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  1. 3 Acta Physica Polonica A

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  1. 1 1996

  2. 2 1994

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  1. 3 Mejdani R.

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Ising Model on Tangled Chain. I. Free Energy and Entropy

100%
Mejdani R.
Acta Physica Polonica A
|
1994
|
vol. 85
|
issue 6
907-918
EN
In this paper we have considered an Ising model defined on tangled chain, in which more bonds have been added to those of pure Ising chain. To understand their competition, particularly between ferromagnetic and antiferromagnetic bonds, we have studied, using the transfer matrix method, some simple analytical calculations and an iterative algorithm, the behaviour of the free energy and entropy, particularly in the zero-field and zero-temperature limit, for different configurations of the ferromagnetic tangled chain and different types of additional interactions (ferromagnetic or antiferromagnetic). We found that the condition J = -J' between the ferromagnetic interaction J along the chain and the antiferromagnetic interaction J' across the chain is somewhat as a "transition region" condition for this behaviour. Our results indicate also the existence of nonzero entropy at zero temperature.
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Ising Model on Tangled Chain. II. Magnetization and Susceptibility

100%
Mejdani R.
Acta Physica Polonica A
|
1994
|
vol. 85
|
issue 6
919-935
EN
In the preceding paper we have considered an Ising model defined on tangled chain to study the behaviour of the free energy and entropy, particularly in the zero-field and zero-temperature limit. In this paper, following the main line and basing on some results of the previous work, we shall study in the "language" of state configurations the behaviour of the magnetization and the susceptibility for different conditions of the model, to understand better the competition between the ferromagnetic bonds along the chain and the antiferromagnetic additional bonds across the chain. Particularly interesting is the behaviour of the susceptibility in the zero-field and zero-temperature limit. Exact solutions for the magnetization and susceptibility, generated by analytical calculations and iterative algorithms, are described. The additional bonds, introduced as a form of perfect disorder, indicate a particular effect on the spin correlation. We found that the condition J = -J' between the ferromagnetic interaction J along the chain and the antiferromagnetic interaction J' across the chain is somewhat as a "transition-region" condition for this behaviour.
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Orientational Ordering for System of Ellipsoidal Molecules Dispersed in Cubic Lattice

100%
Mejdani R.
Acta Physica Polonica A
|
1996
|
vol. 90
|
issue 6
1127-1141
EN
We represent here some results for a model of migrating and nonmigrating ellipsoidal molecules dispersed in a cubic lattice, describing: the orientational ordering in nematics, the possibility of a biaxial ordered liquid state, the orientational ordering in solid hydrogen (orto-H_{2} or para-D_{2}) or the orientational glassy structure (the "higher rank"glass state). Through this kind of molecular ensemble (in the case of the system of ellipsoidal and spherical molecules) it is also possible to study the orientational and positional orderings in binary mixtures and to obtain the so-called plastic state.
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