Critical phenomena and renormalization-group theory

http://www.bio21.bas.bg/ibf/PC_review.pdf

Phases and phase transitions of the phosphatidylcholines

The direct conversion of electrical energy to mechanical work by a material is relevant to a number of applications. This is illustrated by ferroelectric 'relaxors' such as Pb(Mg(1/3)Nb(2/3))O(3)-PbTiO(3) (PMN-PT; refs 5, 6): these materials exhibit a giant electromechanical (piezoelectric) response that is finding use in ultrasonic and medical applications, as well as in telecommunications. The origins of this effect are, however, still unclear. Here we show that the giant electromechanical response in PMN-PT (and potentially other ferroelectric relaxors) is the manifestation of critical points that define a line in the phase diagram of this system. Specifically, in the electric-field-temperature-composition phase diagram of PMN-PT (the composition being varied by changing the PT concentration), a first-order paraelectric-ferroelectric phase transition terminates in a line of critical points where the piezoelectric coefficient is maximum. Above this line, supercritical evolution is observed. On approaching the critical point, both the energy cost and the electric field necessary to induce ferroelectric polarization rotations decrease significantly, thus explaining the giant electromechanical response of these relaxors.

The giant electromechanical response in ferroelectric relaxors as a critical phenomenon

Electrocaloric materials for future solid-state refrigeration technologies

Since the discovery of the liquid-crystalline state of matter 125 years ago, this field has developed into a scientific area with many facets. This Review presents recent developments in the molecular design and self-assembly of liquid crystals. The focus is on new exciting soft-matter structures distinct from the usually observed nematic, smectic, and columnar phases. These new structures have enhanced complexity, including multicompartment and cellular structures, periodic and quasiperiodic arrays of spheres, and new emergent properties, such as ferroelctricity and spontaneous achiral symmetry-breaking. Comparisons are made with developments in related fields, such as self-assembled monolayers, multiblock copolymers, and nanoparticle arrays. Measures of structural complexity used herein are the size of the lattice, the number of distinct compartments, the dimensionality, and the logic depth of the resulting supramolecular structures.

Development of structural complexity by liquid-crystal self-assembly.

Measurements were made of the dielectric constant along the ferroelectric axis e a , the spontaneous polarization P s and the heat capacity c p of Rb 2 ZnCl 4 crystals in the temperature range from -150^° C to 150°C. The normal-incommensurate transition point determined as the peak position of c p lies at 30.1°C, which is about 6°C lower than the temperature for the maximum of e a . It was found that e a and P s show a characteristic thermal hysteresis over a wide temperature range including the incommensurate-commensurate trarasition point (-80^° C ). The thermal hysteresis occurs even when heating and cooling runs are cycled within the incommensurate phase. The observed thermal hysteresis is understood if we assume that defects such as impurities act as obtacles to diffusion of discommensurations and prevent the crystal from reaching thermal equilibrium.

Critical Phenomena and Anomalous Thermal Hysteresis Accompanying the Normal-Incommensurate-Commensurate Phase Transitions in Rb 2 ZnCl 4

A high-resolution computerized calorimeter capable of fully automatic operation in either ac or relaxation modes is described. Emphasis is given to a new version of the relaxation technique in which the heater power is ramped linearly in time. This improvement results in superior performance and convenience in studying both first- and second-order phase transitions and allows quantitative evaluation of latent heats as well as pretransitional heat capacity variations. Examples are given for the use of this calorimeter in the study of liquid crystal phase transitions.

Nonadiabatic scanning calorimeter

An amorphous blue phase III with low and wide thermal range (∼20 °C) including room temperature is induced by doping a bent-core nematic with a strong chiral material. We confirm that the electrooptical response is due to the Kerr effect, with the Kerr constant being up to two orders of magnitude larger than conventional Kerr materials such as nitrobenzene.

Liquid crystalline amorphous blue phase and its large electrooptical Kerr effect

The thermal hysteresis accompanying the incommensurate-commensurate phase transitions in Rb2ZnCl4, K2ZnCl4 and Rb2ZnBr4 is assumed to be caused by pinning of the incommensurate wave by defects in the crystal. The results obtained for (Rb1-xKx)2ZnCl4 mixed crystal system are also well explained in terms of such pinning effect of defects.

Effect of impurities on the incommensurate-commensurate phase transitions in Rb2ZnCl4, K2ZnCl4 and Rb2ZnBr4

The kinetic process of the commensurate-to-incommensurate phase transition in Rb 2 ZnCl 4 has been studied by tracing the time variations of the polarization and dielectric constant during the transition induced by removing the applied high dc electric field from the sample. The polarization shows an abrupt dropat the instant of field removal and then decays monotonically. The dielectric constant first increases, reaches a maximum and then decreases toward the thermal equilibrium value. The relaxation process slows down as T c is approached from above. A qualitative explanation of the observed transient characteristics is presented on the basis of a nucleation model. It is suggested that at an early stage of the transition the nucleated discommensurations are spaced more widely than in the final thermal equilibrium state.

Kenji Ema

Papers

Critical Phenomena and Anomalous Thermal Hysteresis Accompanying the Normal-Incommensurate-Commensurate Phase Transitions in Rb 2 ZnCl 4

Nonadiabatic scanning calorimeter

Liquid crystalline amorphous blue phase and its large electrooptical Kerr effect

Effect of impurities on the incommensurate-commensurate phase transitions in Rb2ZnCl4, K2ZnCl4 and Rb2ZnBr4

Transient Dielectric Behavior during the Commensurate-to-Incommensurate Phase Transition in Rb2ZnCl4