Un enfoque conductual sobre "datos fenomenológicos"

Parker E. Lichtenstein: Psicólogo de nacionalidad norteamericana. Nació en 1915 y murió en 2011. Estudió en la Universidad de Indiana. Empezó su carrera como profesor universitario en la Universidad de Denison, en Granville, Ohio. Para el momento de la publicación en español del trabajo aquí presentado, impartía clases en Newark, Ohio. Fue editor y miembro del consejo editorial de la revista "The Psychological Record"."Un enfoque conductual sobre «datos fenomenológicos»" es la traducción, hecha por el Dr. Enerio Rodríguez Arias, del artículo de Parker E. Lichtenstein titulado "Behavioral Approach to «phenomenological data»", el cual fue publicado originalmente en 1971. En este texto se evalúa la importancia del reciente renacimiento de la fenomenología poniendo énfasis tanto en sus fuerzas como en sus debilidades. Se hace un esfuerzo por colocar a la fenomenología dentro del contexto de una psicología conductual ampliada

On the mechanism for orbital-ordering in KCuF3

The Mott insulating perovskite KCuF3 is considered the archetype of an orbitally-ordered system. By using the LDA+dynamical mean-field theory (DMFT) method, we investigate the mechanism for orbital-ordering (OO) in this material. We show that the purely electronic Kugel-Khomskii super-exchange mechanism (KK) alone leads to a remarkably large transition temperature of T_KK about 350 K. However, orbital-order is experimentally believed to persist to at least 800 K. Thus Jahn-Teller distortions are essential for stabilizing orbital-order at such high temperatures.Comment: 4 pages, 5 figure

Nature of the Mott transition in Ca2RuO4

We study the origin of the temperature-induced Mott transition in Ca2RuO4. As a method we use the local-density approximation+dynamical mean-field theory. We show the following. (i) The Mott transition is driven by the change in structure from long to short c-axis layered perovskite (L-Pbca to S-Pbca); it occurs together with orbital order, which follows, rather than produces, the structural transition. (ii) In the metallic L-Pbca phase the orbital polarization is ~0. (iii) In the insulating S-Pbca phase the lower energy orbital, ~xy, is full. (iv) The spin-flip and pair-hopping Coulomb terms reduce the effective masses in the metallic phase. Our results indicate that a similar scenario applies to Ca_{2-x}Sr_xRuO_4 (x<0.2). In the metallic x< 0.5 structures electrons are progressively transferred to the xz/yz bands with increasing x, however we find no orbital-selective Mott transition down to ~300 K.Comment: 4 pages, 3 figures; published versio

Excitonic Instability and Pseudogap Formation in Nodal Line Semimetal ZrSiS

Electron correlation effects are studied in ZrSiS using a combination of first-principles and model approaches. We show that basic electronic properties of ZrSiS can be described within a two-dimensional lattice model of two nested square lattices. High degree of electron-hole symmetry characteristic for ZrSiS is one of the key features of this model. Having determined model parameters from first-principles calculations, we then explicitly take electron-electron interactions into account and show that at moderately low temperatures ZrSiS exhibits excitonic instability, leading to the formation of a pseudogap in the electronic spectrum. The results can be understood in terms of Coulomb-interaction-assisted pairing of electrons and holes reminiscent to that of an excitonic insulator. Our finding allows us to provide a physical interpretation to the unusual mass enhancement of charge carriers in ZrSiS recently observed experimentally.Comment: 6 pages, 4 figures. Final versio

Hydrogen on graphene: Electronic structure, total energy, structural distortions, and magnetism from first-principles calculations

Density functional calculations of electronic structure, total energy, structural distortions, and magnetism for hydrogenated single-layer, bilayer, and multi-layer graphene are performed. It is found that hydrogen-induced magnetism can survives only at very low concentrations of hydrogen (single-atom regime) whereas hydrogen pairs with optimized structure are usually nonmagnetic. Chemisorption energy as a function of hydrogen concentration is calculated, as well as energy barriers for hydrogen binding and release. The results confirm that graphene can be perspective material for hydrogen storage. Difference between hydrogenation of graphene, nanotubes, and bulk graphite is discussed.Comment: 8 pages 8 figures (accepted to Phys. Rev. B

Orbital Kondo effect in Cobalt-Benzene sandwich molecules

We study a Co-benzene sandwich molecule bridging the tips of a Cu nanocontact as a realistic model of correlated molecular transport. To this end we employ a recently developed method for calculating the correlated electronic structure and transport properties of nanoscopic conductors. When the molecule is slightly compressed by the tips of the nanocontact the dynamic correlations originating from the strongly interacting Co 3d shell give rise to an orbital Kondo effect while the usual spin Kondo effect is suppressed due to Hund's rule coupling. This non-trivial Kondo effect produces a sharp and temperature-dependent Abrikosov-Suhl resonance in the spectral function at the Fermi level and a corresponding Fano line shape in the low bias conductance

Mott transition and suppression of orbital fluctuations in orthorhombic 3d1d^{1} perovskites

Using $t_{2g}$ Wannier-functions, a low-energy Hamiltonian is derived for orthorhombic $3d^{1}$ transition-metal oxides. Electronic correlations are treated with a new implementation of dynamical mean-field theory for non-cubic systems. Good agreement with photoemission data is obtained. The interplay of correlation effects and cation covalency (GdFeO$_{3}$-type distortions) is found to suppress orbital fluctuations in LaTiO$_{3},$ and even more in YTiO$_{3}$, and to favor the transition to the insulating state.Comment: 4 pages, 3 figures; revised manuscrip