J.J. Castro

Nonadiabatic structure instability of planar hexagonal gold cluster cation Au7 and its spectral signature

Determining the structure for gold nanoclusters at which the 2D–3D transition occurs has been a challenging problem for both theoretical and experimental studies. For gold cation clusters, quantum chemistry calculations have produced controversial results concerning the last most stable planar structure, some indicating that corresponds to n = 7, whereas others conclude that at this value, the structure is already 3D. Experimentally, ion mobility spectroscopy measurements, assisted by DFT calculations, have been interpreted as a planar structure for Au7+. Here, we show that the usually assumed 2D non-degenerate ground state cluster structure for Au7+ with D6h symmetry is unstable against a puckering distortion, ending in a 3D stable structure with D3d symmetry. The analysis is based on the pseudo Jahn–Teller effect through vibronic coupling between the ground state and two excited states. This structure presents, in average, the same surface area than the 2D, being therefore compatible with experimental results. We discuss the effects the vibronic coupling would have on Raman, IR and UV-vis spectra. The study is based on a scalar and spin orbit relativistic DFT calculation in the Zero Order Regular Approximation (ZORA).

A THEORETICAL-ANALYSIS OF PHONON-SPECTRA IN HIGH-TC SUPERCONDUCTORS

Abstract The normal modes of vibration of the superconductors Y 1 Ba 2 Cu 3 O 7 and related compounds are calculated using the harmonic aproximation. The conditions for the Jahn-Teller effect and its influence on the phonon density of states are analysed.

Theoretical analysis of the Jahn-Teller effect in the LaBaCuO superconductor

Abstract We just commemorated the 10 years anniversary of the discovery of the high Tc superconductivity of Bednorz and Muller (BM). In their celebrated paper BM mentioned the possible role of the Jahn-Teller (JT) effect in the enhancement of the transition temperature to the superconducting state. In the present work we analize, under the quasi-molecular approximation, the conditions for the existence of the JT effect in the CuO6 cluster belongin to the LaBaCuO superconductor of BM. The electronic part is obtained from self consistent ab initio calculations and the force constants are calculated taking the second derivatives of the energy using the Hartree-Fock approximation for the electronic part. Both, the electronic and vibrational parts are classified accordingly to the irreducible representations of the D4h group in order to establish the JT theorem. The physical consecuences of the presence of the JT effect in CuO6 cluster are discussed.

A theoretical phonon calculation in the Bi2Sr2CaCu2O8 superconductor

Abstract The normal modes of vibration of the Bi 2 Sr 2 CaCu 2 O 8 superconducting material (with Tc near 85 K) are calculated within the harmonic approximation. The theoretical group analysis of the normal modes and the phonon density of states are reported.

Jahn-Teller effect in the LaSrCuO superconductor

The CuO 6 octahedron in the T structure of the La-Sr-Cu-O superconductor is considered within the quasi-molecular approximation in order to calculate the electronic structure and the vibrational modes that could participate in the Jahn-Teller effect. The electronic calculations are made within the extended Huckel model and the vibrations are taken from previously reported results. Once the electronic and vibrational participants are determined using group theory analysis, the intensity of the electron-phonon interaction is calculated to establish the Jahn-Teller deformation

Surface properties for α-cluster nuclear matter

We introduce a new microscopic model for α-cluster matter, which simulates the properties of ordinary nuclear matter and α-clustering in a curved surface of a large but finite nucleus. The model is based on a nested icosahedral fullerene-like multiple-shell structure, where each vertex is occupied by a microscopic α-particle. The novel aspect of this model is that it allows a consistent description of nuclear surface properties from microscopic parameters to be made without using the leptodermous expansion. In particular, we show that the calculated surface energy is in excellent agreement with the corresponding coefficient of the Bethe-Weizacker semi-empirical mass formula. We discuss the properties of the surface α-cluster state, which resembles an ultra cold bosonic quantum gas trapped in an optical lattice. By comparing the surface and interior states we are able to estimate the α preformation probability. Possible extensions of this model to study nuclear dynamics through surface vibrations and departures from approximate sphericity are mentioned.

Structural Properties of the Bimetallic Cluster Au 12 Ag 6 on the NO x (x=1,2,3) Adsorption Processes

It has been shown that Au and Ag nanoparticles present catalytic properties of pollutant gases, product of the hydrocarbons combustion processes, whose efficiency is strongly dependant on their size, morphology and supporting material. The activity of Ag subnanometric particles supported on alumina in the NO x reduction mechanism, for the reaction HC-SCR (Hydrocarbon Selective Catalytic Reaction) is a good example of these catalytic properties. It is also well established that both Au and Ag nanoparticles are oxygen poisoning resistant and that in certain reactions the bimetallic combination may be more efficient than their isolated parts. In this work we present a DFT calculation of the NO x (x =1, 2, 3) adsorption energies on the Au 12 Ag 6 cluster surface. It is also discussed the possible synergetic effect of the different configurations, forming small Ag islands composed of one to five atoms, on the surface of the bimetallic cluster. The analysis was done using DFT in the ZORA approximation.

Numerical Simulation Of A Voltage-Dependent Surface-Induced Molecular Reorientation In A Confined Nematic Liquid Crystal

We present a computer simulation of the voltage-dependent orientational ordering of a nematic liquid crystal within the lattice approximation, confined between two substrates with different anchoring for each surface. The system is studied by analyzing the response of the molecular reorientation near the surfaces as a function of the applied voltage. The model considers a molecular interaction responsible for the nematic phase that includes a superposition of the anisotropic induced-dipole-induced-dipole and isotropic Maier-Saupe interactions, whereas for the nematic-surface intermolecular interaction we assume the Rapini-Papoular form. The response to the external electric field is studied by calculating the dielectric constant tensor, based on a model through the molecular polarizability. We particularly discussed the case for negative dielectric anisotropy that could have relevance for the voltage controlled twist effect. The simulation is carried through a numerical relaxation method for the total energy of the system.

Molecular Dynamics and Relaxation Methods in the Stability Calculations for the Study of Distortions of Confined Nematic Liquid Crystals

We present a calculation using a relaxation method for the study of the orientational ordering of a nematic liquid crystal near the surfaces confining the system. We comment on the advantage of using this method as compared with molecular dynamics simulation. The system is simulated by a lattice model with a superposition of isotropic and anisotropic intermolecular interaction of the Maier-Saupe and induced dipole-induced dipole type force for the bulk nematic phase. For the nematic confining surface we consider a Rapini-Papoular interaction. We present simulations for negative dielectric anisotropy.

Relaxation Method Simulation of Confined Polymer Dispersed Liquid Crystals in an External Field

Polymer dispersed liquid crystals (PDLC) are materials formed by nematic liquid crystals droplets with radii of a few hundred A embedded in a polymer matrix. We discuss the use of relaxation methods for the study of the response of the director of a PDLC under the switching of an external electric field. We simulate the confining system by considering different boundary conditions at the droplet surface.