25,794 research outputs found
Longwall shearer tracking system
A tracking system for measuring and recording the movements of a longwall shearer vehicle includes an optical tracking assembly carried at one end of a desired vehicle path and a retroreflector assembly carried by the vehicle. Continuous horizontal and vertical light beams are alternately transmitted by means of a rotating Dove prism to the reflector assembly. A vertically reciprocating reflector interrupts the continuous light beams and converts these to discrete horizontal and vertical light beam images transmitted at spaced intervals along the path. A second rotating Dove prism rotates the vertical images to convert them to a second series of horizontal images while the first mentioned horizontal images are left unrotated and horizontal. The images are recorded on a film
Realistic Magnetohydrodynamical Simulation of Solar Local Supergranulation
Three-dimensional numerical simulations of solar surface magnetoconvection
using realistic model physics are conducted. The thermal structure of
convective motions into the upper radiative layers of the photosphere, the main
scales of convective cells and the penetration depths of convection are
investigated. We take part of the solar photosphere with size of 60x60 Mm in
horizontal direction and by depth 20 Mm from level of the visible solar
surface. We use a realistic initial model of the Sun and apply equation of
state and opacities of stellar matter. The equations of fully compressible
radiation magnetohydrodynamics with dynamical viscosity and gravity are solved.
We apply: 1) conservative TVD difference scheme for the magnetohydrodynamics,
2) the diffusion approximation for the radiative transfer, 3) dynamical
viscosity from subgrid scale modeling. In simulation we take uniform
two-dimesional grid in gorizontal plane and nonuniform grid in vertical
direction with number of cells 600x600x204. We use 512 processors with
distributed memory multiprocessors on supercomputer MVS-100k in the Joint
Computational Centre of the Russian Academy of Sciences.Comment: 6 pages, 5 figures, submitted to the proceedings of the GONG 2008 /
SOHO XXI conferenc
Superconducting atomic contacts under microwave irradiation
We have measured the effect of microwave irradiation on the dc
current-voltage characteristics of superconducting atomic contacts. The
interaction of the external field with the ac supercurrents leads to replicas
of the supercurrent peak, the well known Shapiro resonances. The observation of
supplementary fractional resonances for contacts containing highly transmitting
conduction channels reveals their non-sinusoidal current-phase relation. The
resonances sit on a background current which is itself deeply modified, as a
result of photon assisted multiple Andreev reflections. The results provide
firm support for the full quantum theory of transport between two
superconductors based on the concept of Andreev bound states
Network of recurrent events for the Olami-Feder-Christensen model
We numerically study the dynamics of a discrete spring-block model introduced
by Olami, Feder and Christensen (OFC) to mimic earthquakes and investigate to
which extent this simple model is able to reproduce the observed spatiotemporal
clustering of seismicty. Following a recently proposed method to characterize
such clustering by networks of recurrent events [Geophys. Res. Lett. {\bf 33},
L1304, 2006], we find that for synthetic catalogs generated by the OFC model
these networks have many non-trivial statistical properties. This includes
characteristic degree distributions -- very similar to what has been observed
for real seismicity. There are, however, also significant differences between
the OFC model and earthquake catalogs indicating that this simple model is
insufficient to account for certain aspects of the spatiotemporal clustering of
seismicity.Comment: 11 pages, 16 figure
Adjunctive quetiapine for serotonin reuptake inhibitor-resistant obsessive-compulsive disorder: A meta-analysis of randomised controlled treatment trials
Small studies have shown positive effects from adding a variety of antipsychotic agents in patients with obsessive–compulsive disorder who are unresponsive to treatment with serotonin reuptake inhibitors. The evidence, however, is contradictory. This paper reports a meta-analysis of existing double-blind randomized placebo-controlled studies looking at the addition of the second-generation antipsychotic quetiapine in such cases. Three studies fulfilled the inclusion criteria. Altogether 102 individuals were subjected to analysis using Review Manager (4.2.7). The results showed evidence of efficacy for adjunctive quetiapine (< 400 mg/day) on the primary efficacy criterion, measured as changes from baseline in total Yale–Brown Obsessive Compulsive Scale scores (P = 0.008), the clinical significance of which was limited by between-study heterogeneity. The mechanism underlying the effect may involve serotonin and/or dopamine neurotransmission
Efficient formalism for large scale ab initio molecular dynamics based on time-dependent density functional theory
A new "on the fly" method to perform Born-Oppenheimer ab initio molecular
dynamics (AIMD) is presented. Inspired by Ehrenfest dynamics in time-dependent
density functional theory, the electronic orbitals are evolved by a
Schroedinger-like equation, where the orbital time derivative is multiplied by
a parameter. This parameter controls the time scale of the fictitious
electronic motion and speeds up the calculations with respect to standard
Ehrenfest dynamics. In contrast to other methods, wave function orthogonality
needs not be imposed as it is automatically preserved, which is of paramount
relevance for large scale AIMD simulations.Comment: 5 pages, 3 color figures, revtex4 packag
Breakdown of Strong-Coupling Perturbation Theory in Doped Mott Insulators
We show that doped Mott insulators, such as the copper-oxide superconductors,
are asymptotically slaved in that the quasiparticle weight, , near
half-filling depends critically on the existence of the high energy scale set
by the upper Hubbard band. In particular, near half filling, the following
dichotomy arises: when the high energy scale is integrated out but Z=0
in the thermodynamic limit when it is retained. Slavery to the high energy
scale arises from quantum interference between electronic excitations across
the Mott gap. Broad spectral features seen in photoemission in the normal state
of the cuprates are argued to arise from high energy slavery.Comment: Published versio
Lymphoproliferative malignancies in association with endemic African Kaposi’s Sarcoma
A CAJM study on Lymphoproliferative malignancies observed at a teaching hospital in South Africa among black Africans.The association of classical Kaposi’s sarcoma with lymphoproliferative disorders is well known. However, far less is known about lymphoproliferative malignancies in endemic African Kaposi’s sarcoma. A review of 47 patients with the endemic type of Kaposi’s sarcoma treated at the Johannesburg Teaching Hospital Complex between 1980 and 1992 revealed four patients (8,5 pc) in whom Kaposi’s sarcoma was associated with a malignant lymphoma. Possible pathogenetic mechanisms are suggested and the current literature is reviewed
Conservative Quantum Computing
Conservation laws limit the accuracy of physical implementations of
elementary quantum logic gates. If the computational basis is represented by a
component of spin and physical implementations obey the angular momentum
conservation law, any physically realizable unitary operators with size less
than n qubits cannot implement the controlled-NOT gate within the error
probability 1/(4n^2), where the size is defined as the total number of the
computational qubits and the ancilla qubits. An analogous limit for bosonic
ancillae is also obtained to show that the lower bound of the error probability
is inversely proportional to the average number of photons. Any set of
universal gates inevitably obeys a related limitation with error probability
O(1/n^2)$. To circumvent the above or related limitations yielded by
conservation laws, it is recommended that the computational basis should be
chosen as the one commuting with the additively conserved quantities.Comment: 5 pages, RevTex. Corrected to include a new statement that for
bosonic ancillae the lower bound of the error probability is inversely
proportional to the average number of photons, kindly suggested by Julio
Gea-Banacloch
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