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

TL;DR

This study uses time-resolved photoemission spectroscopy to differentiate bulk and surface electron-phonon interactions in Bi2Se3, revealing distinct phonon couplings and softening effects at the surface, supported by DFT calculations.

Contribution

First band-resolved evidence of A1g phonon mode coupling to the surface state in a topological insulator, highlighting surface-specific phonon softening.

Findings

Oscillatory modulations at 2.23 THz in bulk and surface states

Additional surface-specific phonon frequency at 2.05 THz

DFT calculations confirm surface phonon softening

Abstract

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