129 research outputs found
Sermón de la beata Juana de Aza, madre del santo padre y patriarca Santo Domingo de Guzmán que en el dia primero de las solemnes fiestas del Capítulo General celebrado en el Real Convento de Predicadores de Zaragoza el día 9 de Junio de 1832
Copia digital. Valladolid : Junta de Castilla y León. Consejería de Cultura y Turismo, 2009-201
Interplay of couplings between antiferrodistortive, ferroelectric, and strain degrees of freedom in monodomain PbTiO/SrTiO superlattices
We report first-principles calculations on the coupling between epitaxial
strain, polarization, and oxygen octahedra rotations in monodomain
(PbTiO)/(SrTiO) superlattices. We show how the
interplay between (i) the epitaxial strain and (ii) the electrostatic
conditions, can be used to control the orientation of the main axis of the
system. The electrostatic constrains at the interface facilitate the rotation
of the polarization and, as a consequence, we predict large piezoelectric
responses at epitaxial strains smaller than those that would be required
considering only strain effects. In addition, ferroelectric (FE) and
antiferrodistortive (AFD) modes are strongly coupled. Usual steric arguments
cannot explain this coupling and a covalent model is proposed to account for
it. The energy gain due to the FE-AFD coupling decreases with the periodicity
of the superlattice, becoming negligible for .Comment: 5 pages, 4 figure
Structural and energetic properties of domains in PbTiO 3 /SrTiO 3 superlattices from first principles
We report first-principles calculations, within the density functional theory, on the structural and energetic properties of 180 ∘ stripe domains in (PbTiO 3 ) n /(SrTiO 3 ) n superlattices. For the explored periodicities (n=3 and 6), we find that the polydomain structures compete in energy with the monodomain phases. Our results suggest the progressive transition, as a function of n , from a strong to a weak electrostatic coupling regime between the SrTiO 3 and PbTiO 3 layers. Structurally, they display continuous rotation of polarization connecting 180 ∘ domains. A large offset between [100] atomic rows across the domain wall and huge strain gradients are observed. The domain wall energy is very isotropic, depending very weakly on the stripe orientation.The authors thank P. Zubko for the valuable discussion and
his careful reading of themanuscript. Thisworkwas supported
by the Spanish Ministery of Science and Innovation through
the MICINN Grant FIS2009-12721-C04-02, by the Spanish
Ministry of Education through the FPU fellowship AP2006-
02958 (PAP), and by the European Union through the project
EC-FP7, Grant No. CP-FP 228989-2 “OxIDes.” The authors
thankfully acknowledge the computer resources, technical
expertise, and assistance provided by the Red Española de
Supercomputación. Calculations were also performed at the
ATC group of the University of Cantabria
Effect of intrinsic defects on the thermal conductivity of PbTe from classical molecular dynamics simulations
Despite being the archetypal thermoelectric material, still today some of the
most exciting advances in the efficiency of these materials are being achieved
by tuning the properties of PbTe. Its inherently low lattice thermal
conductivity can be lowered to its fundamental limit by designing a structure
capable of scattering phonons over a wide range of length scales. Intrinsic
defects, such as vacancies or grain boundaries, can and do play the role of
these scattering sites. Here we assess the effect of these defects by means of
molecular dynamics simulations. For this we purposely parametrize a Buckingham
potential that provides an excellent description of the thermal conductivity of
this material over a wide temperature range. Our results show that intrinsic
point defects and grain boundaries can reduce the lattice conductivity of PbTe
down to a quarter of its bulk value. By studying the size dependence we also
show that typical defect concentrations and grain sizes realized in experiments
normally correspond to the bulk lattice conductivity of pristine PbTe
Ferromagneticlike Closure Domains in Ferroelectric Ultrathin Films: First-Principles Simulations
We simulate from first principles the energetic, structural, and electronic properties of ferroelectric domains in ultrathin SrRuO3/BaTiO3/SrRuO3 ferroelectric capacitors in short circuit. The domains are stabilized down to two unit cells at zero temperature, adopting the form of a domain of closure, common in ferromagnetic thin films. The domains are closed by the in-plane relaxation of the atoms in the first SrO layer of the electrode, which behaves more like SrO in highly polarizable SrTiO3 than in metallic SrRuO3. Even if small, these lateral displacements are very important to stabilize the domains and might provide some hints to explain why some systems break into domains while others remain in a monodomain configuration. An analysis of the electrostatic potential reveals preferential points of pinning for charged defects at the ferroelectric-electrode interface, possibly playing a major role in film fatigue
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Two-dimensional electron gas at the PbTi O3/SrTi O3 interface: An ab initio study
In the polar catastrophe scenario, polar discontinuity accounts for the driving force of the formation of a two-dimensional electron gas (2DEG) at the interface between polar and nonpolar insulators. In this paper, we substitute the usual, nonferroelectric, polar material with a ferroelectric thin film and use the ferroelectric polarization as the source for polar discontinuity. We use ab initio simulations to systematically investigate the stability, formation, and properties of the two-dimensional free-carrier gases formed in PbTiO3/SrTiO3 heterostructures under realistic mechanical and electrical boundary conditions. Above a critical thickness, the ferroelectric layers can be stabilized in the out-of-plane monodomain configuration due to the electrostatic screening provided by the free carriers. Our simulations also predict that the system can be switched between three stable configurations (polarization up, down, or zero), allowing the nonvolatile manipulation of the free-charge density and sign at the interface. Furthermore, the link between ferroelectric polarization and free-charge density demonstrated by our analysis constitutes compelling support for the polar catastrophe model that is used to rationalize the formation of 2DEG at oxide interfaces
Lattice screening of the polar catastrophe and hidden in-plane polarization in KNbO3/BaTiO3 interfaces
We have carried out first-principles simulations, based on density functional theory, to obtain the atomic and electronic structure of (001) BaTiO3/KNbO3 interfaces in an isolated slab geometry. We tried different types of structures including symmetric and asymmetric configurations and variations in the thickness of the constituent materials. The spontaneous polarization of the layer-by-layer non-neutral material (KNbO3) in these interfaces cancels out almost exactly the “built-in” polarization responsible for the electronic reconstruction. As a consequence, the so-called polar catastrophe is quenched and all the simulated interfaces are insulating. A model, based on the modern theory of polarization and basic electrostatics, allows an estimation of the critical thickness for the formation of the two-dimensional electron gas between 33 and 36 KNbO3 unit cells. We also demonstrate the presence of an unexpected in-plane polarization in BaTiO3 localized at the p-type TiO2/KO interface, even under in-plane compressive strains. We expect this in-plane polarization to remain hidden due to angular averaging during quantum fluctuations unless the symmetry is broken with small electric fields
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