Switching external field-induced

Figure 10. External field-induced switching as monitored by capetcitance (Cy) mear surements sample A (small dots), sample B (medium dots), and sample C (large...

A significant step in the measurement of MFEs is the development of rapid field switching apparatusModem MOSFET transistors have made the possibility of field switching on the timescale of RP dynamics a reality. This technique was first used in a method called SEMF-CIDNP (switched external magnetic field chemically induced dynamic nuclear polarization). 

Fig. 6.1. A surface state electron hovering at x,y = 0,z = 0) above the surface 5 of a dielectric with dielectric constant e. The electron induces a surface charge density a in S. (a) No external fields present, (b) A homogeneous microwave field parallel to the x axis is switched on.

Ug expresses the electronic response (induced dipoles), oi is associated with the average orientation induced in the distribution of permanent molecular dipoles, and as denotes the total response. These contributions can in principle be monitored experimentally Immediately following a sudden switch-on of an external field T>, the instantaneous locally averaged induced dipole is zero, however after a time large relative to Xg but small with respect to Xn the polarization becomes Pg = ag D. Equation (1.237) is satisfied only after a time long relative to r . Similarly we can define two dielectric constants, Sg and Sg such that 5 = and Pg = [(sg —... 

The role of the external field is clear from the experimental data discussed below. There is no change of structural and thermodynamic characteristics in unoriented polymers in the vicinity of the transition temperature T -j. This means that the transition is induced by the field. On the other hand the transition also occurs when the electric field is switched off at a temperature 10-15 above T -j. and further cooling is carried out in the absence... 

In low-temperature phase, the order parameter is a sum of field-induced and spontaneous parts so that it becomes nonlinear function of external field due to domain structure influence. As a result, the field dependence of order parameters in the ferroics is described by hysteresis loops, schematically depicted in Fig. 1.1. The shape of these loops is close to those observed in specially prepared (e.g. by application of external field during the crystal growth) monodomain ferroics. One can see from Fig. 1.1 that order parameter in the ferroics contains spontaneous part (at zero field, where two opposite values of order parameter exist) and field-induced one, that saturates at large fields. Under the field decrease, order parameter first decreases and at some field called coercive, it becomes zero and then changes sign (so-called switching phenomenon) accompanied by strong nonlinearity. 

Since most of bent-core mesophases (e.g., Bl, B2, B5, B7, B7-like, and B8) possess intrinsic polar order, they are responsive upon applying an external stimulus such as an E-field, exhibiting either AF or EE switching or both. In this section, the author will focus on polar SmCP phases including B2 and DC, SmAP, and polar Col phases and review their field-induced switching, chirality transformation, and structural transformation. 

The preconditions for the use of polymer liquid crystals in display applications are that they exhibit bulk optical properties dependent on the molecular orientation in the mesophase and that this orientation may be altered on application of an external field. In this chapter we shall be concerned with electric or optical fields only. The particular optical property, i.e. (a) the birefringence, (b) the dichroism or (c) the scattering power, defines the display construction in terms of the use of polarized (a and b) or non-polarized (b and c) light, whereas the ability to switch from one orientation to another depends on the anisotropic electric permittivity and the orientational elastic constants. The dynamics of the induced orientation will depend, additionally, on the viscosity constants of the material. 

In experiments with the field-induced SHG we always tried to obtain the phase-matched generation, having in mind to study the zero-field SHG in the same synchronism directions when the external field is switched off. The search for zero-field SHG in the phase-matched conditions increases markedly the sensitivity of an experimental set-up and allows the pump beam intensity to be lowered. The study of the field-induced SHG was also used for the calculation of the number of parameters of the liquid crystalline medium (a cubic non-linear susceptibility a quadratic hyperpolarizability " and the discussion of their connection with molecular structure. In contrast to isotropic liquids, the value of Fy / depends not only on molecular parameters but on the orientational order parameters P- and (P4) of liquid crystal as well. The latter opens the possibility to measure (P2) and (P4). 

In practical implementations or switching devices, the logical thing to do is to involve only one or a small munber of these distortions. If an external field is applied, the field-dependent terms [cf Eq. (4.5a) and (4.5b)] shonld be added to the total free-energy expression. The process of field-induced director axis distortion in SmC is analogous to the nematic case. For example, the first three terms on the right-hand side of Equation (4.70) correspond to the splay term in nematics ... 

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