Lifting D-modules from positive to zero characteristic. Bulletin de la Société Mathématique de France Tome 139 Fasc. 2, 2011, 145--286. See also Berthelot's article "A note on
Frobenius divided modules in mixed characteristics" (link below).
Fundamental group-schemes in positive characteristic. Oberwolfach reports Volume 4, Issue 2, 1514--1516 (2007). (Summary of "Fundamental group schemes for
stratified sheaves". Based on a talk delivered at "Arithmetic and
differential Galois groups", Oberwolfach, May 2007.)