“…. For the truth of the conclusions of physical science, observation is the supreme Court of Appeal….” (Sir Arthur Eddington, The Philosophy of Physical Science.) In this paper we shall review the theoretical and experimental results obtained on simple magnetic model systems. We shall consider the Heisenberg, XY and Ising type of interaction (ferro and antiferromagnetic), on magnetic lattices of dimensionality 1, 2 and 3. Particular attention will be paid to the approximation of these model systems in real crystals, viz. how they can be realized or be expected to exist in nature. A large number of magnetic compounds which, according to the available experimental information, meet the requirements set by one or the other of the various models are considered and their properties discussed. Many examples will be given that demonstrate to what extent experiments on simple magnetic systems support theoretical descriptions of magnetic ordering phenomena and contribute to their understanding. It will a...

Experiments on simple magnetic model systems

This review discusses the physical properties of nematic, cholesteric, and smectic liquid crystals. Molecular theories of the liquid crystal phases are discussed and the molecular field theories of the phase transitions between the various liquid crystal phases are presented. The elastic theory and hydrodynamics of liquid crystals is developed. A wide variety of phenomena in liquid crystals, including elastic distortions, disclinations, flow properties, fluctuations, light scattering, wave propagation, nuclear magnetic resonance, effects of magnetic and electric fields, electrohydrodynamics, and optical properties, is discussed.

Physics of liquid crystals

We assume that (1) the local state of order in the isotropic phase is a symmetric traceless tensor Qαβ, proportional to the anisotropic part of a tensor property such as the magnetic susceptibility; (2) the free energy may be expanded in powers of Qαβ and of its gradients. This allows a unified description covering the anomalous magnetic birefringence, the intensity of light scattering, and the properties of the nematic/isotropic interface. For a cholesteric, although the optical rotation is huge in the ordered phase, we predict that it should not be anomalous just above the transition point Tc. We also investigate the dynamics of fluctuations of Qαβ, and discuss the flow birefringence, the frequency width of the Rayleigh scattering, and the attenuation of ultrasonic shear waves, in terms of 3 viscosity coefficients.

Short Range Order Effects in the Isotropic Phase of Nematics and Cholesterics

Structures and properties of liquid crystalline phases formed by bent-core molecules are reviewed. At least eight phases designated as B1–B8 have been found, being unambiguously distinguished from phases formed by usual calamitic molecules due to a number of remarkable peculiarities. In addition to B1–B8 phases, smectic A-like phases and biaxial nematic phases formed by bent-core molecules are also reviewed. The most attractive aspects of this new class of liquid crystals are in polarity and chirality, despite being formed from achiral molecules. The bent-core mesogens are the first ferroelectric and antiferroelectric liquid crystals realized without introducing chirality. Spontaneous chiral deracemization at microscopic and macroscopic levels occurs and is controllable. Moreover, achiral bent-core molecules enhance system chirality. The interplay between polarity and chirality provides chiral nonlinear optic effects. Further interesting phenomena related to polarity and chirality are also reviewed.

https://iopscience.iop.org/article/10.1143/JJAP.45.597/pdf

Bent-Core Liquid Crystals: Their Mysterious and Attractive World

The phase diagram of the ternary system potassium laurate-1-decanol-D$_{2}\mathrm{O}$ was studied over concentration ranges where nematic phases are likely to occur. Two uniaxial nematic phases which are separated by a biaxial nematic phase are found. In limited concentration range the following phase sequence may be observed reversibly on heating and on cooling: isotropic-uniaxial nematic (positive optical anisotropy)-biaxial nematic-uniaxial nematic (negative optical anisotropy)-biaxial nematic-uniaxial nematic (positive optical anisotropy)-isotropic.

Observation of a Biaxial Nematic Phase in Potassium Laurate-1-Decanol-Water Mixtures

Ordered States of a Nematic Liquid

We wish to report on a simple method for measuring the birefringence and indices of refraction of nematic liquids with an accuracy comparable with or superior to previously reported means.(1–3) The method is exemplified in measurement of the temperature dependences of the indices of refraction of p-methoxybenzylidene p-butylaniline (MBBA) at two wavelengths.

On the Measurement of Indices of Refraction of Nematic Liquids

The general form of the energy of interaction between two asymmetric molecules is considered. This leads to a generalization of the Maier-Saupe model of a nematic liquid. The exact state of minimum internal energy of the system is found to be biaxial. In the molecular field approximation the consistency relations yield two distinct ordered states, a uniaxial state and a biaxial state. It is shown that with decreasing temperature the fluid will have two successive transitions according to the scheme: isotropicuniaxial orderbiaxial order.

Successive Transitions in a Nematic Liquid

Discovery of a Simple Cubic Antiferromagnet: Antiferromagnetic Resonance in RbMnF 3

An electro-optical display device employing a cell comprising a nematic liquid between two glass plates whose inner surfaces have been coated with transparent electrodes. Such coated electrodes are rubbed with a cloth or filter paper so that the liquid crystal film becomes oriented along the direction of rubbing. Such film will be homogeneous and strongly birefringent so that when an electric field is applied to the electrodes of the cell and the latter is viewed between crossed polarizers, a marked contrast in light between the quiescent and active states, respectively, of the cell is observed, even in the presence of ambient illumination of the cell.

Marvin J. Freiser

Papers

Ordered States of a Nematic Liquid

On the Measurement of Indices of Refraction of Nematic Liquids

Successive Transitions in a Nematic Liquid

Discovery of a Simple Cubic Antiferromagnet: Antiferromagnetic Resonance in RbMnF 3

Liquid crystal display element