841 research outputs found
The two-dimensional frustrated Heisenberg model on the orthorhombic lattice
We discuss new high-field magnetization data recently obtained by Tsirlin et
al. for layered vanadium phosphates in the framework of the square-lattice
model. Our predictions for the saturation fields compare exceptionally well to
the experimental findings, and the strong bending of the curves below
saturation agrees very well with the experimental field dependence. Furthermore
we discuss the remarkably good agreement of the frustrated Heisenberg model on
the square lattice in spite of the fact that the compounds described with this
model actually have a lower crystallographic symmetry. We present results from
our calculations on the thermodynamics of the model on the orthorhombic (i.e.,
rectangular) lattice, in particular the temperature dependence of the magnetic
susceptibility. This analysis also sheds light on the discussion of magnetic
frustration and anisotropy of a class of iron pnictide parent compounds, where
several alternative suggestions for the magnetic exchange models were proposed.Comment: 4 pages, 3 figures, accepted for publication in Journal of Physics:
Conference Serie
Ab initio modeling of Bose-Einstein condensation in Pb2V3O9
We apply density functional theory band structure calculations and quantum
Monte Carlo simulations to investigate the Bose-Einstein condensation in the
spin-1/2 quantum magnet Pb2V3O9. In contrast to previous conjectures on the
one-dimensional nature of this compound, we present a quasi-two-dimensional
model of spin dimers with ferromagnetic and antiferromagnetic interdimer
couplings. Our model is well justified microscopically and provides a
consistent description of the experimental data on the magnetic susceptibility,
high-field magnetization, and field vs. temperature phase diagram. The
Bose-Einstein condensation in the quasi-two-dimensional spin system of Pb2V3O9
is largely governed by intralayer interactions, whereas weak interlayer
couplings have a moderate effect on the ordering temperature. The proposed
computational approach is an efficient tool to analyze and predict high-field
properties of quantum magnets.Comment: 6 pages, 6 figures, 1 tabl
Structural distortion and frustrated magnetic interactions in the layered copper oxychloride [CuCl]LaNb(2)O(7)
We present a computational study of the layered copper oxychloride
[CuCl]LaNb(2)O(7) that has been recently proposed as a spin-1/2 frustrated
square lattice compound. Our results evidence an orbitally degenerate ground
state for the reported tetragonal crystal structure and reveal a
Jahn-Teller-type structural distortion. This distortion heavily changes the
local environment of copper -- CuO(2)Cl(2) plaquettes are formed instead of
CuO(2)Cl(4) octahedra -- and restores the single-orbital scenario typical for
copper oxides and oxyhalides. The calculated distortion is consistent with the
available diffraction data and the experimental results on the electric field
gradients for the Cu and Cl sites. The band structure suggests a complex
three-dimensional spin model with the interactions up to the fourth neighbors.
Despite the layered structure of (CuCl)LaNb(2)O(7), the spin system has
pronounced one-dimensional features. Yet, sizable interchain interactions lead
to the strong frustration and likely cause the spin-gap behavior. Computational
estimates of individual exchange couplings are in qualitative agreement with
the experimental data.Comment: 13 pages, 9 figures, 3 table
Average Relaxations of Extremal Problems
In this paper extremal problems that include averaging operation in constraints and objective are considered. The relaxation caused by a replacement of a problem without averaging with a problem that includes averaging is formally defined and investigated. Canonical form for nolinear programming problem with averaging is constructed and its conditions for optimality are derived. It is shown how optimality conditions for optimal control problems with various types of objectives and constraints can be derived using its averaged relaxation.averaging; constraint relaxation; nonlinear programming; optimal control problem; optimality conditions
Optimal Dispatch in Electricity Markets
The problem of calculating the optimal dispatch and prices in a single-period electricity auction in a wholesale electricity market is considered here. The novel necessary and sufficient conditions of optimality for this problem are derived and computational algorithms for solving these conditions are constructed.optimal dispatch; electricity market; nonlinear programming; non-convex problems; dynamic programming
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