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O: Fachverband Oberflächenphysik

O 86: Surface dynamics

O 86.9: Talk

Thursday, March 12, 2026, 17:15–17:30, HSZ/0403

Manipulation of Optical Phonons by Thermal Expansion Mismatch in Bi/Si(111) Heterostructures — •Fabian Thiemann, Maja Penkl, and Michael Horn-von Hoegen — University of Duisburg-Essen, Lotharstr. 1, 47057 Duisburg, Germany

We investigate how substrate-induced biaxial strain controls the temperature behaviour of the A_1g phonon frequency Ω(T) in epitaxial Bi(111) films on Si(111) in the time domain. Bismuth’s anisotropic A7 structure, strong electron-phonon coupling and its Peierls’ distortion make the out-of-plane c-axis vibrations highly sensitive to strain. Using in situ SPA-LEED and broadband femtosecond transient reflectivity on the same samples, we measured temperature-dependent lattice parameters and coherent-phonon frequencies. Strained Bi films on Si(111) exhibit a larger thermal redshift ∂ Ω / ∂ T compared to relaxed bulk or polycrystalline Bi. Applying an extended phonon-shift model reveals that the anharmonic multi-phonon term depends solely on the value of the c-axis lattice parameter. This identifies c-axis expansion as the key structural factor governing the A_1g phonon, enabling targeted strain-engineering of phonon properties in bismuth and related anisotropic materials.

Keywords: coherent phonons; bismuth; strain; heterostructures; femtosecond pump-probe