Bandgap modulation in photoexcited topological insulator Bi₂Te₃ via atomic displacements

The atomic and electronic dynamics in the topological insulator (TI) Bi₂Te₃ under strong photoexcitation were characterized with time-resolved electron diffraction and time-resolved mid-infrared spectroscopy. Three-dimensional TIs characterized as bulk insulators with an electronic conduction surface band have shown a variety of exotic responses in terms of electronic transport when observed under conditions of applied pressure, magnetic field, or circularly polarized light. However, the atomic motions and their correlation between electronic systems in TIs under strong photoexcitation have not been explored. The artificial and transient modification of the electronic structures in TIs via photoinduced atomic motions represents a novel mechanism for providing a comparable level of bandgap control. The results of time-domain crystallography indicate that photoexcitation induces two-step atomic motions: first bismuth and then tellurium center-symmetric displacements. These atomic motions in Bi₂Te₃ trigger 10% bulk bandgap narrowing, which is consistent with the time-resolved mid-infrared spectroscopy results.