2,816 research outputs found
Modulated nematic structures induced by chirality and steric polarization
What kind of one-dimensional modulated nematic structures (ODMNS) can form
nonchiral and chiral bent-core and dimeric materials? Here, using
Landau-deGennes theory of nematics, extended to account for molecular steric
polarization, we study a possibility of formation of ODMNS, both in nonchiral
and intrinsically chiral liquid crystalline materials. Besides nematic and
cholesteric phases, we find four bulk ODMNS for nonchiral materials, two of
which have not been reported so far. These new structures are longitudinal
() and transverse () periodic waves where the polarization
field being periodic in one dimension stays parallel and perpendicular,
respectively, to the wave vector. The other two phases have all characteristic
features of the twist-bend nematic phase () and the splay-bend nematic
phase (), but their fine structure appears more complex than that
considered so far. The presence of molecular chirality converts nonchiral
and into new phases. Interestingly, the nonchiral
phase can stay stable even in the presence of intrinsic molecular
chirality. Exemplary phase diagrams provide further insights into the relative
stability of these new modulated nematic structures.Comment: 5 pages, 5 figure
Tetrahedratic mesophases, chiral order, and helical domains induced by quadrupolar and octupolar interactions
We present an exhaustive account of phases and phase transitions that can be stabilized in the recently introduced generalized Lebwohl-Lasher model with quadrupolar and octupolar microscopic interactions [ L. Longa, G. Pająk and T. Wydro Phys. Rev. E 79 040701 (2009)]. A complete mean-field analysis of the model, along with Monte Carlo simulations allows us to identify four distinct classes of the phase diagrams with a number of multicritical points where, in addition to the standard uniaxial and biaxial nematic phases, the other nematic like phases are stabilized. These involve, among the others, tetrahedratic (T), nematic tetrahedratic (NT), and chiral nematic tetrahedratic (NT*) phases of global Td, D2d, and D2 symmetry, respectively. Molecular order parameters and correlation functions in these phases are determined. We conclude with generalizations of the model that give a simple molecular interpretation of macroscopic regions with opposite optical activity (ambidextrous chirality), observed, e.g., in bent-core systems. An estimate of the helical pitch in the NT* phase is also given
Identidades colectivas y nuevas demandas sociales : Las generaciones políticas y la cuestión de género en el Frente Popular Darío Santillán
Fil: Longa, Francisco. Universidad Nacional de La Plata. Facultad de Humanidades y Ciencias de la Educación; Argentina
Beads and warriors : the cemetery at Hagar el-Beida 2 (Sudan)
A complex of tumuli cemeteries dating to the first centuries AD was discovered in the
vicinity of Hagar el-Beida village located in the region of the Fourth Nile Cataract. In two seasons
in 2005, 24 tombs were explored on one of these sites (HB2). The bodies of the deceased were
richly equipped, including bracelets and finger rings, but primarily strings of beads wound around
the necks and hips. The beads were made of a variety of materials: faience, various stones, glass and
ostrich eggshell
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