997 research outputs found
Conductivity, weak ferromagnetism and charge instability in single crystal
The temperature dependence of resistivity, magnetization and electron-spin
resonance of the single crystal were measured in temperature
range of . Magnetization hysteresis in applied magnetic field
up to 0.7 T at , irreversible temperature behavior of
magnetization and resistivity were found . The obtained data were explained in
terms of degenerate tight binding model using random phase approximation. The
contribution of holes in and bands of manganese ions to the
conductivity, optical absorbtion spectra and charge instability in were studied. Charge susceptibility maxima resulted from the competition of
the on-site Coulomb interaction between the holes in different orbitals and
small hybridization of sub-bands were calculated at .Comment: 6 pages, 12 figure
Accretion dynamics in the classical T Tauri star V2129 Oph
We analyze the photometric and spectroscopic variability of the classical T
Tauri star V2129 Oph over several rotational cycles to test the dynamical
predictions of magnetospheric accretion models. The photometric variability and
the radial velocity variations in the photospheric lines can be explained by
rotational modulation due to cold spots, while the radial velocity variations
of the He I (5876 \AA) line and the veiling variability are due to hot spot
rotational modulation. The hot and cold spots are located at high latitudes and
about the same phase, but the hot spot is expected to sit at the chromospheric
level, while the cold spot is at the photospheric level. Using the
dipole+octupole magnetic-field configuration previously proposed in the
literature for the system, we compute 3D MHD magnetospheric simulations of the
star-disk system. We use the simulation's density, velocity and scaled
temperature structures as input to a radiative transfer code, from which we
calculate theoretical line profiles at all rotational phases. The theoretical
profiles tend to be narrower than the observed ones, but the qualitative
behavior and the observed rotational modulation of the H\alpha and H\beta
emission lines are well reproduced by the theoretical profiles. The
spectroscopic and photometric variability observed in V2129 Oph support the
general predictions of complex magnetospheric accretion models with
non-axisymmetric, multipolar fields.Comment: Accepted by Astronomy and Astrophysic
MHD Simulations of Magnetospheric Accretion, Ejection and Plasma-field Interaction
We review recent axisymmetric and three-dimensional (3D) magnetohydrodynamic
(MHD) numerical simulations of magnetospheric accretion, plasma-field
interaction and outflows from the disk-magnetosphere boundary.Comment: 11 pages, 8 figures, conference proceedings: "Physics at the
Magnetospheric Boundary", Geneva, Switzerland, 25-28 June, 201
Electron Bloch Oscillations and Electromagnetic Transparency of Semiconductor Superlattices in Multi-Frequency Electric Fields
We examine phenomenon of electromagnetic transparency in semiconductor
superlattices (having various miniband dispersion laws) in the presence of
multi-frequency periodic and non-periodic electric fields. Effects of induced
transparency and spontaneous generation of static fields are discussed. We paid
a special attention on a self-induced electromagnetic transparency and its
correlation to dynamic electron localization. Processes and mechanisms of the
transparency formation, collapse, and stabilization in the presence of external
fields are studied. In particular, we present the numerical results of the time
evolution of the superlattice current in an external biharmonic field showing
main channels of transparency collapse and its partial stabilization in the
case of low electron density superlattices
Optimal clustering of a pair of irregular objects
Cutting and packing problems arise in many fields of applications and theory. When dealing with irregular objects, an important subproblem is the identification of the optimal clustering of two objects. Within this paper we consider a container (rectangle, circle, convex polygon) of variable sizes and two irregular objects bounded by circular arcs and/or line segments, that can be continuously translated and rotated. In addition minimal allowable distances between objects and between each object and the frontier of a container, may be imposed. The objects should be arranged within a container such that a given objective will reach its minimal value. We consider a polynomial function as the objective, which depends on the variable parameters associated with the objects and the container. The paper presents a universal mathematical model and a solution strategy which are based on the concept of phi-functions and provide new benchmark instances of finding the containing region that has either minimal area, perimeter or homothetic coefficient of a given container, as well as finding the convex polygonal hull (or its approximation) of a pair of objects
Warps, bending and density waves excited by rotating magnetized stars: results of global 3D MHD simulations
We report results of the first global three-dimensional magnetohydrodynamic
simulations of the waves excited in an accretion disc by a rotating star with a
dipole magnetic field misaligned from the star's rotation axis (which is
aligned with the disc axis). The main results are the following: (1) If the
magnetosphere of the star corotates approximately with the inner disc, then we
observe a strong one-armed bending wave (a warp). This warp corotates with the
star and has a maximum amplitude between corotation radius and the radius of
the vertical resonance. The disc's center of mass can deviate from the
equatorial plane up to the distance of z_w\approx 0.1 r. However, the effective
height of the warp can be larger, h_w \approx 0.3 r due to the finite thickness
of the disc. Stars with a range of misalignment angles excite warps. However,
the amplitude of the warps is larger for misalignment angles between 15 and 60
degrees. (2) If the magnetosphere rotates slower, than the inner disc, then a
bending wave is excited at the disc-magnetosphere boundary, but does not form a
large-scale warp. Instead, high-frequency oscillations become strong at the
inner region of the disc. These are (a) trapped density waves which form inside
the radius where the disc angular velocity has a maximum, and (b) inner bending
waves which appear in the case of accretion through magnetic Raleigh-Taylor
instability. These two types of waves are connected with the inner disc and
their frequencies will vary with accretion rate. Bending oscillations at lower
frequencies are also excited including global oscillations of the disc. In
cases where the simulation region is small, slowly-precessing warp forms.
Simulations are applicable to young stars, cataclysmic variables, and accreting
millisecond pulsars.Comment: 26 pages, 25 figure
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