Journal
PHYSICAL REVIEW LETTERS
Volume 113, Issue 15, Pages -Publisher
AMER PHYSICAL SOC
DOI: 10.1103/PhysRevLett.113.157401
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Funding
- U.S. Department of Energy, Office of Science, Basic Energy Sciences, Materials Sciences and Engineering Division [DE-AC02-76SF00515]
- Swiss National Science Foundation
- Stanford Graduate Fellowship
- Laboratory Directed Research and Development Program of Lawrence Berkeley National Laboratory under the U.S. Department of Energy [DE-AC02-05CH11231]
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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 A(1g) 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. Density functional theory calculations quantitatively reproduce the magnitude of the surface phonon softening. These results represent the first band-resolved evidence of the A(1g) phonon mode coupling to the surface state in a topological insulator.
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