9,252 research outputs found
Hamiltonian of a many-electron system with single-electron and electron-pair states in a two-dimensional periodic potential
Based on the metastable electron-pair energy band in a two-dimensional (2D)
periodic potential obtained previously by Hai and Castelano [J. Phys.: Condens.
Matter 26, 115502 (2014)], we present in this work a Hamiltonian of many
electrons consisting of single electrons and electron pairs in the 2D system.
The electron-pair states are metastable of energies higher than those of the
single-electron states at low electron density. We assume two different
scenarios for the single-electron band. When it is considered as the lowest
conduction band of a crystal, we compare the obtained Hamiltonian with the
phenomenological model Hamiltonian of a boson-fermion mixture proposed by
Friedberg and Lee [Phys. Rev. B 40, 6745 (1989)]. Single-electron-electron-pair
and electron-pair-electron-pair interaction terms appear in our Hamiltonian and
the interaction potentials can be determined from the electron-electron Coulomb
interactions. When we consider the single-electron band as the highest valence
band of a crystal, we show that holes in this valence band are important for
stabilization of the electron-pair states in the system
Passaging of a Newcastle disease virus pigeon variant in chickens results in selection of viruses with mutations in the polymerase complex enhancing virus replication and virulence
Some Newcastle disease virus (NDV) variants isolated from pigeons (pigeon paramyxovirus type 1; PPMV-1) do not show their full virulence potential for domestic chickens but may become virulent upon spread in these animals. In this study we examined the molecular changes responsible for this gain of virulence by passaging a low-pathogenic PPMV-1 isolate in chickens. Complete genome sequencing of virus obtained after 1, 3 and 5 passages showed the increase in virulence was not accompanied by changes in the fusion protein – a well known virulence determinant of NDV – but by mutations in the L and P replication proteins. The effect of these mutations on virulence was confirmed by means of reverse genetics using an infectious cDNA clone. Acquisition of three amino acid mutations, two in the L protein and one in the P protein, significantly increased virulence as determined by intracerebral pathogenicity index tests in day-old chickens. The mutations enhanced virus replication in vitro and in vivo and increased the plaque size in infected cell culture monolayers. Furthermore, they increased the activity of the viral replication complex as determined by an in vitro minigenome replication assay. Our data demonstrate that PPMV-1 replication in chickens results in mutations in the polymerase complex rather than the viral fusion protein, and that the virulence level of pigeon paramyxoviruses is directly related to the activity of the viral replication complex
Up-down symmetry of the turbulent transport of toroidal angular momentum in tokamaks
Two symmetries of the local nonlinear delta-f gyrokinetic system of equations
in tokamaks in the high flow regime are presented. The turbulent transport of
toroidal angular momentum changes sign under an up-down reflection of the
tokamak and a sign change of both the rotation and the rotation shear. Thus,
the turbulent transport of toroidal angular momentum must vanish for up-down
symmetric tokamaks in the absence of both rotation and rotation shear. This has
important implications for the modeling of spontaneous rotation.Comment: 15 pages, 2 figure
Wigner crystallization in quantum electron bilayers
The phase diagram of quantum electron bilayers in zero magnetic field is
obtained using density functional theory. For large electron densities the
system is in the liquid phase, while for smaller densities the liquid may
freeze (Wigner crystallization) into four different crystalline phases; the
lattice symmetry and the critical density depend on the the inter-layer
distance. The phase boundaries between different Wigner crystals consist of
both first and second order transitions, depending on the phases involved, and
join the freezing curve at three different triple points.Comment: To appear in Europhys. Lett. (11 pages in REVTEX + 2 figures in
postscript
Fluxonic Cellular Automata
We formulate a new concept for computing with quantum cellular automata
composed of arrays of nanostructured superconducting devices. The logic states
are defined by the position of two trapped flux quanta (vortices) in a 2x2
blind-hole-matrix etched on a mesoscopic superconducting square. Such small
computational unit-cells are well within reach of current fabrication
technology. In an array of unit-cells, the vortex configuration of one cell
influences the penetrating flux lines in the neighboring cell through the
screening currents. Alternatively, in conjoined cells, the information transfer
can be strengthened by the interactions between the supercurrents in adjacent
cells. Here we present the functioning logic gates based on this fluxonic
cellular automata (FCA), where the logic operations are verified through
theoretical simulations performed in the framework of the time-dependent
Ginzburg-Landau theory. The input signals are defined by current loops placed
on top of the two diagonal blind holes of the input cell. For given
current-polarization, external flux lines are attracted or repelled by the
loops, forming the '0' or '1' configuration. The read-out technology may be
chosen from a large variety of modern vortex imaging methods, transport and
LDOS measurements.Comment: Featured on the cover page of APL, November 2007 issu
The split-operator technique for the study of spinorial wavepacket dynamics
The split-operator technique for wave packet propagation in quantum systems
is expanded here to the case of propagating wave functions describing
Schr\"odinger particles, namely, charge carriers in semiconductor
nanostructures within the effective mass approximation, in the presence of
Zeeman effect, as well as of Rashba and Dresselhaus spin-orbit interactions. We
also demonstrate that simple modifications to the expanded technique allow us
to calculate the time evolution of wave packets describing Dirac particles,
which are relevant for the study of transport properties in graphene.Comment: 19 pages, 4 figure
Superconducting films with antidot arrays - novel behavior of the critical current
Novel behavior of the critical current density of a regularly
perforated superconducting film is found, as a function of applied magnetic
field . Previously pronounced peaks of at matching fields were
always found to decrease with increasing . Here we found a {\it reversal of
this behavior} for particular geometrical parameters of the antidot lattice
and/or temperature. This new phenomenon is due to a strong ``caging'' of
interstitial vortices between the pinned ones. We show that this vortex-vortex
interaction can be further tailored by an appropriate choice of the
superconducting material, described by the Ginzburg-Landau parameter .
In effective type-I samples we predict that the peaks in at the
matching fields are transformed into a {\it step-like behavior}.Comment: 5 pages, 4 figure
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