Ultrafast Carrier–Lattice Interactions and Interlayer Modulations of Bi<sub>2</sub>Se<sub>3</sub> by X-ray Free-Electron Laser Diffraction
Sungwon Kim, Youngsam Kim, Jaeseung Kim, Sungwook Choi, Kyuseok Yun, Dong-Jin Kim, Soo Yeon Lim, Sunam Kim, Sae Hwan Chun, Jaeku Park, Intae Eom, Kyung Sook Kim, T. Y. Koo, Yunbo Ou, Ferhat Katmis, Haidan Wen, Anthony D. DiChiara, Donald A. Walko, Eric C. Landahl, Hyeonsik Cheong, Eunji Sim, Jagadeesh S. Moodera, Hyunjung Kim
Abstract
As a 3D topological insulator, bismuth selenide (Bi2Se3) has potential applications for electrically and optically controllable magnetic and optoelectronic devices. Understanding the coupling with its topological phase requires studying the interactions of carriers with the lattice on time scales down to the subpicosecond regime. Here, we investigate the ultrafast carrier-induced lattice contractions and interlayer modulations in Bi2Se3 thin films by time-resolved diffraction using an X-ray free-electron laser. The lattice contraction depends on the carrier concentration and is followed by an interlayer expansion accompanied by oscillations. Using density functional theory and the Lifshitz model, the initial contraction can be explained by van der Waals force modulation of the confined free carrier layers. Our theoretical calculations suggest that the band inversion, related to a topological phase transition, is modulated by the expansion of the interlayer distance. These results provide insights into the topological phase control by light-induced structural change on ultrafast time scales.