Role of Thermal Equilibrium Dynamics in Atomic Motion during Nonthermal Laser-Induced Melting

Abstract

This study shows that initial atomic velocities as given by thermodynamics play an important role in the dynamics of phase transitions. We tracked the atomic motion during nonthermal laser-induced melting of InSb at different initial temperatures. The ultrafast atomic motion following bond breaking can in general be governed by two mechanisms: the random velocity of each atom at the time of bond breaking (inertial model), and the forces acting on the atoms after bond breaking. The melting dynamics was found to follow the inertial model over a wide temperature range.

Superionic states formation in group III oxides irradiated with ultrafast lasers

“…Combining obtained information, we can deduce the following path of nonthermal transitions into a superionic state in the investigated compounds. The transition starts from a fast displacement of O atoms because of shallow and highly asymmetric potential well 35 . Moving oxygen atoms change the potential energy surface profile of metal atoms “dragging” them away from the equilibrium positions of an ambient crystal.…”

The seeds and homogeneous nucleation of photoinduced nonthermal melting in semiconductors due to self-amplified local dynamic instability

“…The idea behind the inertial model is that the unbound atoms move at the random velocity they had at the time when the bonds were broken. This model is further supported by the finding of temperaturedependent melting rates because the initial random velocities are set by the thermodynamic temperature (29). However, this has also been disputed by Zijlstra et al (28) based on an ab initio density functional theory (DFT) prediction that dense electron-hole plasma softens only the acoustic phonon rather than all modes, as assumed in the inertial model.…”

“…Ultrashort laser pulses are now used to manipulate the structure and function of materials at far from equilibrium states (1)(2)(3)(4)(5)(6)(7)(8)(9), with the corresponding ultrafast dynamics being one of the ultimate problems in modern science and technology. In terms of applications, the femtosecond and nanosecond pulsed laser was first used to deal with the annealing of the amorphous layer of ion-implanted silicon (Si) in the late 1970s (10) and then extended to annealing the lattices of other semiconductors, such as Si (11)(12)(13)(14)(15)(16)(17)(18)(19)(20)(21)(22)(23), GaAs (2,15,24,25), InSb (1,3,(26)(27)(28)(29), and Ge (30,31). Soon after the discovery of the so-called pulsed laser annealing, it was established that this laser annealing is an ultrafast nonthermal melting process (32)(33)(34)(35) in which the photoexcited electrons are hot and the ions are still cold (in terms of kinetic energy) because the lattice disordering starts and finishes well before the completion of carrier-lattice thermalization via electron-phonon coupling.…”

“…It is commonly believed that, on the subpicosecond time scale, the excitation of a large fraction of electrons from bonding valence bands to antibonding conduction bands could weaken the lattice and induce a repulsive interatomic force to quickly disorder the lattice without remarkably increasing its thermal energy ( 1 – 3 ). The debate now focuses on the mechanisms and microscopic picture for such a disorder occurs, whether driven by softened phonon modes with an imaginary frequency [corresponding to a saddle point on the potential energy surface (PES)] ( 14 , 15 , 20 , 28 ) or by the random velocity of each atom while there is a flat PES ( 3 , 29 , 40 ). The former is termed electron-hole plasma-induced phonon instability theory ( 11 ), and the latter is the so-called inertial model ( 3 ).…”

Using Diffuse Scattering to Observe X-Ray-Driven Nonthermal Melting

“…This is calculated by subtracting the t=0 signal, normalized to the pump pulse intensity of each shot, and then normalizing the sum of this pump-subtracted signal to the probe pulse intensity. Also shown is the signal simulated by DFT-MD, described above, and that from a purely inertial molecular dynamics model -this simulation was performed with Lammps [30,31], using a 1000 Si atoms intialized in a cubic diamond structure at 300 K, and evolved with only a Lennard-Jones potential between the particles, such that their evolution is dominated by the room temperature thermal motion [32]. As expected, the DFT-MD simulation disorders more quickly, due to the Coulomb forces induced between the ionized silicon, driving them towards a closer-packed liquid state [4].…”