Delayed Onset of Nonthermal Melting in Single-Crystal Silicon Pumped with Hard X Rays

Area of Science:

• Materials Science
• Condensed Matter Physics
• Ultrafast Phenomena

Background:

• Nonthermal melting is a non-equilibrium phase transition.
• Understanding the dynamics of melting is crucial for materials processing and fundamental physics.

Purpose of the Study:

• To investigate the onset and dynamics of nonthermal melting in single-crystal silicon.
• To determine the temporal evolution of atomic order during ultrafast laser-induced melting.

Main Methods:

• Utilized an X-ray pump-X-ray probe spectroscopy scheme.
• Employed ultrashort X-ray pulses from the Linac Coherent Light Source (SLAC).
• Implemented a custom-built split-and-delay line for high temporal resolution.

Main Results:

• Observed no loss of long-range atomic order for 150±40 femtoseconds (fs) after photoabsorption.
• Interpreted this delay as the time for the electronic system to reach critical nonthermal melting temperature.
• Found that the loss of long-range order occurs inertially, completing within 315±40 fs.

Conclusions:

• The electronic system equilibrates before the onset of nonthermal melting.
• Atomic disordering proceeds ballistically/inertially once the critical temperature is reached.
• This study provides critical insights into the fundamental mechanisms of ultrafast phase transitions in solids.