Density evolution in formation of swift heavy ion tracks in insulators

Swift heavy ion irradiation leaves a latent ion track around the ion path in many materials. Here we report computational molecular dynamics (MD) simulation results on track formation in several insulating materials, quartz, amorphous silica (a-SiO2), zinc oxide and diamond, concentrating especially in mass transport leading to density variations in the track volume during the initial stages of track formation. These details are largely unobservable in experiments due to the picosecond timescale and very local nature, and also in many computational models of track formation. Earlier a low-density core - high-density shell fine structure has been observed in latent tracks in amorphous silica, and here we study if other materials than silica show similar behavior. The results highlight the dynamical nature of track formation, that includes competing effects of heat and mass transport, rapid quenching of the heated area and recrystallization.