Twisted sheaves and the period-index problem

We use twisted sheaves and their moduli spaces to study the Brauer group of a scheme. In particular, we (1) show how twisted methods can be efficiently used to re-prove the basic facts about the Brauer group and cohomological Brauer group (including Gabber’s theorem that they coincide for a separated union of two affine schemes), (2) give a new proof of de Jong’s periodindex theorem for surfaces over algebraically closed fields, and (3) prove an analogous result for surfaces over finite fields. We also include a reduction of all period-index problems for Brauer groups of function fields over algebraically closed fields to characteristic 0, which (among other things) extends de Jong’s result to include classes of period divisible by the characteristic of the base field. Finally, we state a conjecture about the geometry of moduli spaces of semistable vector bundles on curves which implies the conjectured period-index relation for the Brauer groups of function fields.