192 research outputs found
Supercritical Conversion Of The 3rd Blue Phase To The Isotropic-Phase In A Highly Chiral Liquid-Crystal
The results of two independent experiments in the vicinity of the “transition” from the third blue phase ( BPIII) to isotropic phase ( I) are reported for a highly chiral liquid crystal. Heat capacity measurements using a high-resolution calorimeter and dynamic light-scattering measurements using circularly polarized light have been performed. The data show a continuous evolution of BPIII into I with no critical fluctuations. This is strong evidence that the BPIII-I transition in this compound is supercritical, indicating that the BPIII and I phases possess the same macroscopic symmetry
Determination of optimal reversed field with maximal electrocaloric cooling by a direct entropy analysis
Application of a negative field on a positively poled ferroelectric sample
can enhance the electrocaloric cooling and appears as a promising method to
optimize the electrocaloric cycle. Experimental measurements show that the
maximal cooling does not appear at the zero-polarization point, but around the
shoulder of the P-E loop. This phenomenon cannot be explained by the theory
based on the constant total entropy assumption under adiabatic condition. In
fact, adiabatic condition does not imply constant total entropy when
irreversibility is involved. A direct entropy analysis approach based on work
loss is proposed in this work, which takes the entropy contribution of the
irreversible process into account. The optimal reversed field determined by
this approach agrees with the experimental observations. This study signifies
the importance of considering the irreversible process in the electrocaloric
cycles
History-Dependent Patterns in Randomly Perturbed Nematic Liquid Crystals
We study the characteristics of nematic structures in a randomly perturbed nematic liquid crystal (LC) phase. We focus on the impact of the samples history on the universal behavior. The obtained results are of interest for every randomly perturbed system exhibiting a continuous symmetry-breaking phase transition. A semimicroscopic lattice simulation is used where the LC molecules are treated as cylindrically symmetric, rod-like objects interacting via a Lebwohl-Lasher (LL) interaction. Pure LC systems exhibit a first order phase transition into the orientationally ordered nematic phase at T=Tc on lowering the temperature T. The orientational ordering of LC molecules is perturbed by the quenched, randomly distributed rod-like impurities of concentration p. Their orientation is randomly distributed, and they are coupled with the LC molecules via an LL-type interaction. Only concentrations below the percolation threshold are considered. The key macroscopic characteristics of perturbed LC structures in the symmetry-broken nematic phase are analyzed for two qualitatively different histories at T≪Tc. We demonstrate that, for a weak enough interaction among the LC molecules and impurities, qualitatively different history-dependent states could be obtained. These states could exhibit either short-range, quasi-long-range, or even long-range order
Quasi-long range order in glass states of impure liquid crystals, magnets, and superconductors
In this review we consider glass states of several disordered systems:
vortices in impure superconductors, amorphous magnets, and nematic liquid
crystals in random porous media. All these systems can be described by the
random-field or random-anisotropy O(N) model. Even arbitrarily weak disorder
destroys long range order in the O(N) model. We demonstrate that at weak
disorder and low temperatures quasi-long range order emerges. In
quasi-long-range-ordered phases the correlation length is infinite and
correlation functions obey power dependencies on the distance. In pure systems
quasi-long range order is possible only in the lower critical dimension and
only in the case of Abelian symmetry. In the presence of disorder this type of
ordering turns out to be more common. It exists in a range of dimensions and is
not prohibited by non-Abelian symmetries.Comment: 32 page
Evidence For A Supercritical ‘Transition’ To The Isotropic Phase In A Highly Chiral Liquid Crystal
The results of two independent experiments in the vicinity of the “transition” from the third blue phase (BPIII) to isotropic phase (I) are reported for a highly chiral liquid crystal. Heat capacity measurements using a high-resolution calorimeter and dynamic light-scattering measurements using circularly polarized light have been performed. The data show a continuous evolution of BPIII into I with no critical fluctuations. This is strong evidence that the BPIII-I transition in this compound is supercritical, indicating that the BPIII and I phases possess the same macroscopic symmetry
Effective index of refraction, optical rotation, and circular dichroism in isotropic chiral liquid crystals
This paper concerns optical properties of the isotropic phase above the
isotropic-cholesteric transition and of the blue phase BP III. We introduce an
effective index, which describes spatial dispersion effects such as optical
rotation, circular dichroism, and the modification of the average index due to
the fluctuations. We derive the wavelength dependance of these spatial
dispersion effects quite generally without relying on an expansion in powers of
the chirality and without assuming that the pitch of the cholesteric is
much shorter than the wavelength of the light , an approximation which
has been made in previous studies of this problem. The theoretical predictions
are supported by comparing them with experimental spectra of the optical
activity in the BP III phase.Comment: 15 pages and 7 figures. Submitted to PR
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