigger the eruption,
thus making a major addition to the traditional magnetic-reconnection paradigm.Comment: 13 pages, 3 figure
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