Ultrafast carrier-lattice interactions and interlayer modulations of Bi2Se3 by X-ray free electron laser diffraction

TL;DR

This study uses X-ray free-electron laser diffraction to observe ultrafast carrier-lattice interactions and interlayer modulations in Bi2Se3, revealing how these dynamics influence topological phase transitions on sub-picosecond timescales.

Contribution

It demonstrates the first direct measurement of ultrafast carrier-induced lattice dynamics and topological phase modulation in Bi2Se3 using time-resolved X-ray diffraction.

Findings

Carrier concentration affects lattice contraction magnitude.

Interlayer expansion exhibits oscillations post-excitation.

Topological phase transition is influenced by interlayer distance changes.

Abstract

As a 3D topological insulator, bismuth selenide (Bi2Se3) has potential applications for electrically and optically controllable magnetic and optoelectronic devices. How the carriers interact with lattice is important to understand the coupling with its topological phase. It is essential to measure with a time scale smaller than picoseconds for initial interaction. Here we use an X-ray free-electron laser to perform time-resolved diffraction to study ultrafast carrier-induced lattice contractions and interlayer modulations in Bi2Se3 thin films. The lattice contraction depends on the carrier concentration and is followed by an interlayer expansion accompanied by oscillations. Using density functional theory (DFT) and the Lifshitz model, the initial contraction can be explained by van der Waals force modulation of the confined free carrier layers. Band inversion, related to a topological phase transition, is modulated by the expansion of the interlayer distance. These results provide insight into instantaneous topological phases on ultrafast timescales.