2,471 research outputs found
Dynamics of mistuned radial turbine wheels
This paper presents investigations carried out at Holset into the dynamics of mistuned
radial turbine wheels, including a literature review, a lumped parameter model,
identification of the most responsive blade, distribution of the peak maximum order
response and a method of mistiming identification
Knowledge-based graphical interfaces for presenting technical information
Designing effective presentations of technical information is extremely difficult and time-consuming. Moreover, the combination of increasing task complexity and declining job skills makes the need for high-quality technical presentations especially urgent. We believe that this need can ultimately be met through the development of knowledge-based graphical interfaces that can design and present technical information. Since much material is most naturally communicated through pictures, our work has stressed the importance of well-designed graphics, concentrating on generating pictures and laying out displays containing them. We describe APEX, a testbed picture generation system that creates sequences of pictures that depict the performance of simple actions in a world of 3D objects. Our system supports rules for determining automatically the objects to be shown in a picture, the style and level of detail with which they should be rendered, the method by which the action itself should be indicated, and the picture's camera specification. We then describe work on GRIDS, an experimental display layout system that addresses some of the problems in designing displays containing these pictures, determining the position and size of the material to be presented
Electronic origin of magnetic and orbital ordering in insulating LaMnO_3
We derive a spin-orbital model for insulating LaMnO_3 which fulfills the
SU(2) symmetry of S=2 spins at Mn^{3+} ions. It includes the complete e_g and
t_{2g} superexchange which follows from a realistic Mn^{2+} multiplet structure
in cubic site symmetry, and the Jahn-Teller induced orbital interactions. We
show that the magnetic ordering observed in LaMnO_3 is stabilized by a purely
electronic mechanism due to the e_g-superexchange alone, and provide for the
first time a quantitative explanation of the observed transition temperature
and the anisotropic exchange interactions.Comment: 4 pages, ReVTeX, 4 figure
Temperature effects in a spin-orbital model for manganites
We study a two-dimensional effective orbital superexchange model derived for
strongly correlated e_g electrons coupled to t_{2g} core spins in layered
manganites. One finds that the ferromagnetic and antiferromagnetic correlations
closely compete, and small changes of parameters can switch the type of
magnetic order. For the same reason, spin order is easily destroyed with rising
temperature, while alternating orbital correlations can persist to temperatures
where FM order has already melted. A scenario for the AF phase observed in
LaSrMnO_4 is presented.Comment: 4 pages, 3 figures, contribution to the PM05-meeting in Poznan,
Polan
Quantum melting of magnetic long-range order near orbital degeneracy. Classical phases and Gaussian fluctuations
We study the effective spin-orbital model derived for the d9 ions in a
three-dimensional perovskite lattice, as in KCuF_3, where at each site the
doubly degenerate eg orbitals contain a single hole. The model describes the
superexchange interactions that depend on the pattern of orbitals occupied. We
present the ground state properties of this model, depending on the splitting
between the eg orbitals E_z, and the Hund's rule coupling in the excited d8
states, J_H. The classical phase diagram consists of six magnetic phases which
all have different orbital ordering: two antiferromagnetic (AF) phases with
G-AF order and either x2-y2 or 3z2-r2 orbitals occupied, two phases with mixed
orbital (MO) patterns and A-AF order, and two other MO phases with either C-AF
or G-AF order. All of them become degenerate at the multicritical point
M=(E_z,J_H)=(0,0). Using a generalization of linear spin-wave theory we study
both the transverse excitations which are spin-waves and
spin-and-orbital-waves, as well as the longitudinal (orbital) excitations. The
transverse modes couple to each other, and the spin-and-orbital-wave turns into
a soft mode near the M point. Therefore, quantum corrections to the
long-range-order parameter are drastically increased near the orbital
degeneracy, and classical order is suppressed in a crossover regime between the
G-AF and A-AF phases in the (E_z,J_H) plane. This behavior is reminiscent of
that found in frustrated spin models, and we conclude that orbital degeneracy
provides a new and physically realizable mechanism which stabilizes a spin
liquid ground state due to inherent frustration of magnetic interactions. We
also point out that such a disordered magnetic phase is likely to be realized
in LiNiO_2.Comment: 33 pages, 19 figure
Onset of metallic ferromagnetism in a doped spin-orbital chain
Starting from a spin-orbital model for doped manganites, we investigate a
competition between ferromagnetic and antiferromagnetic order in a
one-dimensional model at finite temperature. The magnetic and orbital order at
half filling support each other and depend on a small antiferromagnetic
superexchange between t_{2g} spins and on an alternating Jahn-Teller potential.
The crossover to a metallic ferromagnetic phase found at finite doping is
partly suppressed by the Jahn-Teller potential which may localize e_g
electrons.Comment: 6 pages, 4 figures, contribution to ICTP Ustron, 2004, see phys.
stat. sol. at http://www.interscience.wiley.co
Classical frustration and quantum disorder in spin-orbital models
The most elementary of all physical spin-orbital models is the Kugel-Khomskii
model describing a S=1/2, degenerate Mott-insulator. Recent theoretical
work is reviewed revealing that the classical limit is characterized by a point
of perfect dynamical frustration. It is suggested that this might give rise to
a quantum disordered ground state.Comment: 7 pages Revtex, 3 ps figures, proceedings 1998 NEC symposium, Nasu,
Japa
Warsztaty doskonalenia umiejętności dydaktycznych nauczycieli akademickich
The results of the author’s study over the stimulation of the auto-creative processes and the development of didactic abilities in the young university teachers are discussed in the article. The work resulted in the system of university teachers training, focused on the improvement of their teaching skills, which is implemented in the Akademia Gorniczo-Hutnicza (Mining and Steelworks Academy) and the Akademia Roinicza (Agricultural Academy) in Krakow in the form of Courses in Teaching Excellence. The article contains the description of the workshop, its theoretical assumptions, targets, curriculum and its particular components.Artykuł prezentuje wyniki prac Autora nad pobudzaniem procesów autokreacyjnych i rozwijaniem kompetencji dydaktycznych młodych nauczycieli akademickich. Dotychczasowym efektem tych prac jest system doskonalenia umiejętności dydaktycznych nauczycieli akademickich, który jest realizowany w Akademii Górniczo-Hutniczej i Akademii Rolniczej w Krakowie w formie Studium Doskonalenia Pedagogicznego dla Asystentów. Artykuł zawiera opis systemu (warsztatów), założeń teoretycznych, celów, programu oraz poszczególnych jego elementów
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