144,759 research outputs found
Milne-Eddington Solutions for Relativistic Plane-Parallel Flows
Radiative transfer in a relativistic plane-parallel flow, e.g., an accretion
disk wind, is examined in the fully special relativistic treatment. Under the
assumption of a constant flow speed, for the relativistically moving atmosphere
we analytically obtain generalized Milne-Eddington solutions of radiative
moment equations; the radiation energy density, the radiative flux, and the
radiation pressure. In the static limit these solutions reduce to the
traditional Milne-Eddington ones for the plane-parallel static atmosphere,
whereas the source function nearly becomes constant as the flow speed
increases. Using the analytical solutions, we analytically integrate the
relativistic transfer equation to obtain the specific intensity. This specific
intensity also reduces to the Milne-Eddinton case in the static limit, while
the emergent intensity is strongly enhanced toward the flow direction due to
the Doppler and aberration effects as the flow speed increases (relativistic
peaking).Comment: 1o pages, 5 figure
The physical origin of the electron-phonon vertex correction
The electron-phonon vertex correction has a complex structure both in
momentum and frequency. We explain this structure on the basis of physical
considerations and we show how the vertex correction can be decomposed into two
terms with different physical origins. In particular, the first term describes
the lattice polarization induced by the electrons and it is essentially a
single-electron process whereas the second term is governed by the
particle-hole excitations due to the exchange part of the phonon-mediated
electron-electron interaction. We show that by weakening the influence of the
exchange interaction the vertex takes mostly positive values giving rise to an
enhanced effective coupling in the scattering with phonons. This weakening of
the exchange interaction can be obtained by lowering the density of the
electrons, or by considering only long-ranged (small q) electron-phonon
couplings. These findings permit to understand why in the High-Tc materials the
small carrier density and the long ranged electron-phonon interaction may play
a positive role in enhancing Tc.Comment: 11 pages, 5 postscript figure
Multiple scattering of classical waves: from microscopy to mesoscopy and diffusion
A tutorial discussion of the propagation of waves in random media is
presented. In first approximation the transport of the multiple scattered waves
is given by diffusion theory, but important corrections are present. These
corrections are calculated with the radiative transfer or Schwarzschild-Milne
equation, which describes intensity transport at the ``mesoscopic'' level and
is derived from the ``microscopic'' wave equation. A precise treatment of the
diffuse intensity is derived which automatically includes the effects of
boundary layers. Effects such as the enhanced backscatter cone and imaging of
objects in opaque media are also discussed within this framework. In the second
part the approach is extended to mesoscopic correlations between multiple
scattered intensities which arise when scattering is strong. These correlations
arise from the underlying wave character. The derivation of correlation
functions and intensity distribution functions is given and experimental data
are discussed. Although the focus is on light scattering, the theory is also
applicable to micro waves, sound waves and non-interacting electrons.Comment: Review. 86 pages Latex, 32 eps-figures included. To appear in Rev.
Mod. Phy
Foerster resonance energy transfer rate and local density of optical states are uncorrelated in any dielectric nanophotonic medium
Motivated by the ongoing debate about nanophotonic control of Foerster
resonance energy transfer (FRET), notably by the local density of optical
states (LDOS), we study an analytic model system wherein a pair of ideal dipole
emitters - donor and acceptor - exhibit energy transfer in the vicinity of an
ideal mirror. The FRET rate is controlled by the mirror up to distances
comparable to the donor-acceptor distance, that is, the few-nanometer range.
For vanishing distance, we find a complete inhibition or a four-fold
enhancement, depending on dipole orientation. For mirror distances on the
wavelength scale, where the well-known `Drexhage' modification of the
spontaneous-emission rate occurs, the FRET rate is constant. Hence there is no
correlation between the Foerster (or total) energy transfer rate and the LDOS.
At any distance to the mirror, the total energy transfer between a
closely-spaced donor and acceptor is dominated by Foerster transfer, i.e., by
the static dipole-dipole interaction that yields the characteristic
inverse-sixth-power donor-acceptor distance dependence in homogeneous media.
Generalizing to arbitrary inhomogeneous media with weak dispersion and weak
absorption in the frequency overlap range of donor and acceptor, we derive two
main theoretical results. Firstly, the spatially dependent Foerster energy
transfer rate does not depend on frequency, hence not on the LDOS. Secondly the
FRET rate is expressed as a frequency integral of the imaginary part of the
Green function. This leads to an approximate FRET rate in terms of the LDOS
integrated over a huge bandwidth from zero frequency to about 10 times the
donor emission frequency, corresponding to the vacuum-ultraviolet. Even then,
the broadband LDOS hardly contributes to the energy transfer rates. We discuss
practical consequences including quantum information processing.Comment: 17 pages, 9 figure
Phonons, electronic charge response and electron-phonon interaction in the high-temperature superconductors
We investigate in the framework of linear response theory the complete phonon
dispersion, phonon induced electronic charge response, electron-phonon
interaction and dielectric and infrared properties of the high-temperature
superconductors (HTSC's). In particular the experimentally observed strong
renormalization of the in-plane oxygen bond-stretching modes (OBSM) which
appear upon doping in the HTSC's is discussed. It is shown that the
characteristic softening, indicating a strong EPI, is most likely a generic
effect of the CuO plane and is driven by a nonlocal coupling of the displaced
ions to the localized charge-fluctuations (CF's) at the Cu and O ions. The
different behaviour of the OBSM during the insulator-metal transition via the
underdoped phase is calculated and from a comparison of these modes conclusions
about the electronic state in the HTSC's are drawn. The underdoped state is
modelled in terms of a charge response which is insulator-like at the Cu and is
competing with a metallic charge response at the O-network in the CuO plane.
For the non-cuprate HTSC Ba-Bi-O also a strong renormalization of the OBSM is
predicted. C-axis polarized infrared and Raman-active modes of the HTSC's are
calculated in terms of CF's and anisotropic dipole-fluctuations and the problem
of a metallic character of the BiO planes is studied.Interlayer phonons and
their accompanying charge response are investigated. Depending on the
interlayer coupling calculations are performed from the static, adiabatic- to
the non-adiabatic regime.It is shown that phonon-plasmon mixing and a strong
long-ranged non-adiabatic EPI becomes evident within a certain region around
the c-axis. Both the OBSM and the non-adiabatic coupled c-axis phonon-plasmon
modes are found to be important for pairing in the HTSC's.Comment: 65 pages,20 figures. Extended version to appear in Physica Status
Solidi (b) 2004; figure 20 has been corrected; references have been adde
Radiative Heat Transfer and Effective Transport Coefficients
The theory of heat transfer by electromagnetic radiation is based on the
radiative transfer equation (RTE) for the radiation intensity, or equivalently
on the Boltzmann transport equation (BTE) for the photon distribution. We focus
in this review article, after a brief overview on different solution methods,
on a recently introduced approach based on truncated moment expansion. Due to
the linearity of the underlying BTE, the appropriate closure of the system of
moment equations is entropy production rate minimization. This closure provides
a distribution function and the associated effective transport coefficients,
like mean absorption coefficients and the Eddington factor, for an arbitrary
number of moments. The moment approach is finally illustrated with an
application of the two-moment equations to an electrical arc
Neglecting the porosity of hot-star winds can lead to underestimating mass-loss rates
Context: The mass-loss rate is a key parameter of massive stars. Adequate
stellar atmosphere models are required for spectral analyses and mass-loss
determinations. Present models can only account for the inhomogeneity of
stellar winds in the approximation of small-scale structures that are optically
thin. This treatment of ``microclumping'' has led to reducing empirical
mass-loss rates by factors of two and more. Aims: Stellar wind clumps can be
optically thick in spectral lines. We investigate how this ``macroclumping''
impacts on empirical mass-loss rates. Methods: The Potsdam Wolf-Rayet (PoWR)
model atmosphere code is generalized in the ``formal integral'' to account for
clumps that are not necessarily optically thin. Results: Optically thick clumps
reduce the effective opacity. This has a pronounced effect on the emergent
spectrum. Our modeling for the O-type supergiant zeta Puppis reveals that the
optically thin H-alpha line is not affected by wind porosity, but that the PV
resonance doublet becomes significantly weaker when macroclumping is taken into
account. The reported discrepancies between resonance-line and
recombination-line diagnostics can be resolved entirely with the macroclumping
modeling without downward revision of the mass-loss rate. Conclusions:
Mass-loss rates inferred from optically thin emission, such as the H-alpha line
in O stars, are not influenced by macroclumping. The strength of optically
thick lines, however, is reduced because of the porosity effects. Therefore,
neglecting the porosity in stellar wind modeling can lead to underestimating
empirical mass-loss rates.Comment: A&A (in press), see full abstract in the tex
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