26,664 research outputs found
Complex Dynamics of Correlated Electrons in Molecular Double Ionization by an Ultrashort Intense Laser Pulse
With a semiclassical quasi-static model we achieve an insight into the
complex dynamics of two correlated electrons under the combined influence of a
two-center Coulomb potential and an intense laser field. The model calculation
is able to reproduce experimental data of nitrogen molecules for a wide range
of laser intensities from tunnelling to over-the-barrier regime, and predicts a
significant alignment effect on the ratio of double over single ion yield. The
classical trajectory analysis allows to unveil sub-cycle molecular double
ionization dynamics.Comment: 5 pages, 5 figures. to appear in Phys. Rev. Lett.(2007
A model of rotating hotspots for 3:2 frequency ratio of HFQPOs in black hole X-ray binaries
We propose a model to explain a puzzling 3:2 frequency ratio of high
frequency quasi-periodic oscillations (HFQPOs) in black hole (BH) X-ray
binaries, GRO J1655-40, GRS 1915+105 and XTE J1550-564. In our model a
non-axisymmetric magnetic coupling (MC) of a rotating black hole (BH) with its
surrounding accretion disc coexists with the Blandford-Znajek (BZ) process. The
upper frequency is fitted by a rotating hotspot near the inner edge of the
disc, which is produced by the energy transferred from the BH to the disc, and
the lower frequency is fitted by another rotating hotspot somewhere away from
the inner edge of the disc, which arises from the screw instability of the
magnetic field on the disc. It turns out that the 3:2 frequency ratio of HFQPOs
in these X-ray binaries could be well fitted to the observational data with a
much narrower range of the BH spin. In addition, the spectral properties of
HFQPOs are discussed. The correlation of HFQPOs with jets from microquasars is
contained naturally in our model.Comment: 8 pages, 4 figures. accepted by MNRA
Precision spectroscopy and density-dependent frequency shifts in ultracold Sr
By varying the density of an ultracold Sr sample from cm
to cm, we make the first definitive measurement of the
density-related frequency shift and linewidth broadening of the -
optical clock transition in an alkaline earth system. In addition, we
report the most accurate measurement to date of the Sr
optical clock transition frequency. Including a detailed analysis of systematic
errors, the frequency is () Hz.Comment: 4 pages, 4 figures, 1 table. submitte
Momentum Distribution of Near-Zero-Energy Photoelectrons in the Strong-Field Tunneling Ionization in the Long Wavelength Limit
We investigate the ionization dynamics of Argon atoms irradiated by an
ultrashort intense laser of a wavelength up to 3100 nm, addressing the momentum
distribution of the photoelectrons with near-zero-energy. We find a surprising
accumulation in the momentum distribution corresponding to meV energy and a
\textquotedblleft V"-like structure at the slightly larger transverse momenta.
Semiclassical simulations indicate the crucial role of the Coulomb attraction
between the escaping electron and the remaining ion at extremely large
distance. Tracing back classical trajectories, we find the tunneling electrons
born in a certain window of the field phase and transverse velocity are
responsible for the striking accumulation. Our theoretical results are
consistent with recent meV-resolved high-precision measurements.Comment: 5 pages, 4 figure
Classical Trajectory Diagnosis of Finger-Like Pattern in the Correlated Electron Momentum Distribution for Helium Double Ionization
With a semiclassical quasistatic model we identify the distinct roles of
nuclear Coulomb attraction, final state electron repulsion and electron-field
interaction in forming the finger-like (or V-shaped) pattern in the correlated
electron momentum distribution for Helium double ionization [Phys. Rev. Lett.
\textbf{99}, 263002; \emph{ibid}, 263003 (2007)]. The underlying microscopic
trajectory configurations responsible for asymmetric electron energy sharing
after electron-electron collision have been uncovered and corresponding
sub-cycle dynamics are analyzed. The correlation pattern is found to be
sensitive to the transverse momentum of correlated electrons.Comment: 5pages 3figure
A Memory Bandwidth-Efficient Hybrid Radix Sort on GPUs
Sorting is at the core of many database operations, such as index creation,
sort-merge joins, and user-requested output sorting. As GPUs are emerging as a
promising platform to accelerate various operations, sorting on GPUs becomes a
viable endeavour. Over the past few years, several improvements have been
proposed for sorting on GPUs, leading to the first radix sort implementations
that achieve a sorting rate of over one billion 32-bit keys per second. Yet,
state-of-the-art approaches are heavily memory bandwidth-bound, as they require
substantially more memory transfers than their CPU-based counterparts.
Our work proposes a novel approach that almost halves the amount of memory
transfers and, therefore, considerably lifts the memory bandwidth limitation.
Being able to sort two gigabytes of eight-byte records in as little as 50
milliseconds, our approach achieves a 2.32-fold improvement over the
state-of-the-art GPU-based radix sort for uniform distributions, sustaining a
minimum speed-up of no less than a factor of 1.66 for skewed distributions.
To address inputs that either do not reside on the GPU or exceed the
available device memory, we build on our efficient GPU sorting approach with a
pipelined heterogeneous sorting algorithm that mitigates the overhead
associated with PCIe data transfers. Comparing the end-to-end sorting
performance to the state-of-the-art CPU-based radix sort running 16 threads,
our heterogeneous approach achieves a 2.06-fold and a 1.53-fold improvement for
sorting 64 GB key-value pairs with a skewed and a uniform distribution,
respectively.Comment: 16 pages, accepted at SIGMOD 201
Enhancement of Friction between Carbon Nanotubes: An Efficient Strategy to Strengthen Fibers
Interfacial friction plays a crucial role in the mechanical properties of
carbon nanotube based fibers, composites, and devices. Here we use molecular
dynamics simulation to investigate the pressure effect on the friction within
carbon nanotube bundles. It reveals that the intertube frictional force can be
increased by a factor of 1.5 ~ 4, depending on tube chirality and radius, when
all tubes collapse above a critical pressure and when the bundle remains
collapsed with unloading down to atmospheric pressure. Furthermore, the overall
cross-sectional area also decreases significantly for the collapsed structure,
making the bundle stronger. Our study suggests a new and efficient way to
reinforce nanotube fibers, possibly stronger than carbon fibers, for usage at
ambient conditions.Comment: revtex, 5 pages, accepted by ACS Nano 10 Dec 200
Adsorption of oxygen-containing functional groups on free and supported graphene using point contact
First-principles electronic structure calculations based on spin-polarized density functional theory were carried out to study the adsorption of oxygen-containing functional groups -OH, -CHO, and -COOH on a two-dimensional (2D) infinite graphene sheet without edge states and defects. We find that the energy gain of adsorption can be significantly improved when the graphene sheet is supported via a point contact, a prototype for graphene sheet supported by catalysts, nanoparticles or nanopillars, or a surface with steps, edges, adatoms, or defects. This was modeled by placing a single atom of Fe, Co, and Ni under the graphene surface. Thus supported graphene not only becomes magnetic, but the carbon atoms in contact with the metal atoms become chemically more active as well. The use of point contact support to improve adsorption has advantages over that of introducing defects: (1) It does not destroy the intrinsic 2D geometry of the graphene sheet. (2) Patterned structures can be created by tailoring the position of the metal atoms supporting the graphene sheet. (3) The geometry distortion created by point contact can be made more uniform and lead to better adsorption energies. (4) Reduction of the work function of supported graphene makes the manipulation of electrons more flexible and controllable in tuning their electronic structure and magnetic properties
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