Gregorio H. Cocoletzi

Refractive index of colored films of molybdenum trioxide

We apply a film thickness-independent technique to measure the index of refraction n=n+ik for amorphous thin films of molybdenum trioxide. The films were grown by thermal evaporation and then colored by light irradiation for different exposure times. Data for the reflectivity and differential reflectivity yield n and show no dependence on the density of color centers. We find that small variations of the absorption coefficient induce changes in the differential reflectance line shape at angles different from the Brewster angle.

Wannier states and optical transitions in a ‘‘diatomic’’ semiconductor superlattice

We study the effect of several structural parameters on the band structure and Wannier functions of a superlattice with a complex period. Our calculations include nonparabolic‐band corrections and different effective masses. As an interesting and physical example, we study the evolution of a ‘‘molecular lattice’’—with two wells per period—with respect to the connectivity between different wells. We find strong dependence of important physical quantities, such as bandwidths, wave functions, and interband oscillator strength. We discuss how these effects should be observable in transport and photoluminescence experiments.

First-principles calculations of the indigo encapsulation and adsorption by MgO nanotubes

We have performed ab-initio calculations to investigate the structural and electronic properties of (m,m) chiral magnesium oxide nanotubes, (m,m)MgONTs, to explore the encapsulation, inclusion, and adsorption of dyes (organic molecules) such as Indigo (IND). Studies start by determining the structural parameters of the MgO nanotubes with different diameters and the IND. The indigo encapsulation into the MgONT is studied considering four (m,m) chiralities which yield 4 different NT diameters. In the endohedral functionalization, the indigo is within the NT at a tilt angle as in previous theoretical studies of organic molecules inside carbon and boron-nitride nanotubes. Results show that the encapsulation is a strong exothermic process with the m = 6 case exhibiting the largest encapsulation energy. It is also explored the indigo adsorption on the NT surface in the parallel and perpendicular configurations. The perpendicular configuration of the IND adsorption on the (8,8)MgONT exhibits the largest energy. ...

FIRST PRINCIPLES CALCULATIONS OF THE ADSORPTION OF GROUP III METALS ON Si(001) AT HIGH TEMPERATURE

First principles total energy calculations are performed to investigate the Si(001)-(3 × 4) reconstruction induced by the adsorption of group III metals (Ga, Al and In) when deposited at high temperatures. We have considered different models in our study and we have found that the pyramid-like structure proposed by Bunk et al. for In on Si(001) yields the most stable atomic configuration in all cases. We present a detailed description of this surface reconstruction, and compare the structural parameters for Al, Ga and In.

Optical response of a strongly anisotropic thin film as a nonmagnetic negative phase velocity material

We study the optical response of a strongly anisotropic nonmagnetic material. As a first case, it is considered an anisotropic nonmagnetic thin film (medium 2) sandwiched between medium 1 of refraction index n1 and medium 3 of refraction index n3. As a second case, the anisotropic thin film and medium 3 are interchanged. These geometries correspond to the Kretchmann [E. Kretschmann and H. Raether, Z. Naturforsch. A 23, 2135 (1968); E. Kretschmann, Z. Phys. 241, 313 (1971)] and Otto [A. Otto, Z. Phys. 216, 398 (1968)] attenuated total reflectivity arrays, respectively. Our main goal is to obtain a negative phase velocity propagation. To achieve this we choose an index of refraction n1>1 and show how the appropriate angle of incidence allows us to induce a negative phase velocity nonmagnetic material. We focused our attention in two distinct regions that emerge in the reflectivity curves when varying the angle of incidence. In the first region, the minima of the reflectivity are interpreted in terms of the ...

Acoustic wave propagation in multilayer systems

Theoretical studies are performed on the propagation of acoustic waves in multilayer systems using a transfer matrix approach similar to that developed for the studies of optical filters [L. I. Epstein, J. Opt. Soc. Am. 42, 806–810 (1952)] and metallic superlattices [E. L. Olazagasti et al. , Solid State Commun. 78, 9–12 (1991)]. The formalism presented here is used to calculate the reflectivity R of sound waves on multilayer structures. Comparisons are made with experimental results [B. G. Martin, J. Acoust. Soc. Am. 91, 1469–1473 (1992)] for a three‐layer structure, in the normal incidence case. The calculated resonances are in good agreement with the experimental ones. For the same system, in the non‐normal incidence of sound waves, the spectra of R suggests surface waves coupling. Finally, to show the usefulness of the theory, a Kronig–Penney type structure is studied, calculating the dispersion relation of the normal modes of an infinite system and the reflectivity for the semiinfinite case.