2009-2010
14 July Manuel Feito Guzman, Universidad Complutense de Madrid
"Ratchets brownianas retroalimentadas"
Las ratchets o motores brownianos pueden verse como controladores que actúan sobre sistemas estocásticos con el objetivo de inducir movimiento neto a través de la rectificación de las fluctuaciones. En particular, las ratchets de ciclo cerrado o retroalimentadas son aquéllas cuyo mecanismo de rectificación depende explícitamente del estado del sistema. Mostraremos nuestros trabajos sobre la dinámica y el rendimiento de estas ratchets bajo diversos protocolos de actuación, incluso en la presencia de retardos temporales y ruidos en el control. Veremos finalmente como el uso de la información sobre el estado del sistema permite incrementar el rendimiento sobre las ratchets de ciclo abierto, tal y como se ha comprobado en recientes trabajos teóricos y experimentales.
30 June Sang Wook Kim, Pusan National University
"Maxwell's demon in quantum wonderland"
The Szilard engine, a physically serious version of Maxwell's demon, is a machine which extracts useful work from information entropy. I will talk about its fully quantum mechanical description and discuss manifestation of the quantum statistical nature of identical particles, i.e., bosons and fermions.
16 June Alejandro Bermúdez, Universidad Complutense de Madrid
"2D and 3D Topological Insulators in Condensed Matter and High Energy"
Topological insulators (TIs) comprise exotic states of matter with an insulating bulk, but metallic boundaries. These phases, such as the well-known quantum Hall effect, support dissipationless edge currents protected by some topological order. In this talk, I will introduce the concept of TIs, discuss different examples that arise in condensed-matter and high-energy physics, and finally focus on their realization with ultracold fermions in optical lattices.
26 May Filip Beunis, Ghent University and ICFO
"A Modern Version of Millikan's Oil-drop Experiment"
Almost a hundred years ago, Robert Millikan found that measurements of the charge on aerosol oil droplets were all integer multiples of a constant value. He concluded that there must be some indivisible, elementary charge and went on to calculate its value. This work earned him the Nobel Prize for Physics in 1923, and was recently elected (by readers of the New York Times) to be the third most beautiful experiment ever performed.
Performing a similar experiment for colloidal particles in a liquid is even more challenging than for oil drops in air. This is mainly due to the higher viscosity of liquids, and to the fact that (surface) charge on a particle is much less stable in a liquid than in air. Such experiments, in which we can monitor the elementary events of a charging reaction, could however provide valuable information about processes on solid/liquid interfaces, many of which are not completely understood yet.
In this presentation, measurements of the charge on a single optically trapped polymer particle in dodecane are presented. The accuracy of the experiments is high enough to resolve the elementary charge, while the time between measurements is still small enough to see the elementary events when
the charge jumps from one multiple of the electron charge to another. Because of the optical tweezers, the charging process can be followed during thousands of seconds, which allows making detailed statistics of the elementary reaction steps.
Note: This seminar is on a Monday
17 May András Jánossy, Budapest University of Technology and Economics
"Two dimensional spin dynamics in layered organic conductors"
The family of organic layered conductors k-(BEDT-TTF)2X has a rich phase diagram of metallic, superconducting and magnetically ordered phases. In these sandwich materials single atom thick polymer layers, (denoted by X in the chemical formula), separate conducting or magnetically oredered single molecule thick BEDTT-TTF layers. In the high temperature metallic phases, transport perpendicular to the layers is effectively blocked. Estimates of the anisotropy between in-plane and perpendicular conductivities exceed 105 at ambient temperatures. The measured dc conductivity anisotropies are however, orders of magnitude smaller. We measure the interlayer spin hopping quantitatively with a novel electron spin resonance method in X=Cu[N(CN)2]Cl crystals and find that the anisotropy of spin diffusion is extremely large. Spin diffusion is two-dimensional at ambient temperatures and pressures; electrons are confined to a single molecular layer within the relatively long spin life time. The inter layer hopping rate increases rapidly under pressure. The ground state at zero pressure is a weak ferromagnet, with very weak coupling between the magnetic molecular layers. Under moderate pressures, the system is superconducting below 12 K.The high temperature interplanar hopping rate correlates with the low temperature state.
12 May Luis Vina, Universidad Autonoma de Madrid
"Polaritones en microcavidades semiconductoras: condensados fuera de equilibrio"
Las microcavidades de semiconductores son un sistema único para estudiar la física de bosones débilmente interactuantes. Sus excitaciones elementales, los polaritones — mezclas de excitones y fotones —s e pueden acumular en estados macroscópicos degenerados para formar condensados.
En esta charla presentamos las características de condensados de polaritones, creados bajo excitación resonante, e ilustramos la observación de comportamiento superfluido que se manifiesta como movimiento sin fricción y circulación permanente en presencia de vórtices.
“Collective fluid dynamics of a polariton condensate in a semiconductor microcavity”. Nature 457, 291 (2009)
“Persistent currents and quantised vortices in a polariton superfluid”, Nature Physics (in press)
5 May Thorsten Emig, Institut für Theoretische Physik, Universität zu Köln
"Casimir effects"
The properties of fluctuation induced interactions like van der Waals and Casimir-Lifshitz forces are of interest in a plethora of fields ranging from biophysics to nanotechnology. Recent high precision experiments and emerging micro-electromechanical applications have generated great interest in the properties and the selective manipulation of these interactions. In this talk, I will present a powerful method to compute Casimir forces and torques between various shapes and materials. The approach is based on a combination of methods from statistical physics and scattering theory.
7 April Manuel Donaire, Centro de Física do Porto, Portugal
"The quantum vacuum of complex media and the Casimir energy"
In this talk, I will explain how the electromagnetic vacuum gets polarized in a dielectric medium made of single dipoles. The construction of the dielectric constant will be addressed following a field theory diagrammatic approach. I will argue that the Lamb shift, the spontaneous emission rate and the van-der-Waals pressure derive from variations of the self-energy of the dielectric matter. In addition, a zero-temperature pressure is found as a result of variations of the zero-point-energy. The Casimir energy spectrum will be discussed and possible experimental setups proposed. Some remarks on the cosmological consequences will be made.
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24 March Tobias Salger, University of Bonn, Germany
"Directed transport of atoms in a Hamiltonian quantum ratchet"
Ratchets are a tool that generate a directed motion of particles in the absence of any gradients or net forces. The ratchet effect can be realised, for instance, in a fluctuating environment as a physical mechanism of microbiological motion. In order to observe a directed transport of atoms one has to break the spatiotemporal symmetries of the system. We demonstrate the first experimental realisation of a quantum ratchet, working in the absence of dissipation (Hamiltonian regime) within the observation time. A directed motion of atoms can be observed by exposing an 87-Rb Bose-Einstein condensate to a sawtooth-like optical lattice potential, the amplitude of which is modulated periodically in time. The ratchet potential is built up by superimposing a conventional optical standing wave with 2λ spatial periodicity and a lattice with 4λ spatial periodicity, created with the same laser, using the dispersive properties of the ground state atom. The ratchet transport arises from broken spatiotemporal symmetries of the driving potential, leading to a desymmetrisation of the transporting eigenstates (Floquet states).
24 February M.A. Gonzalez, Institut Laue Langevin, Grenoble, France
"La dispersión de neutrones aplicada al estudio de sistemas desordenados"
La determinación de la estructura y la dinámica a nivel microscópico de un sistema desordenado constituye una tarea extremadamente compleja debido a la imposibilidad de aplicar las numerosas herramientas matemáticas utilizadas para describir un cristal. Por esta razón la investigación de un sistema sin orden de largo alcance requiere habitualmente el uso de varias técnicas experimentales y su combinación con la simulación u otros tipos de modelización. Entre ellas, la dispersión de neutrones constituye un método particularmente poderoso y bien adaptado a dicho estudio, además de complementar perfectamente las técnicas de simulación habitualmente usadas. En este seminario se presentarán brevemente los fundamentos básicos de la teoría de la dispersión de neutrones y los distintos tipos de instrumentos existentes para el estudio de las propiedades estructurales y dinámicas de sistemas desordenados. A continuación, mediante una serie de ejemplos se mostrará el tipo de información que los neutrones pueden proporcionar sobre las propiedades microscópicas de líquidos, vidrios y sistemas biofísicos.
27 January Mathias Albert, University of Geneva
"Mesoscopic physics with cold atoms: from superfluidity to Anderson localization"
Phase coherence is a key ingredient of many characteristic quantum effects in transport phenomena, some of the most striking ones being superfluidity, conductance quantization, or the quantum Hall effect. In particular, interference effects have a prominent role in presence of disorder, resulting in weak or strong Anderson localization.
In this talk I will discuss transport properties of a one dimensional Bose-Einstein condensate moving through a disordered region of finite extent. I will show that the interplay between disorder and interactions leads to a quite rich physics where superfluidity and Anderson localization compete. Motivated by recent experiments, I will also discuss the effect of disorder on dipole oscillations as well as the localization properties of the quasi-periodic Aubry-Andre model.
References: T. Paul et al PRL 98, 210602 (2007)
M. Albert et al PRL 100, 250405 (2008)
M. Albert et al arXiv:0905.2331 (2009)
20 January Andrey Malyshev, Universidad Complutense
"Localzation properties of a 1D tight-binding model with non-random long-range intractions"
We focus on localization properties of the one-dimensional tight-binding Hamiltonian model with stochastic uncorrelated on-site energies and non-fluctuating long-range hopping integrals. It was argued [A. Rodriguez at al., J. Phys. A 33, L161 (2000)] that this model undergoes a localization-delocalization transition with respect to the disorder magnitude, provided 1 < μ < 3/2. The transition occurs at one of the band edges. The states at the other band edge are always localized, which suggests the existence of a single mobility edge. We analyze the mobility edge and show that, although the number of delocalized states tends to infinity, they form a set of null measure in the thermodynamic limit, i.e. the mobility edge tends to the band edge. The critical magnitude of disorder for the band edge states is calculated numerically versus the interactionexponent μ by making use of the conjecture on the universality of the normalized participation number distribution at criticality. A simple qualitative criterion of the existence of delocalized states in disordered models will also be discussed.
13 January Elsa Prada, Instituto de Ciencia de Materiales de Madrid (CSIC)
"Quantum pumping in graphene"
We show that graphene-based quantum pumps can tap into evanescent modes, which penetrate deeply into the device as a consequence of Klein tunneling. The evanescent modes dominate pumping at the Dirac point, and give rise to a universal response under weak driving for short and wide pumps, in close analogy to their role for the minimal conductivity in ballistic transport. In contrast, evanescent modes contribute negligibly to normal pumps. Our findings add a new incentive for the exploration of graphene-based nanoelectronic devices.
16 December Oliver Morsch,University of Pisa
"Coherent control of dressed matter waves"
Strong driving of a periodic potential can lead to a reduction or even complete suppression of tunneling between the lattice wells. In our experiments with Bose-Einstein condensates in strongly driven ("shaken") optical lattices we have
demonstrated this tunneling suppression and found that the coherence of the wavefunction can be preserved if the parameters of the driving are chosen appropriately. This allowed us to load the condensate adiabatically into a single
Floquet state of the driven system and to explore the properties of the resulting "dressed matter waves". In particular, in a three-dimensional lattice we have seen strong evidence that the superfluid to Mott insulator quantum phase transition can be induced through the strong driving.
25th November Fernando Cucchietti, ICFO, Barcelona.
"Quantum simulations of critical systems"
Quantum phase transitions are among the most interesting many body phenomena in condensed matter physics. By studying it from a quantum information point of view, we stand to gain not only a deep understanding of the properties of states near quantum criticality, but also the possibility of controlling an arbitrary quantum phase transition directly in the laboratory -- by means of a quantum simulator. This approach, still in its infancy, has already been demonstrated in a variety of systems. I will discuss the theory and experimental implementation of two quantum simulations: in the first one, we measured the critical points of an antiferromagnetic Ising chain with transverse and longitudinal fields. In the second one, we study the geometric phase of a simple open system in presence of an environment near quantum criticality. I will discuss how even if the details of our experiments depend on the nuclear magnetic resonance hardware we chose, the theoretical methods and observables we study are general and should scale well to other quantum simulations.