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

We compare the experimentally measured generalized vibrational density of s tates (DOS) in Al70Co15Ni15 and the related crystalline Al13Co4, with the p honon spectra resulting from the numerical simulation based on realistic bi nary Al-Co pair potentials [R. Phillips, J. Zou, A.E. Carlsson, M. Widom, P hys. 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 Angstrom cluster s decorating vertices of a Penrose tiling. The experimentally measured low- omega part of the VDOS is appreciably higher than the Debye VDOS with the m ean 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 V DOS calculated in the harmonic approximation using pair potentials has acou stics modes with the corresponding mean sound velocity close to the experim ental value, but shows an even bigger excess DOS at low omega than the expe rimental neutron generalized VDOS, due to the non-acoustic character of a s ignificant fraction of modes at low omega. (C) 2000 Elsevier Science B.V. A ll rights reserved.