Ultrafast Dynamics of Excited Carries in Graphene

Condensed Matter Seminars

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 [1]. 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 [1]. Electron-phonon scattering is expected to play a central role in the relaxation processes. Nonetheless, the microscopic  mechanisms are still under-debate [2].

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 [5]. 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.

References:

[1] A. H. C. Neto et al., Rev. Mod. Phys. 81, 109 (2009)
[2] J. C. W. Song et al., Phys. Rev. Lett. 109, 106602 (2012)
[3] A. C. Betz et al., Nat. Phys. 9, 109 (2013)
[4] M. W. Graham et al., Nat. Phys. 9, 103 (2013)
[5] J. C. Johannsen  et al., Phys. Rev. Lett.  111, 027403 (2013)