Distinguishing bulk and surface electron-phonon coupling in the topological insulator Bi2Se3 using time-resolved photoemission spectroscopy

摘要

We report time- and angle-resolved photoemission spectroscopy measurements onthe topological insulator Bi2Se3. We observe oscillatory modulations of theelectronic structure of both the bulk and surface states at a frequency of 2.23THz due to coherent excitation of an A1g phonon mode. A distinct, additionalfrequency of 2.05 THz is observed in the surface state only. The lower phononfrequency at the surface is attributed to the termination of the crystal andthus reduction of interlayer van der Waals forces, which serve as restorativeforces for out-of-plane lattice distortions. DFT calculations quantitativelyreproduce the magnitude of the surface phonon softening. These resultsrepresent the first band-resolved evidence of the A1g phonon mode coupling tothe surface state in a topological insulator.