Low-energy phonon excitations in the decagonal quasicrystal Al70Co15Ni15 and in crystalline Al13Co4 phases

We compare the experimentally measured generalized vibrational density of states (DOS) in Al70Co15Ni15 and the related crystalline Al13Co4, with the phonon spectra resulting from the numerical simulation based on realistic binary Al–Co pair potentials [R. Phillips, J. Zou, A.E. Carlsson, M. Widom, Phys. Rev. B 49 (1994) 9322], the Al13Co4 structure model after Grin et al. [J. Grin, U. Burkhardt, M. Ellner, K. Peters, J. Alloys Compounds 206 (1994) 243], and an Al70Co15Ni15 atomic structure model with 20 Å clusters decorating vertices of a Penrose tiling. The experimentally measured low-ω part of the VDOS is appreciably higher than the Debye VDOS with the mean sound velocity measured by Bianchi et al. [M.A. Chernikov, H.R. Ott, A. Bianchi, A. Migliori, T.W. Darling, Phys. Rev. Lett. 80 (1998) 321]. The VDOS calculated in the harmonic approximation using pair potentials has acoustics modes with the corresponding mean sound velocity close to the experimental value, but shows an even bigger excess DOS at low ω than the experimental neutron generalized VDOS, due to the non-acoustic character of a significant fraction of modes at low ω.