1,510,243 research outputs found
Pressure dependence of the Curie temperature in Ni2MnSn Heusler alloy: A first-principles study
The pressure dependence of electronic structure, exchange interactions and
Curie temperature in ferromagnetic Heusler alloy Ni2MnSn has been studied
theoretically within the framework of the density-functional theory. The
calculation of the exchange parameters is based on the frozen--magnon approach.
The Curie temperature, Tc, is calculated within the mean-field approximation by
solving the matrix equation for a multi-sublattice system. In agrement with
experiment the Curie temperature increased from 362K at ambient pressure to 396
at 12 GPa. Extending the variation of the lattice parameter beyond the range
studied experimentally we obtained non-monotonous pressure dependence of the
Curie temperature and metamagnetic transition. We relate the theoretical
dependence of Tc on the lattice constant to the corresponding dependence
predicted by the empirical interaction curve. The Mn-Ni atomic interchange
observed experimentally is simulated to study its influence on the Curie
temperature.Comment: 8 pages, 8 figure
An Imaging Fabry-Perot System for the Robert Stobie Spectrograph on the Southern African Large Telescope
We present the design of the Fabry-Perot system of the Robert Stobie
Spectrograph on the 10-meter class Southern African Large Telescope and its
characterization as measured in the laboratory. This system provides
spectroscopic imaging at any desired wavelength spanning a bandpass 430 - 860
nm, at four different spectral resolving powers ranging from 300 to 9000. Our
laboratory tests revealed a wavelength dependence of the etalon gap and
parallelism with a maximum variation between 600 - 720 nm that arises because
of the complex structure of the broadband multi-layer dielectric coatings. We
also report an unanticipated optical effect of this multi-layer coating
structure that produces a significant, and wavelength dependent, change in the
apparent shape of the etalon plates. This change is caused by two effects: the
physical non-uniformities or thickness variations in the coating layers, and
the wavelength dependence of the phase change upon refection that can amplify
these non-uniformities. We discuss the impact of these coating effects on the
resolving power, finesse, and throughput of the system. This Fabry-Perot system
will provide a powerful tool for imaging spectroscopy on one of the world's
largest telescopes.Comment: 17 pages, 14 figures, accepted for publication in The Astronomical
Journa
Bragg spectroscopy of a cigar shaped Bose condensate in optical lattices
We study properties of excited states of an array of weakly coupled
quasi-two-dimensional Bose condensates by using the hydrodynamic theory. We
calculate multibranch Bogoliubov-Bloch spectrums and its corresponding
eigenfunctions. The spectrum of the axial excited states and its eigenfunctions
strongly depends on the coupling among various discrete radial modes within a
given symmetry. This mode coupling is due to the presence of radial trapping
potential. The multibranch nature of the Bogoliubov-Bloch spectrum and its
dependence on the mode-coupling can be realized by analyzing dynamic structure
factor and momentum transferred to the system in Bragg spectroscopy
experiments. We also study dynamic structure factor and momentum transferred to
the condensate due to the Bragg spectroscopy experiment.Comment: 7 pages, 5 figures, to appear in Journal of Physics B: Atomic,
Molecular & Optical Physic
Highly Dispersive Scattering From Defects In Non-Collinear Magnets
We demonstrate that point-like defects in non-collinear magnets give rise to
a highly dispersive structure in the magnon scattering, violating a standard
paradigm of its momentum independence. For a single impurity spin coupled to a
prototypical non-collinear antiferromagnet we find that the resolvent is
dominated by a distinct dispersive structure with its momentum-dependence set
by the magnon dispersion and shifted by the ordering vector. This feature is a
consequence of umklapp scattering off the impurity-induced spin texture, which
arises due to the non-collinear ground state of the host system. Detailed
results for the staggered and uniform magnetization of this texture as well as
the T-matrix from numerical linear spin-wave theory are presented.Comment: 5+5 pages, 4+5 fig
Extended Fokker Planck model: properties and solutions
In the current paper Fokker Planck model of random walks has been extended to
non conservative cases characterized by explicit dependence of diffusion and
energy on time. A given generalization allows describing of such non
equilibrium processes as Levy flights in a classical differential form without
use of fractal PDE. Besides it takes into account mixing properties that are
obligatory for a certain class of chaotic systems such as Kolmogorov K system.
It was shown that an abnormal transport is a consequence of the equilibrium
distortion and not stationary diffusion. The particular case of fixed
boundaries was considered. According to the received solutions it was shown
that a system structure can resist a weak disturbance in the vicinity of the
discrete regimes, defined by a system scale and its nonlinear properties. These
regimes correspond to the exponential increase of quasi regular structure
fluctuations. Only fast disruption of regime is possible for other states of
the system. It leads to an immediate transition to the chaos.Comment: 6 page
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