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DY: Fachverband Dynamik und Statistische Physik
DY 33: Critical Phenomena and Phase Transitions
DY 33.11: Talk
Wednesday, March 18, 2015, 17:45–18:00, BH-N 334
Thermal and nonthermal phase transitions in silicon induced by femtosecond free-electron laser pulse — •Nikita Medvedev1, Zheng Li1,2, and Beata Ziaja1,3 — 1CFEL at DESY, Notkestr. 85, 22607 Hamburg, Germany — 2University of Hamburg, 20355, Hamburg, Germany — 3Institute of Nuclear Physics, Polish Academy of Sciences, Radzikowskiego 152, 31-342 Krakow, Poland
Silicon under irradiation with intense femtosecond laser pulses can undergo a phase transition via two different channels: thermal and nonthermal. The first one occurs if the lattice is heated strongly enough with electron-phonon coupling to trigger silicon melting, while the second one results from the modification of the interatomic potential energy surface by excitation of electrons from the valence to the conduction band. We developed a model to include both channels. Tight-binding molecular dynamics (TBMD) is used to model atomic dynamics with the potential dependent on the state of electronic system. Simultaneously, electronic state is traced with the Boltzmann equation for low-energy electrons (the valence and the bottom of the conduction band), and with a Monte Carlo model for photoabsorbtion, high-energy electrons, and deep shell holes.
Our results show that electron-phonon coupling triggers phase transition into a low-density liquid phase for the deposited doses > 0.65 eV/atom. For deposited doses of over 0.9 eV/atom, silicon undergoes a phase transition into high-density liquid phase triggered via interplay of thermal heating and the nonthermal change of the atomic potential.