Ultrafast laser-induced structure transition plays important roles in a number of applications, such as micro/nano processing, device fabrication, and data memory. Time-dependent density functional theory molecular dynamics (TDDFT-MD) is a computational technique that can capture a real-space atomic picture of materials in real time, and in particular can also unveil real-time interactions between electrons and lattice. In this presentation, we will focus on germanium telluride (GeTe), a kind of ferroelectric material and phase-change memory material, under optical excitation. First, a TDDFT-MD reveals an unexpected effect of optical excitation in the experimentally observed rhombohedral-to-cubic transition of GeTe. The excitation can induce coherent forces along [001], which is attributed to the energy landscape of Peierls distorted solids. The forces drive the A<sub>1g</sub> optical phonon mode in which Ge and Te move out of phase. Upon damping of the A<sub>1g</sub> mode, phase transition takes place. Second, an ultrafast nonvolatile ferroelectric polarization switching is demonstrated in GeTe by a TDDFT-MD. While optical tuning of the transition barrier and initiation of a directional atomic motion are crucial, the dephasing of the excited state holds the key for the realization of nonvolatility. Therefore, the TDDFT-MD studies demonstrate GeTe has an ability of realizing fast structure transition by optical excitation. These processes happen within 1 ps and are with no atomic diffusion, defect formation, or the nucleation and growth. The present report will benefit the application of GeTe in optical data memory with fast speed and nonvolatility.

Ultrafast structure transition of GeTe under optical excitation: TDDFT-MD studies

ACS Spring Meeting 2021

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