Homogeneity assessment of YBa_2Cu_3O_(7-x) single crystals by cathodoluminescence microscopy

Cathodoluminescence (CL) of YBa_2Cu_3O_(7-x) single crystals has been investigated in the scanning electron microscope, Some regions of the crystals, generally related to steps or other growth features, show enhanced CL Intensity. Spectra show the presence of two emission bands centered at about 2.85 and 2.3-2.4 eV, respectively. In the regions of higher CL intensity, the 2.3 eV band, which has been attributed to defects in the oxygen sublattice, shows a higher relative intensity. (C) 1996 Americans Institute of Physics

Shape-controlled synthesis and cathodoluminescence properties of elongated α-Fe_2O_3 nanostructures

α-Fe_2O_3 (hematite) nanostructures with various morphologies have been grown by thermal oxidation of compacted iron powder at temperatures between 700 and 900 degrees C. Different thermal treatments have been found to induce the growth of single-crystalline nanowires, nanobelts, nanoplates and featherlike structures, free and caped nanopillars, and pyramidal microcrystals or cactuslike microstructures. The experimental conditions leading to the different morphologies have been systematically investigated, as well as the possible growth mechanisms. The obtained nanostructures have been characterized by scanning electron microscopy (SEM), high-resolution transmission electron microscopy, x-ray diffraction, and cathodoluminescence (CL) spectroscopy in the SEM. The formation of the nanostructures induces changes in the intensity and spectral distribution of the CL emission, as compared with the bulk material. Ligand to metal charge transfer transitions as well as Fe^(3+) ligand field transitions are thought to be involved in the observed luminescence. The evolution of the panchromatic CL intensity in the visible range as a function of temperature shows some anomalies that may be induced by magnetic ordering effects

Anomalies in the cathodoluminescence of the antiferromagnetic oxides NiO and CoO

Cathodoluminescence (CL) of the antiferromagnetic oxides NiO and CoO has been investigated. Both compounds show luminescence bands in the visible and near infrared spectral ranges, tentatively attributed to intra-ionic transitions. Dependence of the visible CL intensity with temperature presents anomalies at about half of the Neel temperature, which are explained by the existence of magnetic ordering effects. The results are briefly compared with previous reports on the CL from YBa_2Cu-3O-(7-x)

Magnetic transitions in alpha-Fe_2O_3 nanowires

Magnetic transitions in single-crystal alpha-F_2O_3 (hematite) nanowires, grown by thermal oxidation of iron powder, have been studied in the range of 5-1023 K with a superconducting quantum interference device below room temperature and with a vibrating sample magnetometer at higher temperatures. The broad temperature range covered enables us to compare magnetic transitions in the nanowires with the transitions reported for bulk hematite. Morin temperatures (T-M) of the nanowires and of hematite bulk reference powder were found to be 123 and 263 K, respectively. Also the Neel temperature (T-N) of the nanowires, 852 K, was lower than the bulk T-N value. Measurements of the magnetization as a function of temperature show an enhanced signal in the nanowires, which suggests a decrease in the anti ferromagnetic coupling. A coercive field observed below T-M in the hysteresis loops of the nanowires is tentatively explained by the presence of a magnetic phase

Electron beam induced current and scanning tunnelling spectroscopy correlative study of Cd-xHg_(1-x)Te and CdTe crystals

A combined scanning electron microscope-scanning tunnelling microscope (SEM-STM) system has been used to characterize CdxHg1-xTe and CdTe crystals, The electron beam induced current (EBIC) mode of the SEM shows the existence of inhomogeneities in the electronic behaviour of the samples, mainly related to the presence of subgrain boundaries and precipitates. Current imaging tunnelling spectroscopy images and the related normalized differential conductance curves, obtained with the STM, reveal the electronic inhomogeneities at a finer scale. In particular, local variations of the band gap were shown by the conductance curves in regions with strong EBIC contrast. SEM-and STM-based techniques in a combined instrument appear to be complementary characterization techniques

Cathodoluminescence characterization of InGaSb crystals

The nature and the spatial distribution of radiative defects in In(X)Ga(1-x)Sb grown by the vertical Bridgman method have been studied by cathodoluminescence (CL) in a scanning electron microscope. The CL results have been complemented by X-ray microanalysis and backscattered electron imaging to relate the local luminescence properties to the chemical composition. Measurements of the band gap energy from the CL spectra, supported by X-ray compositional mappings, reveal an effective incorporation of In in the matrix, leading to the fori-nation of the ternary alloy in the whole volume of the ingot. A band often observed in the CL spectra, peaked at about 20 meV below the band gap energy, is attributed to the presence in the ternary alloy of an acceptor level that would correspond to the V(Ga)-Ga(Sb) acceptor in GaSb

Properties of Hα-selected star-forming galaxies from z ∼ 0.8 to now

We present a long-term project to study the characteristics and evolution of current star-forming galaxies at different redshifts. Our results support the idea that the higher Star Formation Density measured at redshift z=0.24 is due to an increase of the density of bursting galaxies and not to an intrinsic change on overall galaxy properties

Análisis de las actitudes de los estudiantes hacia el aprendizaje colaborativo y en línea

El presente proyecto aborda dos aspectos muy importantes y estrechamente ligados a la adquisición de competencias básicas de cualquier titulación de grado actual y altamente demandadas por los empleadores: la adquisición de habilidades para desarrollar trabajo colaborativo y la capacidad de desarrollar actividades de aprendizaje en línea. Estas últimas tienen una doble función. Por una parte, deben preparar a los estudiantes para ser capaces de gestionar su trabajo, de manera autónoma, con vistas al posible desarrollo de trabajo no presencial, cada vez más importante en muchos entornos laborales. Por otra parte, deber permitirles adquirir las habilidades necesarias para continuar se formación a lo largo de su vida (LLL, Long Life Learning) a través de entornos más informales, pero no por ello menos efectivos, como puede ser el caso de los Massive Online Open Courses (MOOC), ofertados cada vez por más instituciones, con cursos de calidad, impartidos por grandes expertos y con posibilidades de certificación del trabajo realizado

Anisotropy of the electric field gradient in two-dimensional alpha-MoO_(3) investigated by (57)^Mn((57)^Fe) emission mossbauer spectroscopy

Van der Waals alpha-MoO_(3) samples offer awide range of attractive catalytic, electronic, and optical properties. We present herein an emission Mossbauer spectroscopy (eMS) study of the electric-field gradient (EFG) anisotropy in crystalline free-standing alpha-MoO_(3) samples. Although alpha-MoO3 is a twodimensional (2D) material, scanning electron microscopy shows that the crystals are 0.5-5-mu m thick. The combination of X-ray diffraction and micro-Raman spectroscopy, performed after sample preparation, provided evidence of the phase purity and crystal quality of the samples. The eMS measurements were conducted following the implantation of (57)^Mn (t(1/ 2) = 1.5 min), which decays to the (57)^Fe, 14.4 keV Mossbauer state. The eMS spectra of the samples are dominated by a paramagnetic doublet (D1) with an angular dependence, pointing to the Fe^(2+) probe ions being in a crystalline environment. It is attributed to an asymmetric EFG at the eMS probe site originating from strong in-plane covalent bonds and weak out-of-plane van derWaals interactions in the 2D material. Moreover, a second broad component, D2, can be assigned to Fe^(3+) defects that are dynamically generated during the online measurements. The results are compared to ab initio simulations and are discussed in terms of the in-plane and out-of-plane interactions in the syste

Estudio de propiedades de recombinación electrónica en superconductores de alta temperatura crítica mediante técnicas de microscopía electrónica de barrido y microscopía túnel

En este trabajo se han utilizado distintas técnicas de caracterización basadas en el microscopio electrónico de barrido (SEM) y el microscopio de efecto túnel (STM) con el propósito de investigar las propiedades de recombinación electrónica de diferentes superconductores de alta temperatura. En el caso del SEM dichas técnicas son fundamentalmente la catodoluminiscencia (CL) y la corriente inducida por el haz de electrones en modo remoto (REBIC), aunque también se utilizan con frecuencia el microanálisis de rayos X en dispersión de energías (EDX) o longitudes de onda (WDX), el modo de emisión de electrones secundarios (EES) y la fotoluminiscencia (PL) con resolución temporal. En lo que al STM se refiere, además del modo usual de corriente constante, hacemos uso de la espectroscopía túnel en el modo CITS (Current Induced Tunnelling Spectroscopy) y desarrollamos un dispositivo experimental con el cual se extiende la técnica SEM-REBIC al STM. Se investiga la CL de cerámicas y/o monocristales de YBa2Cu3O7-x (YBCO), Bi2Sr2CaCu2O8+x(Bi-2212), TI2Ba2CuO6+8 (TI-2201) y Ba2Cu5Ca6(CuxCay)Oz (BCCO), y se examina la relación entre la distribución espacial de la emisión y la morfología de cada muestra, analizando las características comunes de la luminiscencia detectada en los distintos superconductores y estudiando la evolución de la CL con la temperatura. Asimismo se analizan los cambios microestructurales y composicionales inducidos en los superconductores por diferentes tratamientos capaces de alterar la distribución o el contenido de oxígeno de las muestras, tales como la irradiación electrónica en el SEM, la deformación mecánica o el ataque químico. Por otro lado se demuestra por primera vez la capacidad de la técnica REBIC en el estudio de las propiedades de conducción electrónica de superconductores de YBCO y Bi-2212. Mediante el uso de un microscopio combinado SEM / STM se lleva a cabo un estudio correlativo de la topografía y propiedades electrónicas de muestras monocristalinas de YBCO y Bi-2212 en el que se comparan los resultados de las medidas de CITS efectuadas en las zonas en las que previamente se ha observado contraste SEM-REBIC con los obtenidos en áreas alejadas de estas últimas. Se investiga asimismo la modificación a escala nanométrica de la superficie de ambos tipos de superconductores mediante la aplicación de pulsos de voltaje con el STM, un proceso de interés tecnológico relacionado con el desarrollo de dispositivos electrónicos basados en superconductores de alta Tc. Por último, se desarrolla un dispositivo experimental con el cual se extiende el uso de la técnica REBIC al STM, y se investiga posteriormente su posible aplicación al estudio de los defectos cargados observados en los monocristales de YBCO y Bi-2212 mediante SEM-REBIC