Field-Induced Periodic Structures

Thus, this calculation of the magnitude of the field, induced at the site by a single ion, and its dependence on distance explain the experimental observation of the jump-like magnetization changes at h = h and h = hi. As a result of the first phase transition, the reversed spins form a close-packed structure with a period ro 10 7cm. 

However, these very low-frequency oscillations periodic in jB could be successfully explained by the standard model for field induced spin density waves [75, 76], see also Figs. 2.8 and 2.9 in Sect. 2.2.3. Although this effect is strongly related to the quasi-lD band structure and is by itself highly interesting no direct conclusions concerning the FS topology can be extracted from the measurements. 

Fig. 11.31 Flexoelectric instability. Periodic structure of the field induced director distribution along the y-axis represented by projections and...

Both cholesteric and smectic mesophases are layered. In the former case, the periodicity arises from a natural twist to the director field, and in the latter, from a center-of-mass correlation in one dimension. There are many types of smectic phases distinguished by their symmetry and order. The set of field-induced phenomena is quite different for these two materials, owing primarily to the very different layer compressibility. That is, the cholesteric pitch can be unwound by an external field, whereas the smectic layering is typically too strong to be altered significantly. However, because of the common layered structure, there are also strong similarities. 

Lonberg, F., Fraden, S., Hurd, A. J., and Meyer, R. E., Field-induced transient periodic structures in nematic liquid crystals the twist-Freedericksz transition, Phy.s. Rev. Lett., 52, 1903-1906 (1984). 

Even in the thermodynamic equilibrium, 0 z) can form a periodic soliton-like structure with a period incommensurate with the helical pitch qo = 2itfPo [41]. When the field is applied the conical distortion can appear, especially for certain values of the twist elastic constants, K22 > Kzz [29, 42]. Such conical distortion can be accompanied by a compression of the hehcal pitch. The results of some earlier controversial observations were discussed in [6] and [1]. Probably, the conical deformation has been observed in a ternary cholesteric mixture by measuring the field-induced changes in the... 

By combining classical samplings with quantum chemistry semiempirical TDHF calculations the impact of dynamic fluctuations on the first hyperpolarizability of helical strands has been evidenced . In particular, these fluctuations are responsible for relative variations of 20% in the hyper-Rayleigh responses in both pyridine-pyrimidine (py-pym) and hydrazone-pyrimidine (hy-pym) strands. Dynamical disorder has an even more important impact on the electric field-induced second harmonic generation responses, whose variations can reach 2 (py-pym) or 5 (hy-pym) times their mean value. These results demonstrate that geometrical fluctuations have to be taken into account for a reliable description of the second-order NLO properties in flexible structures such as helical strands. This work has also highlighted the relationships between the nature of the unit cell and the helical conformation of foldamers and their second-order NLO responses. In particular, the value of the hyper-Rayleigh depolarization ratio, which is characteristics of octupolar symmetry, is related to the helix periodicity, of three unit cells per turn in both compounds. 

Several cases of dielectric, hydrodynamic, and flexoelectric instabilities and domain structures have been observed and extensively studied in CLCs. Their appearance depends on the initial orientation of molecules, the physical parameters of the material, and the applied electric field. In CLCs with positive dielectric anisotropy Ae > 0, an electric field applied along the helix axis of a planar (Grandjean) texture can induce a two-dimensional spatially periodic deformation which has the form of a square grid [96], The period and threshold voltage of this field-induced instability depend on the elastie constants, the dieleetric anisotropy, and the sample thickness [97],... 

The action of electric fields on structured enzyme systems leads to a great variety of behaviors it is possible to obtain regulations of enzyme activity or commutations in addition it is possible to induce oscillatory behaviors in such systems with two chemical parameters direct electric fields are able to induce periodical oscillations, whereas alternating electric fields are able to induce aperiodical oscillations (chaotic oscillations) in wide domains of the different parameters. 

HPDLCs are fabricated by photo-polymerization induced phase separation (PIPS) method by exposing a mixture of LC(s) and photo-sensitive monomer(s) to a laser optical interference field [38 1]. The laser optical field is created by two or multiple coherent laser beams. The periodic structure in HPDLCs is composed of alternating lamellas of polymer-rich layer and phase-separated LC-rich layer, in... 

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