Dipolar glasses crystals

The frustration effects are implicit in many physical systems, as different as spin glass magnets, adsorbed monomolecular films and liquid crystals [32, 54, 55], In the case of polar mesogens the dipolar frustrations may be modelled by a spin system on a triangular lattice (Fig, 5), The corresponding Hamiltonian consists of a two particle dipolar potential that has competing parallel dipole and antiparallel dipole interactions [321, The system is analyzed in terms of dimers and trimers of dipoles. When the dipolar forces between two of them cancel, the third dipole experiences no overall interaction. It is free to permeate out of the layer, thus frustrating smectic order. 

We applied the hole digging method to several samples of molecular clusters and quantum spin glasses. The most detailed study has been done on the Fe8 system. We found the predicted square-root t relaxation, Eq. (11), in experiments on fully saturated Fe8 crystals [16, 79] and on nonsaturated samples [34]. Figure 22 analyzes the dipolar distributions, revealing a remarkable structure due to next-nearest-neighbor effects [34]. These results are in good agreement with simulations [80, 55]. 

At the onset of the ferroelectric phase transition the Cu+ ions are frozen and no longer constitute the seed of the relaxation process. In this respect the ferroelectric phase transition quenches the glass-forming liquid. Below the phase transition temperature, the dipolar clusters surrounding the Nbs+ ions merge to yield the spontaneous polarization of the (now ferroelectric) crystal, and due to the strong crystal field, the off-center potential minima of the Cu+ ions are no longer symmetrical. 

Thus, one may summarize the physical picture of the relaxation dynamics in KTN crystal-doped with Cu+ ions in the following way In the paraelectric phase, as the ferroelectric phase transition is approached, the Nb5+ ions form dipolar clusters around the randomly distributed Cu+ impurity ions. The interaction between these clusters gives rise to a cooperative behavior according to the AG theory of glass-forming liquids. At the ferroelectric phase transition the cooperative relaxation of the Cu+ ions is effectively frozen. ... 

Figure 8. Dipolar interactions between polar groups of liquid crystals and silanol groups of glass surfaces (after reference [36]).

The laser diffraction method [57] was employed to experimentally determine the crystal structure within the flbrillated columns by using a uniform glass microsphere/silicone oil system, and a bet structure was observed as predicted. The diffraction pattern is shown in Figure 7 for monodispersed glass beads of various sizes. The structure constants determined from the laser diffraction experiment were found to agree very well with the theoretical calculation based on dipolar interaction energy. Table 4 lists the experimentally determined and theoretically calculated structure constants for the bet structure formed by the silica spheres. The experimental data are consistent witli the proposed bet structure. 

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