34,183 research outputs found
The anatomy of the Gunn laser
A monopolar GaAs Fabry–Pérot cavity laser based on the Gunn effect is studied both experimentally and theoretically. The light emission occurs via the band-to-band recombination of impact-ionized excess carriers in the propagating space-charge (Gunn) domains. Electroluminescence spectrum from the cleaved end-facet emission of devices with Ga1−xAlxAs (x = 0.32) waveguides shows clearly a preferential mode at a wavelength around 840 nm at T = 95 K. The threshold laser gain is assessed by using an impact ionization coefficient resulting from excess carriers inside the high-field domain
Non-equilibrium spatial distribution of Rashba spin torque in ferromagnetic metal layer
We study the spatial distribution of spin torque induced by a strong Rashba
spin-orbit coupling (RSOC) in a ferromagnetic (FM) metal layer, using the
Keldysh non-equilibrium Green's function method. In the presence of the s-d
interaction between the non-equilibrium conduction electrons and the local
magnetic moments, the RSOC effect induces a torque on the moments, which we
term as the Rashba spin torque.
A correlation between the Rashba spin torque and the spatial spin current is
presented in this work, clearly mapping the spatial distribution of Rashba Spin
torque in a nano-sized ferromagnetic device. When local magnetism is turned on,
the out-of-plane (Sz) Spin Hall effect (SHE) is disrupted, but rather
unexpectedly an in-plane (Sy) SHE is detected. We also study the effect of
Rashba strength (\alpha_R) and splitting exchange (\Delta) on the
non-equilibrium Rashba spin torque averaged over the device. Rashba spin torque
allows an efficient transfer of spin momentum such that a typical switching
field of 20 mT can be attained with a low current density of less than 10^6
A/cm^2
Vocal learning promotes patterned inhibitory connectivity.
Skill learning is instantiated by changes to functional connectivity within premotor circuits, but whether the specificity of learning depends on structured changes to inhibitory circuitry remains unclear. We used slice electrophysiology to measure connectivity changes associated with song learning in the avian analog of primary motor cortex (robust nucleus of the arcopallium, RA) in Bengalese Finches. Before song learning, fast-spiking interneurons (FSIs) densely innervated glutamatergic projection neurons (PNs) with apparently random connectivity. After learning, there was a profound reduction in the overall strength and number of inhibitory connections, but this was accompanied by a more than two-fold enrichment in reciprocal FSI-PN connections. Moreover, in singing birds, we found that pharmacological manipulations of RA's inhibitory circuitry drove large shifts in learned vocal features, such as pitch and amplitude, without grossly disrupting the song. Our results indicate that skill learning establishes nonrandom inhibitory connectivity, and implicates this patterning in encoding specific features of learned movements
Molecular Gas Content of HI Monsters and Implications to Cold Gas Content Evolution in Galaxies
We present 12CO (J=1-0) observations of a sample of local galaxies
(0.04<z<0.08) with a large neutral hydrogen reservoir, or "HI monsters". The
data were obtained using the Redshift Search Receiver on the FCRAO 14 m
telescope. The sample consists of 20 HI-massive galaxies with M(HI)>3e10Msun
from the ALFALFA survey and 8 LSBs with a comparable M(HI) (>1.5e10Msun). Our
sample selection is purely based on the amount of neutral hydrogen, thereby
providing a chance to study how atomic and molecular gas relate to each other
in these HI-massive systems. We have detected CO in 15 out of 20 ALFALFA
selected galaxies and 4 out of 8 LSBs with molecular gas mass M(H2) of
(1-11)e9Msun. Their total cold gas masses of (2-7e10Msun make them some of the
most gas-massive galaxies identified to date in the Local Universe. Observed
trends associated with HI, H2, and stellar properties of the HI massive
galaxies and the field comparison sample are analyzed in the context of
theoretical models of galaxy cold gas content and evolution, and the importance
of total gas content and improved recipes for handling spatially differentiated
behaviors of disk and halo gas are identified as potential areas of improvement
for the modeling.Comment: 18 pages, 11 figures, 2 tables; Accepted for publication in MNRA
Discovering the Higgs Bosons of Minimal Supersymmetry with Muons and a Bottom Quark
We investigate the prospects for the discovery at the CERN Large Hadron
Collider of a neutral Higgs boson produced with one bottom quark followed by
Higgs decay into a muon pair. We work within the framework of the minimal
supersymmetric model. The dominant physics background from the production of , , j=g,u,d,s,c, and is calculated
with realistic acceptance cuts. Promising results are found for the CP-odd
pseudoscalar () and the heavier CP-even scalar () Higgs bosons with
masses up to 600 GeV. This discovery channel with one energetic bottom quark
greatly improves the discovery potential of the LHC beyond the inclusive
channel .Comment: Version to appear in Phys. Rev. Let
Labeling Schemes for Bounded Degree Graphs
We investigate adjacency labeling schemes for graphs of bounded degree
. In particular, we present an optimal (up to an additive
constant) adjacency labeling scheme for bounded degree trees.
The latter scheme is derived from a labeling scheme for bounded degree
outerplanar graphs. Our results complement a similar bound recently obtained
for bounded depth trees [Fraigniaud and Korman, SODA 10], and may provide new
insights for closing the long standing gap for adjacency in trees [Alstrup and
Rauhe, FOCS 02]. We also provide improved labeling schemes for bounded degree
planar graphs. Finally, we use combinatorial number systems and present an
improved adjacency labeling schemes for graphs of bounded degree with
Adjacency labeling schemes and induced-universal graphs
We describe a way of assigning labels to the vertices of any undirected graph
on up to vertices, each composed of bits, such that given the
labels of two vertices, and no other information regarding the graph, it is
possible to decide whether or not the vertices are adjacent in the graph. This
is optimal, up to an additive constant, and constitutes the first improvement
in almost 50 years of an bound of Moon. As a consequence, we
obtain an induced-universal graph for -vertex graphs containing only
vertices, which is optimal up to a multiplicative constant,
solving an open problem of Vizing from 1968. We obtain similar tight results
for directed graphs, tournaments and bipartite graphs
Spectral Analysis of Protein-Protein Interactions in Drosophila melanogaster
Within a case study on the protein-protein interaction network (PIN) of
Drosophila melanogaster we investigate the relation between the network's
spectral properties and its structural features such as the prevalence of
specific subgraphs or duplicate nodes as a result of its evolutionary history.
The discrete part of the spectral density shows fingerprints of the PIN's
topological features including a preference for loop structures. Duplicate
nodes are another prominent feature of PINs and we discuss their representation
in the PIN's spectrum as well as their biological implications.Comment: 9 pages RevTeX including 8 figure
Disentanglement of two harmonic oscillators in relativistic motion
We study the dynamics of quantum entanglement between two Unruh-DeWitt
detectors, one stationary (Alice), and another uniformly accelerating (Rob),
with no direct interaction but coupled to a common quantum field in (3+1)D
Minkowski space. We find that for all cases studied the initial entanglement
between the detectors disappears in a finite time ("sudden death"). After the
moment of total disentanglement the correlations between the two detectors
remain nonzero until late times. The relation between the disentanglement time
and Rob's proper acceleration is observer dependent. The larger the
acceleration is, the longer the disentanglement time in Alice's coordinate, but
the shorter in Rob's coordinate.Comment: 16 pages, 8 figures; typos added, minor changes in Secs. I and
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