10 research outputs found
Origin of the orbital and spin orderings in rare-earth titanates
Rare-earth titanates RTiO are Mott insulators displaying a rich physical
behavior, featuring most notably orbital and spin orders in their ground state.
The origin of their ferromagnetic to antiferromagnetic transition as a function
of the size of the rare-earth however remains debated. Here we show on the
basis of symmetry analysis and first-principles calculations that although
rare-earth titanates are nominally Jahn-Teller active, the Jahn-Teller
distortion is negligible and irrelevant for the description of the ground state
properties. At the same time, we demonstrate that the combination of two
antipolar motions produces an effective Jahn-Teller-like motion which is the
key of the varying spin-orbital orders appearing in titanates. Thus, titanates
are prototypical examples illustrating how a subtle interplay between several
lattice distortions commonly appearing in perovskites can produce orbital
orderings and insulating phases irrespective of proper Jahn-Teller motions.Comment: Accepted in Physical Review
Energetics of oxygen-octahedra rotations in perovskite oxides from first principles
We use first-principles methods to study oxygen-octahedra rotations in ABO3
perovskite oxides. We focus on the short-period, perfectly antiphase or
in-phase, tilt patterns that characterize most compounds and control their
physical (e.g., conductive, magnetic) properties. Based on an analytical form
of the relevant potential energy surface, we discuss the conditions for the
stability of polymorphs presenting different tilt patterns, and obtain
numerical results for a collection of thirty-five representative materials. Our
results reveal the mechanisms responsible for the frequent occurrence of a
particular structure that combines antiphase and in-phase rotations, i.e., the
orthorhombic Pbnm phase displayed by about half of all perovskite oxides and by
many non-oxidic perovskites. The Pbnm phase benefits from the simultaneous
occurrence of antiphase and in-phase tilt patterns that compete with each
other, but not as strongly as to be mutually exclusive. We also find that
secondary antipolar modes, involving the A cations, contribute to weaken the
competition between different tilts and play a key role in their coexistence.
Our results thus confirm and better explain previous observations for
particular compounds. Interestingly, we also find that strain effects, which
are known to be a major factor governing phase competition in related (e.g.,
ferroelectric) perovskite oxides, play no essential role as regards the
relative stability of different rotational polymorphs. Further, we discuss why
the Pbnm structure stops being the ground state in two opposite limits, for
large and small A cations, showing that very different effects become relevant
in each case. Our work thus provides a comprehensive discussion on these
all-important and abundant materials, which will be useful to better understand
existing compounds as well as to identify new strategies for materials
engineering
Imaging flux distributions around superconductors: Geometrical susceptibility in the Meissner state
Under the terms of the Creative Commons Attribution 3.0 Unported License.Experiment and analytical calculations show that the demagnetizing field of a superconductor is a sensitive probe of quantities otherwise difficult to measure, such as the sample-probe distance in flux-density imaging experiments and the field of first flux penetration Hp. In particular, the ratio of the maximum field measured above the superconductor edge and the applied field can be determined unambiguously so as to define a linear >geometric> susceptibility. The evolution of this susceptibility with field depends on the regime of flux penetration and can be used as a means to determine Hp and the effect of a parallel field component in magneto-optical imaging experiments. © 2013 AIP Publishing LLC.A. Badía acknowledges funding by the Spanish MINECO and the European FEDER Program (Projects MAT2011-22719, ENE2011-29741) and by Gobierno de Aragón (Research group T12). We are grateful to D. Colson and A. Forget of the CEA-IRAMIS-SPEC for providing the Ba(Fe0.925Co0.075)2As2 crystals. C. J. van der Beek acknowledges support from the MagCorPnic grant of the Triangle de la Physique du Plateau de Saclay.Peer Reviewe
Imaging flux distributions around superconductors: Geometrical susceptibility in the Meissner state
Metal-insulator-transition engineering by modulation tilt-control in perovskite nickelates for room temperature optical switching
Metal–insulator-transition engineering by modulation tilt-control in perovskite nickelates for room temperature optical switching
Cohort profile. the ESC-EORP chronic ischemic cardiovascular disease long-term (CICD LT) registry
The European Society of cardiology (ESC) EURObservational Research Programme (EORP) Chronic Ischemic Cardiovascular Disease registry Long Term (CICD) aims to study the clinical profile, treatment modalities and outcomes of patients diagnosed with CICD in a contemporary environment in order to assess whether these patients at high cardiovascular risk are treated according to ESC guidelines on prevention or on stable coronary disease and to determine mid and long term outcomes and their determinants in this population