Seminars and Events
Theory and Simulation of Materials
Coordinators: Eduardo Hernández, Rafael Roldán
25 January 2018, 12:00 h. Sala de Seminarios, 182
Exploiting polaritonic chemistry to manipulate molecular structure and dynamics
IFIMAC & Departamento de Física Teórica de la Materia Condensada, UAM
Strong coupling is achieved when the coherent energy exchange between a confined electromagnetic field mode and material excitations becomes faster than the decay and decoherence of either constituent. This creates a paradigmatic hybrid quantum system with eigenstates that have mixed light-matter character (polaritons). It has recently been realized that polariton formation in organic molecules also affects their internal nuclear degrees of freedom, opening the possibility to manipulate and control reactions through polaritonic chemistry. I will first discuss our theoretical approach towards modeling such systems, based on extending the well-known Born-Oppenheimer approximation to describe polaritonic potential energy surfaces. I will then show various applications, including the possibility to completely suppress reactions such as photoisomerization, which surprisingly works more efficiently when many molecules are coupled to a single light mode due to a “collective protection” effect. Finally, I will show how polaritonic chemistry can be exploited to allow many-molecule reactions triggered by a single photon. Here, the collective nature of polaritons and the resulting formation of a "supermolecule", in which a single excitation is distributed over many molecules, can enable a reaction involving the nuclear degrees of freedom of most or even all coupled molecules. This process can overcome the Stark-Einstein law that applies for most common photochemical reactions, which states that a single photon will only induce a reaction in a single molecule.
Finally, I will discuss future perspectives, open questions, and remaining challenges to fully exploit the potential of polaritonic chemistry.
18 January 2018, 12:00 h. Sala de Seminarios, 182
Topological π-junctions from crossed Andreev reflection in the Quantum Hall regime
Instituto de Ciencia de Materiales de Madrid ICMM-CSIC and IMDEA Nanociencia
In this talk I will review some of the current achievements in the pursuit of Majorana fermions, along with the major drawbacks that stand in the way of exploiting them as fundamental qubits in the field of topological quantum computation. I will introduce a novel setting for creating Majorana bound states and performing protected non-Abelian single-qubit operations without real space braiding of Majoranas and with no fine-tuning of the control parameters. The proposed platform is a two-dimensional electron gas in the Quantum Hall regime in the presence of a Zeeman field, with the Fermi level tuned to filling factor 1. I will show that, in the presence of spin-orbit coupling, contacting the 2DEG to a narrow strip of an s-wave superconductor produces a topological superconducting gap along the contact as a result of crossed Andreev reflection processes across the strip. The sign of the topological gap depends periodically on the Fermi wavelength and strip width and can be externally tuned. An interface between two halves of a long strip with topological gaps of opposite sign implements a robust π-junction that hosts a pair of protected Majorana zero modes. Such a configuration can be exploited to perform non-Abelian tunnel-braid operations that are much simpler to execute than Majorana braidings in real space, and are nevertheless topologically protected.
11 January 2018, 12:00 h. Sala de Seminarios, 182
Graphene on rhodium from first principles: Tailoring electronic, structural and chemical properties
Carlos Romero Muñiz
Universidad Autónoma de Madrid
We demonstrate the possibility of modeling recent experiments in complex graphene-metal (G-M) systems by means of Density Functional Theory (DFT) calculations. Since different modification techniques are currently applied on G -and related materials- to get new functionalities, our work does not restrict to the mere description of the G-M interface but we have to reproduce all these surface modifications. More precisely, we reveal the multi-domain structure of G grown on Rh(111), an archetypical strongly interacting substrate, in which G adopts a rippled structure with corrugations larger than 1 Å . Additionally, we will present different examples of surface modification; like the evolution of G properties as a function of the oxygen coverage in the interface ; the atomistic mechanisms involved in this intercalation process ; and the tailoring of the electronic properties by means of ion implantation nitrogen doping [4,5]. Finally, we will briefly discuss about some methodological aspects of DFT calculations in this kind of G-M systems and their current limitations when dealing with very large systems (like grain boundaries, G-covered metallic steps or under-cover chemical reactions).To overcome this issue, we present preliminary results of a new approach based on highly optimized localized orbital basis set to reach high-accuracy descriptions at quantum level of systems with thousands of atoms.
 A. Martín-Recio et al. Nanoscale 7 (2015) 11300
 C. Romero-Muñiz et al. Carbon 101 (2016) 129
 C. Romero-Muñiz et al. (submitted)
 A. Martín-Recio et al. Nanoscale 8 (2016) 17686
 A. Martín-Recio et al. (submitted)