59,832 research outputs found
Quantum Transport Calculations Using Periodic Boundary Conditions
An efficient new method is presented to calculate the quantum transports
using periodic boundary conditions. This method allows the use of conventional
ground state ab initio programs without big changes. The computational effort
is only a few times of a normal ground state calculation, thus it makes
accurate quantum transport calculations for large systems possible.Comment: 9 pages, 6 figure
Calibration and Irradiation Study of the BGO Background Monitor for the BEAST II Experiment
Beam commissioning of the SuperKEKB collider began in 2016. The Beam Exorcism
for A STable experiment II (BEAST II) project is particularly designed to
measure the beam backgrounds around the interaction point of the SuperKEKB
collider for the Belle II experiment. We develop a system using bismuth
germanium oxide (BGO) crystals with optical fibers connecting to a multianode
photomultiplier tube (MAPMT) and a field-programmable gate array (FPGA)
embedded readout board for monitoring the real-time beam backgrounds in BEAST
II. The overall radiation sensitivity of this system is estimated to be
Gy/ADU (analog-to-digital unit) with the standard
10 m fibers for transmission and the MAPMT operating at 700 V. Our -ray
irradiation study of the BGO system shows that the exposure of BGO crystals to
Co -ray doses of 1 krad has led to immediate light output
reductions of 25--40%, and the light outputs further drop by 30--45% after the
crystals receive doses of 2--4 krad. Our findings agree with those of the
previous studies on the radiation hard (RH) BGO crystals grown by the low
thermal gradient Czochralski (LTG Cz) technology. The absolute dose from the
BGO system is also consistent with the simulation, and is estimated to be about
1.18 times the equivalent dose. These results prove that the BGO system is able
to monitor the background dose rate in real time under extreme high radiation
conditions. This study concludes that the BGO system is reliable for the beam
background study in BEAST II
Entrainment and chaos in a pulse-driven Hodgkin-Huxley oscillator
The Hodgkin-Huxley model describes action potential generation in certain
types of neurons and is a standard model for conductance-based, excitable
cells. Following the early work of Winfree and Best, this paper explores the
response of a spontaneously spiking Hodgkin-Huxley neuron model to a periodic
pulsatile drive. The response as a function of drive period and amplitude is
systematically characterized. A wide range of qualitatively distinct responses
are found, including entrainment to the input pulse train and persistent chaos.
These observations are consistent with a theory of kicked oscillators developed
by Qiudong Wang and Lai-Sang Young. In addition to general features predicted
by Wang-Young theory, it is found that most combinations of drive period and
amplitude lead to entrainment instead of chaos. This preference for entrainment
over chaos is explained by the structure of the Hodgkin-Huxley phase resetting
curve.Comment: Minor revisions; modified Fig. 3; added reference
Study the Heavy Molecular States in Quark Model with Meson Exchange Interaction
Some charmonium-like resonances such as X(3872) can be interpreted as
possible molecular states. Within the quark model, we study
the structure of such molecular states and the similar
molecular states by taking into account of the light meson exchange (,
, , and ) between two light quarks from different
mesons
Development and degeneration of cone bipolar cells are independent of cone photoreceptors in a mouse model of retinitis pigmentosa
Retinal photoreceptors die during retinal synaptogenesis in a portion of retinal degeneration. Whether cone bipolar cells establish regular retinal mosaics and mature morphologies, and resist degeneration are not completely understood. To explore these issues, we backcrossed a transgenic mouse expressing enhanced green fluorescent protein (EGFP) in one subset of cone bipolar cells (type 7) into rd1 mice, a classic mouse model of retinal degeneration, to examine the development and survival of cone bipolar cells in a background of retinal degeneration. Our data revealed that both the development and degeneration of cone bipolar cells are independent of the normal activity of cone photoreceptors. We found that type 7 cone bipolar cells achieved a uniform tiling of the retinal surface and developed normal dendritic and axonal arbors without the influence of cone photoreceptor innervation. On the other hand, degeneration of type 7 cone bipolar cells, contrary to our belief of central-to-peripheral progression, was spatially uniform across the retina independent of the spatiotemporal pattern of cone degeneration. The results have important implications for the design of more effective therapies to restore vision in retinal degeneration.published_or_final_versio
Market valuation of decreases in R&D expenditures
While many studies report that R&D investments significantly contribute to firm value, little existing research investigates the effect of the reduction in R&D expenditures on firm value. This paper examines the long-term performance following significant R&D decreases. We find that, contrary to conventional wisdom, R&D decreases enhance rather than destroy shareholder value. We explore three potential economic motives behind R&D decreases -- R&D spillover, managerial myopia, and overinvestment. We find no compelling evidence to support either the spillover or myopia explanation. However, our results suggest that operating performance deteriorates immediately preceding R&D decreases and firms with low or decreasing investment opportunities outperform; these findings strongly support the overinvestment hypothesis. We also show that the cost of capital declines after R&D decreases. However, the market seems to underestimate the improvement in cost of capital following R&D reductions.postprintThe 2010 China International Conference in Finance (CICF 2010), Beijing, China, 4-7 July 2010
Imaginary phonon modes and phonon-mediated superconductivity in Y2C3
For YC with a superconducting critical temperature (T) 18
K, zone-center imaginary optical phonon modes have been found for the
high-symmetry - structure due to C dimer wobbling motion and electronic
instability from a flat band near Fermi energy. After lattice distortion to the
more stable lowest symmetry structure, these stabilized low-energy phonon
modes with mixed C and Y characters carry a strong electron-phonon coupling to
give arise to the observed sizable T. Our work shows that compounds with
the calculated dynamical instability should not be simply excluded in
high-throughput search for new phonon-mediated superconductors.Comment: 6 pages, 4 figures and 1 tabl
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