107,494 research outputs found
Dynamics of multiply charged ions in intense laser fields
We numerically investigate the dynamics of multiply charged hydrogenic ions
in near-optical linearly polarized laser fields with intensities of order 10^16
to 10^17 W/cm^2. Depending on the charge state Z of the ion the relation of
strength between laser field and ionic core changes. We find around Z=12
typical multiphoton dynamics and for Z=3 tunneling behaviour, however with
clear relativistic signatures. In first order in v/c the magnetic field
component of the laser field induces a Z-dependent drift in the laser
propagation direction and a substantial Z-dependent angular momentum with
repect to the ionic core. While spin oscillations occur already in first order
in v/c as described by the Pauli equation, spin induced forces via spin orbit
coupling only appear in the parameter regime where (v/c)^2 corrections are
significant. In this regime for Z=12 ions we show strong splittings of resonant
spectral lines due to spin-orbit coupling and substantial corrections to the
conventional Stark shift due to the relativistic mass shift while those to the
Darwin term are shown to be small. For smaller charges or higher laser
intensities, parts of the electronic wavepacket may tunnel through the
potential barrier of the ionic core, and when recombining are shown to give
rise to keV harmonics in the radiation spectrum. Some parts of the wavepacket
do not recombine after ionisation and we find very energetic electrons in the
weakly relativistic regime of above threshold ionization.Comment: submitte
Charge fluctuation induced dephasing of exchange coupled spin qubits
Exchange coupled {\it spin} qubits in semiconductor nanostructures are shown
to be vulnerable to dephasing caused by {\it charge noise} invariably present
in the semiconductor environment. This decoherence of exchange gate by
environmental charge fluctuations arises from the fundamental Coulombic nature
of the Heisenberg coupling, and presents a serious challenge to the scalability
of the widely studied exchange gate solid state spin quantum computer
architectures. We estimate dephasing times for coupled spin qubits in a wide
range (from 1 ns up to s) depending on the exchange coupling strength
and its sensitivity to charge fluctuations.Comment: 4 pages, 3 figure
Semantic modelling of learning objects and instruction
We introduce an ontology-based semantic modelling framework that addresses subject domain modelling, instruction modelling, and interoperability aspects in the development of complex reusable learning objects. Ontologies are knowledge representation frameworks, ideally suited to support knowledge-based modelling of these learning objects. We illustrate the benefits of semantic modelling for learning object assemblies within the context of standards such as SCORM Sequencing and Navigation and Learning Object Metadata
Thermalization and Lyapunov Exponents in the Yang-Mills-Higgs Theory
We investigate thermalization processes occurring at different time scales in
the Yang-Mills-Higgs system at high temperatures. We determine the largest
Lyapunov exponent associated with the gauge fields and verify its relation to
the perturbatively calculated damping rate of a static gauge boson.Comment: 33 pages (revtex), 4 PS figures, submitted to PR
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