Speaker: Alberto Crepaldi, Elettra-Sincrotrone Trieste
Date & Time: January 16, 2014 14:00 - 15:00
Location: UBC, Hennings 318
Local Contact: Andrea Damascelli
Intended Audience: Graduate
The transport and optical properties of graphene are made unique by the linear dispersion of the Dirac particles at the K point of its Brillouin zone . Exploiting graphene for opto-electronic and light-harvesting devices requires a detailed knowledge of the physics describing the electron-hole pair generation and recombination, after optical perturbation . Electron-phonon scattering is expected to play a central role in the relaxation processes. Nonetheless, the microscopic mechanisms are still under-debate .
In particular, the role of optical and acoustic phonons in determining the relaxation of excited carriers is not universally established [2-4]: there is increasing evidence that supercollisions, three body scattering between electrons and phonons mediated by defects, are the dominating mechanism for the relaxation acting at the picosecond timescale [3,4]
Time and angle resolved photo-electron spectroscopy (tr-ARPES), with extreme UV photons (33 eV) generated by high harmonics in gas, offers a direct insight in the hot-charge carrier dynamics of photo-excited graphene. We investigated the out-of-equilibrium Fermi Dirac distribution, and we analyze the electronic temperature with a three-temperatures model, including supercollisions . This model allows us to extract the electron-phonon coupling constants. Furthermore, we infer the photo-induced electron-hole pairs density, which stays under the threshold of carrier multiplication. This suggests that carrier multiplication is not achieved with our experimental conditions.
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