Exotic Dehn twists on 4-manifolds

A self-diffeomorphism of a smooth manifold is said to be exotic if it is topologically isotopic to the identity but smoothly not. We provide the first examples of exotic diffeomorphisms (in a relative sense) of contractible 4-manifolds, more generally of definite 4-manifolds. Such examples are given as Dehn twists along certain Seifert homology 3-spheres, which also give new examples of exotic diffeomorphisms that survive after one stabilization. The proof uses families Seiberg-Witten theory over RP^2. This is joint work with Abhishek Mallick and Masaki Taniguchi.