Landau theory

About: Landau theory is a research topic. Over the lifetime, 2882 publications have been published within this topic receiving 57078 citations.

Spontaneous strain and the ferroelastic phase transition in As2O5

Abstract: The tetragonal-orthorhombic phase transition in arsenic pentoxide has been studied by X-ray powder diffraction and is found to be a proper ferroelastic transition. The spontaneous strain behaves as the order parameter following Landau theory with epsilon s varies as mod Tc-T mod beta , beta =1/2, Tc=578 K. The order parameter susceptibility has been determined from line broadening and follows mod Tc-T mod -y, gamma =1. No deviation from Landau behavior has been observed experimentally.

Landau theory for biaxial nematic liquid crystals with two order parameter tensors

Abstract: We present a phenomenological theory for the homogeneous phases of nematic liquid crystals constituted by biaxial molecules. We propose a general polynomial potential in two macroscopic order parameter tensors that reproduces the mean-field phase diagram confirmed by Monte Carlo simulations [De Matteis et al. in Phys Rev E 72:041706 (2005)] and recently recognized to be universal [Bisi et al. in Phys Rev E 73:051709 (2006)] for dispersion force molecular pair-potentials enjoying the D 2h symmetry. The requirement that the phenomenological theory comply uniquely with this phase diagram reduces considerably the admissible phenomenological coefficients, both in their number and in the ranges where they can vary.

Mechanism of ferroelectric phase transition in ammonium Rochelle salt

Abstract: The phase transition in ammonium Rochelle salt is interpreted from the group-theoretical point of view following the Landau theory of phase transition and shown to be improper since it is induced by a two-dimensional representation at k =(1/2) b 1 which correctly describes the observed phase change from the space group D 2 3 to C 2 2 . The expression of the free energy is given in terms of the doubly-degenerate transition parameter, besides the macroscopic polarization and deformation.

Harmonic corrections to the mean‐field phase diagram for block copolymers

Abstract: Allowance is made for harmonic corrections within mean‐field theory for weakly segregated block copolymers. The theory of Marques and Cates [C. M. Marques and M. E. Cates, Europhys. Lett. 13, 267 (1990)] for the stability of phases near the ordering transition is applied to diblock copolymer melts. A wave‐vector dependence of the coefficients in the Landau mean‐field free energy is included. We find that the face centered cubic (fcc) and square phases predicted by Marques and Cates to be stable near the ordering transition depending on the structure factor for the system are not stable for diblock copolymers. In addition, hexagonal close packed (hcp) and ordered bicontinuous double diamond (OBDD) structures are found to be unstable with respect to the classical lamellar, cylinder, and body centered cubic phases. The hexagonal intermediate phases and complex bicontinuous structures recently observed near the order–disorder transition are not accounted for in this theory.

Theory of the Splitting of Discommensurations in the Charge-Density-Wave State of 2 H - TaSe 2

Abstract: The commensurate charge-density-wave state of $2H$-${\mathrm{TaSe}}_{2}$ can have hexagonal ($H$) or orthorhombic ($O$) symmetry. With use of a Landau theory it is shown that near the boundary between $H$ and $O$ states a discommensuration splits into two $H\ensuremath{-}O$ interfaces and thereby greatly lowers its energy. This leads to a large suppression of the incommensurate-commensurate transition temperature and a reentrant commensurate state under pressure.