Seminars 2012

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.

Material science and catalytic systems play a very important role in many man-kind activities as well as in nature. This can be easily understood if we consider that more than 90 catalysis-related 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 in-situ 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 in-situ characterization methodologies, to study catalytic systems and the synthesis of nanomaterials under real conditions will be presented regarding bio-mimetic and metal oxide catalysts. The advantages on understanding the underlying mechanism and establishing structural/functional relationship will be discussed.

We analyze spin dynamics in one dimensional systems and find that despite simplicity they show interesting surprises.
We concentrate on two effects: spin-dependent tunneling and spin relaxation and noise.
First, we analyze spin dynamics in the tunneling decay of a localized particle in the presence of spin-orbit 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 diffraction-in-time 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 one-dimensional 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 spin-orbit 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.

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 Ginzburg-Landau equation and the unsteady, laminar flow in a 2D driven cavity.

Magnetic Resonance Imaging (MRI) is a well-established 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 phase-sensitivity of the MR signal to particle motion. In the last decade the full potential of MRI-based 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: three-dimensional diffusers, jets in cross-flow, turbine blade cooling configurations, compact heat exchangers, and flow in porous media. The advantages of the technique emerge: the capability of providing three-dimensional 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.

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 one-dimensional lattice with a non-trivial topology, threaded by a uniform magnetic flux, and study the effect of adding a time-periodic ac electric field. We will show that the dynamical phases produced by the driving field can combine with the familiar Aharonov-Bohm 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. Gómez-León 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).