606 research outputs found
Coherence of Currents in Mesoscopic Cylinders
The persistent currents driven by the pure Aharonov-Bohm type magnetic field
in mesoscopic normal metal or semiconducting cylinders are studied. A
two-dimensional (2D) Fermi surfaces are characterized by four parameters.
Several conditions for the coherence and enhancement of currents are discussed.
These results are then generalized to a three-dimensional (3D) thin-walled
cylinder to show that under certain geometric conditions on the Fermi surface,
a novel effect - the appearance of spontaneous currents is predicted.Comment: 17 pages, Latex, 8 figures available on request, to be published in
Z.Physik
Radiative return at NLO and the measurement of the hadronic cross-section in electron-positron annihilation
Electron-positron annihilation into hadrons plus an energetic photon from
initial state radiation allows the hadronic cross-section to be measured over a
wide range of energies. The full next-to-leading order QED corrections for the
cross-section for e^+ e^- annihilation into a real tagged photon and a virtual
photon converting into hadrons are calculated where the tagged photon is
radiated off the initial electron or positron. This includes virtual and soft
photon corrections to the process e^+ e^- \to \gamma +\gamma^* and the emission
of two real hard photons: e^+ e^- \to \gamma + \gamma + \gamma^*. A Monte Carlo
generator has been constructed, which incorporates these corrections and
simulates the production of two charged pions or muons plus one or two photons.
Predictions are presented for centre-of-mass energies between 1 and 10 GeV,
corresponding to the energies of DAPHNE, CLEO-C and B-meson factories.Comment: 13 pages, 15 figure
Flux qubit on mesoscopic nonsuperconducting ring
The possibility of making a flux qubit on nonsuperconducting mesoscopic
ballistic quasi 1D ring is discussed. We showed that such ring can be
effectively reduced to a two-state system with two external control parameters.
The two states carry opposite persistent currents and are coupled by tunneling
which leads to a quantum superposition of states. The qubit states can be
manipulated by resonant microwave pulses. The flux state of the sample can be
measured by a SQUID magnetometer. Two or more qubits can be coupled by the flux
the circulating currents generate. The problem of decoherence is also
discussed.Comment: Phys. Rev. B. (accepted
Persistent currents in carbon nanotubes
Persistent currents driven by a static magnetic flux parallel to the carbon
nanotube axis are investigated. Owing to the hexagonal symmetry of graphene the
Fermi contour expected for a 2D-lattice reduces to two points. However the
electron or hole doping shifts the Fermi energy upwards or downwards and as a
result, the shape of the Fermi surface changes. Such a hole doping leading to
the Fermi level shift of (more or less) 1eV has been recently observed
experimentally. In this paper we show that the shift of the Fermi energy
changes dramatically the persistent currents and discuss the electronic
structure and possible currents for zigzag as well as armchair nanotubes.Comment: 8 text pages, 6 figures, to appear in Physics Letters
On the possibility of spontaneous currents in mesoscopie systems
It is shown that a mesoscopic metallic system can exhibit a phase transition
to a low temperature state with a spontaneous orbital current if it is
sufficiently free of elastic defect scattering. The interaction among the electrons,
which is the reason of the phase transition, is of the magnetic origin
and it leads to an ordered state of the orbital magnetic moments
Possibility of long-range order in clean mesoscopic cylinders
A microscopic Hamiltonian of the magnetostatic interaction is discussed. This
long-range interaction can play an important role in mesoscopic systems leading
to an ordered ground state.
The self-consistent mean field approximation of the magnetostatic interaction
is performed to give an effective Hamiltonian from which the spontaneous,
self-sustaining currents can be obtained.
To go beyond the mean field approximation the mean square fluctuation of the
total momentum is calculated and its influence on self-sustaining currents in
mesoscopic cylinders with quasi-1D and quasi-2D conduction is considered. Then,
by the use of the microscopic Hamiltonian of the magnetostatic interaction for
a set of stacked rings, the problem of long-range order is discussed. The
temperature below which the system is in an ordered state is
determined.Comment: 14 pages, REVTeX, 5 figures, in print in Phys. Rev.
Exact zero-point energy shift in the , many modes dynamic Jahn-Teller systems at strong coupling
We find the exact semiclassical (strong coupling) zero-point energy shifts
applicable to the and dynamic Jahn-Teller
problems, for an arbitrary number of discrete vibrational modes
simultaneously coupled to one single electronic level. We also obtain an
analytical formula for the frequency of the resulting normal modes, which has
an attractive and apparently general Slater-Koster form. The limits of validity
of this approach are assessed by comparison with O'Brien's previous
effective-mode approach, and with accurate numerical diagonalizations.
Numerical values obtained for with and coupling
constants appropriate to C are used for this purpose, and are
discussed in the context of fullerene.Comment: 20 pages, 4 ps figure
Multi-model simulations of the impact of international shipping on Atmospheric Chemistry and Climate in 2000 and 2030
The global impact of shipping on atmospheric chemistry and radiative forcing, as well as the associated uncertainties, have been quantified using an ensemble of ten state-of-the-art atmospheric chemistry models and a predefined set of emission data. The analysis is performed for present-day conditions ( year 2000) and for two future ship emission scenarios. In one scenario ship emissions stabilize at 2000 levels; in the other ship emissions increase with a constant annual growth rate of 2.2% up to 2030 ( termed the "Constant Growth Scenario" (CGS)). Most other anthropogenic emissions follow the IPCC ( Intergovernmental Panel on Climate Change) SRES ( Special Report on Emission Scenarios) A2 scenario, while biomass burning and natural emissions remain at year 2000 levels. An intercomparison of the model results with observations over the Northern Hemisphere (25 degrees - 60 degrees N) oceanic regions in the lower troposphere showed that the models are capable to reproduce ozone (O-3) and nitrogen oxides (NOx= NO+ NO2) reasonably well, whereas sulphur dioxide (SO2) in the marine boundary layer is significantly underestimated. The most pronounced changes in annual mean tropospheric NO2 and sulphate columns are simulated over the Baltic and North Seas. Other significant changes occur over the North Atlantic, the Gulf of Mexico and along the main shipping lane from Europe to Asia, across the Red and Arabian Seas. Maximum contributions from shipping to annual mean near-surface O-3 are found over the North Atlantic ( 5 - 6 ppbv in 2000; up to 8 ppbv in 2030). Ship contributions to tropospheric O3 columns over the North Atlantic and Indian Oceans reach 1 DU in 2000 and up to 1.8 DU in 2030. Tropospheric O-3 forcings due to shipping are 9.8 +/- 2.0 mW/m(2) in 2000 and 13.6 +/- 2.3 mW/m(2) in 2030. Whilst increasing O-3, ship NOx simultaneously enhances hydroxyl radicals over the remote ocean, reducing the global methane lifetime by 0.13 yr in 2000, and by up to 0.17 yr in 2030, introducing a negative radiative forcing. The models show future increases in NOx and O-3 burden which scale almost linearly with increases in NOx emission totals. Increasing emissions from shipping would significantly counteract the benefits derived from reducing SO2 emissions from all other anthropogenic sources under the A2 scenario over the continents, for example in Europe. Globally, shipping contributes 3% to increases in O-3 burden between 2000 and 2030, and 4.5% to increases in sulphate under A2/CGS. However, if future ground based emissions follow a more stringent scenario, the relative importance of ship emissions will increase. Inter-model differences in the simulated O-3 contributions from ships are significantly smaller than estimated uncertainties stemming from the ship emission inventory, mainly the ship emission totals, the distribution of the emissions over the globe, and the neglect of ship plume dispersion
Development of an approximate method for quantum optical models and their pseudo-Hermicity
An approximate method is suggested to obtain analytical expressions for the
eigenvalues and eigenfunctions of the some quantum optical models. The method
is based on the Lie-type transformation of the Hamiltonians. In a particular
case it is demonstrated that Jahn-Teller Hamiltonian can
easily be solved within the framework of the suggested approximation. The
method presented here is conceptually simple and can easily be extended to the
other quantum optical models. We also show that for a purely imaginary coupling
the Hamiltonian becomes non-Hermitian but -symmetric. Possible generalization of this approach is outlined.Comment: Paper prepared fo the "3rd International Workshop on Pseudo-Hermitian
Hamiltonians in Quantum Physics" June 2005 Istanbul. To be published in
Czechoslovak Journal of Physic
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