Abstract (EN)

The presence of frozen-in or quenched disorder in a system can often modifythe nature of its phase transition. A particular instance ofthis phenomenon is the so-called rounding effect: it has been shown in many cases that the free-energy curve ofthe disordered system at its critical point is smoother thanthat of the homogenousone. In particular some disordered systems do not allow first-order transitions. Westudy this phenomenon for the pinning of a renewal with stretched-exponential tails on adefect line (the distributionKof the renewal increments satisfiesK(n)∼cKexp(−nα),α∈(0,1)) which has a first order transition when disorder is not present. We showthat the critical behavior of the disordered system dependson the value ofα: whenα >1/2 the transition remains first order, whereas the free-energy diagram is smoothedforα≤1/2. Furthermore we show that the rounding effect is getting stronger whenαdiminishes.