  We present theoretical and experimental data on the threshold behaviour of
nanodot creation with swift heavy ions. A model calculation based on a
two-temperature model taking the spatially resolved electron density into
account gives a threshold of 12 keV/nm below which the energy density at the
end of the track is no longer high enough to melt the material. In the
corresponding experiments we irradiated SrTiO$_3$ surfaces under grazing
incidence with swift heavy ions. The resulting chains of nanodots were analyzed
by atomic force microscopy. In addition, samples irradiated under normal
incidence were analyzed by transmission electron microscopy. Both experiments
show two thresholds, connected to the appearance of tracks and to the creation
of fully developed tracks, respectively. The threshold values are similar for
surface and bulk tracks, suggesting that the same processes occur at glancing
and normal incidence. The experimental threshold for the formation of fully
developed tracks compares well to the value obtained by the theoretical
description.
