Simulating electronically driven structural changes in silicon with two-temperature molecular dynamics

Abstract

Radiation can drive the electrons in a material out of thermal equilibrium with the nuclei, producing hot, transient electronic states that modify the interatomic potential energy surface. We present a rigorous formulation of two-temperature molecular dynamics that can accommodate these electronic effects in the form of electronic-temperature-dependent force fields. Such a force field is presented for silicon, which has been constructed to reproduce the ab initio-derived thermodynamics of the diamond phase for electronic temperatures up to 2.5 eV, as well as the structural dynamics observed experimentally under nonequilibrium conditions in the femtosecond regime. This includes nonthermal melting on a subpicosecond timescale to a liquidlike state for electronic temperatures above ∼1 eV. The methods presented in this paper lay a rigorous foundation for the large-scale atomistic modeling of electronically driven structural dynamics with potential applications spanning the entire domain of radiation damage.

Modelling radiation effects in solids with two-temperature molecular dynamics

“…This can be corrected for, however, by dynamically varying the electronic heat capacity in a way that compensates for the energy gain or loss. A rigorous Hamiltonian-based formulation for this energy conserving correction has recently been derived [55].…”

“…As described in section 2.4, varying the interatomic potentials will necessarily change the energy of the system, which must be compensated for. A method for conserving energy has recently been derived and should form the basis for future studies that employ dependent potentials [55].…”

Correction of Density-Functional-Theory based polynomial interatomic potentials to reproduce experimental melting properties

“…Femtosecond laser pules excite the electrons in matter to high electronic temperatures T e 's inducing significant ultrafast changes in the interatomic bonding whereas the ions remain mostly unaffected until electron-phonon interactions become active [12]. In order to address the short lived changes in interatomic bonding due to the hot electrons in large scale MD simulations, T e -dependent interatomic potentials were introduced [13][14][15][16][17][18][19][20], which depend beside the atomic coordinates also on the electronic temperature T e . The hot electrons cause many ultrafast phenomena like bond hardening or softening [21][22][23], structural solidsolid and solid-liquid phase transitions [24][25][26], phonon squeezing or antisqueezing [27,28], excitation of coherent phonons [29,30], which can be well described by T e -dependent density functional theory (DFT).…”

Self-Learning Method for Construction of Analytical Interatomic Potentials to Describe Laser-Excited Materials

“…Notice also, that previously developed potentials for Si at high T e 's [22,26,40] exhibit a very inaccurate description of the atomic RMSD during thermal phonon squeezing and nonthermal melting in bulk Si and of the atomic RMSD perpendicular to the surface of the thin film [41]. Fitting the coefficients of the hereby used [42] and of several widely used [1,2] classical analytical potentials to our thin-film DFT-MD simulations lead to a better description of the latter for the resulting potentials, but the accuracy of our Φ(T e ) is not reached [41].…”

“…Such an interatomic potential should correctly describe, apart from the structural properties of the laser-excited material, the evolution of bulk and surface after excitation at low computational cost, which makes them suitable for being used in large-and ultralarge scale MD simulations. In spite of intensive research in this direction [21][22][23][24][25][26], interatomic potentials fulfilling the above mentioned requirements could so far not be reliably constructed, partly due to the lack of sufficient microscopic data.…”