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

Abstract

In this work, we monitor the onset of nonthermal melting in single-crystal silicon by implementing an x-ray pump-x-ray probe scheme. Using the ultrashort pulses provided by the Linac Coherent Light Source (SLAC) and a custom-built split-and-delay line for hard x rays, we achieve the temporal resolution needed to detect the onset of the transition. Our data show no loss of long-range order up to 150±40 fs from photoabsorption, which we interpret as the time needed for the electronic system to equilibrate at or above the critical nonthermal melting temperature. Once such equilibration is reached, the loss of long-range atomic order proceeds inertially and is completed within 315±40 fs from photoabsorption.

Femtosecond phase-transition in hard x-ray excited bismuth

“…Given that x-ray diffraction is mainly sensitive to the changes in core electron density, it should be noted that Bragg diffraction signal is, in principle, not directly sensitive to the PES. Indeed, for hard x-ray excited Si with similar absorbed doses to our case 5 , the start of the Bragg diffraction signal decay was observed after ~110 fs from the incidence of the pump. The similar excitation condition in Bi in our case also implies a significantly low ionisation degree, thus low photoelectron population, despite the absorbed dose is up to a few times higher than its damage threshold 13 .…”

“…3 (shaded green area) has been observed at a comparable timescale in x-ray irradiated diamond within its transient transmission of optical pulses 25 . An exponential fitting to extrapolate the decay time constant is possible for reflections from lattice insensitive to optical phonon-modes 5,13 . In our case, however, due to the sensitivity of (111) reflection to the A 1g phonon mode, we cannot uniquely exclude the possible origin of the plateau in Fig.…”

“…Meanwhile the free energy of the sample increases by the amount of the absorbed energy, which is stored in high energy (>keV) photoelectrons. In a conventional non-thermal melting processes, the following dynamics has been discussed, especially for the case of semiconductors 5,32,33 . A fast electron distributes its energy through collisional interactions repetitively until it settles down into bound states.…”

“…The typical response time of the internal microscopic degrees of freedom in a solid, such as the arrangement of the electrons and atoms, ranges between few fs to few ps. Ultrashort laser pulses can excite materials on time scales faster than those response times, often revealing unique behaviour 1–5 and furthering the understanding of the interactions between electrons and atomic lattice 6–8 . In the case of ultrafast melting, photoexcitation drives the material in a highly non-equilibrium state where the electrons are excited while the lattice is still cold.…”

Impact of free electron degeneracy on collisional rates in plasmas

“…We define this delay as the difference in time between the XFEL pulse and bulk electron heating, meaning a time delay of zero would correspond to instant thermalization of the XFEL energy into the bulk electrons. We have used delay times in our calculations which approximately agree with recently reported thermalization times of XFEL-generated hot electrons in silicon [55].…”