Seminars 2012
Seminario
Viernes, 30 de noviembre de 2012
Analysing the structure of graphene at the atomic level
J. H. Warner
Department of Materials, University of Oxford
Abstract
Defects in graphene influence its electronic, chemical,
magnetic and mechanical properties. In this talk I will
discuss how we can study defects and their impact on
the structure of graphene at the single atom level. We
produce synthetic graphene by chemical vapour
deposition and transfer it to TEM grids for analysis.
Using Oxford's JEOL 2200 HRTEM fitted with spherical
aberration correctors and a monochromator for the
electron gun, we can achieve 80pm spatial resolution at
a low accelerating voltage of 80kV. We have developed
techniques to introduce defects in a defined spatial
location with 10nm precision and study there stability
and dynamics. Edge dislocation pairs are formed by
sputtering carbon atoms along the zig sag direction and
lead to substantial distortion of the lattice. We map
out the strain sensors from these dislocation using
geometric phase analysis. These results provide some of
the most detailed knowledge to date on the true atomic
form of defects in graphene.
El Seminario tendrá lugar a las 12:30 horas en la Sala
2.1.C19 (Edificio Sabatini) Universidad Carlos III
Seminario
Viernes, 23 de noviembre de 2012
Structural and functional relationship in materials,
catalysts and environmental systems
Konstantinos Christoforidis
UC3M
Abstract
Material science and catalytic systems play a very
important role in many mankind activities as well as
in nature. This can be easily understood if we consider
that more than 90 catalysisrelated processes. In
nature, enzymes are the commonest and most efficient
catalysts found. Taking this into account, the idea of
transferring principles from nature to a chemistry lab
and mimicking enzymatic reactions by synthetic
catalysts looks very promising in an effort to produce
highly active and selective catalysts. On the other
hand solid materials used as heterogeneous catalysts
(i.e. semiconductors, metal oxides), have advantages
towards industrial applications. Howevee, in order to
achieve the ultimate goal of producing materials with
improved properties and to understand in depth
environmental systems, analysis in atomic scale and
electronic level are considered mandatory. Towards this
objective, the utilization of insitu spectroscopic
characterization techniques under real working
conditions and computational chemistry have gained
significance with respect to the most commonly used
classical characterization methodology.
In this seminar, the application of insitu
characterization methodologies, to study catalytic
systems and the synthesis of nanomaterials under real
conditions will be presented regarding biomimetic and
metal oxide catalysts. The advantages on understanding
the underlying mechanism and establishing
structural/functional relationship will be discussed.
El Seminario tendrá lugar a las 12:30 horas en la Sala
2.1.C19 (Edificio Sabatini) Universidad Carlos III
Seminario
Miércoles, 7 de noviembre de 2012
Spin dynamics in one dimension: any surprises?
Eugene Sherman
Universidad del País Vasco
Abstract
We analyze spin dynamics in one dimensional systems and
find that despite simplicity they show interesting
surprises.
We concentrate on two effects: spindependent tunneling
and spin relaxation and noise.
First, we analyze spin dynamics in the tunneling decay
of a localized particle in the presence of spinorbit
coupling. The spin polarization at a short time scale
is affected by initial state while at long times both
the probability and the spin density exhibit
diffractionintime phenomenon. We find that tunneling
in general can be characterized by a new parameter, the
tunneling length, which can be seen in the spin
precession.
Next, we consider the effects of random potentials in
onedimensional nanosystems and develop a theory of
spin relaxation there. A theory of spin noise in
semiconductor nanowires considered as prospective
elements for spintronics will be presented. In these
structures spinorbit coupling can be realized as a
random function of coordinate. We demonstrate that the
spin relaxation can be very slow and the resulting
noise power spectrum diverges as frequency goes to
zero.
El Seminario tendrá lugar a las 12:30 horas en la Sala
2.1.D03 (Edificio Sabatini) Universidad Carlos III
Seminario
Viernes, 26 de octubre de 2012
PODbased reduced order models to speed up the
numerical integration of unsteady problems
Filippo Terragni
UC3M
Abstract
Various ideas and methods involving local proper
orthogonal decomposition (POD) and Galerkin projection
are presented aiming at accelerating the numerical
integration of nonlinear time dependent parabolic
problems.
The proposed approach combines, in interspersed time
intervals, short runs with a given numerical solver and
reduced order models constructed by expanding the
solution of the problem into appropriate POD modes
(which span a POD manifold) and Galerkin projecting
some evolution equations onto that linear basis. The
POD manifold is completely calculated from the outset
and only updated as time proceeds according to the
dynamics, which yields an adaptive and flexible
procedure.
In addition, some properties concerning the weak
dependence of the POD modes on time and possible
parameters in the problem are exploited in order to
increase the flexibility and efficiency of the low
dimensional models, which turns out to be especially
interesting in the computation of bifurcations.
In this talk, the results obtained by applying the
developed techniques to the approximation of transient
dynamics and the simulation of attractors in
bifurcation problems are presented and discussed. The
test problems considered to illustrate the various
ideas are the 1D complex GinzburgLandau equation and
the unsteady, laminar flow in a 2D driven cavity.
El Seminario tendrá lugar a las 12:30 horas en la Sala
2.2.C17 (Edificio Sabatini) Universidad Carlos III
Seminario
Jueves, 28 de junio de 2012
Magnetic Resonance Imaging: a tool for investigating
fluid flows
Filippo Coletti
Stanford University
Abstract
Magnetic Resonance Imaging (MRI) is a wellestablished
technique in the medical community, able to produce
tomographic and volumetric images of the human body.
MRI can also be used to perform accurate velocimetry in
fluid flows, thanks to the phasesensitivity of the MR
signal to particle motion. In the last decade the full
potential of MRIbased techniques to investigate
engineering flows has been demonstrated. In this
seminar recent applications will be presented in which
mean velocity and scalar fields are measured with high
spatial resolution. Those include: threedimensional
diffusers, jets in crossflow, turbine blade cooling
configurations, compact heat exchangers, and flow in
porous media. The advantages of the technique emerge:
the capability of providing threedimensional fields in
complex geometries, with high data yield and without
the need of optical access. The potential for
developments in areas such as environmental and
biomedical engineering is discussed.
El Seminario tendrá lugar a las 12:30 horas en la Sala
2.3.B04 (Edificio Sabatini) Universidad Carlos III
Seminario
Miércoles, 8 de febrero de 2012
Coherent dynamics of electrons in ac driven quantum dot
arrays
Gloria Platero
Instituto de Ciencia de Materiales de Madrid (CSIC)
Abstract
A powerful method of manipulating the coherent dynamics
of quantum particles is to control the phase of their
tunnelling. We will show how such phases can be
produced in two distinct and complementary ways. We
have considered the dynamics of two interacting
electrons hopping on a quasi onedimensional lattice
with a nontrivial topology, threaded by a uniform
magnetic flux, and study the effect of adding a
timeperiodic ac electric field. We will show that the
dynamical phases produced by the driving field can
combine with the familiar AharonovBohm phases arising
from the magnetic flux to give precise control over the
dynamics and localization of the particles, even in the
presence of strong particle interactions [1].
Recent electron spin resonance experiments measure
coherent spin rotations of one single electron, a
fundamental ingredient for quantum operations. We will
show how it is possible to manipulate electron charge
and spin dynamics in double and triple quantum dots by
means of ac magnetic fields. We demonstrate that by
tuning the the field intensity, frequency and the phase
difference between the fields within each dot, charge
localization can be achieved. Furthermore, ac magnetic
fields are also able to induce spin locking, i.e., to
freeze the electronic spin, at certain field parameters
and symmetry configurations [2]. Spin Blockade has been
measured in transport experiments through double
quantum dots. We will discuss the effect of ac magnetic
fields on spin blockade and we will show that ac
magnetic fields can not only remove spin blockade, but
also restore it due to collective rotations of the two
spins at certain parameters of the field [3].
[1] C.E. Creffield and G. Platero, Phys. Rev. Lett.
105, 086804 (2010).
[2] A. GómezLeón and G. Platero, Phys. Rev. B (RC)
84, 121310(R) (2011).
[3] M. Busl et al., Phys. Rev. B 81, 121306(R)
(2010); R. Sánchez et al., Phys. Rev. B
77 165312 (2008).
El Seminario tendrá lugar a las 12:30 horas en la Sala
2.3.B04 (Edificio Sabatini) Universidad Carlos III
