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Journal Article

Thermal and nonthermal melting of silicon under femtosecond x-ray irradiation


Li,  Zheng
International Max Planck Research School for Ultrafast Imaging & Structural Dynamics (IMPRS-UFAST), Max Planck Institute for the Structure and Dynamics of Matter, Max Planck Society;
Center for Free-Electron Laser Science, Deutsches Elektronen-Synchrotron DESY, Notkestrasse 85, D-22607 Hamburg, Germany;
Department of Physics, University of Hamburg, D-20355, Hamburg, Germany;

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Medvedev, N., Li, Z., & Ziaja, B. (2015). Thermal and nonthermal melting of silicon under femtosecond x-ray irradiation. Physical Review B, 91(5): 054113. doi:10.1103/PhysRevB.91.054113.

Cite as: http://hdl.handle.net/11858/00-001M-0000-002B-250B-C
As is known from visible-light experiments, silicon under femtosecond pulse irradiation can undergo so-called “nonthermal melting” if the density of electrons excited from the valence to the conduction band overcomes a certain critical value. Such ultrafast transition is induced by strong changes in the atomic potential energy surface, which trigger atomic relocation. However, heating of a material due to the electron-phonon coupling can also lead to a phase transition, called “thermal melting.” This thermal melting can occur even if the excited-electron density is much too low to induce nonthermal effects. To study phase transitions, and in particular, the interplay of the thermal and nonthermal effects in silicon under a femtosecond x-ray irradiation, we propose their unified treatment by going beyond the Born-Oppenheimer approximation within our hybrid model based on tight-binding molecular dynamics. With our extended model we identify damage thresholds for various phase transitions in irradiated silicon. We show that electron-phonon coupling triggers the phase transition of solid silicon into a low-density liquid phase if the energy deposited into the sample is above ∼0.65 eV per atom. For the deposited doses of over ∼0.9 eV per atom, solid silicon undergoes a phase transition into high-density liquid phase triggered by an interplay between electron-phonon heating and nonthermal effects. These thresholds are much lower than those predicted with the Born-Oppenheimer approximation (∼2.1 eV/atom), and indicate a significant contribution of electron-phonon coupling to the relaxation of the laser-excited silicon. We expect that these results will stimulate dedicated experimental studies, unveiling in detail various paths of structural relaxation within laser-irradiated silicon.