Journées théorie

24 mai 2012, 08:30 → 25 mai 2012, 17:30 Europe/Paris

Amphithéatre LPSC (Polygone Scientifique)

Julia Meyer

Le CPTG vous invite à ses 4èmes "Journées de Physique Théorique". Le but de ces journées est de permettre des échanges entre physiciens théoriciens travaillant dans des domaines très variés. Pendant deux jours, quatre thèmes seront abordés: - Physique des particules - Systèmes intégrables - Matière Molle / Biophysique - Energie & Thermoélectricité En plus, il y aura une table ronde sur les activités futures du CPTG. Tous les physiciens, théoriciens et expérimentateurs, sont les bienvenus. Les présentations seront en anglais. Le programme sur le site web est provisoire. Il sera complété au fur et à mesure. __________________________________________________________________________ The CPTG invites you to its 4th "Journées de Physique Théorique". The purpose of the workshop is to enable exchanges between theoretical physicists working in various fields. During two days, four topics will be addressed: - Particle Physics - Integrable Systems - Soft Matter/Biophysics - Energy & Thermoelectricity Furthermore, there will be a round table discussion on future CPTG activities. All physicists, theorists and experimentalists, are welcome. The talks will be given in English. The program on the website is preliminary. It will be completed as information becomes available.

jeudi 24 mai

08:30 → 09:00 Reception, Coffee

09:00 → 10:30 Physique des particules

Président de session: Ingo Schienbein

09:00 Parton distribution functions

Parton distribution functions (PDFs) are of supreme importance in modern high energy physics. Firstly, they encode fundamental information on the structure of hadrons as probed in high energy reactions. Moreover, PDFs are an indispensable tool for the computation of cross sections at hadron colliders such as the LHC providing the link between the perturbatively calculable parton level cross sections for quark and gluon scattering processes and the observable cross sections in hadron-hadron collisions. In this talk, I will give a basic introduction to PDFs in nucleons and nuclei and describe how they are determined in so-called global analyses of a large variety of different scattering processes.

Orateur: Tzvetalina Stavreva (LPSC)

Transparents stavreva.pdf

09:20 Aspects of QCD studies at LAPTH

In this brief talk I will present various facets of the research carried out at LAPTh on the theory of the strong interaction (QCD), namely: - Loop calculations involving many external legs and the GOLEM library; - Prompt photon production at hadronic colliders: NLO calculations and phenomenology; - Studies on QCD at finite temperature and quark-gluon plasma formation in heavy-ion collisions.

Orateur: Francois Arleo (LAPTH)

Transparents arleo_ctpg.pdf

09:40 Physics Beyond the Standard Model

We highlight some of the shortcomings of the Standard Model and explain why we expect to find new physics around the TeV scale at the Large Hadron Collider (LHC) at CERN. We present some well-motivated scenarios for physics beyond the Standard Model like supersymmetry, grand unification, new gauge symmetries, extra dimensions and string theory, and show how our work at LPSC contributes to unraveling the theoretical concepts behind the new phenomena that the LHC is poised to discover.

Orateur: Akin Wingerter (LPSC)

Transparents talk.pdf

10:10 The life and times of Supersymmetry

The ongoing quest for finding what exists beyond the Standard Model of particle physics is driven by many of the observed phenomena like the dark matter, the origin of neutrino masses or the origin of the matter-antimatter asymmetry in the early universe. One of the most elegant extensions of the Standard Model of particle physics is Supersymmetry. It is theoretically well motivated and provides solutions to many of the puzzles in particle physics like the dark matter. Search for Supersymmetry is ongoing on the earth via looking for signals of new particles at colliders like the Large Hadron Collider and in the sky via finding the properties of dark matter with direct and indirect dark matter detection experiments like Xenon and CDMS. In this talk, I give an overview of the status of Supersymmetry in the light of current astrophysical and collider experiments.

Orateur: Suchita Kulkarni (LPSC)

Transparents Kulkarni_CPTG.key Kulkarni_CPTG.pdf

10:30 → 11:00 Coffee & Tea

11:00 → 12:30 Physique des particules

Président de session: Sabine Kraml

11:00 Research in Astrophysics and Cosmology at LAPTh

I will briefly review the composition and the activities of the astrophysics and cosmology group at LAPTh. A sketch will be given of two representative topics of research in the two areas, namely indirect dark matter searches (and connections with cosmic ray astrophysics problems) and neutrino cosmology.

Orateur: Pasquale Serpico (LAPTH)

Transparents SerpicoGrenoble.pdf

11:20 Higgs physics at the LHC

Orateur: Abdelhak Djouadi (CERN)

Transparents djouadi_lpsc.pdf

12:10 Higgs and Dark Matter in SUSY

Supersymmetry, being one of the most mainstream extensions of the Standard Model, is now being probed to a high level in different experimental studies. Though the direct searches for superpartners at the LHC are still confined to put lower bounds on squarks, it may not be the case in the Higgs sector, and on the other side the experimental searches for dark matter are getting sensitive to the bulk of parameters of many supersymmetric theories. We will see how the different tools developped in the particles physics group in Annecy are used to tackle the issue of the constraints on supersymmetric theories, taking both the case of minimal supersymmetry (MSSM) and extended set-up (BMSSM).

Orateur: Guillaume Drieu la Rochelle (LAPTH)

Transparents drieu.pdf

12:30 → 14:00 Lunch

14:00 → 15:30 Systemes integrables

The notion of integrability is based on an essential property, namely the existence of an "adequate" number of conserved quantities in the system. This "adequate" number then allows for the full resolution of the model, i.e., to calculate exactly (without using approximations or perturbative techniques) all its physical quantities, such as energies, momenta, correlation functions, etc... Analytical and algebraic developments have grown in the 80s (starting with the study of spin chains) and there exists now a huge arsenal of mathematical tools to study such systems. Of course, not all systems are integrable, but, surprisingly enough, there are many integrable systems in physics, even if integrable techniques are not always used to solve them.

Integrable systems take place in many areas, in physics or in mathematics. In physics, they are involved in field theory and symmetries (especially in elementary particle physics, in string theories and in supersymmetric Yang-Mills theories), in statistical mechanics (for example in so-called ASEP models, for asymmetric exclusion principle), or condensed matter physics (e.g. in models used to describe nano-technology materials such as carbon nano-tubes). In mathematics, integrable systems are themselves the basis for the development of a particularly rich set of new mathematical structures (quantum groups, deformed algebras, Hopf and quasi-Hopf structures, etc...).

Président de session: Eric Ragoucy (LAPTh)

14:00 An introduction to integrable systems

Orateur: Jean-Michel Maillet (ENS Lyon)

Transparents JMM-Grenoble.pdf

15:00 Dynamical reflection algebras: examples from Calogero-Moser model

A major feature of quantum integrable systems is the quantum group structure encapsulated in the Yang Baxter equation RTT = TTR. The presence of boundaries to a quantum integrable system imposes to complement it by the boundary equation RKRK = KRKR. In parallel consistent deformations of the YB algebra have been defined, leading to so-called dynamical Yang Baxter equations. The work presented here aims at exploring the association of both generalizations as dynamical boundary algebras. Three such structures are known at this time. We will describe these algebraic structures and unravel their connections with the famous integrable N-body Calogero-Moser model, focusing on the rational potential case v(r) = 1/r².

Orateur: Jean Avan (Université de Cergy)

Transparents JA-grenoble6.pdf

15:30 → 16:00 Coffee & Tea

16:00 → 17:30 Systemes integrables

The notion of integrability is based on an essential property, namely the existence of an "adequate" number of conserved quantities in the system. This "adequate" number then allows for the full resolution of the model, i.e., to calculate exactly (without using approximations or perturbative techniques) all its physical quantities, such as energies, momenta, correlation functions, etc... Analytical and algebraic developments have grown in the 80s (starting with the study of spin chains) and there exists now a huge arsenal of mathematical tools to study such systems. Of course, not all systems are integrable, but, surprisingly enough, there are many integrable systems in physics, even if integrable techniques are not always used to solve them.

Integrable systems take place in many areas, in physics or in mathematics. In physics, they are involved in field theory and symmetries (especially in elementary particle physics, in string theories and in supersymmetric Yang-Mills theories), in statistical mechanics (for example in so-called ASEP models, for asymmetric exclusion principle), or condensed matter physics (e.g. in models used to describe nano-technology materials such as carbon nano-tubes). In mathematics, integrable systems are themselves the basis for the development of a particularly rich set of new mathematical structures (quantum groups, deformed algebras, Hopf and quasi-Hopf structures, etc...).

Président de session: Eric Ragoucy (LAPTh)

16:00 Integrability in Super-Yang-Mills theories

Nowadays, the notion of integrability is also used in string theories. The celebrated AdS / CFT correspondence conjecture between conformal field theories in four dimensions and string theory in a 11 dimensional space entered a predictive phase, following a tremendous breakthrough made in recent years. This conjecture predicts equivalence between operators in conformal field theories (eg N = 4 SYM) on the one hand, and the excited states of a superstring propagating on a curved anti-de Sitter spacetime on the other hand. Originally, the AdS / CFT correspondence applied to quasi-classical strings (relatively well studied) and conformal field theory in the strong coupling regime, virtually inaccessible by current methods. Most recently, this situation has completely changed after the discovery that the two theories are probably based on very similar integrability principles: integrable systems of Heisenberg spin chain type were found in the N = 4 SYM theory and also on the side of the quasi-classical string. This allowed for the first time, to compare (with success) quantitative predictions (and not merely qualitative, as early in the AdS / CFT) of string theory and perturbative results in conformal field theories. Together with this integrability appoach appeared infinite dimensional symmetry algebras, such as Yangians of superalgebras. These quantum groups encompass the superconformal symmetry algebra and allow to put severe constraints on the possible form of e.g. Wilson loops.

Orateur: James Drummond (LAPTh & CERN)

Transparents James-Grenobletalk.pdf

16:30 Positive current cross-correlations in a highly transparent normal-superconducting beam splitter

The possibility of realizing in the future a source of spin-entangled pairs of electrons based on three-terminal normal metal-superconductor-normal metal structures has aroused considerable interest recently, both theoretical and experimental. The topic of available experiments is to measure either the current flowing in one of the normal electrodes in response to a voltage on the other normal electrode, or to evaluate current-current noise cross-correlations. I will present our theoretical results showing that, surprisingly, positive current-current cross-correlations can be obtained for highly transparent contacts [1,2]. These positive cross-correlations at high transparency have nothing to do with Cooper pair splitting. [1] R. Melin, C. Benjamin and Th. Martin, Phys. Rev. B 77, 094512 (2008). [2] A. Freyn, M. Flöser and R. Melin, Phys. Rev. B 82, 014510 (2010).

Orateur: Régis Melin (Institut Néel)

Transparents melin.pdf

17:00 Exact solution for the stirring of a one-dimensional interacting Bose gas on a ring trap

Recent experimental activities of boson trapping on a ring geometry open the way to explore this novel topology. We focus on a tight ring trap with strong transverse confinement leading to an effectively one-dimensional motion along its circumference. We consider a strongly interacting Bose gas on the ring subjected to a localized barrier potential which is suddenly set into motion. The Bose-Fermi (BF) mapping allows to obtain an exact solution for the many-body wavefunction in the impenetrable-boson (Tonks-Girardeau) limit of infinitely strong interactions between the particles with arbitrary external potential, not treatable with the Bethe Ansatz. Using the time-dependent extension of the BF mapping we obtain an exact solution for the dynamical evolution of the many-body wavefunction. The exact solution allows to explore the possibility of transferring angular momentum to the system through the barrier motion. In particular we calculate the particle current, the particle current fluctuations and the drag force acting on the barrier. In the weak barrier limit the stirring drives the system into a state with net zero current and vanishingly small current fluctuations for velocities smaller than a critical velocity v_c. The existence of a velocity threshold for current generation indicates superfluid-like behavior of the mesoscopic Tonks-Girardeau gas, different from the non-superfluid behavior predicted for the TG gas in an infinite tube. At velocities approaching integer multiples of v_c angular momentum can be transferred to the fluid and a nonzero drag force arises. At these velocities we predict the formation of a macroscopic superposition of a rotating and a nonrotating Fermi sphere of the mapped Fermi gas. We calculate some observable of experimental interest, as the momentum distribution and time of flight images. Furthermore, we demonstrate the nonclassical nature of the superposition by studying its Wigner function.

Orateur: Christoph Schenke (LPMMC)

Transparents schenke.pdf

17:30 → 18:00 Table Ronde CPTG

17:30 Table Ronde

Orateur: Julia Meyer

vendredi 25 mai

08:30 → 09:00 Accueil, Coffee

09:00 → 10:30 Matiere molle/Biophysique

This session will focus on applications of soft matter concepts and other methods from statistical physics to problems of biological interest, from cell adhesion to chromosome dynamics or protein structures.

Président de session: Jean-Louis Barrat (LIPhy)

09:00 Structure and Dynamics of Interphase Chromosomes

During interphase chromosomes decondense, but fluorescent in situ hybridization experiments reveal the existence of distinct territories occupied by individual chromosomes inside the nuclei of most eukaryotic cells. We use computer simulations to show that the existence and stability of territories is a kinetic effect that can be explained without invoking an underlying nuclear scaffold or protein-mediated interactions between DNA sequences. In particular, we show that the experimentally observed territory shapes, FISH observations on spatial distances between marked chromosome sites as well as 3-/Hi-C data for contact probabilities for human, Drosophila, and budding yeast chromosomes can be reproduced by a parameter-free minimal model of decondensing chromosomes. Our results suggest that the observed interphase structure and dynamics are due to generic polymer effects: confined Brownian motion conserving the local topological state of long chain molecules and segregation of mutually unentangled chains due to topological constraints. Understanding the statistical physics of the corresponding equilibrium system, unentangled melts of unconcatenated ring polymers, remains a challenge.

Orateur: Prof. Ralf Everaers (Laboratoire de Physique et Centre Blaise Pascal, ENS-Lyon et CNRS)

09:45 Physical modeling of cell adhesion mecanosensitivity

Cell adhesion has attracted the attention of physicists and engineers since it has become apparent that (i) managing it is a prerequisite to organize cells in a synthetic matrix for bioengineering applications, and (ii) that it could not be described with theories designed for dead matter, albeit sophisticated they are. The adhesion of cells to their surrounding medium begins with the formation of ligand/receptor bonds. Bond formation induces the generation of intracellular contraction forces, which in turn regulate the size of the adhesion clusters. The amplitude of these muscle like, contraction forces adapts to the mechanical properties of the environment, and so does the size of the adhesion clusters. However, the adaptation of cell adhesion to the mechanical properties of their environment follows an unexpected trend if referring to the physical description of passive nucleation and growth of clusters of adsorbed molecules onto a surface: adhesion patches preferentially grow onto rigid environments, although passive anchoring of proteins should be favored on soft, deformable substrates. Here we present a thermodynamic approach that captures cell living feature as its ability to provide energy to balance the losses in substrate deformation. We compare this approach to other molecular scale, stochastic descriptions, and conclude on the possibility that cell adhesion mechanosensitivity could be governed by general, physical principles.

Orateur: Dr Alice NICOLAS (Laboratoire des Technologies de la Microelectronique, CEA-CNRS-UJF)

10:30 → 11:00 Coffee & Tea

11:00 → 12:30 Matiere molle/Biophysique

This session will focus on applications of soft matter concepts and other methods from statistical physics to problems of biological interest, from cell adhesion to chromosome dynamics or protein structures.

Président de session: Jean-Louis Barrat (LIPhy)

11:00 Proteins in the light of evolution

I will present work aimed at deciphering the relation between the sequence and function of proteins based on the principle of co-evolution. By analyzing statistically large numbers of related sequences, we can infer from this principle the patterns of functional couplings between amino acids in a protein. This analysis reveals a decomposition of proteins into evolutionary units that differs from the usual decomposition of proteins into a hierarchy of structural units. I will describe this new decomposition, explain how it is obtained, and show some of the experimental evidence.

Orateur: Dr Olivier RIVOIRE (Laboratoire Interdisciplinaire de Physique Université Grenoble I et CNRS)

11:45 Graphs and statistics for protein interfaces.

Living organisms are made of nucleotides (DNA and RNA), lipids and proteins. All biological activities consist in interactions between those different entities. Identifying and rationalizing the interaction modes between partners is therefore crucial for investigating any biological problem. The interactions are encoded by patterns that are not easily inferred due to the large number of possible combinations. We focus on interactions between chains, or subunits, in oligomeric proteins, frequently encountered biomolecules in cells. Our approach is based on a symmetrization process that finds the strongest interface interactions and describe them by graphs (interaction networks). The graph topology contains important information on the interface geometry and on the involvement of each amino acid. The interface geometry is a key feature as it shapes most of the physico-chemical aspects of the interface itself. Thus, we need to manage it and and primarily investigate within each geometry class. Large dataset statistics have allowed us to extract aspects of the graph topology in the interface geometry given by an intermolecular beta sheet. Interesting specific features have been revealed: the presence of two typical subgraphs, a peculiar organization of the graph, a different use of amino acids in the two subgraphs, and so on. A careful geometrical study of the symmetrization has shown that the interface interactions are ranked in levels and can be organized in clusters, according to the local topology and geometry. A hierarchy of graphs emerges.

Orateur: Dr Giovanni FEVERATI (Laboratoire d'Annecy de physique théorique, CNRS et Université de Savoie)

Transparents protein_interfaces_2012.pdf

12:30 → 14:00 Lunch

14:00 → 15:30 Energie et thermoelectricite

Présidents de session: Serge Florens (Institut Neel), Xavier Blase (Institut Neel)

14:00 Exciton dynamics in complex environment structures: non-equilibirum dynamics of vibrational modes and the appearance of spontaneous electronic coherence in photosynthesis

The interactions between the environment and excitons generated by light harvesting in photosynthetic pigment-protein complexes (PPC) is a key part of the remarkable, often close to 100%, quantum efficiency of solar energy transduction in natural photosynthesis. The environmental fluctuations spectra in PPCs are known to be have significant frequency structure, including strong contributions from vibrational modes with frequencies comparable to the energy differences between excitonic excited states in PPCs. In this talk, the novel non-equilibirum dynamics induced in these resonant modes by the excitation of excitons will be shown to exert a non-trivial back action on the exciton dynamics which acts to generate or regenerate electronic coherences. Importantly, it will be shown how these back actions support picosecond electronic coherences during energy transport and that these coherences may also appear spontaneously from incoherent initial conditions. This suggests that recent proposals for a quantum mechanical role in efficient, natural light harvesting may indeed be valid. This talk will introduce the basic experimental results supporting these ideas and the theoretical many-body methods which enable highly non-Markovian and correlated open quantum systems to be simulated. A more conceptual discussion of systems - as exemplified by PPCs - where the quantum sub-system and environment are not easily separated into distinct entities will also be presented.

Orateur: Alex Chin (University of Cambridge)

14:45 Carrier multiplication in quantum dots for more efficient solar cells

Carrier-multiplication (CM) is a process in which more than one electron-hole pair is created as the result of the absorption of a single photon. CM in semiconductor quantum dots (QDs) has recently received considerable attention both experimentally and theoretically. But the results are subjects of important controversies and therefore it is still not clear whether CM in QDs has the potential to improve the performance of solar cells. The main origin of CM is the relaxation of photo-excited carriers by impact ionization which, above a certain energy threshold, becomes more efficient than the intraband relaxation by emission of phonons. In this presentation, we review recent theoretical works on the physics of CM in QDs. Critical assessment of these works will be done. We present calculations of impact ionization rates, carrier-multiplication yields and solar-power conversion efficiencies in solar cells based on quantum dots (QDs) of α-Sn and HgTe. Using these results and previous ones on PbSe and PbS QDs, we discuss a strategy to select QDs with the highest carrier-multiplication rate for more efficient solar cells. We show that the improvement of the maximum solar-power conversion efficiency remains a challenging task.

Orateur: Christophe Delerue (IEMN, Université de Lille)

Transparents carrier_multiplication_grenoble.pdf

15:30 → 16:00 Coffee & Tea

16:00 → 17:30 Energie et thermoelectricite

Présidents de session: Serge Florens (Institut Neel), Xavier Blase (Institut Neel)

16:00 Molecular fingerprints in the electronic properties of crystalline organic semiconductors: from experiment to theory

Because of the weakness of van der Waals inter-molecular bonds, organic semiconductors are commonly believed to stand in an intermediate region between the molecular limit and the extended band picture, calling for concepts that go beyond the standard paradigms of inorganic semiconductors. Experimental charge transport and optical studies of organic semiconductors over the past few years have indeed given contrasting indications on the nature of electronic carriers. The existence of low-mass quasiparticles and the apparently “band-like” temperature dependent electronic mobilities observed in the best crystalline systems are difficult to reconcile — within the framework of band theory alone — with absolute values of the mobility that hardly exceed few tens of cm2/V s. A number of photoemission studies of high-mobility organic semiconductors have recently appeared, providing evidence that even the most accurate ab initio band-structure calculations predict bandwidths that are significantly smaller than measured experimentally. Based on a combined theoretical/experimental analysis of the photoemission spectra of pentacene, we demonstrate that this discrepancy stems from the interaction of electrons with the internal vibrations of the molecules. Unambiguous signatures are identified, showing that the nature of extended electronic states in organic materials is deeply modified by the electron-molecular vibration coupling. Such features should be general to broad classes of molecular systems including organic semiconductors, doped organic conductors and charge-transfer organic salts.

Orateur: Simone Fratini (Institut Néel)

Transparents CPTG-Fratini.pdf

16:45 Thermoelectric transport in Nanoparticle Embedded in Alloy (NEAT) materials and other related systems

Several novel types of nanostructured materials are emerging as potentially suitable for the development of high ZT thermoelectrics. Amongst them, Nanoparticle Embedded in Alloy Thermoelectric (NEAT) materials, offer a good opportunity for theoretical computations to predict thermoelectric properties and compare with experimental results. In this talk I will first discuss the theoretical approaches to compute the thermal conductivity of NEAT materials, including ab-initio methods. I will present concrete results for SiGe [1], Mg2SiSn, and (BiSb)2Te3 [2] based composites, discussing various expected limits. Finally, the thermoelectric properties of sintered nano-grained composites will be discussed within a simplified model, and a high throughput computational study of the power factor for 3000 sintered compounds will be presented [3]. [1] A. Kundu, N. Mingo, D. A. Broido, and D. A. Stewart, The role of lighter and heavier embedded nanoparticles on the thermal conductivity of SiGe alloys, Phys. Rev. B, 84, 125426 (2011). [2] N. Ayape-Katcho, N. Mingo and D. A. Broido, Lattice thermal conductivity of (BixSb1−x)2Te3 alloys with embedded nanoparticles, PRB 85, 115208 (2012). [3] S. Wang, Z. Wang, W. Setyawan, N. Mingo, and S. Curtarolo, Assessing the thermoelectric properties of sintered compounds via high-throughput ab initio calculations, Phys. Rev. X, 1, 021012 (2011).

Orateur: Natalio Mingo (LITEN/LCH, CEA Grenoble)