Seminars and Events
Materials Science Factory
30 November 2017, 12:00 h. Salón de Actos
SOME APPLICATIONS OF THE PHYSICS OF EXCEPTIONAL POINTS IN 3D TOPOLOGICAL SEMIMETALS
José González Carmona
Instituto de Estructura de la Materia CSIC
We will reexamine the topological protection of surface states in Dirac, Weyl and nodal-line semimetals by characterizing them as evanescent waves for complex values of the momentum. We will find in this way a sequence of exceptional points —that is, branch points with real energy in the complex spectrum— allowing us to identify the set of surface states decaying into the 3D semimetal. The topological protection of the surface states stems in this approach from the fact that the branch cuts in the spectrum cannot be closed by small perturbations. In the case of Weyl semimetals under circularly polarized light, we will see that new exceptional points arise within the gap opened between Floquet bands, as a signature of new surface states produced by the radiation. We will also show that the exceptional points survive when nodal-line semimetals are placed in a strong magnetic field, leading to Landau surface states and the appearance of chiral currents at the surface of the 3D material.
*Work in collaboration with R. A. Molina
25 October 2017, 09:00 h. Salón de Actos
1st Workshop of the Materials Science Factory
The Material Sciences Factory is a scientific unit that includes several scientists and research teams of the ICMM with the goal of promoting the collaboration and a quantifiable scientific excellence in the areas of Material Sciences and Nanotechnology.
The first Workshop of the Materials Science Factory is intended to give an overview of the research background, expertise, know-how and recent activities of the different research groups integrating the Materials Science Factory.
11 July 2017, 12:00 h. Sala de Seminarios, 182
Nanophotonics Research at Sandia: Nano light sources and topological photonics
Ganapathi (Ganesh) Subramania
Semiconductor materials and device sciences, Sandia National Laboratories
Nanophotonic architectures such as photonic crystals and metamaterials have become key players in modern photonics. They offer hitherto unprecedented capabilities combined with great versatility to control various properties of light¬ -propagation, polarization, emission and photon statistics. They have become increasingly important for chipscale photonics. In this talk, I will present research carried out along with my colleagues in this area using photonic crystals and metamaterials at Sandia. In particular, I will provide a broad overview of our work covering three-dimensional photonic crystals operating in the visible, light emission from three-nitride nanowire two-dimensional photonic crystal arrays, metal-dielectric epsilon-near-zero metamaterials at visible wavelengths and non-resonant, broadband ultrasubwavelength light confinement structures. I will follow this overview with two of our more recent efforts. One, is on fabrication and spectroscopy of site-selective III-nitride quantum dots for quantum light sources using photo-electro-chemical etch. This approach for deterministic placement can potentially lead to quantum light sources with deterministic properties, important for quantum information processing. The other is on our efforts on achieving topologically non-trivial photonic structures. Topological photonic structures exhibit one-way scatter-free light transport that can have important applications in optical and quantum communications.
25 April 2017, 11:30 h. Salón de Actos
NETWORKS THEORY: HOW THINGS ARE CONNECTED
Yeshiva University, NY USA
In this seminar I will present some basic ideas of networks’ theory and their areas of current application in different fields of science
In particular I will present some of main theoretical models of networks and of network formation , and I will present some applications to robustness and to spreading and diffusion of processes in networks.
16 March 2017, 11:00 h. Salón de Actos
Organic field effect transistors in biosensing and neurosciences
Life Sciences Dept., Università di Modena e Reggio Emilia, Modena , Italy
Organic bioelectronics is an emerging platform with impact in diagnostics,
loco-regional treatments and theranostics. It is largely based on organic field effect transistors (OFET) that can be operated as ultra-sensitive biosensors, transducers of electrical and electrochemical signals from cells, and stimulators for electroactive cells. Their applications range from detection of biomarkers in bodily fluids to implants for bidirectional communication with the central nervous system. Several OFET layouts have been demonstrated to be effective in aqueous operations, which are distinguished either by their architecture or by the respective mechanism of doping by the ions in the electrolyte solution. In this work we discuss first some fundamental aspects that concern the coupling mechanism of these devices with the biological systems, in particular we elucidate the role of
the different interfaces into greatly enhancing the sensitivity of these devices and their capability to amplify very small potential variations at the interfaces. We also show that this device, operated as a biosensor for a primary inflammatory citokine, i.e. TNF-alpha, responds
super-exponentially, and not linearly, in current vs analyte concentration in the sub-nM range. We unify the super-exponential and linear regimes by means of an alaysis of the density of states of the organic semiconductor channel upon the change of the electro-chemical potential caused by the adsorption of TNF-alpha. We finally show that the response is modulated by the gate voltage applied, and that is possible to measure the association binding constant of the antibody-antigen recognition, the molar free energy, and the electrostatic contribution to the
21 February 2017, 15:00 h. Sala de Seminarios, 182
Atomic Force Microscopy in controlled environment: a powerful tool for original experiments on hybrid systems, biomaterials, and biocomposites
Université Bretagne Sud, CNRS Centre de Recherche C. Huygens
There is a growing research interest on hybrid systems, biomaterials, and biocomposites because of their industrial application demands. Owing unique properties of these materials make more attractive in electronic, medical and transport applications.
Whatever the application field, a constant need of miniaturization, performance, structure lightening necessitates working and exploring the material properties at nanoscale. To do this, one of the most powerful tool is Atomic Force Microscopy (AFM). By using AFM nanoscale tips, which interact with matter due to attractive and repulsive forces, numerous properties can be studied systematically. Due to its strong impact on properties and behaviour, temperature is considered as a key parameter. Indeed, its variations induce a significant change in material behaviour which can be positive (adhesion for example) or negative (electrical change). Consists of tip, micro-lever, photodetector, laser and control loop, the AFM can be used in different modes i.e. tapping, contact, non-contact, and PeakForce QNM. Consequently, a wide selection of measurements can be made under controlled environment (humidity, liquids, temperature, etc.,), and with different purposes such as topography, electrical measurements, force measurements, PeakForce-QNM.
To illustrate the contribution of the technics, different applications are presented. In the first part, a combination of AFM and RX reflectivity is used to understand the structure and interfaces of hybrid nanofilms (Polystyren/Gold) at different temperatures (-20°C to 220°C). In this way, AFM has been used to highlight different properties which are not possible to study by other technics such as electrical behaviour, adhesion and surface characterization with a new method
31 January 2017, 12:00 h. Sala de Seminarios, 182
Unveiling Natural Optical Activity of Disordered Media
F. A. Pinheiro
Instituto de Física, Universidade Federal do Rio de Janeiro, Brasil
The concept of chirality, introduced by Lord Kelvin in order to describe geometrical objects that cannot superimposed with their mirror image, is ubiquitous in the natural world. Despite substantial efforts to understand the optical properties of naturally occurring chiral media and to design artificial ones, disordered media remain an overlooked class of chiral systems. Since disordered systems lack centre and plane of mirror symmetry, they should exhibit natural optical activity. However, previous experimental evidence of natural optical activity in random media has never been attributed to the intrinsic chirality of a random system, but rather to alternative explanations, such as surface contamination by unwanted chiral substances.
Here, we demonstrate natural optical activity due to intrinsic geometric chirality in disordered, diffusive scattering systems, consisting of plasmonic resonators. We employ a microscopic electromagnetic wave transport theory, and derive an expression for the rotatory power and the spatial dichroism of a medium consisting of randomly distributed pointlike scatterers. By means of a systematic statistical analysis of natural optical activity in random media, we argue that the standard deviation of both rotatory power and spatial dichroism are strongly dependent on the density of scatterers and the scattering mean free path. We independently confirm our results by full-wave finite element simulations and show that disordered ensembles of plasmonic nanoparticles can exhibit dichroism more than an order of magnitude higher than in helical configurations with the same particle density.