43 research outputs found
Aplicación del Mentoring (Mentor) en el proceso de Enseñanza-Aprendizaje en la Educación Superior
En el ámbito educativo, el mentoring podría ser muy útil en periodos donde la adaptación de los estudiantes sea clave para poder lograr un éxito posterior. Puede ayudar a entender la importancia de determinadas normas y a asumir determinados objetivos de gestión de rendimiento. El proceso termina cuando el estudiante se ve suficientemente capaz para continuar con sus tareas sin supervisión. El mentoring puede ser utilizado como una herramienta educativa y de fomento del liderazgo en la medida en que algunos estudiantes pueden ejercer de mentores de otros en la incorporación al Centro Educativo o a determinados programas, lógicamente con la supervisión correspondiente del tutor
Desorption kinetics and interaction of Xe with single-wall carbon nanotube bundles
We present a study on the kinetics of xenon desorption from single-wall
carbon nanotube (SWNT) bundles using thermal desorption spectroscopy (TDS).
TD-spectra from SWNT samples show a broad desorption feature peaked at
significantly higher temperature than the corresponding low-coverage desorption
feature on graphite. The observations are explained using a coupled
desorption-diffusion (CDD) model, which allows the determination of the
low-coverage Xe binding energy for adsorption on SWNT bundles, 27 kJ/mol. This
energy is about 25% higher than the monolayer binding energy on graphite, 21.9
kJ/mol. By comparison with molecular mechanics calculations we find that this
increase of the binding energy is consistent with adsorption in highly
coordinated groove-sites on the external bundle surface.Comment: 8 pages, 5 figure
Inversion of two-band superconductivity at the critical electron doping of (Mg,Al)B-2
Electron energy-loss spectroscopy (EELS) was combined with heat capacity measurements to probe changes of electronic structure and superconductivity in Mg(1-x)AlxB2. A simultaneous decrease of EELS intensity from sigma-band hole states and the magnitude of the sigma gap was observed with increasing x, thus verifying that band filling results in the loss of strong superconductivity. These quantities extrapolated to zero at x approximate to 0.33 as inferred from the unit cell volume. However, superconductivity was not quenched completely, but persisted with T-c< 7 K up to about x approximate to 55. Only the pi band had detectable density of states for 0.33 less than or similar to x less than or similar to 0.55, implying an inversion of the two-band hierarchy of MgB2 in that regime. Since pi-band superconductivity is active in other materials such as intercalated graphite, implications for new materials with high T-c are discussed
Perpendicular-current Studies of Electron Transport Across Metal/Metal Interfaces
We review what we have learned about the scattering of electrons by the
interfaces between two different metals (M1/M2) in the
current-perpendicular-to-plane (CPP) geometry. In this geometry, the intrinsic
quantity is the specific resistance, AR, the product of the area through which
the CPP current flows times the CPP resistance. We describe results for both
non-magnetic/non-magnetic (N1/N2) and ferromagnetic/non-magnetic (F/N) pairs.
We focus especially upon cases where M1/M2 are lattice matched (i.e., have the
same crystal structure and the same lattice parameters to within ~ 1%), because
in these cases no-free-parameter calculations of 2AR agree surprisingly well
with measured values. But we also list and briefly discuss cases where M1/M2
are not lattice matched, either having different crystal structures, or lattice
parameters that differ by several percent. The published calculations of 2AR in
these latter cases do not agree so well with measured values.Comment: 6 pages, 2 figures, 2 tables. In Press: Applied Surface Scienc
Scattering theory of interface resistance in magnetic multilayers
The scattering theory of transport has to be applied with care in a diffuse
environment. Here we discuss how the scattering matrices of heterointerfaces
can be used to compute interface resistances of dirty magnetic multilayers.
First principles calculations of these interface resistances agree well with
experiments in the CPP (current perpendicular to the interface plane)
configuration.Comment: submitted to J. Phys. D (special issue at the occasion of Prof. T.
Shinjo's 60th birthday
Phonons and specific heat of linear dense phases of atoms physisorbed in the grooves of carbon nanotube bundles
The vibrational properties (phonons) of a one-dimensional periodic phase of
atoms physisorbed in the external groove of the carbon nanotube bundle are
studied. Analytical expressions for the phonon dispersion relations are
derived. The derived expressions are applied to Xe, Kr and Ar adsorbates. The
specific heat pertaining to dense phases of these adsorbates is calculated.Comment: 4 PS figure
Dimensional Crossover of Dilute Neon inside Infinitely Long Single-Walled Carbon Nanotubes Viewed from Specific Heats
A simple formula for coordinates of carbon atoms in a unit cell of a
single-walled nanotube (SWNT) is presented and the potential of neon (Ne)
inside an infinitely long SWNT is analytically derived under the assumption of
pair-wise Lennard-Jones potential between Ne and carbon atoms. Specific heats
of dilute Ne inside infinitely long (5, 5), (10, 10), (15, 15) and (20, 20)
SWNT's are calculated at different temperatures. It is found that Ne inside
four kinds of nanotubes exhibits 3-dimensional (3D) gas behavior at high
temperature but different behaviors at low temperature: Ne inside (5, 5)
nanotube behaves as 1D gas but inside (10, 10), (15, 15), and (20, 20)
nanotubes behaves as 2D gas. Furthermore, at ultra low temperature, Ne inside
(5, 5) nanotube still displays 1D behavior but inside (10, 10), (15, 15), and
(20, 20) nanotubes behaves as lattice gas.Comment: 10 pages, 5 figure
Electron doping in MgB2 studied by electron energy-loss spectroscopy
The electronic structure of electron-doped polycrystalline Mg1-xAlx(B1-yCy)(2) was examined by electron energy-loss spectroscopy (EELS) in a scanning transmission electron microscope (STEM) and first-principle electronic structure calculations. We found significant changes in the boron K edge fine structure, suggesting the two bands of the B K edge, the sigma and the pi band, are being simultaneously filled as the electron doping concentration of Mg1-xAlx(B1-yCy)(2) was increased. Our density-functional theory calculations confirm the filling of the sigma band states close to the Fermi level, which is believed to cause the loss of superconductivity in highly doped MgB2, since the electron-phonon coupling of these states is thought to be responsible for the high superconducting transition temperature. Our results do not show significant differences in the electronic structure for electron doping on either the Mg or the B site, although many superconducting properties, such as T-c or H-c2 differ considerably for C- and Al-doped MgB2. This behavior cannot be satisfactorily explained by band filling alone, and effects such as interband scattering are considered
Specific heats of dilute neon inside long single-walled carbon nanotube and related problems
An elegant formula for coordinates of carbon atoms in a unit cell of a
single-walled nanotube (SWNT) is presented and the potential of neon (Ne)
inside an infinitely long SWNT is analytically derived out under the condition
of the Lennard-Jones potential between Ne and carbon atoms.
Specific heats of dilute Ne inside long (20, 20) SWNT are calculated at
different temperatures. It is found that Ne exhibits 3-dimensional (3D) gas
behavior at high temperature but behaves as 2D gas at low temperature.
Especially, at ultra low temperature, Ne inside (20, 20) nanotubes behaves as
lattice gas. A coarse method to determine the characteristic temperature
for low density gas in a potential is put forward. If
, we just need to use the classical statistical
mechanics without solving the Shr\"{o}dinger equation to consider the thermal
behavior of gas in the potential. But if , we
must solve the Shr\"{o}dinger equation. For Ne in (20,20) nanotube, we obtain
K.Comment: 14 pages, 7 figure