Rafael Rey

Followers

Following

Public Views

Interests

Uploads

Papers by Rafael Rey

Diffusive-like minibands in finite superlattices of disordered quantum wells

Brazilian Journal of Physics, 2002

Download

La Dinámica De Canales Disipativos Lorentzianos Es No Markoviana

We consider the dissipative dynamics of an exciton in a Lorentzian reservoir, relevant in the des... more We consider the dissipative dynamics of an exciton in a Lorentzian reservoir, relevant in the description of some photosynthetic complexes. Although this dynamics have been frequently described as non-Markovian, we show that this characterization may be misleading. We describe an approach within which it is possible to decide if this dynamics is or not Markovian. We apply this approach to the Lorentzian reservoir.

Insulator-to-Semiconductor-to-Conductor Phase-Like Transition of DNA Chains

Journal of Nanoscience and Nanotechnology, 2018

Using a tight-binding model, we study the transport of charge carriers through DNA molecular wire... more Using a tight-binding model, we study the transport of charge carriers through DNA molecular wires. In double-stranded DNA chains, according to Chargaff's rules, only Adenine-Thymine (AT) and Cytosine-Guanine (CG) pairs are allowed. In our model, a decimation procedure allows us to represent each pair of bases by a single site with one localized electronic state. We consider chains of different lengths with only AT (CG) sites, and ordered and disordered chains with both types of sites. Disordered chains may include short range correlation. Additionally, hydration is considered in the form of a change of the site energy. We find a conductor-to-semiconductor-to-insulator transition as a function of the three effects taken into account: chain size, intrinsic disorder of CG and AT pairs, and hydration. This model predicts that an appropriate choice of chain size and concentration of AT pairs can be used to tailor the electrical behavior of DNA strands.

format_quoteThe study models electric conductance in hydrated DNA chains using an Anderson tight-binding model with diagonal disorder.format_quote

Download

Short-range order in linear binary alloys: Delocalization of states versus memory of ordered band structures

Physical Review B, 1996

We discuss the electronic properties of a one-dimensional tight-binding model for binary alloys w... more We discuss the electronic properties of a one-dimensional tight-binding model for binary alloys with correlated disorder. The correlation inhibits the bonds between atoms of one of the atomic species, leading to what is called repulsive binary alloys. As it is well known, the transmission probability of repulsive binary alloys shows resonances due to the delocalization of states in this peculiar disordered one-dimensional system. We show that this delocalization can be related to the memory of the band structures of different ordered chains, since short segments of these ordered chains constitute the repulsive binary alloy. This memory effect is revealed in the localization length as a function of energy. The same argument is used to show that competing correlations in the same linear chain destroy delocalization, as can be inferred from the properties of binary alloys with no correlations at all. We also show that, complementary to delocalized bands, smooth transmission gaps should also exist at specific energy ranges only for correlated disorder cases. ͓S0163-1829͑96͒10133-8͔

Download

Evidence of iridescence in TiO2 nanostructures: An approximation in plane wave expansion method

Photonics and Nanostructures - Fundamentals and Applications, 2016

Abstract Titanium dioxide nanotubes, TiO2 NTs, can be obtained by electrochemical anodization of ... more Abstract Titanium dioxide nanotubes, TiO2 NTs, can be obtained by electrochemical anodization of Titanium sheets. After nanotubes are removed by mechanical stress, residual structures or traces on the surface of titanium sheets can be observed. These traces show iridescent effects. In this paper we carry out both experimental and theoretical study of those interesting and novel optical properties. For the experimental analysis we use angle resolved UV–vis spectroscopy while in the theoretical study is evaluated the photonic spectra using numerical simulations into the frequency-domain and the framework of the wave plane approximation. The iridescent effect is a strong property and independent of the sample. This behavior can be important to design new materials or compounds for several applications such as, cosmetic industry, optoelectronic devices, photocatalysis, sensors, among others.

From sequential to correlated tunneling of two bosons

Journal of Physics: Conference Series, 2016

The sequential to correlated-tunneling transition of two identical bosons confined in a symmetric... more The sequential to correlated-tunneling transition of two identical bosons confined in a symmetrical double well potential is studied using a time-dependent variational formalism. Increasing the strength of the short-range boson-boson interaction, the probability of finding both bosons in the same well allow to identify three regimes. For small values of the boson- boson coupling constante, this probability display beatings. Then, the probability of finding both bosons in the same well becomes quasi-periodical in time, for almost all values of the coupling constant; however, there is a zero-measure set, which contains an infinity of points, for which this probability is time-periodic. Finally, for larger values of the coupling constant, this probability changes periodically. Based on the dynamics of the probability of finding one boson on each well and increasing the boson-boson interaction, we find that tunnelling changes from a regime where one boson tunnels after the other to a regime in which they tunnel together.

Statistical Characterization of the Energy Spectrum in 1D Disordered Multilayer Systems

physica status solidi (b), 2002

Models of one dimensional systems with short range correlated disorder have predicted the existen... more Models of one dimensional systems with short range correlated disorder have predicted the existence of an energy region where the states are delocalized. This is contrary to the earlier belief that all the eigenstates are localized in 1D disordered systems. We study the statistical properties of the spectra of finite superlattices made up of short chains of random binary alloy which present correlated disorder.

Download

Quantum transport through aromatic molecules

Journal of Applied Physics, 2013

ABSTRACT In this paper, we study the electronic transport properties through aromatic molecules c... more ABSTRACT In this paper, we study the electronic transport properties through aromatic molecules connected to two semi-infinite leads. The molecules are in different geometrical configurations including arrays. Using a nearest neighbor tight-binding approach, the transport properties are analyzed into a Green&#39;s function technique within a real-space renormalization scheme. We calculate the transmission probability and the Current-Voltage characteristics as a function of a molecule-leads coupling parameter. Our results show different transport regimes for these systems, exhibiting metal-semiconductor-insulator transitions and the possibility to employ them in molecular devices.

Stabilization and interaction energies of non planar DNA base pairs Guanine-Cytosine and Adenine-Thymine. A study based on the SIESTA method

Not Available Bibtex entry for this abstract Preferred format for this abstract (see Preferences)... more

format_quoteThe optimal geometries show AT pair relaxation took 135 steps and GC took 91, with bond distances approximately 10% shorter than benchmarks.format_quote

Download

Photonic Effects On Nanostructures In The Ti- TiO2 Interphase

Advanced Materials Letters, 2016

In this work we present a theoretical-experimental study of optics effects in nanostructures form... more In this work we present a theoretical-experimental study of optics effects in nanostructures formed during the growth of titania nanotubes. Titania nanotubes were fabricated via electrochemical anodization method using Ethylene glycol. Traces or prints left by the nanotubes on the titanium surface are observed thought X-ray diffraction measurements. The pattern shows a polycrystalline structure with the presence of Anatase and titanium phases. The reflectance and absorbance spectra reveal in the traces optical phenomena similar to iridescence. The iridescence behavior is commonly found in nature and is traditionally associated to photonic properties. Using the plane wave method and modelling the nanostructures observed on the interphase like two-dimensional photonic crystals, the photonic dispersion relation was calculated in function of geometrical parameters. Enlargement of the photonic band gap was found as spatial contrast is increased. These results suggest that these types of nanostructures can be promissory candidates to develop photonic devices.

Download

Propiedades Electrónicas De Sistemas Tipo Panal De Abejas en Dos Dimensiones. Un Estudio Ab Initio

The novel properties of graphene have motivated the search for materials with similar characteris... more The novel properties of graphene have motivated the search for materials with similar characteristics. In this work, we study the chemical stability and the band structures of GaAs and GaN hexagonal monolayers, using a first principles formalism. Results obtained with SIESTA code, as a DFT implementation, show GaAs (GaN) is an indirect (a direct) bandgap semiconductor.

Download

¿El adn es un hilo conductor de tamaño nanométrico? Is dna a nanometric conductor wire?

In the last years have grown the interest to use nucleic acids and proteins in nanodevices. Howev... more In the last years have grown the interest to use nucleic acids and proteins in nanodevices. However, the experimental results of its electronic properties are contradictory, being necessary to improve the theoretical models that permit to understand the behavior of the DNA as conductor, semiconductor or insulator. We addressed the problem of electronic transport across a finite segment of DNA, using a linear chain model into tight binding framework with nearest neighbor interaction and one orbital per site to represent the DNA. Hydrations effects are taken into account as random variations of the self-energy. The electronic current is calculated starting from the Landauer’s formulation as function of applied bias, chain length, disorder, or hydration concentration. We find insulator behavior for long chains or high uncorrelated disordered cases while short or low disordered chains are ohmic-like materials.

Download

Mecánica cuántica: Un reto inaplazable para nuestro país

TecnoLógicas, 2011

Actualmente, desde que los niños son muy pequeños, manejan dispositivos como los teléfonos celula... more Actualmente, desde que los niños son muy pequeños, manejan dispositivos como los teléfonos celulares y los computadores, los cuales tienen como uno de sus elementos básicos el transistor. No es sorprendente, por lo tanto, que se pregunten cómo funcionan, de qué están hechos. Y muchas veces los padres de estos pequeños somos incapaces de dar respuestas bien informadas a estas inquie-tudes, respuestas que ya no deberían ser una cuestión de especia-listas, sino conocimiento básico de cualquier ciudadano. El propó-sito de esta comunicación es doble: por un lado esclarecer el papel de la mecánica cuántica tanto en las tecnologías presentes y futu-ras como en otras áreas del conocimiento y la cultura, y por otro, hacer una reflexión acerca de la pertinencia de la misma en nues-tro país y en nuestras instituciones.

Download

Vibrational propeties of disordered double chains: an application to DNA

Los recientes desarrollos en el área de la nanoelectrónica ha impulsado el estudio de sistemas cu... more Los recientes desarrollos en el área de la nanoelectrónica ha impulsado el estudio de sistemas cuasi uno dimensionales tales com el ADN y sus aplicaciones a nuevos dispositivos. En etse trabajo usamos un modelo de una doble cadena lineal para estudiar las propiedades vibracionales del ADN. Para el caso puramente unidimensional, el desorden induce un comportamiento diatómico.

Electronic Structure of a 2D Array of Quantum Dots

ABSTRACT In this work we study the electronic structure of a 2D array of quantum dots within the ... more ABSTRACT In this work we study the electronic structure of a 2D array of quantum dots within the tight binding approximation. The confining potential is emulated by a modulation in the hopping parameters of the system, which can be changed from weak to strong coupling between the dots. The obtained electronic structure shows arising of flat bands when the antidot region is increased. Such a behaviour points out the presence of discret energy levels of single dots, that should be present in the case of isolated quantum dots. INTRODUCTION The current development of crystal growth techniques have allowed the fabrication of two dimensional nanostructure arrays, and even lattices, such as quantum dots (QD) lattices, which constitutes a new generation of artificial structured materials with controlable electronic and optical properties [1][2]. Lee et al. has shown that it is possible to tune the size, orientation and the number of QDs in the basis of the QD crystal unit cell. This is achieved by means of epitaxially self-assembling of QDs on a coherently strained layer [1]. A good understanding on the transport and optical properties of this new systems is necessary to make their possible applications in quantum effect devices a reality. The electronic structure of QD lattices has been found using a variety of methods, for example augmented plane wave calculations (APW) [4] and envelope function approximation [3], where QDs are depicted shematically. As in real crystals, coupling between artificial atoms (QDs) leads to formation of minibands due to the splitting of discrete energy levels of single dots, generating an artificial crystal.

Norm conserving pseudopotentials for c, n and o atoms

ABSTRACT En este trabajo hacemos una breve revisión de la teoría de los pseudopotenciales que con... more ABSTRACT En este trabajo hacemos una breve revisión de la teoría de los pseudopotenciales que conservan la norma. A partir de las auto energías y funciones propias de c\&#39;alculos autoconsistentes realizados para los \&#39;atomos C, N y O se obtienen los correspondientes pseudopotenciales y pseudofunciones de onda.

Excitones en anillos cuánticos bidimensionales

ABSTRACT The technology advances during the last years have allowed the growth of semiconductor s... more ABSTRACT The technology advances during the last years have allowed the growth of semiconductor structures such as quantum wires, arrangement of quantum dots or bidimensional quantum disks, among others. We solve the hamiltonian of this system using the Fractal Dimension Method. In this method the exciton trial wave function is taken as a product of free electron and free hole wave functions and a correlation function. It depends only on electron-hole separation. In particular, we consider a confinement potential shape that model quantum rings. We present results of the exciton binding energy for ground and some excited states as function of the width of ring and the applied magnetic field. Los desarrollos tecnológicos alcanzados en las últimas décadas han permitido el crecimiento de estructuras semiconductoras con dimensiones del orden de los nanómetros, tales como hilos, arreglos de puntos o discos cuánticos bidimensionales, entre otros. El Hamiltoniano del sistema, se resuelve usando el método variacional de Dimensión Fractal. En este, la función de prueba del excitón se toma como el producto de las funciones de ondas del electrón y del hueco no ligados en el sistema cuántico y una función de correlación arbitraria que depende solamente de la separación electrón-hueco. En particular, consideraremos una forma de potencial de confinamiento que modela anillos cuánticos. Se presentan resultados de la energía de enlace del excitón, del estado base y algunos estados excitados como función del ancho del anillo y del campo magnético.

Excitones Confinados en Puntos Cuanticos Tipo II

ABSTRACT Recent advances in the area of the nanotecnology have allowed to development new nanostr... more ABSTRACT Recent advances in the area of the nanotecnology have allowed to development new nanostructures [?]. In this work, we study the efects of the spatial confinement on the energy of the base state of an exciton in a type II quantum dot [?]. For this system, the Hamiltonian is described into approaches of efective mass and free electron. The problem is solved using the variational method. Two-dimensional spherical quantum dots are studied for diferent radii (sizes) and the efect of form of the confinement potencial between electron-hole is analyzed. Los recientes trabajos en el área de la nanotecnología han permitido el desarrollo de nuevas heteroestructuras de tamaños nanométricas [?]. En este trabajo estudiamos el efecto del confinamiento espacial sobre la energía del estado base de un exciton confinado en un punto cuántico tipo II [?]. Para este sistema el hamiltoniano se describe dentro de las aproximaciones de masa efectiva y de electrón independiente. El problema se resuelve utilizando el método variacional. Se estudian puntos cuánticos esféricos bidimensionales con diferentes radios (tamaños) y se analiza el efecto de la forma del potencial de confinamiento entre el electrón y el hueco.

Editorial: Mecánica cuántica: Un reto inaplazable para nuestro país

Is the dynamics of dissipative lorentzian channels non markovian?

Resumen Consideramos la dinámica disipativa de un excitón en un reservorio Lorentziano, relevante... more Resumen Consideramos la dinámica disipativa de un excitón en un reservorio Lorentziano, relevante en la descripción de algunos complejos fotosintéticos. Aunque esta dinámica se ha descrito con frecuencia como no Markoviana, mostramos que esta puede ser una caracterización equivocada. Describimos un enfoque en el cual es posible decidir si la dinámica es o no Markoviana y lo aplicamos al reservorio Lorentziano.

Download

Rafael Rey

Uploads

Papers by Rafael Rey

Log In