Abrupt transition in a sandpile model
  We present a fixed energy sandpile (FES) model which, by increasing the initial energy,undergoes, at the level of individual configurations, a discontinuous transition.The model is obtained by modifying the toppling procedure in the BTW rules: the energy transfer from a toppling site takes place only to neighbouring sites with less energy (negative gradient constraint) and with a time ordering (asynchronous). The model is minimal in the sense that removing either of the two above mentioned constraints (negative gradient or time ordering) the abrupt transition goes over to a continuous transition as in the usual BTW case. Therefore the proposed model offers an unique possibility to explore at the microscopic level the basic mechanisms underlying discontinuous transitions. 
Y.F. Contoyiannis and F.K. Diakonos (University of Athens, Athens,   Greece)