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HL: Fachverband Halbleiterphysik
HL 76: Poster IIIC
HL 76.19: Poster
Donnerstag, 19. März 2020, 15:00–17:30, P2/4OG
Transient negative thermal expansion in HgTe/CdTe heterostructures by heating transverse phonons — •Matthias Rössle1, Marc Herzog2, Jan Pudell2, Wolfram Leitenberger2, Maximilian Mattern1,2, Lukas Lunczer3, Claus Schumacher3, Harmut Buhmann3, Laurens Molenkamp3, and Matias Bargheer1,2 — 1Helmholtz-Zentrum Berlin für Materialien und Energie, Germany — 2Institut für Physik und Astronomie, Universität Potsdam, Germany — 3Physikalisches Institut EP3, Universität Würzburg, Germany
We investigate the transient negative thermal expansion of semimetallic HgTe and semiconducting CdTe by using synchrotron-based time-resolved X-ray diffraction. At T = 20 K, far below the Debye temperature of both materials, the selective optical excitation of the HgTe top layer with an ultrashort near-infrared laser pulse leads to a rapid expansion of HgTe that is followed by a long lasting contraction. The CdTe substrate is compressed by the HgTe thin film expansion, and subsequently CdTe contracts due to thermally excited transverse phonon modes. This shows that negative thermal expansion is manifest on ultrafast timescales, consistent with the negative Grüneisen coefficient for transverse phonons in semiconducting materials with sphalerite crystal structure. At T = 200 K, far above the Debye temperature of both materials, the expansion driven by longitudinal acoustic phonons is prevalent. We simulate the lattice dynamics in an elastic model where transient thermal stresses are calculated via heat diffusion based on equilibrium thermoacoustic properties.