The purposes of the reported computer simulation of the normal (high-temperature) phase of rubi-dium tetrachlorozincate are to understand the disordered structure in that phase and to investigate the possibility that the transition, upon cooling, from the normal phase to one with an incommensu-rate modulation is associated with a change from the disordered structure to an ordered one. The simulation of the dynamics of 168 ions in a periodic structure begins from a slight perturbation of a structure that is determined by minimization of the potential energy within the constraints of the experimentally determined average symmetry. Rigid ions with short-range interactions described by the electron-gas model (with a qualification) are assumed. We find both zinc-induced and rubidium-induced instabilities in the chloride sublattices of the average experimental structure. The zinc-destabilized chloride ions move to a new sublattice in the simulation; however, a crude estimate indicates that this is caused by neglect of ionic polarizability and that these chlorides should either remain at their original sites or be disordered with chains of correlated positions. The rubidium-destabilized chloride ions form two-dimensional ordered networks in the disordered structure. We suggest that the inevitable freezing-out of disorder among the chains of zinc-destabilized chloride ions and among the networks of rubidium-destabilized chloride ions is the mechanism for the transition to the incommensurate phase.
The contribution to the vortex lattice energy which is due to the vortex-induced strains is calculated covering all the magnetic field range which defines the vortex state. This contribution is compared with previously reported ones what shows that, in the most part of the vortex state, it has been notably underestimated until now. The reason of such underestimation is the assumption that only the vortex cores induce strains. In contrast to what is generally assumed, both core and non-core regions are important sources of strains in high-κ superconductors. 74.25.Qt
When studying the influence of quenched defects on first-order structural phase transitions, one has to take into account the elastic deformations which are induced by heterogeneous nucleation of the new phase. In the present paper the influence of both 'random local transition temperature' and 'random local field' defects are considered within a first-order perturbation Landau theory. Both the order parameter (o.p.) fluctuations and the defects are treated perturbatively, which is consistent for structural phase transitions close to a tricritical point and for low enough defect concentrations. In such systems the o.p. fluctuations tend to induce the first-order character of the transition, whereas both types of defect play the opposite role and tend to decrease the jump of the o.p. at the transition. Estimates of the concentration of defects which can lead to complete smearing of the transition are given.
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