12,266 research outputs found
Vacuum birefringence and dichroism in a strong plane-wave background
In the present study, we consider the effects of vacuum birefringence and
dichroism in strong electromagnetic fields. According to quantum
electrodynamics, the vacuum state exhibits different refractive properties
depending on the probe photon polarization and one also obtains different
probabilities of the photon decay via production of electron-positron pairs.
Here we investigate these two phenomena by means of several different
approaches to computing the polarization operator. The external field is
assumed to be a linearly polarized plane electromagnetic wave of arbitrary
amplitude and frequency. Varying the probe-photon energy and the field
parameters, we thoroughly examine the validity of the locally-constant field
approximation (LCFA) and techniques involving perturbative expansions in terms
of the external-field amplitude. Within the latter approach, we develop a
numerical method based on a direct evaluation of the weak-field Feynman
diagrams, which can be employed for investigating more complex external
backgrounds. It is demonstrated that the polarization operator depends on two
parameters: classical nonlinearity parameter and the product of the laser field frequency and the photon energy
( is the electron mass). The domains of validity of the approximate
techniques in the plane are explicitly identified.Comment: 11 pages, 6 figure
Performance of the modified Becke-Johnson potential
Very recently, in the 2011 version of the Wien2K code, the long standing
shortcome of the codes based on Density Functional Theory, namely, its
impossibility to account for the experimental band gap value of semiconductors,
was overcome. The novelty is the introduction of a new exchange and correlation
potential, the modified Becke-Johnson potential (mBJLDA). In this paper, we
report our detailed analysis of this recent work. We calculated using this
code, the band structure of forty one semiconductors and found an important
improvement in the overall agreement with experiment as Tran and Blaha [{\em
Phys. Rev. Lett.} 102, 226401 (2009)] did before for a more reduced set of
semiconductors. We find, nevertheless, within this enhanced set, that the
deviation from the experimental gap value can reach even much more than 20%, in
some cases. Furthermore, since there is no exchange and correlation energy term
from which the mBJLDA potential can be deduced, a direct optimization procedure
to get the lattice parameter in a consistent way is not possible as in the
usual theory. These authors suggest that a LDA or a GGA optimization procedure
is used previous to a band structure calculation and the resulting lattice
parameter introduced into the 2011 code. This choice is important since small
percentage differences in the lattice parameter can give rise to quite higher
percentage deviations from experiment in the predicted band gap value.Comment: 10 pages, 2 figures, 5 Table
Equation of state of cubic boron nitride at high pressures and temperatures
We report accurate measurements of the equation of state (EOS) of cubic boron
nitride by x-ray diffraction up to 160 GPa at 295 K and 80 GPa in the range
500-900 K. Experiments were performed on single-crystals embedded in a
quasi-hydrostatic pressure medium (helium or neon). Comparison between the
present EOS data at 295 K and literature allows us to critically review the
recent calibrations of the ruby standard. The full P-V-T data set can be
represented by a Mie-Gr\"{u}neisen model, which enables us to extract all
relevant thermodynamic parameters: bulk modulus and its first
pressure-derivative, thermal expansion coefficient, thermal Gr\"{u}neisen
parameter and its volume dependence. This equation of state is used to
determine the isothermal Gr\"{u}neisen mode parameter of the Raman TO band. A
new formulation of the pressure scale based on this Raman mode, using
physically-constrained parameters, is deduced.Comment: 8 pages, 7 figure
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