989 research outputs found
Spin induced nonlinearities in the electron MHD regime
We consider the influence of the electron spin on the nonlinear propagation
of whistler waves. For this purpose a recently developed electron two-fluid
model, where the spin up- and down populations are treated as different fluids,
is adapted to the electron MHD regime. We then derive a nonlinear Schrodinger
equation for whistler waves, and compare the coefficients of nonlinearity with
and without spin effects. The relative importance of spin effects depend on the
plasma density and temperature as well as the external magnetic field strength
and the wave frequency. The significance of our results to various plasmas are
discussed.Comment: 5 page
Dynamics of spin 1/2 quantum plasmas
The fully nonlinear governing equations for spin 1/2 quantum plasmas are
presented. Starting from the Pauli equation, the relevant plasma equations are
derived, and it is shown that nontrivial quantum spin couplings arise, enabling
studies of the combined collective and spin dynamics. The linear response of
the quantum plasma in an electron--ion system is obtained and analyzed.
Applications of the theory to solid state and astrophysical systems as well as
dusty plasmas are pointed out.Comment: 4 pages, 2 figures, to appear in Physical Review Letter
The application of Item Response Theory on a teaching strategy profile questionnaire
<p>Abstract</p> <p>Background</p> <p>In medical education research, various questionnaires are often used to study possible relationships between strategies and approaches to teaching and learning and the outcome of these. However, judging the applicability of such questionnaires or the interpretation of the results is not trivial.</p> <p>Methods</p> <p>As a way to develop teacher thinking, teaching strategy profiles were calculated for teachers in a research intensive department at Karolinska Institutet. This study compares the sum score, that was inherent in the questionnaire used, with an Item Response Theory (IRT) approach. Three teaching dimensions were investigated and the intended sum scores were investigated by IRT analysis.</p> <p>Results</p> <p>Agreements as well as important differences were found. The use of the sum score seemed to agree reasonably with an IRT approach for two of the dimensions, while the third dimension could not be identified neither by a the sum score, nor by an IRT approach, as the items included showed conflicting messages.</p> <p>Conclusions</p> <p>This study emphasizes the possibilities to gain better insight and more relevant interpretation of a questionnaire by use of IRT. A sum score approach should not be taken for granted. Its use has to be thoroughly evaluated.</p
Graviton mediated photon-photon scattering in general relativity
In this paper we consider photon-photon scattering due to self-induced
gravitational perturbations on a Minkowski background. We focus on four-wave
interaction between plane waves with weakly space and time dependent
amplitudes, since interaction involving a fewer number of waves is excluded by
energy-momentum conservation. The Einstein-Maxwell system is solved
perturbatively to third order in the field amplitudes and the coupling
coefficients are found for arbitrary polarizations in the center of mass
system. Comparisons with calculations based on quantum field theoretical
methods are made, and the small discrepances are explained.Comment: 5 pages, 3 figure
A possibility to measure elastic photon--photon scattering in vacuum
Photon--photon scattering in vacuum due to the interaction with virtual
electron-positron pairs is a consequence of quantum electrodynamics. A way for
detecting this phenomenon has been devised based on interacting modes generated
in microwave waveguides or cavities [G. Brodin, M. Marklund and L. Stenflo,
Phys. Rev. Lett. \textbf{87} 171801 (2001)]. Here we materialize these ideas,
suggest a concrete cavity geometry, make quantitative estimates and propose
experimental details. It is found that detection of photon-photon scattering
can be within the reach of present day technology.Comment: 7 pages, 3 figure
Parametric excitation of plasma waves by gravitational radiation
We consider the parametric excitation of a Langmuir wave and an
electromagnetic wave by gravitational radiation, in a thin plasma on a
Minkowski background. We calculate the coupling coefficients starting from a
kinetic description, and the growth rate of the instability is found. The
Manley-Rowe relations are fulfilled only in the limit of a cold plasma. As a
consequence, it is generally difficult to view the process quantum
mechanically, i.e. as the decay of a graviton into a photon and a plasmon.
Finally we discuss the relevance of our investigation to realistic physical
situations.Comment: 5 pages, REVTe
Fast electrochemical doping due to front instability in organic semiconductors
The electrochemical doping transformation in organic semiconductor devices is
studied in application to light-emitting cells. It is shown that the device
performance can be significantly improved by utilizing new fundamental
properties of the doping process. We obtain an instability, which distorts the
doping fronts and increases the doping rate considerably. We explain the
physical mechanism of the instability, develop theory, provide experimental
evidence, and perform numerical simulations. We further show how improved
device design can amplify the instability thus leading to a much faster doping
process and device kinetics.Comment: 4 pages, 4 figure
Photon-graviton pair conversion
We consider the conversion of gravitons and photons as a four-wave mixing
process. A nonlinear coupled systems of equations involving two gravitons and
two photons is obtained, and the energy exchange between the different degrees
of freedom is found. The scattering amplitudes are obtained, from which a
crossection for incoherent processes can be found. An analytical example is
given, and applications to the early Universe are discussed.Comment: 5 pages, slightly modified as compared to v1, to appear in Class.
Quantum Grav. as a Letter to the Edito
Detection of QED vacuum nonlinearities in Maxwell's equations by the use of waveguides
We present a novel method for detecting nonlinearities, due to quantum
electrodynamics through photon-photon scattering, in Maxwell's equation. The
photon-photon scattering gives rise to self-interaction terms, which are
similar to the nonlinearities due to the polarisation in nonlinear optics.
These self-interaction terms vanish in the limit of parallel propagating waves,
but if instead of parallel propagating waves the modes generated in wavesguides
are used, there will be a non-zero total effect. Based on this idea, we
calculate the nonlinear excitation of new modes and estimate the strength of
this effect. Furthermore, we suggest a principal experimental setup.Comment: 4 pages, REVTeX3. To appear in Phys. Rev. Let
Plasma waves driven by gravitational waves in an expanding universe
In a Friedmann-Robertson-Walker (FRW) cosmological model with zero spatial
curvature, we consider the interaction of the gravitational waves with the
plasma in the presence of a weak magnetic field. Using the relativistic
hydromagnetic equations it is verified that large amplitude magnetosonic waves
are excited, assuming that both, the gravitational field and the weak magnetic
field do not break the homogeneity and isotropy of the considered FRW
spacetime.Comment: 14 page
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