On the nature of spin-lattice relaxation in nematic MBBA

The measurements of the frequency dependence of the proton T₁ in nematic MBBA have been extended to 270 MHz, and the results have been compared with the predictions of the order fluctuation and self-diffusion relaxation theories. Both the frequency and the temperature dependences of T₁ are shown to be characteristic of a mechanism controlled by molecular self-diffusion.     

Pretransition and critical phenomena in the nematic phase of MBBA

The pretransition and critical phenomena in the nematic phase of p-methoxy-benzylidene p-n-butyl-anilline (MBBA) were studies by means of precision volume measurements. The relationship between the volume change and the order parameter of MBBA to yield the critical index ß is discussed.     

Temperature dependence of the self-diffusion tensor of para-azoxyanisole

The temperature dependence of the self-diffusion tensor of the nematic liquid crystal para-azoxy-anisole (PAA) has been measured by a neutron scattering method. It is found that the self-diffusion is anisotropic and that its temperature variation is apparently dominated by variation of the order parameter.     

Anisotropy of diffusion in nematic alkyloxybenzoic acids

The existence of monomer molecules in the nematic phase of substituted benzoic acids permits the measurement of the anisotropy of diffusion coefficients by a single NMR-method.     

The twist elastic constant as a function of temperature for nematic MBBA

The quantity K_twist/X_anisotropy has been measured as a function of temperature. The results show, that near the clearing temperature this quantity is not proportional to the long range order parameter.     

Anisotropy of the turbidity of a nematic liquid crystal

The absorption coefficient of a nematic liquid present an important anisotropy. The measurement of this coefficient in three simple geometries allows the determination of the three Frank elastic constants on the same sample. We give the results obtained for MBBA at room temperature.     

Anisotropy of the elastic double force tensor of oxygen interstitials in vanadium

The anisotropyA-B of the double force tensor of O atoms on octahedral interstitial sites in V was determined in a neutron diffraction study. The applied experimental method bases on the fact that elastic stresses change, depending on the value ofA-B the O populations on octahedral sites with different spatial orientations The population changes were determined from the variations in the Bragg intensities. The anisotropy of the dobule-force tensor was found to beA-B=(7.5±1.5)eV.     

Compressibility of MBBA

Three isotherms p-V slightly above the nematic-isotropic transition temperature of 46.6°C at p = 1 atm and one at 60°C are reported. The compressibility of the isotropic phase of p-methoxybenzylidene-(p-n-butyl) aniline is very small and comparable to that for liquids close to their resp. triple points. Constancy of pressure in the two-phase region is found to be a very sensitive indication of purity.     

Effects of anisotropy,order parameter,and viscosity on translational self-diffusion in nematic liquid crystals

A theory developed for the translational diffusion in nematic liquids crystals shows a dependence on the viscosities, order parameter, and molecular structure. Theoretical results for self-diffusion in p-azoxyanisole at 125°C are D^{∥ = 4.3 × 10?6, D⊥ = 3.1 × 10?6 c2/s}, andD^∥/D^⊥ = 1.4.     

Anisotropy of the oscillation dynamics of a breather on a trap in the electroconvective twist structure of a nematic

The oscillation dynamics of dislocations with a dissociated nucleus in the electroconvective twist structure of a nematic liquid crystal has been studied. The initial state of the roll twist structure is unstable. One of the origins of this is the helical flows of the nematic in the neighboring rolls with the antiparallel axial velocity components. As a result, oscillating or “breathing” defects are formed, which have the properties of breathers and are described by the sine-Gordon equation. It has been demonstrated that the space-time anisotropy of the character of motion of the dislocations in opposite directions orthogonal to the roll structure takes place in such a nonstationary system.     

Anisotropy of electromagnetically induced left-handedness in atomic three-level media based upon bianisotropic polarizabilities and tensor character

A three-level atomic system, configured as either a gaseous medium or a solid state material, with a driving field establishing a Rabi frequency of control, is tested by a probe field. The medium has bianisotropic microscopic polarizability and magnetizability, from which the permittivity and permeability tensors are derived. Non-isotropy and polarization dependence for left-handedness (negative index of refraction) is demonstrated through examination of tensor components in the detuning frequency spectrum. These results have important implications for use in optical or electronic devices.     

An investigation of polymorphism in MBBA by positron annihilation

The solid-state behavior of MBBA has been studied by the technique of positron lifetime measurements. The positron data support the exitence of at least two metastable forms of MBBA, one of which may be glassy.     

On the structure of the metastable crystal phase of MBBA

The difference between the melting entropies of the stable and metastable crystal phases of MBBA may be interpreted as being due to the motions of the methoxy and n-butyl “tails” of the molecule.     

Oxygen self-diffusion in alpha-quartz

We have studied the formation energy of the simplest oxygen defects in alpha-quartz, the oxygen vacancy and interstitial, by an ab initio approach based on density functional theory in the local density approximation. We have determined the formation energies and entropies and the migration paths and energies. From our results we can conclude that oxygen diffuses in quartz by an interstitial mechanism: the interstitial has a dumbbell structure; one of the constitutive atoms jumps towards a neighboring oxygen site. The activation energy amounts to 4.7 eV in the intrinsic regime and 2.8 eV in the extrinsic regime.     

On the linearity of the self-diffusion process

Formal arguments are given that the self-diffusion process, understood as the mutual diffusion process in a system which consists of two mechanically similar species of particles, and which is at total equilibrium if the species labels are ignored, is an inherently linear, but nonlocal, transport process. There are no nonlinear Burnett effects, and the nonlocal diffusion coefficient is independent of the composition of the mixture. The present state of knowledge, from theory and from computer experiments, concerning the various quantities which appear in the formal analysis is summarized for both fluid and Lorentz systems.Work performed under contract with the U.S. Department of Energy.