Luis A. Pérez

Low-symmetry structures of Au32Z (Z = +1, 0, -1) clusters.

In this work, we have explored new stable structures of the Au32Z (Z = +1, 0, -1) clusters. Theoretical calculations using density functional theory within the generalized-gradient approximation were performed. Our results show that, in the anion state (Au32-), low-symmetry (disordered) structures are preferred over the caged fullerene-like isomer. In addition, the cationic cluster (Au32+) also exhibits a disordered low-symmetry structure as its lowest energy configuration, but it is much closer in energy to the fullerene-like isomer. These results, obtained at T = 0 K, indicate that disordered structures for the Au32- and Au32+ clusters may be detected not only at room temperature, as was experimentally verified for the Au32- one, but also at much lower temperatures.

STRUCTURAL PROPERTIES OF BIMETALLIC CLUSTERS FROM DENSITY FUNCTIONAL CALCULATIONS

The structural properties and energy ordering of the lowest lying isomers of bimetallic (CuAu)n and (PtPd)n, n=5-22 clusters have been investigated by means of density functional theory (DFT) in the generalized gradient approximation (GGA). The initial cluster geometry optimization is performed by using a genetic algorithm with the many body Gupta potential. This technique provide a distribution of the lowest energy cluster structures, that are further reoptimized using the DFT-GGA methodology. The energy ordering of isomers obtained with the Gupta potential does not agree, in general, with the one obtained using DFT-GGA for the two bimetallic clusters investigated. However, the lowest energy strucutures of the (CuAu)n nanoalloy show icosahedral patterns in agreement with the results obtained with the model potential. For the (PtPd)n clusters segregation effects are found, where the Pt atoms are forming the cluster core and the Pd atoms are on the cluster surface, in agreement with previous calculations using the many body Gupta potential.

Oxygen and the 110 K phase in the BiSrCaCuO superconductor system

Abstract We report results obtained from the study of the superconducting properties of the 110 K phase of the BiSrCaCuO high-Tc superconductor in different oxygen environments. Results of TGA analysis and electrical conductivity at high temperatures of samples with about 50% of the 110 K phase, with closed electrical paths of only the 110 K phase between the electrodes, induced us to anneal them in air and vaccum at 430°C. After this heat treatment the zero resistivity of the samples changes in the temperature interval between 70 K and 108 K. The transition temperature decreases by increasing the oxygen defienciency of the sample.

d-Wave hole superconductivity in low-dimensional Hubbard systems

Abstract Superconducting states with d symmetry in anisotropic hole systems are investigated within a generalized Hubbard model and the BCS framework. The results reveal a key participation of the next-nearest-neighbor correlated-hopping interaction (Δt3) in the appearance of cos k x − cos k y superconducting gap, in spite of its small strength in comparison with other terms of the model. This interaction favors the superconducting state over the phase separation, which is an important obstacle when the d-wave superconducting state is originated from an attractive nearest-neighbor density–density interaction. Furthermore, the superconducting critical temperature is highly enhanced by the low-dimensionality of the system and the gap ratio exhibits a non-BCS behavior.

Capillary condensation of Lennard-Jones fluid in a slitlike pore filled with quenched disordered matrix

We have studied a capillary condensation of a Lennard-Jones fluid in a slitlike pore filled with a quenched disordered hard-sphere matrix using the Born–Green–Yvon (BGY) equation with the Fisher–Methfessel (FM) approximation. The solution of the replica Ornstein–Zernike (ROZ) equation in the Percus–Yevick (PY) approximation for a fluid in a homogeneous matrix is used as an input. The adsorption isotherms exhibit hysteresis loops for matrix-free and highly microporous matrix-filled slitlike pores.

Plasmonic interactions: from molecular plasmonics and Fano resonances to ferroplasmons.

Plasmon interactions are a subject of great interest from both the technological as well as the fundamental points of view. In this Perspective, we outline the great variety of physical phenomena that are produced by the interactions of localized surface plasmon resonance with molecular excitons; with other plasmonic nanostructures, particularly the Fano effect; and with nonplasmonic nanoparticles, such as the just-reported interaction with ferromagnetic nanoparticles. The theoretical as well as experimental challenges remaining to be elucidated are discussed.

dx2-y2 pairing in the generalized Hubbard square-lattice model

The pairing between holes in a square lattice and the two-particle wave-function symmetry are studied within a generalized Hubbard model. The key participation of the next-nearest-neighbor correlated-hopping interaction in the appearance of d-wave two-hole ground state is found, which is enhanced by the on-site repulsive Coulomb interaction. There is a clear pairing asymmetry between electrons and holes, where the hole pairing occurs in a realistic regime of interactions. The two-particle states are analyzed by looking at the binding energy, the coherence length, and the effective mass of the pairs. Finally, the case of a hole-singlet in an antiferromagnetic background is also studied.

Controlling stability and electronic properties of small-diameter SiC nanowires by fluorination

We report results of a density functional theory study of the electronic properties and stability of fluorine–saturated 0.355nm–thick silicon carbide nanowires with a 3C–SiC core and grown along the [111] direction. The electronic band gaps of the fully fluorinated SiC nanowires are lower than that of the corresponding fully hydrogenated ones by up to 1.09 eV. Moreover, the structural stability is found to increase linearly with fluorine surface covering. For mixed fluorination and hydrogenation surface decoration schemes, the band gaps usually lie between the values of the fully fluorinated and the corresponding fully hydrogenated nanowire. Furthermore, the band gap type changes from direct to indirect for fluorine coverings exceeding 16.66%. These results indicate that fluorination of the nanowire surface may be used to control the stability as well as the size and nature of the band gap.

Band-gap engineering of halogenated silicon nanowires through molecular doping

AbstractIn this work, we address the effects of molecular doping on the electronic properties of fluorinated and chlorinated silicon nanowires (SiNWs), in comparison with those corresponding to hydrogen-passivated SiNWs. Adsorption of n-type dopant molecules on hydrogenated and halogenated SiNWs and their chemisorption energies, formation energies, and electronic band gap are studied by using density functional theory calculations. The results show that there are considerable charge transfers and strong covalent interactions between the dopant molecules and the SiNWs. Moreover, the results show that the energy band gap of SiNWs changes due to chemical surface doping and it can be further tuned by surface passivation. We conclude that a molecular based ex-situ doping, where molecules are adsorbed on the surface of the SiNW, can be an alternative path to conventional doping. Graphical abstractMolecular doping of halogenated silicon nanowires

Electronic structure and optical vibrational modes of 3C–SiC nanowires

The electronic structure and vibrational optical modes of silicon carbide nanowires (SiCNWs) were studied using the first principles density functional theory. The nanowires were modelled along the [111] direction using the supercell technique passivating all the surface dangling bonds with H atoms, OH radicals and a combination of both. Results show that the full OH passivation lowers the band gap energy compared to the full H passivation owing to C–OH surface states. A shift of the highest optical vibrational modes of Si and C to lower frequency values compared to their bulk counterparts was observed in accordance with phonon confinement scheme.