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CPP: Fachverband Chemische Physik und Polymerphysik

CPP 10: INTERNAL SYMPOSIUM Scattering Experiments I

CPP 10.5: Vortrag

Dienstag, 27. März 2007, 10:45–11:00, H37

The boson peak: Theory of scattering from vibrational excitations in disordered materials — •Walter Schirmacher¹, Bernhard Schmid¹, Giancarlo Ruocco², and Tullio Scopigno² — ¹Physik-Dept. E13, TUMünchen, D-85747 Garching — ²Dipt. di Fisica, Univ. di Roma, I-00185 Roma

A theory of vibrational excitations in disordered materials¹ and their observation by inelastic light, neutron, X-ray² and nuclear γ-ray scattering³, based on a model with fluctuating shear moduli is presented. The enhancement of the vibrational density of states (DOS) g(ω) over Debye’s g(ω)∝ ω² law (“boson peak”) is shown to be a direct consequence of the disorder. The observed spectra in coherent scattering experiments do not in general reveal the DOS directly. Only in incoherent neutron experiments and in nuclear resonant scattering of synchrotron radiation the scattering law is proportional to g(ω). Raman spectra are shown to reveal the imaginary part Σ″(ω) of the disorder-induced self energy, which is only indirectly related to the DOS, namely to its enhancement over the Debye DOS. The high-frequency sound damping, observed as broadening of the Brillouin line, is also proportional to Σ″(ω). It is shown that the existing spectra can be consistently interpreted in the light of this theory.

[1] W. Schirmacher, Europhys. Lett. 73, 892 (2006)

[2] W. Schirmacher, G. Ruocco, T. Scopigno, submitted to
       Phys. Rev. Lett.

[3] A. I. Chumakov et al. Phys. Rev. Lett. 92, 245508 (2004)