  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.
