J. M. Osorio-Guillén

Electronic and optical properties of lead iodide

The electronic properties and the optical absorption of lead iodide (PbI2) have been investigated experimentally by means of optical absorption and spectroscopic ellipsometry, and theoretically by a full-potential linear muffin-tin-orbital method. PbI2 has been recognized as a very promising detector material with a large technological applicability. Its band-gap energy as a function of temperature has also been measured by optical absorption. The temperature dependence has been fitted by two different relations, and a discussion of these fittings is given.

The structure of the metallic high-pressure Fe3O4 polymorph: experimental and theoretical study

The structure of the metallic high-pressure Fe3O4 polymorph : experimental and theoretical study

Electronic and optical properties of γ-Al2O3 from ab initio theory

We report on a density functional theory calculation of the electronic structure and optical properties of gamma-Al2O3. We have made a comparison between the optical and electronic properties of th ...

Origin of transition metal clustering tendencies in GaAs based dilute magnetic semiconductors

While isovalent doping of GaAs (e.g., by In) leads to a repulsion between the solute atoms, two Cr, Mn, or Fe atoms in GaAs are found to have lower energy than the well-separated pair, and hence attract each other. The strong bonding interaction between levels with t2 symmetry on the transition metal (TM) atoms results in these atoms exhibiting a strong tendency to cluster. Using first-principles calculations, we show that this attraction is maximal for Cr, Mn, and Fe while it is minimal for V. The difference is attributed to the symmetry of the highest occupied levels. While the intention is to find possible choices of spintronic materials that show a reduced tendency to cluster, one finds that the conditions that minimize clustering tendencies also minimize the stabilization of the magnetic state.

Optical properties of 4H-SiC

The optical band gap energy and the dielectric functions of n-type 4H-SiC have been investigated experimentally by transmission spectroscopy and spectroscopic ellipsometry and theoretically by an a ...

Bonding and elastic properties of superconducting MgB2

Abstract Bonding and elastic properties of the recently discovered superconductor MgB2, have been studied from first-principles. Calculated lattice parameters, bulk modulus and its pressure derivative are in excellent agreement with the recently reported experimental data. Values of all independent elastic constants (c11, c12, c13, c33, and c55) as well as bulk moduli in the a and c directions (Ba and Bc, respectively) are predicted. Based on an analysis of the electron localization function we show that the bonding in the system is diverse and anisotropic. This explains the high anisotropy of the calculated elastic moduli, indicating that MgB2 should be rather brittle.

Gd3+ spin?lattice relaxation via multi-band conduction electrons in Y1?xGdxIn3: an?electron spin resonance study

Interest in the electronic structure of the intermetallic compound YIn3 has been renewed with the recent discovery of superconductivity at T ∼ 1 K, which may be filamentary in nature. In this work we perform electron spin resonance (ESR) experiments on Gd(3+) doped YIn3 (Y1-xGdxIn3; 0.001 ⪅ x ⩽̸ 0.08), showing that the spin-lattice relaxation of the Gd(3+) ions, due to the exchange interaction between the Gd(3+) localized magnetic moment and the conduction electrons (ce), is processed via the presence of s-, p- and d-type ce at the YIn3 Fermi level. These findings are revealed by the Gd(3+) concentration dependence of the Korringa-like relaxation rate d(ΔH)/dT and g-shift (Δg = g - 1.993), that display bottleneck relaxation behavior for the s-electrons and unbottleneck behavior for the p- and d-electrons. The Korringa-like relaxation rates vary from 22(2) Oe/K for x ⪅ 0.001 to 8(2) Oe/K for x = 0.08 and the g-shift values change, respectively, from a positive Δg = +0.047(10) to a negative Δg = -0.008(4). Analysis in terms of a three-band ce model allows the extraction of the corresponding exchange interaction parameters Jfs, Jfp and Jfd.

A search for the ground state structure and the phase stability of tantalum pentoxide.

Tantalum pentoxide (Ta2O5) is a wide-gap semiconductor that presents good catalytic and dielectric properties, conferring to this compound promising prospective use in a variety of technological applications. However, there is a lack of understanding regarding the relations among its crystalline phases, as some of them are not even completely characterized and there is currently no agreement about which models better explain the crystallographic data. Additionally, its phase diagram is unknown. In this work we performed first-principles density functional theory calculations to study the structural properties of the different phases and models of Ta2O5, the equation of state and the zone-centered vibrational frequencies. From our results, we conclude that the phases that are built up from only distorted octahedra instead of combinations with pentagonal and/or hexagonal bipyramids are energetically more favorable and dynamically stable. More importantly, this study establishes that, given the pressure range considered, the B-phase is the most favorable structure and there is no a crystallographic phase transition to another phase at high-pressure. Additionally, for the equilibrium volume of the B-phase and the λ-model, the description of the electronic structure and optical properties were performed using semi-local and hybrid functionals.

Assessing photocatalytic power of g-C3N4 for solar fuel production: A first-principles study involving quasi-particle theory and dispersive forces

First-principles quasi-particle theory has been employed to assess catalytic power of graphitic carbon nitride, g-C3N4, for solar fuel production. A comparative study between g-h-triazine and g-h-heptazine has been carried out taking also into account van der Waals dispersive forces. The band edge potentials have been calculated using a recently developed approach where quasi-particle effects are taken into account through the GW approximation. First, it was found that the description of ground state properties such as cohesive and surface formation energies requires the proper treatment of dispersive interaction. Furthermore, through the analysis of calculated band-edge potentials, it is shown that g-h-triazine has high reductive power reaching the potential to reduce CO2 to formic acid, coplanar g-h-heptazine displays the highest thermodynamics force toward H2O/O2 oxidation reaction, and corrugated g-h-heptazine exhibits a good capacity for both reactions. This rigorous theoretical study shows a route to further improve the catalytic performance of g-C3N4.

Conduction electron spin resonance in AlB2

This work reports on electron spin resonance experiments in oriented single crystals of the hexagonal AlB2 diboride compound (P6/mmm, D16h structure) which display conduction electron spin resonance. The X-band electron spin resonance spectra showed a metallic Dysonian resonance with g-value and intensity independent of temperature. The thermal broadening of the anisotropic electron spin resonance linewidth ΔH tracks the T-dependence of the electrical resistivity below T is approximately equal to 100 K. These results confirm the observation of a conduction electron spin resonance in AlB2 and are discussed in comparison with other boride compounds. Based on our main findings for AlB2 and the calculated electronic structure of similar layered honeycomb-like structures, we conclude that any array of covalent B-B layers potentially results in a conduction electron spin resonance signal. This observation may shed new light on the nature of the non-trivial conduction electron spin resonance-like signals of complex f-electron systems such as β-YbAlB4.