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HL: Fachverband Halbleiterphysik
HL 33: Poster II
HL 33.61: Poster
Wednesday, March 29, 2023, 17:00–19:00, P1
Negative thermal expansion in HgTe/CdTe heterostructures on ultrafast timescales — •Marc Herzog1, Matthias Rössle2, Jan-Etienne Pudell3, Maximilian Mattern1, Lukas Lunczer4, Claus Schumacher1, Hartmut Buhmann1, Laurenz Molenkamp1, and Matias Bargheer2,3 — 1Institut für Physik und Astronomie, Universität Potsdam, Germany — 2Helmholtz-Zentrum Berlin, Germany — 3European XFEL, Germany — 4Physikalisches Institut EP3, Universität Würzburg, Germany
Materials that would exhibit an ultrafast negative thermal expansion (NTE) are desirable to generate ultrashort and high-amplitude acoustic waves as unconventional stimulus e.g. in the context of nonlinear acoustics or magnetoelastic effects. Both semimetallic HgTe and semiconducting CdTe exhibt pronounced NTE behaviour in thermal equilibrium below their Debye temperatures ΘD≈ 150 K owing to a negative Grüneisen parameter and correspondingly negative stress by transverse acoustic (TA) phonons.
Using ultrafast x-ray diffraction we investigate the coherent (sound) and incoherent (heat) lattice response in HgTe thin films on a CdTe substrate to ultrashort laser pulse excitation. While above ΘD both materials exclusively expand upon excitation, below ΘD a pronounced NTE is observed. However, the time scale for this NTE to develop is a few 100 ps suggesting a slow excitation of the TA modes. At few-ps time scales, however, a strong expansion of HgTe prevails at all measured excitation densities indicating a fast and dominant positive stress due to non-TA phonon modes and/or hot carriers.