Low temperature dielectric relaxation in ordinary perovskite ferroelectrics: enlightenment from high-energy x-ray diffraction

Ordinary ferroelectrics exhibit a second order phase transition that is characterized by a sharp peak in the dielectric permittivity at a frequency-independent temperature. Furthermore, these materials show a low temperature dielectric relaxation that appears to be a common behavior of perovskite systems. Tetragonal lead zirconate titanate is used here as a model system in order to explore the origin of such an anomaly, since there is no consensus about the physical phenomenon involved in it. Crystallographic and domain structure studies are performed from temperature dependent synchrotron x-ray diffraction measurement. Results indicate that the dielectric relaxation cannot be associated with crystallographic or domain configuration changes. The relaxation process is then parameterized by using the Vogel–Fulcher–Tammann phenomenological equation. Results allow us to hypothesize that the observed phenomenon is due to changes in the dynamic behavior of the ferroelectric domains related to the fluctuation of the local polarization.Postprint (author's final draft

Interaction driven phases in the honeycomb lattice from exact diagonalization

We investigate the fate of interaction driven phases in the half-filled honeycomb lattice for finite systems via exact diagonalization with nearest and next nearest neighbour interactions. We find evidence for a charge density wave phase, a Kekul\'e bond order and a sublattice charge modulated phase in agreement with previously reported mean-field phase diagrams. No clear sign of an interaction driven Chern insulator phase (Haldane phase) is found despite being predicted by the same mean-field analysis. We characterize these phases by their ground state degeneracy and by calculating charge order and bond order correlation functions.Comment: 7 pages, 6 figures, updated reference

Evolución del aprovechamiento del agua y contaminación por sales y nitratos en un regadío tradicional. El caso de la Comunidad nº V de Riegos de Bardenas (Zaragoza)

Los entornos agrícolas tienden a la implantación de nuevos sistemas de gestión sin tener en cuenta la respuesta agroambiental de los cambios efectuados. Este trabajo analiza la evolución de la eficiencia de riego y el impacto agroambiental de la cuenca de regadío tradicional C-XIX-6 (95 ha) en la Comunidad de Regantes nº V de Bardenas (Zaragoza) entre los años 2001 y el periodo 2005-2008 (cambios en la gestión del riego con i. asignación de dotaciones, ii. riego a la demanda, y iii. facturación por consumo) mediante el desarrollo de balances anuales de agua sales y nitrógeno. Tras los cambios, la eficiencia de riego se incrementó un 27%, reduciéndose la masa de sales y nitrato exportadas en un 63% y un 60% respectivamente. Los índices de contaminación por sales y nitratos disminuyeron un 70% y un 24%, resultando más efectivos los cambios en la reducción del impacto salino que por nitrato

Insight into the dynamics of low temperature dielectric relaxation of ordinary perovskite ferroelectrics

The temperature dependence of the dielectric response of ordinary ferroelectric materials exhibits a frequency-independent anomalous peak as a manifestation of the ferroelectric to paraelectric phase transition. A second anomaly in the permittivity has been reported in different ferroelectric perovskite-type systems at low temperatures, often at cryogenic temperatures. This anomaly manifests as a frequency-dependent local maximum, which exhibits similar characteristics to that observed in relaxor ferroelectrics around their phase transition. The origin of this unexpected behavior is still controversial. In order to clarify this phenomenon, a model-free route solution is developed in this work. Our findings reveal the same critical linear pattern/glass-like freezing behavior previously observed for glass-forming systems. Contrary to current thought, our results suggest that a critical-like dynamic parameterization could provide a more appropriate solution than the conventional Vogel–Fulcher–Tammann equation. The implemented methodology may open a new pathway for analyzing relaxation phenomena in other functional materials like relaxor ferroics.Postprint (published version

The generalized Vogel-Fulcher-Tamman equation for describing the dynamics of relaxor ferroelectrics

Relaxor ferroelectrics (RF) are outstanding materials owing to their extraordinary dielectric, electromechanical, and electro-optical properties. Although their massive applications, they remain to be one of the most puzzling solid-state materials because understanding their structural local order and relaxation dynamics is being a long-term challenge in materials science. The so-called Vogel-Fulcher-Tamman (VFT) relation has been extensively used to parameterize the relaxation dynamics in RF, although no microscopic description has been firmly established for such empirical relation. Here, we show that VFT equation is not always a proper approach for describing the dielectric relaxation in RF. Based on the Adam-Gibbs model and the Grüneisen temperature index, a more general equation to disentangle the relaxation kinetic is proposed. This approach allows to a new formulation for the configurational entropy leading to a local structural heterogeneity related order parameter for RF. A new pathway to disentangle relaxation phenomena in other relaxor ferroics could have opened.Postprint (published version

On quadratic Hom-Lie algebras with equivariant twist maps and their relationship with quadratic Lie algebras

Hom-Lie algebras having non-invertible and equivariant twist maps are studied. Central extensions of Hom-Lie algebras having these properties are obtained and shown how the same properties are preserved. Conditions are given so that the produced central extension has an invariant metric with respect to its Hom-Lie product making its twist map self-adjoint when the original Hom-Lie algebra has such a metric. This work is focused on algebras with these properties and we call them quadratic Hom-Lie algebras. It is shown how a quadratic Hom-Lie algebra gives rise to a quadratic Lie algebra and that the Lie algebra associated to the given Hom-Lie central extension is a Lie algebra central extension of it. It is also shown that if the 2-cocycle associated to the central extension is not a coboundary, there exists a non-abelian and non-associative algebra, the commutator of whose product is precisely the Hom-Lie product of the Hom-Lie central extension. Moreover, the algebra whose commutator realizes this Hom-Lie product is shown to be simple if the associated Lie algebra is nilpotent. Non-trivial examples are provided