Regensburg 2004 – wissenschaftliches Programm

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HL: Halbleiterphysik

HL 12: Poster I

HL 12.98: Poster

Montag, 8. März 2004, 16:30–19:00, Poster A

Determination of the excitonic dispersion in α-PTCDA with electron energy loss spectroscopy and optical spectroscopy — •Reinhard Scholz, Igor Vragović, and Michael Schreiber — Institut für Physik, Technische Universität Chemnitz

In the present work, recent experimental findings concerning electron energy loss spectroscopy, linear optical spectroscopy and low temperature photoluminescence are interpreted with model calculations based on the transfer of Frenkel excitons. It can be shown in detail that the exciton transfer between non-degenerate vibronic sublevels results in deviations of the excitonic dispersion from a simple cosine-shape [1]. Moreover, these off-diagonal parts of the exciton transfer have a strong influence on the oscillator strength of the vibronic sublevels: In optical absorption at the Γ-point of the Brillouin zone, the coupling strength is redistributed to the upper levels [2], whereas at the surface of the Brillouin zone, it accumulates in the vibronic ground state. The wave vector dependence of the lineshape observed with electron energy loss spectroscopy (EELS) reflects this behaviour [3]. The photoluminescence band dominating at low temperatures results from a vertical recombination process starting from the minimum of the lowest branch of the excitonic dispersion [1].

[1] I. Vragović and R. Scholz, Phys. Rev. B 68, 155202 (2003).

[2] I. Vragović, R. Scholz, and M. Schreiber, Europhys. Lett. 57, 288 (2002).

[3] I. Vragović, Frenkel exciton model of excitation and recombination processes in crystalline α-PTCDA, Dissertation, Technische Universität Chemnitz (2003).