Search PubMed⌕ Search

SEARCH · Search PubMed

Results for “FRUSTRATION”

Search indexed PubMed citations on genomics, clinical trials, systematic reviews and public health. Explore titles, authors and supplied subject terms, then open the PubMed record.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 1,117 records · Page 62Linked to original sources
PubMed1999

Looped finger transformation in frustrated cholesteric liquid crystals.

Localized structures named "fingers" form in the vicinity of the unwinding transition of a cholesteric liquid crystal subjected to an electric field and to homeotropic boundary conditions. Several types of fingers exist, with different static and dynamic properties. For instance, cholesteric fingers of the second species (CF-2) can drift perpendicular to their axes and form spirals in ac electric fields, whereas fingers of the first species (CF-1) crawl along their axes. In this article we show that CF-2's are much easier to nucleate in thick samples (with respect to the pitch) than in thin ones and may form loops like the CF-1's, with or without defects. We show that looped CF-1's always collapse in thick samples at increasing voltage, whereas they can form cholesteric bubbles in thin samples. By contrast, we never observe the formation of a bubble from a loop of a CF-2 except when it possesses a point defect. We also recall that CF-1 segments always collapse at increasing voltage, whereas CF-2 segments systematically give cholesteric bubbles in similar conditions. To qualitatively explain these transformations, we use a simplified representation on the unit sphere S2 of the director field within the fingers. While the CF-1's are described within the standard model of Press and Arrot, we use for the CF-2's a recent model of Gil and Gilli, which we prove to explain most observations. We also describe the growth and collapse dynamics of a loop of a CF-2 in close connection with the spiral dynamics. Finally, we show experimentally and numerically that the CF-2's get abruptly thinner when the electric field exceeds the spinodal limit of the CF-1's. This transformation is reversible, but strongly hysteretic.

Journal Article↗
PubMed2002

Critical behavior of the frustrated antiferromagnetic six-state clock model on a triangular lattice.

We study the antiferromagnetic six-state clock model with nearest neighbor interactions on a triangular lattice with extensive Monte Carlo simulations. We find clear indications of two phase transitions at two different temperatures: Below T(I) a chirality order sets in and by a thorough finite-size-scaling analysis of the specific heat and the chirality correlation length we show that this transition is in the Ising universality class (with a nonvanishing chirality order parameter below T(I)). At T(KT) (<T(I)) the spin-spin correlation length as well as the spin susceptibility diverges according to a Kosterlitz-Thouless (KT) form and spin correlations decay algebraically below T(KT). We compare our results to recent x-ray diffraction experiments on the orientational ordering of CF3Br monolayers physisorbed on graphite. We argue that the six-state clock model describes the universal feature of the phase transition in the experimental system and that the orientational ordering belongs to the KT universality class.

Journal Article↗
PubMed2003

Order reconstruction in frustrated nematic twist cells.

Within the Landau-de Gennes theory of liquid crystals, we study the equilibrium configurations of a nematic cell with twist boundary conditions. Under the assumption that the order tensor Q be uniaxial on both bounding plates, we find three separate classes of solutions, one of which contains the absolute energy minimizer, a twistlike solution that exists for all values of the distance d between the plates. The solutions in the remaining two classes exist only if d exceeds a critical value d(c). One class consists of metastable, twistlike solutions, while the other consists of unstable, exchangelike solutions, where the eigenvalues of Q are exchanged across the cell. When d=d(c), the metastable solution relaxes back to the absolute energy minimizer, undergoing an order reconstruction somewhere within the cell. The critical distance d(c) equals, in general, a few biaxial coherence lengths. This scenario applies to all the values of the boundary twist but pi/2, which thus appears as a very special case, though it is the one more studied in the literature. In fact, when the directors prescribed on the two plates are at right angles, two symmetric twistlike solutions merge continuously into an exchangelike solution at the critical value of d where the latter becomes unstable. Our analysis shows how the classical bifurcation associated with this phenomenon is unfolded by perturbing the boundary conditions.

Journal Article↗
PubMed2005

Discrete flexoelectric polarizations and biaxial subphases with periodicities other than three and four layers in chiral smectic liquid crystals frustrated between ferroelectricity and antiferroelectricity.

The subphase for the temperature range that lies in between Sm-C(A)* and the three-layer Sm-C(A)* (1/3)subphase has been confirmed to exist using the measurements of electric-field-induced birefringence, optical rotation, and the characteristic reflection bands in the antiferroelectric liquid-crystalline compound, 1-trifluoromethylundecyl-4-(4'-dodecyloxybiphenyl-4-yl-carbonyloxy)-3-fluorobenzoate (12BIMF10). The measurements of electric-field-induced birefringence and optical rotatory power are made on -thick homeotropic cells, and the characteristic reflection bands are observed in free-standing films of thicknesses ranging from 30 to 50 microm. Several binary mixtures have been prepared by mixing (S)-12BIMF10 with (S)-4-(1-methylheptyloxycarbonyl)-phenyl-4'-octylbiphenyl-4-carboxylate (MHPBC), and the effect of racemization of the compound on the character of the biaxial subphase (other than three and four layers) is also discussed. The results are interpreted in terms of the Emelyanenko-Osipov model [Phys. Rev. E 68, 051703 (2003)]; the effective long-range couplings between the director orientations in separated smectic layers emerge after the minimization of free energy with respect to the total (ordinary spontaneous and discrete flexoelectric) polarizations, lifting the degeneracy and producing the nonplanar structures of the subphases.

Journal Article↗
PubMed2006

Simplifying random satisfiability problems by removing frustrating interactions.

How can we remove some interactions (generate shorter clauses) in a constraint satisfaction problem (CSP) such that it still remains satisfiable? In this paper we study a modified survey propagation algorithm that enables us to address this question for a prototypical CSP, i.e., random K-satisfiability problem. The average number of removed interactions is controlled by a tuning parameter in the algorithm. If the original problem is satisfiable then we are able to construct satisfiable subproblems ranging from the original one to a minimal one with minimum possible number of interactions. The minimal satisfiable subproblems will directly provide the solutions of the original problem.

Journal Article↗