Miryam Rincón Joya

Anharmonicity Study of Semiconductors and Metallic Nanotubes by Inelastic Spectroscopy

In this work, we have studied the temperature dependence from 83 to 1300 K in the change of position of Raman modes in single-walled carbon nanotubes (SWCNTs) and multi-walled carbon nanotubes (MWCNTs). The effects of synthesis on the quality and quantity of carbon nanotubes (CNTs) were analyzed with high-resolution scanning electron microscopy (SEM) and micro-Raman spectroscopy. The behavior of the modes G for CNTs with the application of temperatures was studied, and we find that the (G)-peak position shifts to low frequency with increasing temperature. When reducing the ambient temperature to 83 K, there is displacement of the G-mode but not many changes were observed in the other modes. At high temperatures, there are changes in the position and intensity of the different modes. This behavior cannot be understood by purely thermal effects, it may also be due to uniaxial tension. The analysis of the experimental data showed the temperature dependencies of frequencies (w) and linewidths (FWHM). As a result of FWHM and the thermal expansion theoretical data, we calculated the Grüneisen parameter and the phonon decay rate, which are well described, taking into account the contributions from thermal expansion and anharmonicity.

Magnetic flux quantum in a Superconducting concave/convex disk

The vortex state in a thin mesoscopic superconducting disk with a concave/convex surface is found theoretically. It is assumed that the outer edge of the sample is in contact with a metallic material. This configuration decreases the Bean-Livingston surface barrier energy, that is, it allows the vortex entry into the sample at lower magnetic field. In this work, we solve numerically the Ginzburg Landau equations with the metal/superconducting boundary condition using the link variable method in polar coordinates. It is shown that for a determined value for the deGennes parameter the superconductor becomes a type I superconductor and the irregular surface allows the vortex giant formation. The value of the thermodynamical properties decreases with this boundary condition and kind of surface.

2D barrier in a superconducting niobium square

The presence of barriers changes the vortex structure in superconducting Nb square in presence of a uniform applied magnetic field. The Cooper pair configurations in a mesoscopics superconducting square of Nb with a barrier are calculated within the nonlinear Ginzburg Landau equations. We predict the nucleation of multi-vortex states into the sample and a soft entry of the magnetic field inside and around into the barrier. A novel and non-conventional vortex configurations occurs at determined magnetic field.

Numerical solution of the time-dependent Ginzburg-Landau equation for a superconducting mesoscopic disk: Link variable method

The time dependent GinzburgLandau equations (TDGLE) are used to study the superconducting properties of a disk by taking into account the influence of internal defects. The Link variable algorithm is applied to a circular geometry surrounded by an insulator and immersed in an external magnetic field applied perpendicular to its plane. The TDGLE are used upon taking the magnetic field and the order parameter invariant along z-direction. We show that the magnetic response is substantially modified by the competition between the confinement geometry and the geometric position of the defects leads to vortex configurations which are not compatible with the symmetry of the sample geometry.

Computational simulation of vortex matter in Type-II mesoscopics superconductors

In recent years, algorithms and computer simulation methodologies have developed to the stage where simulation can now play a complementary role with experiment, aiding in interpretation of experimental data. Several different approaches to the study of superconductor samples have been developed and their use demonstrated. In this work, we solve numerically the nonlinear time dependent Ginzburg Landau equations to study the vortex dynamics in a mesoscopic type II cylinder superconductor containing one rectangular hole in the presence of an external field applied perpendicular to the surfaces. We calculate the spatial distribution of the superconducting electron density and the phase of the superconducting order parameter using the numerical method based on the technique of gauge invariant variables. It is assumed that the inner hole edge is in contact with a metallic material while the outer edge is in contact with vacuum. The vortex dynamics and the magnetization curves are studied as a function of the external magnetic field.

Anomalía en el espectro de fonones entre A1(3TO) y E(3LO)+A1(3LO) del Titanato de Plomo dopado con Lantano inducida por temperatura y presión.

Se estudia una anomalia que se observa entre los modos vibracionales opticos A1(3TO) y E(3LO)+A1(3LO) en la ceramica ferroelectrica, Titanato de Plomo dopado con Lantano (PLT). Este material normalmente presenta una transicion de fase estructural por medio de concentracion, temperatura y presion hidrostatica aplicada. Esta anomalia, aparece en bajas temperaturas con el aumento de la concentracion de Lantano (La) y altas presiones hidrostaticas para muestras del PLT. Con el estudio de los efectos de substitucion de cationes, presion y temperatura en el espectro de fonones en las muestras, a traves de medidas de micro-Raman, fue posible observar un nuevo pico en el espectro Raman. Este pico, se puede estar generando por la disminucion de las distancias interatomicas por efecto de las bajas temperaturas y altas presiones, siendo mas evidente con la concentracion de La. Palabras Clave: Anomalia, Fonones, PLT, presion, Raman.

Nucleación de vórtices y antivórtices en películas superconductoras con nanoestructuras magnéticas

En este trabajo investigamos teoricamente la configuracion de vortices y anti vortices en una pelicula superconductora magneticamente nanoestructurada resolviendo numericamente el sistema de ecuaciones diferenciales no lineales Ginzburg-Landau dependientes del tiempo. Interesantes estructuras de vortices y antivortices son encontradas cuando una pelicula fina superconductora es cubierta por una red de dipolos magneticos. Mostramos que debido a los (anti) vortices y la supercorriente inducida por los dipolos magneticos, la corriente critica de la muestra incrementa si es expuesta a un campo magnetico externo, caso contrario al lo que sucede en superconductores convencionales. Palabras clave: Antivortices, Dipolos magneticos, Nanoestructuras, Superconductor.

Defect-Induced Photoluminescence of Powdered Silica Glass

Visible photoluminescence was generated in standard soda-lime-silica glass powder, mechanically milled in a high-energy attrition mill. The broad emission band maximum shows a linear dependence on the exciting wavelength, suggesting the possibility to tune the PL emission. The photoluminescence was attributed to defect generation related to unsatisfied chemical bonds due to the high surface area. Raman scattering and ultraviolet-visible optical reflectance measurements corroborate this assertion. Transmission electron microscopy measurements indicate that the powder is composed by nanocrystallites with about 10-20 nanometers immersed in an amorphous media.