373 research outputs found
One-step model of photoemission for non-local potentials
The one-step model of valence-band photoemission and inverse photoemission
from single-crystal surfaces is reformulated for generalized (non-local,
complex and energy-dependent) potentials. Thereby, it becomes possible to
account for self-energy corrections taken from many-body electronic-structure
calculations. The original formulation due to Pendry and co-workers employs the
KKR multiple-scattering theory for the calculation of the initial state. This
prevents a straightforward generalization of the one-step model to non-local
potentials. We therefore consider the Dyson equation which is set up within a
muffin-tin-orbitals representation as an alternative to obtain the
initial-state Green function. This approach requires a revision of the
transition-matrix elements which is carried out in detail. The final state is
considered as a time-reversed LEED state as usual. The proposed generalization
of the one-step model allows to distinguish between the bare photocurrent
reflecting the (quasi-particle) band structure and the secondary effects due to
the (dipole) selection rules and due to the wave-vector and energy dependence
of the transition-matrix elements.Comment: Revtex, 20 pages, no figures, phys. stat. sol. (b) 203 (in press
Direct URCA Processes in Supernovae and Accretion Disks with Arbitrary Magnetic Field
An effect of a magnetic field of an arbitrary strength on the beta-decay and
reactions related with it by the crossing symmetry (the beta-processes) in
supernovae and accretion disks around black holes is analyzed. Rates of the
beta-processes and the energy and momentum transfered through them to an
optically transparent matter are calculated. It is shown that the macroscopic
momentum transferred to the medium increases linearly with the magnetic field
strength and can substantially affect the dynamics of supernovae and accretion
disks especially when a matter inside is degenerate. It is also demonstrated
that the rates of the beta-processes and the energy deposition in these
reactions for the magnetic field strength G, which is
assumed to be typical in supernovae and accretion disks, are lower than in the
absence of the field. This suppression is more pronounced for reactions with
neutrinos.Comment: 10 pages, 5 figure
Electron-positron pairs in hot plasma of accretion column in bright X-ray pulsars
The luminosity of X-ray pulsars powered by accretion onto magnetized neutron
stars covers a wide range over a few orders of magnitude. The brightest X-ray
pulsars recently discovered as pulsating ultraluminous X-ray sources reach
accretion luminosity above which exceeds the
Eddington value more than by a factor of ten. Most of the energy is released
within small regions in the vicinity of magnetic poles of accreting neutron
star - in accretion columns. Because of the extreme energy release within a
small volume accretion columns of bright X-ray pulsars are ones of the hottest
places in the Universe, where the internal temperature can exceed 100 keV.
Under these conditions, the processes of creation and annihilation of
electron-positron pairs can be influential but have been largely neglected in
theoretical models of accretion columns. In this letter, we investigate
properties of a gas of electron-positron pairs under physical conditions
typical for accretion columns. We argue that the process of pairs creation can
crucially influence both the dynamics of the accretion process and internal
structure of accretion column limiting its internal temperature, dropping the
local Eddington flux and increasing the gas pressure.Comment: 5 pages, 5 figures, accepted for publication in MNRAS Letter
- …