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SUMMARY:Non-equilibrium processes from an ab-initio perspective: generaliz
 ed Hedin's equations and ultra-fast processes in paradigmatic materials - 
 Andrea Marini\, Consiglio Nazionale delle Ricerche
DTSTART:20141030T141500Z
DTEND:20141030T151500Z
UID:TALK54064@talks.cam.ac.uk
CONTACT:Gareth Conduit
DESCRIPTION:Ultra-fast optical spectroscopy is a powerful tool for the obs
 ervation of dynamical processes in several kind of materials. The basic ti
 me-resolved optical experiment is the so-called "pump-probe" where a first
  light pulse\, the "pump"\, resonantly triggers a photo-induced process. T
 he subsequent system evolution can be monitored\, for example\, by the tim
 e-dependent transmission changes of a delayed "probe" pulse. In a pump-pro
 be experiment the pump pulse photon energy\, spectral width and peak inten
 sity creates a certain density of electron-hole pairs in a more or less lo
 calized region of space. After the creation of the initial carrier populat
 ion the time evolution of the single-particle and many-particle excitation
 s is now governed by a non-trivial interplay between electron-electron sca
 tterings and energy relaxation.\n\nIn this talk I will present a novel app
 roach based on the merging of Non-Equilibrium Green's function theory and 
 Density Functional Theory to investigate the dynamics following a pump exc
 itation. I will first shortly discuss the general mathematical framework o
 f the non-equilibrium theory based on a set of integro-differential equati
 ons (Hedin's equations) written out-of-equilibrium. I will show how it is 
 possible to include consistently quantized electron\, photons and phonons.
  I will\, then\, take the purely longitudinal limit in order to introduce 
 the key approximations that make the problem numerically treatable.\n\nI w
 ill take the case of bulk silicon\, a paradigmatic indirect gap semiconduc
 tor\, to discuss the key features of the carrier dynamics. I will compare 
 the results with recent two photon photoemission measurements. I will show
  that the interpretation of the carrier relaxation in terms of L->X inter-
 valley scattering is not correct. The ultrafast dynamics measured experime
 ntally is\, instead\, due to the scattering between degenerate L states th
 at is activated by the non symmetric population of the conduction bands in
 duced by the laser field. This ultrafast relaxation is\, then\, entirely d
 ue to the specific experimental setup and it can be interpreted by introdu
 cing a novel definition of the quasi-particle lifetimes in an out-of-equil
 ibrium context.
LOCATION:TCM Seminar Room\, Cavendish Laboratory
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