17th June Sigmund Kohler, Instituto de Ciencia de Materiales de Madrid (CSIC)
"Dissipative Landau-Zener tunneling"
Abstract:The coupling of a qubit to a harmonic oscillator can induce Landau-Zener transitions of the qubit upon switching its energy splitting. The adiabatic energies of the qubit-oscillator setup are characterized by multiple exact and avoided level crossings, such that the usual two-level Landau-Zener formula is no longer applicable and needs to be generalized. Our generalization is based on selection rules for multi-level transitions and provides an exact expression for the bit-flip probability. Moreover, it provides an idea for solving the dissipative Landau-Zener problem, i.e., the Landau-Zener problem for a two-level system coupled to a bath of harmonic oscillators. An exact solution for zero temperature is presented, as well as the generalization to a spin bath. Aplications range from single-photon generation in circuit-QED to adiabatic quantum computing and the measurement of tunnel splittings in molecular nanomagnets.
10th June Brittany B. Nelson-Cheeseman, University of California - Berkeley
"Magnetism of Complex Oxide Thin Films, Interfaces and Heterostructures"
Abstract; Complex oxide thin films, interfaces and heterostructures provide a unique opportunity to investigate the exciting magnetic properties that arise at reduced dimensions and in the proximity to other materials. In this presentation, I will discuss a broad range of investigations into these complex oxide systems, ranging from magnetotransport device heterostructures to unconventional magnetic interface effects to anomalous
magnetism seen in single thin films. I will begin by introducing a novel spintronic architecture: the hybrid spin filter / magnetic tunnel junction. With the development of this device, I have demonstrated that putting long range magnetism into the barrier layer of a magnetic tunnel junction device can directly influence the spin transport and provide added functionality to these systems. Next, I will show how extensive soft x-ray synchrotron studies have uncovered unconventional magnetic phenomena at the interfaces between a magnetic insulator (NiMn2O4) and two highly spin polarized materials (Fe3O4 and La0.7Sr0.3 MnO3). I will show that at one interface there is strong magnetic coupling, which gives rise to interfacial magnetic phenomena not found in either of the constituent films; while at the other interface, one surprisingly obtains complete magnetic decoupling between two adjacent ferromagnetic films. Finally, I will discuss my investigation of an anomalous magnetic ground state in single thin films of NiMn2O4 by hard and soft synchrotron x-rays. The results reveal that the relevant magnetic exchange interactions are dramatically altered upon modifying the cation properties of this material, which results in the quenching of one of the two magnetic phases found in the bulk form.
3rd June Enrique Ruiz Arriola, University of Granada
"Dispersión múltiple y transporte cuántico"
29th April Javier Garcia de Abajo, Instituto de Optica (CSIC)
"How to control the flow of light at nanometre scales"
Abstract: Manipulating visible and near-infrared light at nanometre scales is important for applications to medicine (biosensing, cancer therapy, etc) and signal processing (optical computing, on-chip wideband communications). This is a challenge because the light wavelength is at least a fraction of a micrometre in this regime. However, metallic nanostructures prepared by colloidal chemistry or lithography can trap light over nanometre dimensions, thus producing extraordinary effects associated with large optical energy concentration. The conspicuous optical excitations supported by metals and known as plasmons have understandably become extremely relevant, and the source of a new subfield within nanophtonics: plasmonics. In this talk we will review in a tutorial fashion some important achievements within this field, including extraordinary optical transmission, total light absorption, highly-integrable plasmon waveguides, imaging and focusing with plasmons, and localised plasmon excitations.
22nd April Rafael Molina, Instituto de Esctructura de la Materia (CSIC), Madrid
"Cables cuánticos iluminados por un láser: localización y transporte" (postponed from 11th March)
Abstract: Estudiamos las propiedades de transporte y localización de cables cuánticos iluminados por un láser. Para ello desarrollamos un formalismo de funciones de Green-Floquet para hamiltonianos periódicos en el tiempo que permite la generalización de la definición de longitud de localización en el caso de sistemas desordenados y del formalismo de Landauer para el cálculo de la conductancia. Modificando los parámetros del láser podemos controlar las propiedades de transporte y localización de los cables cuánticos de forma coherente.
25th March Víctor Gópar, Universidad de Zaragoza
"Conductancia en nanocontactos: un análisis estadístico"
Abstract: La densidad de los autovalores de la transmisión en nanocontactos de Pb ha sido estimada experimentalmente en ¨mechanically controllable break-junctions¨ recientemente. Motivado por estos experimentos, estudiamos la evolución de la densidad de los autovalores de la transmisión como función del grado de desorden y el número de canales en la constricción balística del punto cuántico de contacto. Nuestro análisis teórico se basa en la ecuación de difusión para las transmisiones llamada ecuación de Dorokhov-Mello-Pereyra-Kumar. Hemos encontrado que la densidad de la transmisión evoluciona rápidamente con el número de canales hacia el resultado conocido para tal densidad en régimen metálico (distribución bimodal). Así pues, para unos pocos canales abiertos la densidad adquiere una distribución bimodal, en acuerdo con los análisis experimentales en nanocontactos de Pb.
11th March Daniel Haskel, Argonne National Laboratory
"Squeezing Magnets - and what can we learn from it"
Abstract: The interactions between electrons in a solid, particularly those affecting its magnetic properties, are intimately related to the overlap of their electronic wavefunctions. Altering this overlap by the application of pressure modifies these interactions and provides a powerful playground for studies of magnetic materials. Coupled with the element- and orbital-selectivity of x-ray magnetic circular dichroism (XMCD), high-pressure studies of magnetism with synchrotron
radiation in the diamond anvil cell (DAC) are bound to improve our understanding of magnetism in complex magnetic systems. We describe the recent development of a high-pressure (25 GPa) XMCD capability at the Advanced Photon Source for low-temperature (10 K), high-field (0.7 Tesla) experiments in a DAC featuring perforated diamond anvils [1-3]. These measurements can currently be performed in the 6-13 keV energy range allowing studies of transition metal (3d), rare-earth (4f), and 5d magnetic systems. We illustrate this capability with examples from recent experiments on giant magneto-caloric materials Gd5(SixGe 1-x)4, and magnetic semiconductors EuX (X=O, S, Se, Te).
[1] D. Haskel et al., High Pressure Research 28, 185 (2008)
[2] N. M. Souza Neto, DH et al., PRL 102, 057206 (2009)
[3] Y. Ding, DH et al., PRL 100, 045508 (2008)
[4] Y. C. Tseng, DH et al., PRB 78, 214433 (2008); ibid PRB 76, 014411 (2007).
25th February Juanma Parrondo, Universidad Complutense de Madrid
"Rectificación de paredes magnéticas"
Abstract: Analizamos el movimiento de paredes de dominio en materiales magnéticos delgados en presencia de defectos asimétricos. La asimetría de los defectos induce efectos de rectificación novedosos, debido a que las paredes son objetos extensos. En particular, en ciertas geometrías la rectificación mueve las paredes rectas en una dirección y las curvadas en la opuesta. Esta "ratchet" cruzada ha sido observada experimentalmente y puede ser útil para el diseño de dispositivos de interés tecnológico.
4th February Sebastian Bergeret, Universidad Autonoma de Madrid
"Non local transport through superconducting structures"
Abstract: I will discuss some properties of the non-local transport in superconductor-normal metal structure. In the first part I will consider the case of tunnel S/N interfaces and discuss the role of the interactions in such systems. In particular I will present some examples for which the crossed Andreev reflection can dominate over the elastic cotunelling process. This findings help to clarify some experimental results. In the second part I will focus on the non-equilibrium properties of a NSN system. From a self-consistent model I will show that while for low transparencies and voltages the contributions from elastic cotunneling and crossed Andreev reflection cancel each other, in the opposite limit the non-local conductance exhibit an abrupt jump for a voltage of the order of the self- consistent gap.
14th January Juanjo Saenz, Universidad Autonoma de Madrid
"Light induced interactions between nanoparticles"
Abstract: By fashioning appropriate optical field gradients it is possible to trap and manipulate small particles with optical tweezers. Unfortunately in the absence of absorbing resonances, light forces on nanometre-scale particles are typically very small. In this talk I will first introduce some general concepts of light scattering, radiation pressure, and polarization forces on small particles. I will then show that there is an additional non-conservative contribution to the scattering force in a light field with non-uniform helicity, proportional to the curl of the spin angular momentum of the light field. in quasi one-dimensional waveguides the coupling of the scalar dipolar modes leads to a resonant total reflection close to the threshold of a new propagating mode. I will show how these resonant modes lead to unusual strong optical interaction between particles, and comment on their experimental consequences.
10th December Geli Vozmediano, Instituto de Ciencia de Materiales de Madrid
"Coulomb interactions and disorder in graphene"
Abstract: The recent synthesis of a single layer of graphite (graphene) and the experimental observation of some unusual electronic and structural properties has prompted a real revolution in the theory of condensed matter systems. Under a theoretical point of view the fact, confirmed by the experiments, that the low energy elementary excitations are well described by massless Dirac fermions implies a revision of the Landau fermi liquid paradigm. In this talk we will give a general overview of the graphene physics and address the problem of the Coulomb interactions and disorder. A renormalization group analysis allows to classify graphene as a strange Fermi liquid.