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K: Fachverband Kurzzeitphysik

K 5: Laserstrahlwechselwirkung und Laseranwendungen

K 5.4: Vortrag

Dienstag, 26. Februar 2013, 14:45–15:00, HS 4

Nonthermal phase transitions of semiconductors under femtosecond XUV irradiation — •Nikita Medvedev¹, Harald Jeschke², and Beata Ziaja¹ — ¹Center for Free Eelectron Laser Science (CFEL at DESY), Notkestr. 85, Hamburg — ²Institut für Theoretische Physik, Goethe-Universität Frankfurt am Main

After high energy deposition into a semiconductor, an ultrafast phase transition can occur within ~100 fs, driven by the changes in the interatomic potential. This ultrafast damage takes place much before the electron-phonon coupling heats up the lattice. To study this, we apply a new hybrid model for tracing the XUV laser induced atom dynamics, taking into account non-equilibrium electron kinetics [1]. The atomic motion is modeled with tight binding molecular dynamics. For tracing the electron distribution we used a hybrid model: Monte Carlo method for high energy electrons combined with a temperature equation for the partially thermalized low energy electrons [1,2]. We found that the electron relaxation kinetics in diamond, leading to a transition to graphite, took place within ~50 fs after the exposure with the short laser pulse (10 fs). It was stimulated by a collapse of the band gap, when the number of electrons excited into the conduction band overcame 1.5 % of the valence electrons (corresponds to 0.7 eV/atom dose) [1]. These results demonstrate for the first time the non-thermal melting of semiconductors under the femtosecond XUV irradiation [1].

[1] N. Medvedev, H.O. Jeschke, B. Ziaja, NJP 14 (2012) [2] B. Ziaja, N. Medvedev, HEDP 8 (2012)