47,243 research outputs found
Mass Outflow Rate From Accretion Discs around Compact Objects
We compute mass outflow rates from accretion disks around compact objects,
such as neutron stars and black holes. These computations are done using
combinations of exact transonic inflow and outflow solutions which may or may
not form standing shock waves. Assuming that the bulk of the outflow is from
the effective boundary layers of these objects, we find that the ratio of the
outflow rate and inflow rate varies anywhere from a few percent to even close
to a hundred percent (i.e., close to disk evacuation case) depending on the
initial parameters of the disk, the degree of compression of matter near the
centrifugal barrier, and the polytropic index of the flow. Our result, in
general, matches with the outflow rates obtained through a fully time-dependent
numerical simulation. In some region of the parameter space when the standing
shock does not form, our results indicate that the disk may be evacuated and
may produce quiescence states.Comment: 30 Latex pages and 13 figures. crckapb.sty; Published in Class.
Quantum Grav. Vol. 16. No. 12. Pg. 387
Competition of L21 and XA Ordering in Fe2CoAl Heusler Alloy: A First-Principles Study
The physical properties of Fe2CoAl (FCA) Heusler alloy are systematically
investigated using the first-principles calculations within generalized
gradient approximation (GGA) and GGA+U. The influence of atomic ordering with
respect to the Wyckoff sites on the phase stability, magnetism and half
metallicity in both the conventional L21 and XA phases of FCA is focused in
this study. Various possible hypothetical structures viz., L21, XA-I, and XA-II
are prepared by altering atomic occupancies at their Wyckoff sites. At first,
we have determined the stable phase of FCA considering various non-magnetic (or
paramagnetic), ferromagnetic (FM) and antiferromagnetic (AFM) configurations.
Out of these, the ferromagnetic (FM) XA-I structure is found to be
energetically most stable. The total magnetic moments per cell are not in
agreement with the Slater-Pauling (SP) rule in any phases; therefore, the
half-metallicity is not observed in any configurations. However, FM ordered
XA-I type FCA shows 78% spin polarization at EF. Interestingly, the results of
XA-I type FCA are closely matched with the experimental results.Comment: 15 pages, 6 figure
Transport properties of diluted magnetic semiconductors: Dynamical mean field theory and Boltzmann theory
The transport properties of diluted magnetic semiconductors (DMS) are
calculated using dynamical mean field theory (DMFT) and Boltzmann transport
theory. Within DMFT we study the density of states and the dc-resistivity,
which are strongly parameter dependent such as temperature, doping, density of
the carriers, and the strength of the carrier-local impurity spin exchange
coupling. Characteristic qualitative features are found distinguishing weak,
intermediate, and strong carrier-spin coupling and allowing quantitative
determination of important parameters defining the underlying ferromagnetic
mechanism. We find that spin-disorder scattering, formation of bound state, and
the population of the minority spin band are all operational in DMFT in
different parameter range. We also develop a complementary Boltzmann transport
theory for scattering by screened ionized impurities. The difference in the
screening properties between paramagnetic () and ferromagnetic ()
states gives rise to the temperature dependence (increase or decrease) of
resistivity, depending on the carrier density, as the system goes from the
paramagnetic phase to the ferromagnetic phase. The metallic behavior below
for optimally doped DMS samples can be explained in the Boltzmann theory
by temperature dependent screening and thermal change of carrier spin
polarization.Comment: 15 pages, 15 figure
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