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. Moreover, the cone length of the track was found to be dependent on electronic energy loss. The results indicate that outflow of the thermal spike-induced 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 the various morphologies of tracks in rutile TiO₂ are a consequence of the molten phase outflow and recrystallization during rapid cooling down.