Fine structure of swift heavy ion track in rutile TiO2

We report on the first observation of fine structure of latent tracks in rutile TiO2, which changes from cylinder to dumbbell-shape and then to sandglass-shape as a function of the ion path length. Based on inelastic thermal spike model, we show that Hagen-Poiseuille flow of molten phase produces the hillocks on surface and the void-rich zone near surface after epitaxial recrystallization due to material deficit, while at a deep depth, the lack of efficient outflow and recrystallization result in the absence of tracks. We propose that core-shell duration of transient molten phase induced by swift heavy ion and parabolic distribution of fluid velocity are radial-dependent. Moreover, the various morphologies of tracks are a consequence of the molten phase outflow and recrystallization during rapid cooling down. Our perspective provides a new interpretation in the track formation.