Algorithm advances and applications of time‐dependent first‐principles simulations for ultrafast dynamics

Abstract

Far from equilibrium phenomenon is a central theme of contemporary material research. Such phenomenon can exhibit itself in atomic structure and dynamics, but very often it also happens as non-equilibrium phenomenon in the electronic structure. In ab initio material simulation, density functional theory (DFT) has played an essential role in studying electronic ground state problems. For excited states, besides many-body perturbation theory, another powerful tool is the time dependent DFT (TDDFT) method. In particular, the real-time TDDFT (rt-TDDFT) method can be used to simulate many non-equilibrium phenomena directly. Here we introduce our works on some algorithm advances based on our recently rt-TDDFT method. This method uses the plane-wave basis set, and significantly accelerates its efficiency by increasing the time step from 0.1-1 as in traditional methods to 0.2-0.5 fs. The noncollinear magnetic moments and spin-orbit coupling have also been included in our rt-TDDFT method. Furthermore, a Boltzmann-TDDFT algorithm has been developed to solve the hot carrier overheating problem in Ehrenfest dynamics, and a natural orbital branching algorithm has been developed to overcome the mean-field approximation in Ehrenfest dynamics nuclear trajectory, thus allows stochastic multiple paths in chemical reactions. Utilizing these methods, we have studied the photoinduced ultrafast demagnetization, ultrafast phase transition, energy transfer between plasmon and hot carriers, as well as the high-energy ion implantation and low-energy atomic diffusion in semiconductors.We believe the tools as the ones introduced here can enable us to study a wide range of phenomena which are of great interest in modern day material research.

Inverse Design of Light Manipulating Structural Phase Transition in Solids

“…In this paper, we present a series of recent studies that employ ab initio molecular dynamics (AIMD) methods , and real-time time-dependent density-functional theory (rt-TDDFT) methods − to simulate the ultrafast PIPT processes in various materials. Our findings demonstrate that the real-time distributions of excited charge and atomic motion can be accurately modeled with an excellent agreement to experimental data, , utilizing a newly improved rt-TDDFT method that considers hot carrier relaxation . By performing the sophisticated rt-TDDFT simulations, we have developed a set of theories that explain the physical mechanisms responsible for laser-induced structural transitions, including the photoexcitation-induced atomic driving force, the role of hot carrier cooling, and the self-amplified local dynamic instability in nonthermal melting .…”

“…To solve eq , we employ the adiabatic state basis functions φ l ( t ) to expand the time-dependent wave function φ i ( t ): where …”

“…We obtain the adiabatic state wave functions φ l ( t 2 ) and eigenenergies ε l ( t 2 ) at the t 2 time. The Hamiltonian H ( t ) at t 1 and t 2 can be expressed as a time-dependent linear relationship (LTDH): …”

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

“…Another laser pulse has a wavelength λ = 387 nm, duration √2σ = 25 fs, and photon energy ω = 3.2 eV, which is consistent with the parameters from another experiment (section S1) ( 13 ). The new Boltzmann factor algorithm for the rt-TDDFT can be found in ( 46 ) and ( 47 ) as well as section S3. In our simulations, we mainly use the NVE ensemble, in which the atomic number N , volume V , and total energy E are conserved.…”

“…Here, we reveal that photoinduced ultrafast nonthermal melting occurs via homogeneous nucleation with randomly distributed local seeds rather than simultaneously breaking all the bonds, as suggested by proposed mechanisms. We use newly developed real-time time-dependent DFT (rt-TDDFT) by introducing a Boltzmann factor to restore the detailed balance, which is capable of describing the hot carrier cooling process ( 46 , 47 ) to perform first-principles simulations of the photoexcitation-induced nonthermal melting in Si. Without “ad hoc” hypotheses, our simulations closely reproduce experimental data using both 387-nm (3.2 eV) and 610-nm (2.03 eV) laser pulses.…”