The SEMPARIS seminar webserver hosts annoucements of all seminars taking place in Paris area, in all topics of physics, mathematics and computer science. It allows registered users to receive a selection of announcements by email on a daily or weekly basis, and offers the possibility to archive PDF or Powerpoint files, making it available to the scientific community. [ More information ]
Upcoming Seminars  [Next 30 ]  
[ scheduler view ] 
Monday 25 October 2021, 10:45 at
LPTMC,
salle 523 du LPTMC  Tour 1213 Jussieu ( (Egalement par zoom https://us06web.zoom.us/j/88613589674?pwd=djRyNVN0dUVJMXhMOG1rem9OVG5WUT09 Meeting ID: 886 1358 9674 Passcode: 253305) )  SEMLPTMC (Séminaire du Laboratoire de Physique Théorique de la Matière Condensée)  condmat 



Abstract:  In this talk I will discuss two situations in which longrange effects such as ordering and forces emerge in certain systems of classical statistical mechanics and how such models are handled through exact techniques. The first part of my talk revolves around the following question: can order extend over distances larger than the bulk correlation length? I will show how a network of Ising boxes connected by channels is able to exhibit an extraordinarily longrange ferromagnetic order over distances which grow exponentially with the cross sections of the channels. The emergence of such a new length scale follows from an exact calculation based on the diagonalization of the transfer matrix for the square lattice Ising model. The analytical study is flanked by extensive Monte Carlo simulations [1]. The second phenomenon I will discuss is the critical (or thermodynamical) Casimir effect [2]. Chemically inhomogeneous colloidal particles immersed in a critical binary mixture are subjected to a fluctuationinducedforce known as the critical Casimir effect. By modeling a binary mixture at its demixing critical point by means of the critical Ising model in two dimensions, and exploiting its scaling limit description in terms of a Conformal Field Theory, I determine the exact density profiles and correlation functions around various particles whose boundaries are formed by patches with different chemical structure and preference of the binary mixture components. The formalism encompasses several interesting configurations, including Janus particles, colloidal quadrupoles and needles with inhomogeneous patches of symmetry breaking boundary conditions. Within the framework of the ‘’Small Particle Operator Expansion’’ I determine the exact asymptotic behavior of the interaction free energy between these colloids, and colloids confined by a wedgeshaped wall. The theoretical predictions are confirmed by numerical results available in the literature. References [1] D. B. Abraham, A. Maciolek, AS, and O. Vasilyev, Action at a distance in classical uniaxial ferromagnetic arrays, Phys. Rev. E 96, 042154 (2017). [2] AS, A. Maciolek, E. Eisenriegler, and S. Dietrich, Critical Casimir interaction between colloidal Janustype particles in two spatial dimensions, J. Stat. Mech. 043208 (2020). 
Monday 25 October 2021, 11:00 at IAP, IAP  Salle Entresol (and for Zoom contact fumagall@iap.fr)  SEMGRECO (Séminaire du GReCO : groupe de GRavitation et COsmologie à l'IAP)  astroph 



Abstract:  To date, gravitational wave detections have been limited to those from quasi circular binary mergers. However, a significant percentage of mergers could have measurable residual eccentricities, resulting from either external perturbations of the system or short timescales between formation and merger of the binary. Understanding how the orbits of such binaries evolve could aid in creating useful eccentric gravitational waveform templates as well as provide astrophysical information about the environment and formation channels of these systems. We have analyzed the longterm evolution of nonspinning, highly eccentric binaries of general mass ratios, incorporating gravitational radiation reactions at 4.5 postNewtonian order, including leading tail effects. We have developed accurate analytic expressions for the binary’s eccentricity and total inspiral time as a function of its latetime gravitational wave frequency, and of its initial energy and angular momentum. We will discuss our results and their implications for eccentric binary gravitational wave detection. 
Monday 25 October 2021, 14:00 at IPHT, Salle Claude Itzykson, Bât. 774  IPHTSTA (Séminaire de Physique Statistique, CEA/Saclay)  condmat 


Tuesday 26 October 2021, 11:00 at
LPTMS,
Salle des séminaires du FAST et du LPTMS, bâtiment Pascal n°530 ( For zoom ID and PW, please write to valentina.ros@universiteparissaclay.fr )  LPTMS (Séminaire du Laboratoire de Physique Théorique et Modèles Statistiques (Orsay))  condmat.statmech 



Abstract:  Symmetries are a pillar of modern physics and an evergreen research topic is the characterisation of how the presence of a symmetry influences the properties of a physical system. I will present an analysis of the entanglement entropies and negativities related to different symmetry sectors of systems with an internal U(1) symmetry. The entanglement entropy admits a decomposition according to its total preserved charge thanks to the block diagonal form of the density matrix. Despite this structure becomes nontrivial after the operation of partial transposition on the density matrix, it has been shown that negativity admits a resolution in terms of the charge imbalance between two subsystems. I will focus on the resolution of entanglement and negativity for free Dirac fields in two spacetime dimensions at finite temperature and size. To this end, I use a geometrical construction in terms of an AharonovBohmlike flux inserted in the Riemann surface defining the entanglement. The main interesting finding is that both entanglement entropy and negativity are equally distributed among the different symmetry sectors at leading order. 
Tuesday 26 October 2021, 14:00 at APC, seminar room 483A, contact roperpol@apc.in2p3.fr for Zoom meeting details  APCTH (Seminar of the theory group of APC)  grqc 



Abstract:  In the first half of this talk, I will discuss how binary systems can be used as dynamical detectors of gravitational waves (GWs). Since the passage of GWs through a binary perturbs the trajectories of the two bodies, we can infer the presence of a GW signal by searching for changes in the binary's orbital parameters. In the presence of a stochastic GW background (SGWB) these changes accumulate over time, causing the binary orbit to execute a random walk through parameter space. I will present a powerful new formalism for calculating the full statistical evolution of a generic binary system in the presence of a SGWB, capturing all six of the binary's orbital parameters, and will show how this formalism can be applied to timing of binary pulsars and lunar laser ranging to set powerful upper limits on the SGWB spectrum in the microHz band between LISA and PTAs. In the second half of this talk, I will discuss the nonlinear GW memory effecta fascinating prediction of general relativity, in which oscillatory GW signals are generically accompanied by a monotonicallyincreasing strain which persists in the detector long after the signal has passed. I will present the firstever calculations of the nonlinear memory signal associated with GW bursts from cusps and kinks on cosmic string loops, which are an important target for current and future GW observatories. Surprisingly, the cusp GW signal diverges for sufficiently large loops when all memory contributions are included, which I argue is due to a breakdown in the weakfield treatment of GW emission from the cusp. This implies that previouslyneglected strong gravity effects play an important role near cusps, although the exact mechanism by which they cure the divergence is not currently understood. I will argue that one possible resolution is for these cusps to collapse to form primordial black holes. 
Tuesday 26 October 2021, 16:00 at
IHES,
Zoom Réunion ( https://indico.math.cnrs.fr/event/6914/ )  PTIHES (Séminaire de physique théorique de l'IHES)  condmathepthquantph 



Abstract:  The Berry phase is a wellknown phenomenon in quantum mechanics with many profound implications. It describes the response of the phase of the wavefunction to the adiabatic evolution of system parameters, defining a U(1) connection on the parameter space. In manybody systems described by quantum field theory, we may also allow the parameters to vary in space, and we find a higher group connection generalizing the Berry phase. This connection also describes phenomena such as the Thouless pump and its generalizations. It allows us to constrain the global structure of phase diagrams by probing noncontractible cycles in the space of quantum field theories. In a typical phase diagram drawn in R^n, these cycles surround topologicallyprotected critical loci called diabolical points, in analogy to the quantum mechanical singularities which act as "monopoles" for the Berry connection. I will discuss these concepts in more detail, as well as a bulkboundary correspondence and some recent applications to phase diagrams of topologically ordered systems. This talk is based on https://arxiv.org/abs/2004.10758 w/ PoShen Hsin and Anton Kapustin https://arxiv.org/abs/2110.07599 and its sequel, 2110.xxxx w/ Nathanan Tantivasadakarn, Ashvin Vishwanath, and Ruben Verresen. 
Wednesday 27 October 2021, 10:00 at IHES, Amphithéâtre Léon Motchane  PTIHES (Séminaire de physique théorique de l'IHES)  hepth 



Abstract:  The variational method is a powerful approach to solve manybody quantum problems non perturbatively. However, in the context of relativistic quantum field theory (QFT), it needs to meet 3 seemingly incompatible requirements outlined by Feynman: extensivity, computability, and lack of UV sensitivity. In practice, variational methods usually break one of the 3, which translates into the need to have an IR or UV cutoff. I will explain how a relativistic modification of continuous matrix product states allows us to satisfy the 3 requirements jointly in 1+1 dimensions. Optimizing over this class of states, one can solve scalar QFT without UV cutoff and directly in the thermodynamic limit, and numerics are promising. I will try to cover both the general philosophy of the method, the basics of the computations, and mention the many open problems. 
Wednesday 27 October 2021, 14:00 at
LPENS,
L367 ( onsite+zoom. For zoom meeting id, write to xiangyu.cao@phys.ens.fr )  FORUMENS (Forum de Physique Statistique @ ENS)  condmat.statmech 



Abstract:  Topological states of matter are characterised by a gap in the bulk of the system referring to an insulator or a superconductor and topological edge modes as well which find various applications in transport and spintronics. The bulkedge correspondence is associated to a topological number. The table of topological states include the quantum Hall effect and the quantum anomalous Hall effect, topological insulators and topological superconductors in various dimensions and lattice geometries. Here, we discuss classes of states which can be understood from mapping onto a spin1/2 particle in the reciprocal space of wavevectors. We develop a geometrical approach on the associated Poincare Bloch sphere, developing smooth fields, which shows that the topology can be encoded from the poles only. We show applications for the lightmatter coupling when coupling to circular polarisations and develop a relation with quantum transport and the quantum Hall conductivity. The formalism allows us to include interaction effects. We show our recent developments on a stochastic approach to englobe these interaction effects and discuss applications for the Mott transition of the Haldane and KaneMele models. Then, we develop a model of coupled spheres and show the possibility of fractional topological numbers as a result of interactions between spheres and entanglement allowing a superposition of two geometries, one encircling a topological charge and one revealing a Bell or EPR pair. Then, we show applications of the fractional topological numbers C=1/2 in bilayer honeycomb models describing topological semimetals characterised by a quantised $\pi$ Berry phase at one Dirac point. 
Thursday 28 October 2021, 14:00 at LPTM, Distanciel sur Teams  SEMLPTMUCP (Seminaires du LPTM , Universite de Cergy Pontoise)  qbio.MN 



Abstract:  Population bursts in a large ensemble of coupled elements result from the interplay between the local excitable properties of the nodes and the global network topology. Here, collective excitability and selfsustained bursting oscillations are shown to spontaneously emerge in globally coupled populations of nonexcitable units subject to adaptive coupling. The ingredients to observe collective excitability are the coexistence of states with different degrees of synchronization joined to a global feedback acting, on a slow timescale, against the synchronization (desynchronization) of the oscillators. These regimes are illustrated for two paradigmatic classes of coupled rotators, namely, the Kuramoto model with and without inertia. For the bimodal Kuramoto model we analytically show that the macroscopic evolution originates from the existence of a critical manifold organizing the fast collective dynamics on a slow timescale. Our results provide evidence that adaptation can induce excitability by maintaining a network permanently out of equilibrium. 
Friday 29 October 2021, 10:00 at IPHT, Amphi Claude Bloch, Bât. 774  COURS (Cours)  physics 



Abstract:  One of the most powerful tools for understanding quantum aspects of gravity is the holographic principle, which asserts a duality between a theory of quantum gravity on a given manifold and a field theory living on its boundary. A concrete realization in spacetimes with negative curvature is the AdS/CFT correspondence, but it remains an important open question if and how the holographic principle is realized for general spacetimes. \\ Recently, the holographic dual of quantum gravity in asymptotically flat spacetimes has been conjectured to be a codimensiontwo conformal field theory which lives on the celestial sphere at null infinity, aptly referred to as celestial CFT. A first hint at such a duality is the equivalence between the action of the Lorentz group and global conformal transformations on the celestial sphere. Moreover, when recast in a basis of boost eigenstates, scattering amplitudes transform as conformal correlators of primary operators in the dual celestial CFT. These celestial correlators appear to have some, but not all, of the properties of standard CFT correlators. The goal of this course will be to give an introductory guide to recent advances in celestial holography. \\ From the CFT perspective 2D is special: the global conformal group gets enhanced to local conformal symmetries. Remarkably, this infinite dimensional enhancement also appears in the 4D Smatrix which will be a main protagonist in this course. Even more surprisingly, the symmetry structure is much larger: every soft factorization theorem gives a dual ``current'' thus yielding a rich celestial symmetry algebra. Clearly, the exploration of celestial holography has just begun! \\ Plan of the course: \\ 1. Symmetries of asymptotically flat spacetimes: BMS supertranslations and Virasoro/Diff(S2) superrotations. Connection to soft theorems of the Smatrix and memory effects. \\ 2. Conformal primary wavefunctions, celestial amplitudes and their conformally soft and collinear (OPE) limits. \\ 3. Shadows, light rays and conformal block expansion of celestial correlators. \\ 4. Conformally soft sector of celestial CFT: current algebras, soft charges and dressings. 
Friday 29 October 2021, 13:00 at LPTM, 4.13b  SEMLPTMUCP (Seminaires du LPTM , Universite de Cergy Pontoise)  condmat.disnn 



Abstract:  The concept of causality, stating that physical actions cannot propagate in space at an arbitrary speed, can be captured for qudit systems by the notion of Quantum Cellular Automata (QCA), defined as unitary maps preserving locality of observables. In this talk, I will present recent results aiming at a general characterization of QCA and propose them as useful theoretical laboratories for the study of quantum manybody systems out of equilibrium. I will first show how QCA can be expressed, in any dimension, by tensor network operators with special contraction properties, thus unveiling a general connection between causality and entanglement production. Focusing on 1D systems, I will then review the topological classification of QCA in terms of a quantized chiral index. I will present a physical derivation of the latter and finally work out its ramifications for entanglement growth and other standard probes of manybody quantum chaos for generic systems. 
Tuesday 2 November 2021, 14:00 at APC, seminar room 483A, contact roperpol@apc.in2p3.fr for Zoom meeting details  APCTH (Seminar of the theory group of APC)  grqc 



Abstract:  The existence of magnetic fields is ubiquitous on astrophysical (e.g., planets and stars) as well as on cosmological scales (galaxies and galaxy clusters). Low frequency radio observations are revealing an increasing number of diffuse radio sources in galaxy clusters visible through their synchrotron emission. Recently, the diffuse synchrotron radio emission was also detected in the region connecting the pairs of galaxy clusters which is an indirect probe of magnetisation of the Universe on filamentary scales. On the other hand, a search for extended gamma ray emission from distant blazars provides an intriguing possibility of detecting very weak magnetic fields in cosmic voids. This poses an exciting avenue for studying the generation mechanisms and evolution of observed largescale magnetic fields. In this talk, I will review few observational effects of magnetic fields helping us to explore the magnetized Universe on cosmological scales; I will briefly discuss the astrophysical and primordial scenarios for explaining the observed magnetization of the Universe, including the motivation for studying the primordial magnetic fields (PMFs; the seed fields generated in the early Universe). Finally, I will present our results on the evolution of PMFs during large scale structure formation (in connection with previous work for the radiation dominated epoch). We use cosmological magnetohydrodynamical (MHD) code ENZO to evolve inflationgenerated magnetic fields which might have unlimited correlation length scale and causally, phasetransition generated magnetic fields characterized by the correlation length having an upper limit equal to the Hubble length scale. We study how these seed magnetic fields evolve during structure formation and what can be the observable traces of such fields. Our findings include the distinctive evolution of different seed fields retaining the information of magnetic initial conditions on the largest scales of the Universe. We compare simulated Faraday rotation measures from different seeding scenarios with recent observational data and give future prospects in the search of the origin of cosmic, large scale correlated magnetic fields. 
Friday 5 November 2021, 10:00 at IPHT, Amphi Claude Bloch, Bât. 774  COURS (Cours)  physics 



Abstract:  This course is a lightspeed introduction to the BondiMetznerSachs group in three spacetime dimensions (BMS3) and its application to asymptotically Minkowskian holography. Specifically, we first motivate the study of BMS3 by exhibiting some technical but crucial issues with its better known, fourdimensional parent. Then we define BMS3 as an ultrarelativistic limit of the Virasoro algebra, yielding "superrotations" and "supertranslations" as Carrollian analogues of conformal transformations. We also point out that this limit readily provides an elementary holographic understanding of (classical) flatspace gravity in 3D, by relating the BMS3 stress tensor to gravitational boundary degrees of freedom. Quantization then motivates the study of unitary representations of BMS3, labelled by their mass and spin, which can seen as particles dressed with boundary gravitons. If time allows, we apply this understanding to the matching between characters and gravitational oneloop partition functions, as well as the coincidence between BMS3 geometric actions and effective 2D action functionals for 3D flatspace gravity. \parindent0pt \hrulefill \bigskip \textbf{Note:} This lecture complements the IPhT course "Celestial holography primer" by Andrea Puhm. It will be broadcast and, if possible, held inperson at the IPhT. Please refer to the course page for videoconference registration and other details. \smallskip \textbf{Course page:} \verbhttps://courses.ipht.fr/?q=en/node/271 
Monday 8 November 2021, 13:30 at LPENS, Online  Zoom  ID de réunion : 971 0761 8272  Code secret : 0811  LPENSMDQ (Séminaire Matériaux et Dispositifs Quantiques du LPENS)  condmat 



Abstract:  Van der Waals crystals of twodimensional semiconductors have evolved as an increasingly significant material platform for condensed matter research. Using exfoliated monolayers, a wide range of vertical van der Waals heterostructures can be assembled into rationally designed material systems with tailored optical properties of strongly bound excitons. In my talk, I will discuss the results of our recent studies of various van der Waals heterostructure assemblies and highlight the roles of layer constituents, number, twist angle and atomic registry. 
Monday 8 November 2021, 14:00 at
IHES,
Amphithéâtre Léon Motchane ( Séminaire Géométrie et groupes discrets )  MATHIHES (TBA)  math 



Abstract:  In this talk I will present a general construction in terms of Higgs bundles of the higher Teichmüller components of the character variety of a surface group for a real Lie group admitting a positive structure in the sense of GuichardWienhard. Key ingredients in this construction are the notion of magical sl2triple, that we introduce, and the Cayley correspondence. Basics on Higgs bundle theory will be explained. (Based on joint work with Bradlow, Collier, Gothen and Oliveira). 
Monday 8 November 2021, 16:00 at
IHES,
Amphithéâtre Léon Motchane ( Séminaire Géométrie et groupes discrets )  MATHIHES (TBA)  math 



Abstract:  Two hyperbolic surfaces are said to be (length) isospectral if they have the same collection of lengths of primitive closed geodesics, counted with multiplicity (i.e. if they have the same length spectrum). For closed surfaces, there is an upper bound on the size of isospectral hyperbolic structures depending only on the topology. We will show that the situation is very different for infinitetype surfaces, by constructing large families of isospectral hyperbolic structures on surfaces of infinite genus. 
Tuesday 9 November 2021, 14:00 at APC, seminar room 483A, contact roperpol@apc.in2p3.fr for Zoom meeting details  APCTH (Seminar of the theory group of APC)  grqc 



Abstract:  I will discuss the scattering of two compact objects interacting via gravity, using the socalled worldline Effective Field Theory approach in the post Minkowskian expansion (i.e. expanding in the Newton's constant G but not in the velocities). In particular, I will focus on the computation of classical observables such as the total emitted momentum. This is obtained by phasespace integration of the graviton momentum weighted by the modulo squared of the radiation amplitude. It can be recast as an effective cut twoloop integral that I will solve using techniques borrowed from collider physics. 
Tuesday 9 November 2021, 14:00 at
LPTHE,
Library and Zoom (link in the comments) ( Zoom link: https://us06web.zoom.us/j/83758599086? pwd=aVk3QnVoQmRTK01vSzAwTnJQL1Fydz09 )  LPTHEPPH (Particle Physics at LPTHE)  hepph 



Abstract:  Gamma rays and neutrinos being chargeneutral are useful messengers in search for dark matter. In this talk, we will explore the prospect of recently discovered subPeV diffuse gamma rays by Tibet AS gamma in search of decaying dark matter. While for indirect search through neutrinos we focus on neutrinos produced from captured dark matter annihilation in a galactic distribution of neutron stars. We will see that both of these scenarios may probe unexplored regions of the parameter space for heavy dark matter. 
Monday 15 November 2021, 16:00 at IPHT, Salle Claude Itzykson, Bât. 774  IPHTSTA (Séminaire de Physique Statistique, CEA/Saclay)  condmat 


Tuesday 16 November 2021, 14:00 at APC, seminar room 483A, contact roperpol@apc.in2p3.fr for Zoom meeting details  APCTH (Seminar of the theory group of APC)  grqc 


Thursday 18 November 2021, 14:00 at
LPTHE,
INSP great hall ( Zoom link: https://us06web.zoom.us/j/81191889705?pwd=bGpseThPb2xsYWt2NC9vcFpNcHY0UT09 ID de réunion : 811 9188 9705 Code secret : 563176 )  TQM (Theory of quantum matter)  condmat.mtrlscicondmat.strel 



Abstract:  5d iridium oxides are of high interest due to the potential for new quantum states driven by strong spin orbit coupling. In Sr$_2$IrO$_4$, the lowenergy physics of the material is well described by a socalled $j_{eff}=1/2$ state, which consists of a quantum superposition of the three Ir t$_{2g}$ orbitals. Moreover, the interplay of electronelectron interactions and spinorbit coupling leads to an unconventional Mott insulating state, whose spectral properties strongly resemble those of isostructural cuprates. Despite not being superconducting upon doping down to lowest temperatures, the analogy with cuprates is corroborated by Fermi surface and pseudogap properties of doped Sr$_2$IrO$_4$, which suggests an effective minimal oneband model in terms of the $j_{eff}=1/2$ state. However, the $k$dependent orbital composition of this state and recent measurements of its magnetization density distribution cast the validity of a {\it local} $j_{eff}=1/2$ picture into doubt. In this talk, I will use two complementary quantum cluster techniques to study selected electronic, magnetic and spectral properties of this strongly correlated material. The results of our simulations will be compared to different experimental probes and the validity and limitations of an established oneband model of Sr$_2$IrO$_4$ will be discussed. 
Friday 19 November 2021, 10:00 at IPHT, Amphi Claude Bloch, Bât. 774  IPHTSEM (Séminaire du IPHT)  physics 



Abstract:  The bootstrap approach asserts that certain physical theories can be strongly constrained, and sometimes even be solved, using general principles only. We review old and new results obtained from applying this approach to scattering of relativistic particles in flat space. \\ The Smatrix describes transitions between states of freely moving particles in the far past and the distant future. Conventionally such transitions, or scattering amplitudes, are computed in perturbation theory. The Smatrix bootstrap is a program of constructing scattering amplitudes nonperturbatively, based on the general principles of special relativity and quantum mechanics. \\ The aim of the course is twofold. First, we review the basic concepts of Smatrix theory (analyticity, unitarity, crossing) and their connection to the basic principles of relativistic QFT, such as causality, locality and unitarity. This will lead us to the formulation of the Smatrix bootstrap problem, and to the derivation of various results of Smatrix theory that concern nonperturbative properties of scattering amplitudes. Second, we go over various approaches to constructing scattering amplitudes that satisfy the desired properties, and more humbly obtain rigorous nonperturbative bounds on such amplitudes. These include both ideas from half a century ago (when many of the results were first derived), as well as some of the recent work on the Smatrix bootstrap program. \\ \\ Plan of the course: \\ 1. Introduction to Smatrix theory. \\ 2. Kinematics (Mandelstam plane, unitarity, crossing, partial waves). \\ 3. Analyticity (field theory analyticity, unitarity extension of analyticity, Landau equations, maximal analyticity). \\ 4. Universal Bounds (FroissartGribov formula, Mandelstam kernel, MartinFroissart bound, Gribov's theorem, Dragt bootstrap). \\ 5. Bootstrap Methods (primal/dual problems, fixed point method, coupling maximization). \\ course page : https://courses.ipht.fr/?q=en/node/248 \\ Livestream on youtube.com/IPhTTV: no subscription required \\ Inperson audience: facial mask compulsory. \\ Videoconference: subscribe to the course newsletter to receive links 
Monday 22 November 2021, 13:30 at LPENS, Salle Djebar  29 rue d'Ulm  LPENSMDQ (Séminaire Matériaux et Dispositifs Quantiques du LPENS)  condmat 


Monday 22 November 2021, 16:00 at IPHT, Salle Claude Itzykson, Bât. 774  IPHTSTA (Séminaire de Physique Statistique, CEA/Saclay)  condmat 


Thursday 25 November 2021, 11:00 at LPTHE, bibliothèque du LPTHE, tour 1314, 4eme étage  SEMDARBOUX (Séminaire Darboux  physique théorique et mathématiques)  hepth 



Abstract:  Le cobordisme complexe a été inventé par Milnor à la suite des travaux de Thom. Il donne un cadre topologique très général dans lequel calculer des nombres de Chern. Les variétés hyperKählériennes sont entre autres des variétés à classes de Chern de degré impair triviales. Elles engendrent donc un sousanneau ``pair'' de l'anneau de cobordisme, qui est un anneau de polynômes en une infinité de variables. Je vais introduire les deux notions mentionnées dans le titre et parler d'un travail récent en commun avec Georg Oberdieck et Jieao Song, où nous montrons que les schémas de Hilbert ponctuels de surface K3, ou les variétés de Kummer généralisées, donnent des systèmes de générateurs pour ce sousanneau, au moins sur les nombres rationnels. 
Tuesday 30 November 2021, 14:00 at APC, contact roperpol@apc.in2p3.fr for Zoom meeting details  APCTH (Seminar of the theory group of APC)  grqc 


Tuesday 30 November 2021, 17:15 at DPTPHYSENS, Amphi Jaurès  Ecole normale supérieure 29 rue d'Ulm 75005 PARIS  COLLOQUIUMENS (Colloquium of the Physics Department of ENS)  physics 



Abstract:  TBA 
Thursday 2 December 2021, 10:00 at IHP, Institut Henri Poincaré, Room 314  RENCTHEO (Rencontres Théoriciennes)  hepth 


Thursday 2 December 2021, 14:00 at
LPTHE,
INSP great hall ( Zoom link: https://us06web.zoom.us/j/84801275420?pwd=WWV0cHczeU9UK2NBVE53M2orTEZxUT09 ID de réunion : 848 0127 5420 Code secret : 815705 )  TQM (Theory of quantum matter)  condmat.meshallcondmat.mtrlsciphysics.chemphquantph 



Abstract:  Organic piconjugated polymers have been studied for many years in the context of optoelectronic applications such as solar cells, LEDs and sensors, and remain a highly promising class of materials for cheap, nontoxic and flexible lightmatter coupling devices. Recently, onsurface synthesis techniques have allowed novel quasi1D molecular polymers to be fabricated whose electronic and optical properties can be controlled via the connectivity and composition of the monomer units. For polymers based on acene units, Cirera et al. have shown that varying the size of the acene unit causes the polymer band gap to close and reopen, as the system passes from a trivial to a topologically nontrivial electronic phase [1]. In this talk, I will present our recent exploration of the behaviour of the strongly bound excitonic excitations of acene polymers as they pass through this transition. Using selfconsistent GW and Bethe SaltPeter techniques, we show that the presence of the topological transition and the related band flattening and inversion lead to a range of rich, measureable and functionally exploitable properties of singlet and triplet excitons, including negative dispersions and the possibility of spontaneous fission of singlets into entangled triplet pairs. [1] Cirera et al., Nature Nanotechnology, 15, 437–443 (2020) 
Tuesday 7 December 2021, 14:00 at APC, seminar room 483A, contact roperpol@apc.in2p3.fr for Zoom meeting details  APCTH (Seminar of the theory group of APC)  grqc 



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