5,684 research outputs found
Multiple transonic solutions and a new class of shock transitions in solar and stellar winds
The steady isothermal solar wind equations are shown to admit, under certain circumstances, mutliple transonic solutions when, for example, momentum deposition gives rise to multiplee critical points in the flow. These multiple solutions consist of a continuous solution and solutions which involve shock transitions between critical solutions. The ambiguity arising from the multiplicity of the solutions can be resolved by following the time evolution of a wind profile with one critical point. Results of the numerical integration of the time-dependent equations with momentum addition show that each of these multiple solutions is physically accessible and depends on the rate of change of momentum deposition. These results suggest that standing shocks are likely to be present in the inner solar wind flow
Formation of standing shocks in stellar winds and related astrophysical flows
Stellar winds and other analogous astrophysical flows can be described, to lowest order, by the familiar one dimensional hydrodynamic equations which, being nonlinear, admit in some instances discontinuous as well as continuous transonic solutions for identical inner boundary conditions. The characteristics of the time dependent differential equations of motion are described to show how a perturbation changes profile in time and, under well defined conditions, develops into a stationary shock discontinuity. The formation of standing shocks in wind type astrophysical flows depends on the fulfillment of appropriate necessary conditions, which are determined by the conservation of mass, momentum and energy across the discontinuity, and certain sufficient conditions, which are determined by the flow's history
Theoretical de Haas-van Alphen Data and Plasma Frequencies of MgB2 and TaB2
The de Haas-van Alphen-frequencies as well as the effective masses for a
magnetic field parallel to the crystallographic c-axis are calculated within
the local spin density approximation (LSDA) for MgB2 and TaB2. In addition, we
analyze the plasma frequencies computed for each Fermi surface sheet. We find a
large anisotropy of Fermi velocities in MgB2 in difference to the nearly
isotropic behavior in TaB2. We compare calculations performed within the
relativistic non-full potential augmented-spherical-wave (ASW) scheme and the
scalar-relativistic full potential local orbital (FPLO) scheme. A significant
dependence for small cross sections on the bandstructure method is found. The
comparison with the first available experimental de Haas-van Alphen-data by
Yelland et al. (Ref. 19) shows deviations from the electronic structure
calculated within both L(S)DA approaches although the cross section predicted
by FPLO are closer to the experimental data. The elucidation of the relevant
many-body effects beyond the standard LDA is considered as a possible key
problem to understand the superconductivity in MgB2.Comment: Typos corrected, 3references added. Extended and corrected version of
S. Elgazzar et al., Solid State Comm. v. 121, 99 (2002). 7pages, 4figures,
AIP Conference Proc. "Correlated Electron Systems and High-Tc
Superconductors" (ed. F. Mancini) (October 2001, Salerno, Italy
Forward-Backward Asymmetries in Hadronically Produced Lepton Pairs
It has now become possible to observe appreciable numbers of hadronically
produced lepton pairs in mass ranges where the contributions of the photon and
are comparable. Consequently, in the reaction , substantial forward-backward asymmetries can be seen. These
asymmetries provide a test of the electroweak theory in a new regime of
energies, and can serve as diagnostics for any new neutral vector bosons
coupling both to quarks and to charged lepton pairs.Comment: 11 pages, latex, 4 uuencoded figures sent separately, Fig. 2 revise
Field-induced gapless electron pocket in the superconducting vortex phase of YNi2B2C as probed by magnetoacoustic quantum oscillations
By use of ultrasound studies we resolved magneto-acoustic quantum oscillation
deep into the mixed state of the multiband nonmagnetic superconductor YNi2B2C.
Below the upper critical field, only a very weak additional damping appears
that can be well explained by the field inhomogeneity caused by the flux-line
lattice in the mixed state. This is clear evidence for no or a vanishingly
small gap for one of the bands, namely, the spheroidal alpha band. This
contrasts de Haas--van Alphen data obtained by use of torque magnetometry for
the same sample, with a rapidly vanishing oscillation signal in the mixed
state. This points to a strongly distorted flux-line lattice in the latter case
that, in general, can hamper a reliable extraction of gap parameters by use of
such techniques.Comment: 6 pages, 6 figure
Atomic Parity Violation and Precision Electroweak Physics - An Updated Analysis
A new analysis of parity violation in atomic cesium has led to the improved
value of the weak charge, . The implications
of this result for constraining the Peskin-Takeuchi parameters S and T and for
guiding searches for new Z bosons are discussed.Comment: 8 pages, LaTeX, 3 figures, Submitted to Physical Review D. Updated
experimental inputs and references; clarification of notatio
Tight-binding parameters and exchange integrals of Ba_2Cu_3O_4Cl_2
Band structure calculations for Ba_2Cu_3O_4Cl_2 within the local density
approximation (LDA) are presented. The investigated compound is similar to the
antiferromagnetic parent compounds of cuprate superconductors but contains
additional Cu_B atoms in the planes. Within the LDA, metallic behavior is found
with two bands crossing the Fermi surface (FS). These bands are built mainly
from Cu 3d_{x^2-y^2} and O 2p_{x,y} orbitals, and a corresponding tight-binding
(TB) model has been parameterized. All orbitals can be subdivided in two sets
corresponding to the A- and B-subsystems, respectively, the coupling between
which is found to be small. To describe the experimentally observed
antiferromagnetic insulating state, we propose an extended Hubbard model with
the derived TB parameters and local correlation terms characteristic for
cuprates. Using the derived parameter set we calculate the exchange integrals
for the Cu_3O_4 plane. The results are in quite reasonable agreement with the
experimental values for the isostructural compound Sr_2Cu_3O_4Cl_2.Comment: 5 pages (2 tables included), 4 ps-figure
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