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Theory and computation of electronic excitations in condensed matter systems, and the ETSF project

Theory and computation of electronic excitations in condensed matter systems, and the ETSF project

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## Theory and computation of electronic excitations in condensed matter systems, and the ETSF project

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**INFM**Posters! A initiative Theory and computation of electronic excitations in condensed matter systems, and the ETSF project • Why excited state “ab-initio” calculations? • Theory: State-of-the-art, and recent developements (mostly density-based) • Examples: solids, clusters, surfaces • The European Theoretical Spectroscopy Facility: G. Onida, N. Manini, L. Molinari, E. Mulazzi, A. Bordoni, K. Gaál-Nagy, A. Incze, L. Caramella, M. Cazzaniga, E. Ponzio, and M. Gatti* Dipartimento di Fisica and INFM, Università di Milano *LSI-SESI,Ecole Polytechnique, Palaiseau, France**hn**e- Why excited states? -Spectroscopies (experimental characterization) C20 H. Prinzbach et al. Nature407, 60 (2000)**Why excited states?**-”Useful” response to excitations (1) Photoluminescence in nc-Si:H films RT PL excited with a He-Cd laser c. 2.5 eV c. 0.6 eV**Why excited states?**-”Useful” response to excitations (3)**Why ab-initio?**• “first principles”: no parameters(ingredients: N,Z) • predictivity (new esperiments, new materials) • access to details which are difficult to obtain experimentally • useful to design materials with the desired properties • generality, transferability, accuracy “Just” solve Schroedinger equation!**Surface optical reflectivity - study of anisotropy spectra**Tools to analyse the calculated spectra Layer-by-layer spectrum decomposition example: Si(100)(2x1) C.Hogan, R. Del Sole, and G.Onida, PRB 68, 035405 (2003)**excited electronic states**C.I. (Quantum Chemistry) Green’s functions (1965-->’80-->today) • ground state: • Density Functional Theory (DFT) (1964): Y->r E=E[r] (W.Kohn: Nobel prize 1998) 1984: TDDFT! (Runge, Gross): r = r(r,t)A = A [r(t),t] ab-initio methods “First principles” calculations = theory without free parameters Y=Y(r1,r2,.....,rN) ? Spectroscopy: one needs also the**reflectivity**absorption photoemission e- hn inverse photoemission e- hn e- E,q e- STM (I/V) theory: Which excitations? hn hn optical probe electronic electron energy-loss**Photoemission:**One measures EQP = EN – EN-1 = poles of G e- hn hn • Absorption: hn = optical gap Egap-opt = E’N – EN ≠ EN+1 + EN-1 – 2EN =Egap-QP QP and optical gaps coincide only when excitonic effects are negligible (Independent Quasiparticles approximation). The algebraic sum of the EQP measured in photoemission and inverse photoemission yields the quasiparticle gap (Egap-QP)**Independent quasiparticles and transitions?**c hn v P = P0 = -iGG Im [e] ~ vc|<v|D|c>|2d(Ec-Ev-)**---- LDA**---- RPA GW Absorption spectrum of Solid Argon IP-RPA calculation (Independent Quasiparticles) P = P0 = -iGG Excitons? Im [e] ~ vc|<v|D|c>|2d(Ec-Ev-)**Absorption spectrum of Solid Argon**Calculation with excitonic effects (G2 via the Bethe-Salpeter equation) Im [] ~ |vc<v|D|c> Avc|2d (E-) V. Olevano (2000) ->Mixing of transitions ->Modification of excitation energies Onida Reining Rubio RMP 74, 601 (2002)**Back to density functionals?**dVH(1)/dr(2) d(1,3)/dG(2,4) BSE c= 4c0 +4c0 [ v +xc/Gc TDDFT c= c0 + c0 [ v + fxc ] c Common ingredient dVxc(1)/dr(2) Different “electrons” = + G. Onida, L. Reining, A. Rubio, Reviews of Modern Physics 74, 601 (2002)**Effects of oxidation on small Silicon nanoaggregates:**Oxygen on Si10H16 Ground state equilibrium structure (Density Functional calculation) 16.000 steps13.5 ps M. Gatti and G. Onida, PRB 72, 1 (2005)**Redshift (in eV) of**the optical gap of Si10H16 after oxidation Excited state calculations within TDDFT (adiabatic LDA approximation) Silanone (H2SiO) Silane (SiH4) Absorption spectra: TDLDA works better for clusters (finite systems) than for infinite solids. M. Gatti and G. Onida, PRB 72, 1 (2005)**isodensity surfaces:**HOMO LUMO Stokes shift relaxation H2SiO: FIG. 1. Schematic representation of a Stokes shift relaxation. In position (1), the cluster is in its electronic ground state, and the atomic geometry is relaxed to its lowest energy configuration. On absorption of a photon, the nanocluster undergoes a vertical electronic excitation from (1) to (2). Once in the excited electronic state, the atomic geometry of the cluster relaxes to a lower energy configuration from (2) to (3). Finally, the excited electron and hole recombine via another vertical transition, (3) to (4). The Stokes shift is defined as EA - EE (Degoli et al., PRB 69, 155411, 2004)**Oxydized Si(100) surface**Ground State Calculations**Optical properties of Si(100):O (0.5 ML)**A. Incze, R. De Sole, G. Onida, PRB 71, 035350 (2005)**Surface Optical Spectra of Si (100):O**as a function of O coverage A. Incze, R. De Sole, G. Onida (2005)**Optical properties of Si (113) (3x2) ADI***“Bulk Anisotropy” due to the very asymmetric unit cell and the limited thickness of the slab. Very difficult to get converged spectra (K. Gaal-Nagy, G.O. et al, in preparation) *Structure: from Stekolnikov, Furthmueller and Bechstedt, PRB 68, 205306 (2003); PRB 67, 195332 (2003). In this case, the slicing technique is essential!**Ecole Polyt.**Parigi (Reining) Milano (Onida) York (Godby) Berlino (Gross, Scheffler) Roma (Del Sole) S.Sebastian (Rubio) Jena (Bechstedt) Louvain (Gonze) Lund (Almbladh) Researchers mobility: Post-Doc, Phd, diploma thesis... NANOQUANTANETWORKNanoscale photon absorption and spectroscopy with electrons**European Theoretical Spectroscopy Facility:**A “knowledge center”, lasting after Nanoquanta, to make the integrated resources available**“Lasting integration” is needed!**ETSF (European Theoretical Spectroscopy Facility) will offer: • know-how (e.g., TDDFT theory & implementations) • tools, computer codes • complementarity of groups (methods, systems)**Distributed**• Open KNOWLEDGE (European Theoretical Spectroscopy Facility) Collaborate, Publish Train Motivate Develope and Distribute Undergraduates PhD Students Post Docs Other colleagues Public awareness Papers Reviews Books Formula Computer Codes Let a larger community have access**Conclusions**• Ab-initio “theoretical spectroscopy”: • quantitative and predictive calculations • answers to new needs, due to new experiments • We are living a period of strong and fascinating growth of new (density-based) theoretical tools; • International integration of resources (Theory, knowledge and computer codes) is needed • NANOQUANTA is today a reality; the present challenge is to build ETSF. We are on the way.**Web references:**• users.unimi.it/etsf • google: just search “nanoquanta”: • www.abinit.org Thank you for your attention !**Si10H16**(Ground-state adiabatic dynamics) Microcanonical @ 700°K Car-Parrinello Molecular Dynamics simulation (G.Onida and W. Andreoni, Chem. Phys. Lett. 243, 183 (1995)**Nanotubes are transparent**for light polarized in the direction orthogonal to the tube!! Marinopoulos, Reining, Rubio, Vast, Phys. Rev. Lett. 91, 046402 (2003)**NANOQUANTA Industrial Advisory Board***-Siemens Medical Solutions, Forcheim (Germany): Dr. Martin Petersilka, Dr. Thomas von der Haar; -Thales Research and Technology, Orsay (France): Dr. Nguyen Van Dau, magnetic devices; -Labein Centro Tecnologico, Bilbao (Spain): Dr. Roberto Garcia, General Manager; -Max-Lab, Lund (Sweden), Dr. Nils Martensson; -Materials Design s.a.r.l., Le Mans (France): Dr. Erich Wimmer, president; -Telefonica Moviles, Madrid (Spain): Dr. Igacio Camarero, Exec. director of Technology & Operations Support; -Acreo AB, Kista (Sweden): Dr. Jan Y. Andersson, manager of the Optical Engineering dept; -Innovent Technologieentwicklung, Jena (Germany): Dr. Detlef Stock; -SchottGlas, Mainz (Germany): Dr. Wolfgang Mannstadt, Dr. Dirk Sprenger. *provisional list**How will the ETSF work?**The ETSF will be a large facility It will have “code-and theory-lines” It will have users who present projects**INFM**TOSCA - Tools for OpticalSpectra Calculation and Analysis Web page: users.unimi.it/etsf**Why excited states?**-”Useful” response to excitations (2) Optical properties of Ge-Te alloys Not just “academic” interest!****Trajectory: extrema of the action A Evolution of the system (its density) due to external field: TD-DFT [A] Runge and Gross, 1984 Back to density functionals? Static DFT: minimization of E Ground state: Time-Dependent DFT: