Biaxial Domain Research Articles

Optical signals of uniaxial and biaxial cholesteric lyotropic phases

Lyotropic cholesteric liquid crystalline phases were prepared by doping the ternary mixture of potassium laurate (KL)/decanol (DeOH)/ deuterium oxide (D2O) with brucine sulphate, in two different concentrations. Using the variation of the average color in a heating cycle, with and without the insertion of compensator a gypsum plate (1 λ), in the light path, the optical signal of the cholesteric phases ChD-(ChC+) was confirmed as being negative(positive). When the biaxial cholesteric phase (ChB) is between two uniaxial phases (ChD-ChB-ChD) or (ChD-ChB-ChC), the sign of the ChB phase depends on the uniaxial phase in its vicinity. In the case (ChD--ChB--ChD-), the phase ChB has a negative optical signal (ChB-) across the domain. However, if the ChB phase is between uniaxial cholesteric phases of opposite optical signals, it cannot have just one signal, but must have a change at some point in the biaxial domain showing (ChD--ChB--ChB+-ChC+) phase sequence.

Anisotropic AC Magnetic Susceptibility in (Ga,Mn)As Films

Ferromagnetic semiconductor (Ga, Mn)As has emerged as the most thoroughly studied material for semiconductor spintronic applications, showing excellent magnetic properties that are usually described by the single-domain model [1]. However, certain uncertainties about magnetic phase structure and their dynamic properties of this system still remain unresolved. In this study, we systematically carry out both dc and ac magnetic measurements as a function of temperature and applied magnetic field in (Ga, Mn)As films with nominal Mn concentration of 5.0% grown by molecular beam epitaxy. The ac susceptibility vs. temperature shows similar results to those reported previously [2,3]: two peaks are observed with magnetic field H along [100], one close to T C and one close to T C /2, while one peak close to T C is observed with H along [1-10], and one peak close to T C /2 is seen for H along [110]. These results are consistent with dc magnetization measurements along these three axes. Careful analysis of hysteresis loops and of frequency-dependent ac susceptibility leads to the unambiguous conclusion that the ac susceptibility peak near T C occurs because of transition from paramagnetic to ferromagnetic phase; and the peak near T C /2 occurs because of the onset of a biaxial domain structure [4]. Specifically, ac susceptibility near T C involves a 180 degree magnetization flip along the [1-10] axis (i.e., the uniaxial easy axis), while the peak of the ac susceptibility near T C /2 originates from a magnetization flip between two biaxial easy axes with an angular separation of θ(T), arising from the competition between uniaxial and cubic anisotropy. Due to the difference between these two mechanisms, frequency dependences of ac susceptibilities in these two temperature regions are also quite different, providing additional information on both biaxial and uniaxial domain structures in (Ga, Mn)As films. In this connection, magnetic dynamic properties (such as the relaxation times) are analyzed and discussed using various dynamic models in order to allow us to get better insight into the evolution of magnetic domain transitions in (Ga, Mn)As films. The present observations on magnetic domain transitions and their dynamics provide new information relevant to spintronic applications such as memory and logic devices. This work was supported by the NSF Grant DMR14-00432.

Biaxial coherence length in a nematic π-cell

In a highly frustrated calamitic nematic phase, the strain can be relaxed by lowering the nematic order: the starting uniaxial symmetry can be broken and it can be replaced locally with transient biaxial domains. Using simple optical retardation measurements, we estimate the length scale over which the biaxial disturbance decays in space within a π-cell submitted to a weak electric field. We also characterise the transition cascade from the uniaxial splay texture to a bend texture through slow defect motion.

Lyotropic mixture made of potassium laurate/1-undecanol/K2SO4/water presenting high birefringences and large biaxial nematic phase domain: a laser conoscopy study

The lyotropic liquid crystalline quaternary mixture made of potassium laurate (KL), potassium sulphate, 1-undecanol and water was investigated by experimental optical methods (optical microscopy and laser conoscopy). In a particular temperature and relative concentrations range, the three nematic phases (two uniaxial and one biaxial) were identified. The biaxial domain in the temperature/KL concentration surface is larger when compared to other lyotropic mixtures. Moreover, this new mixture gives nematic phases with higher birefringence than similar systems. The behavior of the symmetric tensor order parameter invariants σ (3) and σ (2) calculated from the measured optical birefringences supports that the uniaxial-to-biaxial transitions are of second order, described by a mean-field theory.

Biaxial surface order dynamics in calamitic nematics

Thermotropic nematic materials relax strong distortions by lowering the nematic order: the uniaxial symmetry is broken and is locally replaced by biaxial domains. We investigated the dynamics of the nematic order near a boundary surface of an asymmetric π-cell submitted to an external electric field, close to the electric order reconstruction threshold. An unexpected phenomenon is observed close, but below the threshold: the biaxial order spreads on the surface inducing a consequent bulk topological behaviour equivalent to the splay-bend fast transition allowed by order reconstruction at higher voltage.

Long- and Short-Range Order in the Mesophases of Laterally Substituted Calamitic Mesogens and their Radial Octapodes

The mesomorphic behavior of a calamitic mesogen (4'-undecyloxybiphenyl-4-yl-4-octyloxy-2-(pent-4-en-1-yloxy)benzoate) and of a supermesogenic octapode formed by the side-on attachment of the mesogen to a octasilsesquioxane central core is studied by X-ray diffraction and polarizing optical microscopy. The calamitic compound is found to have a nematic phase that has biaxial domains (cybotactic clusters) of tilted layers throughout its entire temperature range. Domains of analogous structure are also found in both the nematic and the hexagonal columnar mesophases exhibited by the obctapode compound. The spacing of the layers forming the domains is found to have the same, essentially temperature independent value for the calamitic monomer and for the octapode, in both its mesophases. Comparison with compounds of analogous structure shows that this value is determined by the length of the rigid part of the mesogenic unit. Variation of the latter length is shown to have no effect on the size of the hexagonal lattice of the octapode columnar phase or on the stacking distance within the columns. The presence of the biaxial domains in the nematic phase is discussed in connection with the phase biaxiality that has been observed in structurally related tetrapode compounds and the possibility of field induced macroscopic biaxial nematic order.

Bulk and surface biaxiality in nematic liquid crystals

Nematic liquid crystals possess three different phases: isotropic, uniaxial, and biaxial. The ground state of most nematics is either isotropic or uniaxial, depending on the external temperature. Nevertheless, biaxial domains have been frequently identified, especially close to defects or external surfaces. In this paper we show that any spatially varying director pattern may be a source of biaxiality. We prove that biaxiality arises naturally whenever the symmetric tensor S=(Vn)(Vn)T possesses two distinct nonzero eigenvalues. The eigenvalue difference may be used as a measure of the expected biaxiality. Furthermore, the corresponding eigenvectors indicate the directions in which the order tensor Q is induced to break the uniaxial symmetry about the director n. We apply our general considerations to some examples. In particular we show that, when we enforce homeotropic anchoring on a curved surface, the order tensor becomes biaxial along the principal directions of the surface. The effect is triggered by the difference in surface principal curvatures.

A model for the optical properties of oriented molecular layers

This contribution is concerned with developing procedures for extracting information of the mean orientations of transition dipole moments of molecules that make up an ordered molecular layer, e.g. a Langmuir-Blodgett (LB) film, from polarized reflectance spectra. Previous methods are based on approximations, among which is the neglect of or at best approximate account of local field effects. Thus in general the orientation of a transition moment for a molecule imbedded in an anisotropic dielectric medium does not coincide with that of a free molecule with exactly the same orientation and confirmation. In this paper the question of local field effects for such oriented molecular layers is addressed, first in a general way, then for a film made up of randomly oriented biaxial domains, each of which is regular two-dimensional array of polarizable species. On neglecting the interlayer contribution to the local field, this leads to a dichroic ratio that is proportional to the well-known function 1 2 tan 2θ , where θ is the angle between the molecular transition moment and the surface normal, with the proportionality constant being a simple function of the spacing and polarizability of the moieties. The result provides a practical method for calculating θ that should be reasonably accurate for transition moments located on the head groups of many LB films.