Alfredo Juan

Catalytic oxidation of CH3OH to HCOOCH3 on V2O5 : A theoretical study

Abstract In a previous work, the CH 3 OH adsorption-oxidation process on V 2 O 5 was studied by mass spectrometry analysis of the reaction atmosphere. The following partial oxidation products were reported: H 2 CO, CO, CO 2 , (CH 3 O) 2 CH 2 (methylal), CH 3 OCH 2 OH (hemimethylal) and HCOOCH 3 . The data so obtained allowed a reaction pathway to be proposed. In the present work, we used models of the plane (010) of V 2 O 5 and of CH 3 OH, to test whether the pathway proposed in this work as well as alternative ones presented by other authors were energetically favorable or not. With this purpose, the variation of the total system energy was computed in each step by extended Huckel-type calculations (ASED, atom superposition and electron delocalization). The results showed, on the one hand, that the pathway proposed in this work is better than alternatives pathways, and on the other hand, the possible existence of other equally-favorable adsorption sites in which bridge oxygen may take part.

Statistical repulsion/attraction of electrons in graphene in a magnetic field

Abstract The aim of this work is to describe the thermodynamic properties of an electron gas in graphene placed in a constant magnetic field. The electron gas is constituted by N Bloch electrons in the long wavelength approximation. The partition function is analyzed in terms of a perturbation expansion of the dimensionless constant ( eB L ) − 1 . The statistical repulsion/attraction potential for electrons in graphene is obtained in the respective case in which antisymmetric/symmetric states in the coordinates are chosen. Thermodynamic functions are computed for different orders in the perturbation expansion and the different contributions are compared for symmetric and antisymmetric states, showing remarkable differences between them due to the spin exchange correlation. A detailed analysis of the statistical potential is done, showing that, although electrons satisfy Fermi statistics, attractive potential at some interparticle distances can be found.

Vacancy clustering in pure metals: some first principle calculations of positron lifetimes and momentum distributions

First results of a systematic study on the vacancy clustering process in Al, Cu, Mg and Nb are presented. To this aim first principle calculation of positron lifetimes and positron-electron momentum distributions were performed. We test the reliability of the computational scheme used by comparing some of the calculated results with experimental ones.

Dynamical diffusion and renormalization group equation for the Fermi velocity in doped graphene

Abstract The aim of this work is to study the electron transport in graphene with impurities by introducing a generalization of linear response theory for linear dispersion relations and spinor wave functions. Current response and density response functions are derived and computed in the Boltzmann limit showing that in the former case a minimum conductivity appears in the no-disorder limit. In turn, from the generalization of both functions, an exact relation can be obtained that relates both. Combining this result with the relation given by the continuity equation it is possible to obtain general functional behavior of the diffusion pole. Finally, a dynamical diffusion is computed in the quasistatic limit using the definition of relaxation function. A lower cutoff must be introduced to regularize infrared divergences which allow us to obtain a full renormalization group equation for the Fermi velocity, which is solved up to order O ( ℏ 2 ) .

Adsorption/oxidation of CH3OH on V2O5. A theoretical and experimental study

The adsorption/oxidation of CH3OH on V2O5 was studied in previous work by mass spectrometric analysis of the reaction atmosphere, and by measuring the electrical properties (Hall effect) and total pressure of the system. Such data allowed a reaction pathway to be proposed. In the present work, the variations in the surface oxidation number have been calculated via balances in the C and H atoms in the reaction atmosphere and models of the (010) plane of V2O5 and of CH3OH have also been utilized to find energetically favourable sites for the adsorption of CH3OH and its subsequent oxidation to H2CO. For this purpose, the variation in the total energy of the system was computed by Extended Hückel-type calculations (ASED, Atom Superposition and Electron Delocalization). From such experimental results, it was deduced that the H atoms are removed by the network oxygens and that methanol is adsorbed as the methoxy group. Such removal of H atoms has two consequences: high surface hydroxylation and surface reduction [V(5+) → V(4+)]. In support of these suggestions, the theoretical results have confirmed both the removal of H atoms and methoxy group formation, and have also revealed two possible adsorption sites for CH3OH on V2O5: the terminal oxygens (V=O) and the network bridging oxygens (V-O-V).

Ballistic transport properties in pristine/doped/pristine graphene junctions

Abstract We investigate the ballistic electron transport in a monolayer graphene with configurational averaged impurities, located between two clean graphene leads. It is shown that the electron transmission are strongly dependent on the concentration of impurities and the incident energy. In turn, the conductance computed using the Landauer formalism shows a similar behavior to those found in experimental works as a function of the applied voltage for different concentrations of impurities in the limit of low temperatures. In the limit of zero bias voltage, the conductance shows a minimum value which reduces to zero for high concentration of impurities which disentangle graphene sublattices. These results can be very helpful for exploring the tunneling mechanism of electrons through doped thermodynamically stable graphene.

The observable-state model and nonrenormalizable theories

The aim of this work is to apply the observable-state model for the quantum field theory of a phi^n self- interaction. We show how to obtain finite values for the 2-point and n-point correlation functions without introducing counterterms in the Lagrangian. Also, we show how to obtain the renormalization group equation for the mass and the coupling constant. Finally, we found the dependence of the coupling constant with the energy scale and we discuss the validity of the observable-state model in terms of the projection procedure.

Formation of localized magnetic states in graphene in hollow-site adsorbed adatoms

Abstract By applying tight binding model of adatoms in graphene, we study theoretically the localized aspects of the interaction between transition metal atoms and graphene. Considering the electron-electron interaction by adding a Hubbard term in the mean-field approximation, we find the spin-polarized localized and total density of states. We obtain the coupled system of equations for the occupation number for each spin in the impurity and we study the fixed points of the solutions. By comparing the top site and hollow site adsorption, we show that the anomalous broadening of the latter allows to obtain magnetization for small values of the Hubbard parameter. Finally, we model the magnetic boundaries in order to obtain the range of Fermi energies at which magnetization starts.

Analytic solution for gauged Dirac-Weyl equation in (2 + 1)-dimensions

A gauged Dirac-Weyl equation in (2+1)-dimension is considered. This equation has the particularity to describe the states of a graphene Dirac matter. In particular we are interested in matter interacting with a Chern-Simons gauge fields. We show that exact self-dual solutions are admitted. These solutions are the same as those supported by nonrelativistic matter interacting with a Chern-Simons gauge field.