Observation of collective excitations in the amorphous alloy Ni 67 Zr 33 Toshiya Otomo a, * , Masatoshi Arai a , Yasuhiro Inamura a , Jens-Boie Suck b , Steve M. Bennington c , Kenji Suzuki d a Neutron Science Laboratory, Institute of Materials Structure Science, High Energy Accelerator Research Organization, 1-1 Oho, Tsukuba, Ibaraki 305-0801, Japan b Technical University of Chemnitz, Institute of Physics, Materials Research and Liquids, D-09107 Chemnitz, Germany c ISIS Science division, Rutherford Appleton Laboratory, Didcot, OX11 0RA, UK d Institute for Advanced Materials Research, Moniwadai 2-6-8, Taihaku-ku, Sendai, 982-0252, Japan Abstract The dynamical structure factor, S Q; E, for the amorphous alloy Ni 67 Zr 33 has been measured by neutron inelastic scattering, over a wide range of momentum,  hQ, and energy transfer, E. The collective character of the excitations, i.e. a dispersion relation, is observed up to Q  9  A 1 , which is the limit accessible in the present experiment. For the amor- phous alloy Ni 67 Zr 33 , it is ®rst experimental proof that the dispersion curve oscillates and damps around a mean value of the excitation energy (21 meV). Ó 1998 Elsevier Science B.V. All rights reserved. 1. Introduction Neutron inelastic scattering (NIS) is a very powerful tool to investigate the properties of col- lective excitations, e.g. phonon dispersion relations in condensed matter. The investigation of the dis- persion relation in disordered matter, including liquids, is of great importance in understanding certain physical phenomena that are thought to be characteristic of the disordered state. Quite a few investigations of the dispersion rela- tion in disordered matter have been carried out by NIS experiments on metallic liquids (e.g. Rb [1±3] and Cs [4]), molecular liquids [5] and amorphous Se. However, only a few dispersion relations have been measured in amorphous alloys (Mg 70 Zn 30 [6,7], Ni 24 Zr 76 [8], Fe 85 B 15 [9] and Ca 70 Mg 30 [10]). Although studies of these dispersion curves remain rare, any eort in this direction can provide very valuable results. Recent progress in NIS tech- niques using a direct-geometry chopper spectrom- eter on a pulsed neutron source oers the possibility of obtaining new and more complete in- formation about dispersion relations because of the wide Q ) E range and high neutron ¯ux now available. In the present work, the amorphous alloy Ni 67 Zr 33 was selected for the following reasons: (1) this amorphous alloy has shown strong topo- logical short-range order, which is likely to be im- portant for the existence of a clear dispersion relation in the amorphous state; (2) this alloy has a large coherent cross section for neutron scatter- ing; (3) Suck et al. have already observed a disper- Journal of Non-Crystalline Solids 232±234 (1998) 613±618 * Corresponding author. Tel.: 81 298 645615; fax: 81 298 643202; e-mail: toshiya.otomo@kek.jp. 0022-3093/98/$19.00 Ó 1998 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 3 0 9 3 ( 9 8 ) 0 0 5 5 2 - 3