3,025 research outputs found
Dependence of the flux creep activation energy on current density and magnetic field for MgB2 superconductor
Systematic ac susceptibility measurements have been performed on a MgB
bulk sample. We demonstrate that the flux creep activation energy is a
nonlinear function of the current density , indicating a
nonlogarithmic relaxation of the current density in this material. The
dependence of the activation energy on the magnetic field is determined to be a
power law , showing a steep decline in the activation
energy with the magnetic field, which accounts for the steep drop in the
critical current density with magnetic field that is observed in MgB. The
irreversibility field is also found to be rather low, therefore, the pinning
properties of this new material will need to be enhanced for practical
applications.Comment: 11 pages, 6 figures, Revtex forma
Towards hardware acceleration of neuroevolution for multimedia processing applications on mobile devices
This paper addresses the problem of accelerating large artificial neural networks (ANN), whose topology and weights can evolve via the use of a genetic algorithm. The proposed digital hardware architecture is capable of processing any evolved network topology, whilst at the same time providing a good trade off between throughput, area and power consumption. The latter is vital for a longer battery life on mobile devices. The architecture uses multiple parallel arithmetic units in each processing element (PE). Memory partitioning and data caching are used to minimise the effects of PE pipeline stalling. A first order minimax polynomial approximation scheme, tuned via a genetic algorithm, is used for the activation function generator. Efficient arithmetic circuitry, which leverages modified Booth recoding, column compressors and carry save adders, is adopted throughout the design
Luminosity Functions of Lyman-Break Galaxies at z~4 and 5 in the Subaru Deep Field
We investigate the luminosity functions of Lyman-break galaxies (LBG) at z~4
and 5 based on the optical imaging data obtained in the Subaru Deep Field
Project. Three samples of LBGs in a contiguous 875 arcmin^2 area are
constructed. One consists of 3,808 LBGs at z~4 down to i'=26.85 selected with
the B-R vs R-i' diagram. The other two consist of 539 and 240 LBGs at z~5 down
to z'=26.05 selected with two kinds of two-color diagrams: V-i' vs i'-z' and
R-i' vs i'-z'. The adopted selection criteria are proved to be fairly reliable
by spectroscopic observation. We derive the luminosity functions of the LBGs at
rest-frame ultraviolet wavelengths down to M_{UV}=-19.2 at z~4 and M_{UV}=-20.3
at z~5. We find clear evolution of the luminosity function over the redshift
range of 0<z<6, which is accounted for by a sole change in the characteristic
magnitude, M^*. The cosmic star formation rate (SFR) density at z~4 and z~5 is
measured from the luminosity functions. We examine the evolution of the cosmic
SFR density and its luminosity dependence over 0<z<6. The SFR density
contributed from brighter galaxies is found to change more drastically with
cosmic time. The contribution from brighter galaxies has a sharp peak around
z=3-4, while that from fainter galaxies evolves relatively mildly with a broad
peak at earlier epoch. Combining the observed SFR density with the standard
Cold Dark Matter model, we compute the cosmic SFR per unit baryon mass in dark
haloes, i.e., the specific SFR. The specific SFR is found to scale with
redshift as (1+z)^3 up to z~4, implying that the efficiency of star formation
is on average higher at higher redshift in proportion to the cooling rate
within dark haloes, while this is not simply the case at z>4.Comment: 28 pages, 25 figures, accepted for publication in ApJ, a high
resolution version of Figs.7,8,9 is available at
http://hikari.astron.s.u-tokyo.ac.jp/~yoshida/sdflbglf
Probing the inter-layer exciton physics in a MoS/MoSe/MoS van der Waals heterostructure
Stacking atomic monolayers of semiconducting transition metal dichalcogenides
(TMDs) has emerged as an effective way to engineer their properties. In
principle, the staggered band alignment of TMD heterostructures should result
in the formation of inter-layer excitons with long lifetimes and robust valley
polarization. However, these features have been observed simultaneously only in
MoSe/WSe heterostructures. Here we report on the observation of long
lived inter-layer exciton emission in a MoS/MoSe/MoS trilayer van
der Waals heterostructure. The inter-layer nature of the observed transition is
confirmed by photoluminescence spectroscopy, as well as by analyzing the
temporal, excitation power and temperature dependence of the inter-layer
emission peak. The observed complex photoluminescence dynamics suggests the
presence of quasi-degenerate momentum-direct and momentum-indirect bandgaps. We
show that circularly polarized optical pumping results in long lived valley
polarization of inter-layer exciton. Intriguingly, the inter-layer exciton
photoluminescence has helicity opposite to the excitation. Our results show
that through a careful choice of the TMDs forming the van der Waals
heterostructure it is possible to control the circular polarization of the
inter-layer exciton emission.Comment: 19 pages, 3 figures. Just accepted for publication in Nano Letters
(http://pubs.acs.org/doi/10.1021/acs.nanolett.7b03184
Hot and repulsive traffic flow
We study a message passing model, applicable also to traffic problems. The
model is implemented in a discrete lattice, where particles move towards their
destination, with fluctuations around the minimal distance path. A repulsive
interaction between particles is introduced in order to avoid the appearance of
traffic jam. We have studied the parameter space finding regions of fluid
traffic, and saturated ones, being separated by abrupt changes. The improvement
of the system performance is also explored, by the introduction of a
non-constant potential acting on the particles. Finally, we deal with the
behavior of the system when temporary failures in the transmission occurs.Comment: 22 pages, uuencoded gzipped postscript file. 11 figures include
Critical currents, flux-creep activation energy and potential barriers for the vortex motion from the flux creep experiments
We present an experimental study of thermally activated flux creep in a
superconducting ring-shaped epitaxial YBCO film as well as a new way of
analyzing the experimental data. The measurements were made in a wide range of
temperatures between 10 and 83 K. The upper temperature limit was dictated by
our experimental technique and at low temperatures we were limited by a
crossover to quantum tunneling of vortices. It is shown that the experimental
data can very well be described by assuming a simple thermally activated
hopping of vortices or vortex bundles over potential barriers, whereby the
hopping flux objects remain the same for all currents and temperatures. The new
procedure of data analysis also allows to establish the current and temperature
dependencies of the flux-creep activation energy U, as well as the temperature
dependence of the critical current Ic, from the flux-creep rates measured at
different temperatures. The variation of the activation energy with current,
U(I/Ic), is then used to reconstruct the profile of the potential barriers in
real space.Comment: 12 pages, 13 Postscript figures, Submitted to Physical Review
Energy Conditions in Modified Gravity with Non-minimal Coupling to Matter
In this paper we study a model of modified gravity with non-minimal coupling
between a general function of the Gauss-Bonnet invariant, , and matter
Lagrangian from the point of view of the energy conditions. Such model has been
introduced in Ref. [21] for description of early inflation and late-time cosmic
acceleration. We present the suitable energy conditions for the above mentioned
model and then, we use the estimated values of the Hubble, deceleration and
jerk parameters to apply the obtained energy conditions to the specific class
of modified Gauss-Bonnet models.Comment: 12 pages, no figur, Accepted for publication in Astrophysics and
Space Scienc
Vibrational Spectra of a Mechanosensitive Channel
We report the simulated vibrational spectra of a mechanosensitive membrane channel in different gating states. Our results show that while linear absorption is insensitive to structural differences, linear dichroism and sum-frequency generation spectroscopies are sensitive to the orientation of the transmembrane helices, which is changing during the opening process. Linear dichroism cannot distinguish an intermediate structure from the closed structure, but sum-frequency generation can. In addition, we find that two-dimensional infrared spectroscopy can be used to distinguish all three investigated gating states of the mechanosensitive membrane channel.
Infrared Hall effect in high Tc superconductors: Evidence for non-Fermi liquid Hall scattering
Infrared (20-120 cm-1 and 900-1100 cm-1) Faraday rotation and circular
dichroism are measured in high Tc superconductors using sensitive polarization
modulation techniques. Optimally doped YBCO thin films are studied at
temperatures down to 15 K and magnetic fields up to 8 T. At 1000 cm-1 the Hall
conductivity varies strongly with temperature in contrast to the longitudinal
conductivity which is nearly independent of temperature. The Hall scattering
rate has a T^2 temperature dependence but, unlike a Fermi liquid, depends only
weakly on frequency. The experiment puts severe constraints on theories of
transport in the normal state of high Tc superconductors.Comment: 8 pages, 3 figure
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