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HL: Fachverband Halbleiterphysik

HL 33: Poster II

HL 33.61: Poster

Mittwoch, 29. März 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 Bargheer^2,3 — 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 Θ_D≈ 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 Θ_D both materials exclusively expand upon excitation, below Θ_D 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.