Landau theory

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

Theoretical investigation of magnetoelectric effects in Ba 2 CoGe 2 O 7

Abstract: A joint theoretical approach, combining macroscopic symmetry analysis with microscopic methods (density functional theory and a model cluster Hamiltonian), is employed to shed light on magnetoelectricity in Ba${}_{2}$CoGe${}_{2}$O${}_{7}$. The recently reported experimental behavior of polarization guided by a magnetic field [Murakawa et al., Phys. Rev. Lett. 105, 137202 (2010)] can be predicted on the basis of phenomenological Landau theory. On the microscopic side, two main ingredients are needed for the cross-coupling between magnetic and dipolar degrees of freedom: on-site spin-orbit coupling and the spin-dependent hybridization between O-$p$ and transition metal $d$ states. Structural constraints related to the noncentrosymmetric symmetry and the particular configuration of CoO${}_{4}$ tetrahedrons provide additional features for a peculiar magnetoelectricity to develop.

Theory of magnetic phases of hexagonal rare earth manganites

Abstract: The magnetic phases of hexagonal perovskites RMnO3 (R = Ho, Er, Tm, Yb, Sc, Y) are analysed using group theory and the Landau theory of phase transitions. The competition between various magnetic order parameters is discussed in the context of antiferromagnetic interactions. A phenomenological model based on four one-dimensional magnetic order parameters is developed and studied numerically. It is shown that coupling of the various order parameters leads to a complex magnetic field–temperature phase diagram, and the results are compared to other experiments.

Orientational phase transitions and the assembly of viral capsids.

Abstract: We present a Landau theory for large-l orientational phase transitions and apply it to the assembly of icosahedral viral capsids. The theory predicts two distinct types of ordering transitions. Transitions dominated by the l=6,10,12, and 18 icosahedral spherical harmonics resemble robust first-order phase transitions that are not significantly affected by chirality. The remaining transitions depend essentially on including mixed l states denoted as l=15+16 corresponding to a mixture of l=15 and l=16 spherical harmonics. The l=15+16 transition is either continuous or weakly first-order and it is strongly influenced by chirality, which suppresses spontaneous chiral symmetry breaking. The icosahedral state is in close competition with states that have tetrahedral, D_{5}, and octahedral symmetries. We present a group-theoretic method to analyze the competition between the different symmetries. The theory is applied to a variety of viral shells.

Discontinuous Euler instability in nanoelectromechanical systems

Abstract: We investigate nanoelectromechanical systems near mechanical instabilities. We show that, quite generally, the interaction between the electronic and the vibronic degrees of freedom can be accounted for essentially exactly when the instability is continuous. We apply our general framework to the Euler buckling instability and find that the interaction between electronic and vibronic degrees of freedom qualitatively affects the mechanical instability, turning it into a discontinuous one in close analogy with tricritical points in the Landau theory of phase transitions.