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

Nikita Medvedev, Zheng Li, and Beata Ziaja
Phys. Rev. B 91, 054113 – Published 26 February 2015

Abstract

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.

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  • Received 21 November 2014

DOI:https://doi.org/10.1103/PhysRevB.91.054113

©2015 American Physical Society

Authors & Affiliations

Nikita Medvedev1,*, Zheng Li1,2, and Beata Ziaja1,3

  • 1Center for Free-Electron Laser Science, Deutsches Elektronen-Synchrotron DESY, Notkestrasse 85, D-22607 Hamburg, Germany
  • 2Department of Physics, University of Hamburg, D-20355, Hamburg, Germany
  • 3Institute of Nuclear Physics, Polish Academy of Sciences, Radzikowskiego 152, 31-342 Kraków, Poland

  • *nikita.medvedev@desy.de

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Vol. 91, Iss. 5 — 1 February 2015

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