971 research outputs found
Trap-Assisted Tunneling in the Schottky Barrier
The paper presents a new way how to calculate the currents in a Schottky barrier. The novel phenomeno-logical model extends the Shockley-Read-Hall recombi-nation-generation theory of trap-assisted tunneling. The proposed approach explains the occurrence of large leakage currents in Schottky structures on wide band semi-conductors with a high Schottky barrier (above 1 eV) and with a high density of traps. Under certain conditions, trap-assisted tunneling (TAT) plays a more important role than direct tunneling
Properties of the inner penumbral boundary and temporal evolution of a decaying sunspot
It was empirically determined that the umbra-penumbra boundaries of stable
sunspots are characterized by a constant value of the vertical magnetic field.
We analyzed the evolution of the photospheric magnetic field properties of a
decaying sunspot belonging to NOAA 11277 between August 28 - September 3, 2011.
The observations were acquired with the spectropolarimeter on-board of the
Hinode satellite. We aim to proof the validity of the constant vertical
magnetic-field boundary between the umbra and penumbra in decaying sunspots. A
spectral-line inversion technique was used to infer the magnetic field vector
from the full-Stokes profiles. In total, eight maps were inverted and the
variation of the magnetic properties in time were quantified using linear or
quadratic fits. We found a linear decay of the umbral vertical magnetic field,
magnetic flux, and area. The penumbra showed a linear increase of the vertical
magnetic field and a sharp decay of the magnetic flux. In addition, the
penumbral area quadratically decayed. The vertical component of the magnetic
field is weaker on the umbra-penumbra boundary of the studied decaying sunspot
compared to stable sunspots. Its value seem to be steadily decreasing during
the decay phase. Moreover, at any time of the shown sunspot decay, the inner
penumbra boundary does not match with a constant value of the vertical magnetic
field, contrary to what was seen in stable sunspots. During the decaying phase
of the studied sunspot, the umbra does not have a sufficiently strong vertical
component of the magnetic field and is thus unstable and prone to be
disintegrated by convection or magnetic diffusion. No constant value of the
vertical magnetic field was found for the inner penumbral boundary.Comment: Accepted for publication in Astronomy & Astrophysics, 6 pages, 7
figure
Sixty GHz IMPATT diode development
The objective of this program is to develop 60 GHz GaAs IMPATT Diodes suitable for communications applications. The performance goal of the 60 GHz IMPATT is 1W CW output power with a conversion efficiency of 15 percent and 10 year life time. During the course of the program, double drift (DD) GaAs IMPATT Diodes have been developed resulting in the state of the art performance at V band frequencies. A CW output power of 1.12 W was demonstrated at 51.9 GHz with 9.7 percent efficiency. The best conversion efficiency achieved was 15.3 percent. V band DD GaAs IMPATTs were developed using both small signal and large signal analyses. GaAs wafers of DD flat, DD hybrid, and DD Read profiles using molecular beam epitaxy (MBE) were developed with excellent doping profile control. Wafer evaluation was routinely made by the capacitance versus voltage (C-V) measurement. Ion mass spectrometry (SIMS) analysis was also used for more detailed profile evaluation
beta-Cu2V2O7: a spin-1/2 honeycomb lattice system
We report on band structure calculations and a microscopic model of the
low-dimensional magnet beta-Cu2V2O7. Magnetic properties of this compound can
be described by a spin-1/2 anisotropic honeycomb lattice model with the
averaged coupling \bar J1=60-66 K. The low symmetry of the crystal structure
leads to two inequivalent couplings J1 and J1', but this weak spatial
anisotropy does not affect the essential physics of the honeycomb spin lattice.
The structural realization of the honeycomb lattice is highly non-trivial: the
leading interactions J1 and J1' run via double bridges of VO4 tetrahedra
between spatially separated Cu atoms, while the interactions between structural
nearest neighbors are negligible. The non-negligible inter-plane coupling
Jperp~15 K gives rise to the long-range magnetic ordering at TN~26 K. Our model
simulations improve the fit of the magnetic susceptibility data, compared to
the previously assumed spin-chain models. Additionally, the simulated ordering
temperature of 27 K is in remarkable agreement with the experiment. Our study
evaluates beta-Cu2V2O7 as the best available experimental realization of the
spin-1/2 Heisenberg model on the honeycomb lattice. We also provide an
instructive comparison of different band structure codes and computational
approaches to the evaluation of exchange couplings in magnetic insulators.Comment: 11 pages, 10 figures, 2 tables: revised version, extended description
of simulation result
An ultrastable silicon cavity in a continuously operating closed-cycle cryostat at 4 K
We report on a laser locked to a silicon cavity operating continuously at 4 K
with instability and a median linewidth of 17 mHz at 1542
nm. This is a ten-fold improvement in short-term instability, and a
improvement in linewidth, over previous sub-10 K systems. Operating at low
temperatures reduces the thermal noise floor, and thus is advantageous toward
reaching an instability of , a long-sought goal of the optical clock
community. The performance of this system demonstrates the technical readiness
for the development of the next generation of ultrastable lasers that operate
with ultranarrow linewidth and long-term stability without user intervention.Comment: 5 pages, 4 figure
Frequency analysis of the first-overtone RR Lyrae stars based on the Extended Aperture Photometry from the K2 data
Additional low-amplitude signals are observed in many RR Lyrae stars, beside
the pulsations in radial modes. The most common ones are short-period signals
forming a period ratio of around 0.60--0.65 with the first overtone, or
long-period signals forming a period ratio of around 0.68. The RR Lyrae stars
may also exhibit quasi-periodic modulation of the light curves, known as the
Blazhko effect. We used the extensive sample of the first-overtone RR Lyrae
stars observed by the Kepler telescope during the K2 mission to search for and
characterize these low-amplitude additional signals. K2 data provides
space-based photometry for a statistically significant sample. Hence this data
is excellent to study in detail pulsation properties of RR Lyrae stars. We used
K2 space-based photometry for RR Lyrae candidates from Campaigns 0-19. We
selected RR Lyrae stars pulsating in the first overtone and performed a
frequency analysis for each star to characterize their frequency contents. We
classified 452 stars as first-overtone RR Lyrae. From that sample, we selected
281 RR stars, 67 RR stars, and 68 Blazhko stars. We found
particularly interesting stars which show all of the above phenomena
simultaneously. We detected signals in RR stars that form period
ratios lower than observed for the majority of stars. These signals likely form
a new sequence in the Petersen diagram, around a period ratio of 0.60. In 32
stars we detected additional signals that form a period ratio close to that
expected in RRd stars, but the classification of these stars as RRd is
uncertain. We also report a discovery of additional signals in eight stars that
form a new group in the Petersen diagram around the period ratio of
0.465-0.490. The nature of this periodicity remains unknown.Comment: 29 pages, 29 figures, 4 tables, accepted for publication in A&A, full
tables are available upon request before publicatio
HyperMagNet: A Magnetic Laplacian based Hypergraph Neural Network
In data science, hypergraphs are natural models for data exhibiting multi-way
relations, whereas graphs only capture pairwise. Nonetheless, many proposed
hypergraph neural networks effectively reduce hypergraphs to undirected graphs
via symmetrized matrix representations, potentially losing important
information. We propose an alternative approach to hypergraph neural networks
in which the hypergraph is represented as a non-reversible Markov chain. We use
this Markov chain to construct a complex Hermitian Laplacian matrix - the
magnetic Laplacian - which serves as the input to our proposed hypergraph
neural network. We study HyperMagNet for the task of node classification, and
demonstrate its effectiveness over graph-reduction based hypergraph neural
networks.Comment: 9 pages, 1 figur
Five colour photometry of the RRd star V372 Ser
The first UBV(RI)_C time series photometry of the RRd star V372 Ser is
presented to determine some parameters of the star. In April, May 2007 2812 U,
B, V, R_C, I_C frames were obtained at Konkoly and Teide Observatories, 1508 V
observations were collected from the literature. Fourier fitted light curves
have been derived in all bands. The non-linearly coupled frequencies
f_0=(2.121840+/-.000001) c/day, f_1=(2.851188+/-.000001) c/d, i.e. periods
P_0=0.4712891+/-.0000002 days, P_1=0.3507310+/-.0000001 d, P_1/P_0=0.7441950,
amplitudes A_0(V)=0.15399 mag, A_1(V)=0.20591 mag, and phases have been found.
A_1/A_0=1.319+/-.008 has been found from averaging the amplitude ratio in the
different bands i.e. the first overtone is the dominant pulsation mode. From
the V observations upper limits are given for secular change of the Fourier
parameters. The period ratio and period put V372 Ser among the RRd stars of the
globular clusters M3 and IC 4499, mass, luminosity, and metallicity estimates
are given.Comment: accepted by Astronomy and Astrophysics (5 pages, 3 figures, 4 tables
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