Dresden 2020 – scientific programme
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HL: Fachverband Halbleiterphysik
HL 33: Optical properties
HL 33.8: Talk
Tuesday, March 17, 2020, 15:45–16:00, POT 112
Anomalous Raman Scattering in Lead salts — •Nimrod Benshalom1, Olle Hellman2, and Omer Yaffe1 — 1Weizmann Institute of Science, Rehovot, Israel — 2Linköping University, Linköping, Sweden
I present experimental evidence for the violation of symmetry constraints in the Raman spectra of rock-salt Lead-Chalcogenides.
The theoretical framework describing Raman scattering relies almost entirely on the harmonic approximation. In the standard (harmonic) Raman picture an incident photon inelastically scatters by either absorbing or emitting a quantum of energy into a vibrational normal mode excitation, or phonon. The phonon itself is a construct of the harmonic approximation.
Given well-defined normal modes, group-theory translates symmetry considerations into selection rules for the kind of allowed light-matter scattering events. In some crystal structures, like rock-salt, symmetry forbids any single phonon Raman scattering.
Using polarization dependent measurements, we observe two new low-frequency modes consistent with a reduced effective symmetry. These modes persist across the Lead-Chalcogenides series, following a number of curious spectroscopic trends. I discuss this anomalous Raman activity as the result of anharmonic temperature activated symmetry breaking.