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Landau theory
About: Landau theory is a research topic. Over the lifetime, 2882 publications have been published within this topic receiving 57078 citations.
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TL;DR: In this paper, a detailed analysis of polarization reversal in ferroelectrics has been performed, in the framework of the Landau model for phase transitions, in order to integrate the size effects and inhomogeneity contribution to switching of the global order parameter.
Abstract: A detailed analysis of polarization reversal in ferroelectrics has been performed, in the framework of the Landau model for phase transitions. Some important characteristics of homogeneous switching have been emphasized and later used in studying the more general case of inhomogeneous switching. The two extremes of switching current correspond to the inflexion points of the dielectric hysteresis loop. Hysteresis loops of poled ferroelectric samples are expected to include negative-susceptibility regions, for high-frequency applied electric fields. The switching current minimum is eliminated by the experimental method used for recording the switching responses. Equivalent Landau coefficients and electric fields have been defined, in order to integrate the size effects and inhomogeneity contribution to switching of the global order parameter. We correlated the size effects on the critical parameters of the switching (the coercive field) and the ferroelectric-to-paraelectric phase transition (the Curie temperature). Polarization reversal in small-size ferroelectrics can be regarded as a diffuse phase transition, whereas its character is closer to normal for large-size samples. The size dependencies of the reversal speed and maximum current result from the size dependencies of the equivalent Landau coefficients and electric field inducing reversal.
118 citations
Posted Content•
TL;DR: It is shown that structural vortices induce magnetic ones and that, consistent with a recent experimental report, ferroelectric domain walls can carry a magnetic moment.
Abstract: Topological defects in ordered states with spontaneously broken symmetry often have unusual physical properties, such as fractional electric charge or a quantised magnetic field-flux, originating from their non-trivial topology. Coupled topological defects in systems with several coexisting orders give rise to unconventional functionalities, such as the electric-field control of magnetisation in multiferroics resulting from the coupling between the ferroelectric and ferromagnetic domain walls. Hexagonal manganites provide an extra degree of freedom: In these materials, both ferroelectricity and magnetism are coupled to an additional, non-ferroelectric structural order parameter. Here we present a theoretical study of topological defects in hexagonal manganites based on Landau theory with parameters determined from first-principles calculations. We explain the observed flip of electric polarisation at the boundaries of structural domains, the origin of the observed discrete vortices, and the clamping between ferroelectric and antiferromagnetic domain walls. We show that structural vortices induce magnetic ones and that, consistent with a recent experimental report, ferroelectric domain walls can carry a magnetic moment.
117 citations
Book•
07 Jun 2004
TL;DR: Superconductivity: A brief overview of superconductivity can be found in this paper, where the authors present a diagram of the superconducting phase transition of a supercondensate.
Abstract: Preface.Acknowledgements.I: BASIC TOPICS.1. What is superconductivity? A brief overview.2. Superconducting materials.3. Fermi-liquids and attractive interactions.4.The superconducting state. An electronic condensate.5. Weak Links and Josephson Effects.6. London Approximation to Ginzburg-Landau Theory (constant).7. Applications of Ginzburg- Landau Theory (spatially varying).8. More on the Flux-line System.II: ADVANCED TOPICS.9. Two-dimensional superconductivity. Vortex-paired unbinding.10. Dual description of the superconducting phase transition.III: SELECTED APPLICATIONS.11. Small scale applications.12. Superconducting Wire and Cable Technology.IV: TOPICAL CONTRIBUTIONS.13. Topical Contributions.V: HISTORICAL NOTES.Historical notes on supeconductivity: the Nobel Laureates.References.Author Index.Subject Index.
117 citations
TL;DR: By applying the Landau theory, the appearance of the antiferroelectric phase in smectic liquid crystals and the double hysteresis loop are interpreted Several types of phase sequences are obtained phenomenologically.
Abstract: By applying the Landau theory, the appearance of the antiferroelectric phase in smectic liquid crystals and the double hysteresis loop are interpreted Several types of phase sequences including the antiferroelectric, ferroelectric and/or ferrielectric phase are obtained phenomenologically The D-E curves in the antiferroelectric phase are classified into three types depending on temperature and the coefficients of the fourth-order terms in the free energy
116 citations
TL;DR: In this article, the authors derived thermodynamically the free energy functional of a ferroelectric under an applied field E that contains the depolarization field Ed. They found that it is crucial to recognize the relationship between the total polarization and the applied field due to the degradations of the field, and that this relationship effectively double-counts the field and thus causes serious overestimation of its effects.
Abstract: We derived thermodynamically from first principles the free energy functional of a ferroelectric under an applied field E that contains the depolarization field Ed. We found that it is crucial to recognize the relationship between the total polarization and the applied field due to the depolarization field when expressing the free energy functional in the Landau form. Ad hoc formulations of ferroelectric models that overlooked this relationship effectively double-count the depolarization field, and thus cause serious overestimation of its effects. Examples are given in which the Curie temperature and the critical thickness of ferroelectricity, etc., can be overestimated this way by orders of magnitude.
115 citations