High-temperature atomic diffusion and specific heat in quasicrystals

Nagai, Yuki   ; Iwasaki, Yutaka*; Kitahara, Koichi*; Takagiwa, Yoshiki*; Kimura, Kaoru*; Shiga, Motoyuki   

A quasicrystal is an ordered but non-periodic structure understood as a projection from a higher dimensional periodic structure. An anomalous increase in heat capacity at high temperatures has been discussed for over two decades as a manifestation of a hidden high dimensionality of quasicrystals. A theoretical study of the heat capacity of realistic quasicrystals or their approximants has yet to be conducted because of the huge computational complexity. To bridge this gap between experiment and theory, we show experiments and cutting-edge machine-learning molecular simulations on the same material, an Al-Pd-Ru quasicrystal, and its approximants. We show that at high temperatures, aluminum atoms diffuse with discontinuous-like jumps, and the diffusion paths of the aluminum can be understood in terms of jumps corresponding to hyperatomic fluctuations in six-dimensional space.