705 research outputs found
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. PajÄ…k 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
Structure Constant of the Yang-Lee Edge Singularity
This paper studies the Yang-Lee singularity of the 2-dimensional Ising model
on the cylinder via transfer matrix and finite-size scaling techniques. These
techniques enable a measurement of the 2-point and 3-point correlations and a
comparison of a measurement of a corresponding universal amplitude with a
prediction for the amplitude from the (A4,A1) minimal conformal field theory.Comment: 1 figur
Excitation Spectrum at the Yang-Lee Edge Singularity of 2D Ising Model on the Strip
At the Yang-Lee edge singularity, finite-size scaling behavior is used to
measure the low-lying excitation spectrum of the Ising quantum spin chain for
free boundary conditions. The measured spectrum is used to identify the CFT
that describes the Yang-Lee edge singularity of the 2D Ising model for free
boundary conditions.Comment: 7 pages, 1 figur
Stability of Biaxial Nematic Phase for Systems with Variable Molecular Shape Anisotropy
We study the influence of fluctuations in molecular shape on the stability of
the biaxial nematic phase by generalizing the mean field model of Mulder and
Ruijgrok [Physica A {\bf 113}, 145 (1982)]. We limit ourselves to the case when
the molecular shape anisotropy, represented by the alignment tensor, is a
random variable of an annealed type. A prototype of such behavior can be found
in lyotropic systems - a mixture of potassium laurate, 1-decanol, and ,
where distribution of the micellar shape adjusts to actual equilibrium
conditions. Further examples of materials with the biaxial nematic phase, where
molecular shape is subject to fluctuations, are thermotropic materials composed
of flexible trimeric- or tetrapod-like molecular units. Our calculations show
that the Gaussian equilibrium distribution of the variables describing
molecular shape (dispersion force) anisotropy gives rise to new classes of the
phase diagrams, absent in the original model. Depending on properties of the
shape fluctuations, the stability of the biaxial nematic phase can be either
enhanced or depressed, relative to the uniaxial nematic phases. In the former
case the splitting of the Landau point into two triple points with a direct
phase transition line from isotropic to biaxial phase is observed.Comment: 18 pages containing 6 figure
Critical Excitation Spectrum of Quantum Chain With A Local 3-Spin Coupling
This article reports a measurement of the low-energy excitation spectrum
along the critical line for a quantum spin chain having a local interaction
between three Ising spins and longitudinal and transverse magnetic fields. The
measured excitation spectrum agrees with that predicted by the (D, A)
conformal minimal model under a nontrivial correspondence between translations
at the critical line and discrete lattice translations. Under this
correspondence, the measurements confirm a prediction that the critical line of
this quantum spin chain and the critical point of the 2D 3-state Potts model
are in the same universality class.Comment: 7 pages, 2 figure
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