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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: