13,881 research outputs found
Dephasing time in graphene due to interaction with flexural phonons
We investigate decoherence of an electron in graphene caused by
electron-flexural phonon interaction. We find out that flexural phonons can
produce dephasing rate comparable to the electron-electron one. The problem
appears to be quite special because there is a large interval of temperature
where the dephasing induced by phonons can not be obtain using the golden rule.
We evaluate this rate for a wide range of density () and temperature ()
and determine several asymptotic regions with temperature dependence crossing
over from to when
temperature increases. We also find to be a non-monotonous
function of . These distinctive features of the new contribution can provide
an effective way to identify flexural phonons in graphene through the
electronic transport by measuring the weak localization corrections in
magnetoresistance.Comment: 13 pages, 8 figure
Spectral properties, generation order parameters and luminosities for spin-powered X-ray pulsars
We show the spectral properties of 15 spin-powered X-ray pulsars, and the
correlation between the average power-law photon index and spin-down rate.
Generation order parameters (GOPs) based on polar-cap models are introduced to
characterize the X-ray pulsars. We calculate three definitions of generation
order parameters due to the different effects of magnetic and electric fields
on photon absorption during cascade processes, and study the relations between
the GOPs and spectral properties of X-ray pulsars. There exists a possible
correlation between the photon index and GOP in our pulsar sample. Furthermore,
we present a method due to the concept of GOPs to estimate the non-thermal
X-ray luminosity for spin-powered pulsars. Then X-ray luminosity is calculated
in the context of our polar-cap accelerator model which is well consistent with
the most observed X-ray pulsar data. The ratio between X-ray luminosity
estimated by our method and the pulsar's spin-down power is well consistent
with the feature.Comment: 20 pages, 8 figures, 1 table, revised version for the publication in
Ap
Optimisation of size-controllable centroidal voronoi tessellation for FEM simulation of micro forming processes
© 2014 The Authors. Published by Elsevier Ltd. Voronoi tessellation has been employed to characterise material features in Finite Element Method (FEM) simulation, however, a poor mesh quality of the voronoi tessellations causes problems in explicit dynamic simulation of forming processes. Although centroidal voronoi tessellation can partly improve the mesh quality by homogenisation of voronoi tessellations, small features, such as short edges and small facets, lead to an inferior mesh quality. Further, centroidal voronoi tessellation cannot represent all real micro structures of materials because of the almost equal tessellation shape and size. In this paper, a density function is applied to control the size and distribution of voronoi tessellations and then a Laplacian operator is employed to optimise the centroidal voronoi tessellations. After optimisation, the small features can be eliminated and the elements are quadrilateral in 2D and hexahedral in 3D cases. Moreover, the mesh quality is significantly higher than that of the mesh generated on the original voronoi or centroidal voronoi tessellation. This work is beneficial for explicit dynamic simulation of forming processes, such as micro deep drawing processes
Electric field and tip geometry effects on dielectrophoretic growth of carbon nanotube nanofibrils on scanning probes
Single-wall carbon nanotube (SWNT) nanofibrils were assembled onto a variety
of conductive scanning probes including atomic force microscope (AFM) tips and
scanning tunnelling microscope (STM) needles using positive dielectrophoresis
(DEP). The magnitude of the applied electric field was varied in the range of
1-20 V to investigate its effect on the dimensions of the assembled SWNT
nanofibrils. Both length and diameter grew asymptotically as voltage increased
from 5 to 18 V. Below 4 V, stable attachment of SWNT nanofibrils could not be
achieved due to the relatively weak DEP force versus Brownian motion. At
voltages of 20 V and higher, low quality nanofibrils resulted from
incorporating large amounts of impurities. For intermediate voltages, optimal
nanofibrils were achieved, though pivotal to this assembly is the wetting
behaviour upon tip immersion in the SWNT suspension drop. This process was
monitored in situ to correlate wetting angle and probe geometry (cone angles
and tip height), revealing that probes with narrow cone angles and long shanks
are optimal. It is proposed that this results from less wetting of the probe
apex, and therefore reduces capillary forces and especially force transients
during the nanofibril drawing process. Relatively rigid probes (force constant
>= 2 N/m) exhibited no perceivable cantilever bending upon wetting and
de-wetting, resulting in the most stable process control
Anomalies of upper critical field in the spinel superconductor LiTiO
High-field electrical transport and point-contact tunneling spectroscopy were
used to investigate superconducting properties of the unique spinel oxide,
LiTiO films with various oxygen content. We find that the
upper critical field gradually increases as more oxygen
impurities are brought into the samples by carefully tuning the deposition
atmosphere. It is striking that although the superconducting transition
temperature and energy gap are almost unchanged, an astonishing isotropic
up to 26 Tesla is observed in oxygen-rich sample, which
is doubled compared to the anoxic sample and breaks the Pauli limit. Such
anomalies of were rarely reported in other three dimensional
superconductors. Combined with all the anomalies, three dimensional spin-orbit
interaction induced by tiny oxygen impurities is naturally proposed to account
for the remarkable enhancement of in oxygen-rich
LiTiO films. Such mechanism could be general and therefore
provides ideas for optimizing practical superconductors with higher
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Proportional-Integral Observer Design for Uncertain Time-Delay Systems Subject to Deception Attacks: An Outlier-Resistant Approach
National Natural Science Foundation of China (Grant Number: 61873148, 61873169, 61933007 and 62103281); Royal Society of the U.K.; Alexander von Humboldt Foundation of Germany
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