Potential energy threshold for nano-hillock formation by impact of slow   highly charged ions on a CaF$_2$(111) surface
  We investigate the formation of nano-sized hillocks on the (111) surface of CaF$_2$ single crystals by impact of slow highly charged ions. Atomic force microscopy reveals a surprisingly sharp and well-defined threshold of potential energy carried into the collision of about 14 keV for hillock formation. Estimates of the energy density deposited suggest that the threshold is linked to a solid-liquid phase transition (``melting'') on the nanoscale. With increasing potential energy, both the basal diameter and the height of the hillocks increase. The present results reveal a remarkable similarity between the present predominantly potential-energy driven process and track formation by the thermal spike of swift ($\sim$ GeV) heavy ions. 
A.S. El-Said, W. Meissl, M.C. Simon, J.R. Crespo L\'opez-Urrutia, C.   Lemell, J. Burgd\"orfer, I.C. Gebeshuber, HP. Winter,...