A Topos-Theoretic Approach to the Logarithmic Cartier Transform

This article is the second of three articles whose goal is to generalize the Cartier transform of Ogus and Vologodsky to the logarithmic setting. We generalize a topos-theoretic version of this transform, due to Oyama. Let $k$ be a perfect field of positive characteristic $p$ and equip $S=\mathrm{Spec}k$ with the trivial log structure. For a log smooth morphism of logarithmic schemes $X\to S,$ we construct crystalline-like ringed topoi ${\mathcal{E}}^{\mathrm{\prime }}$ and $\underset{‾}{\mathcal{E}}$ and subcategories of crystals of quasi-coherent modules ${\mathcal{C}}^{\mathrm{\prime }}$ and $\underset{‾}{\mathcal{C}},$ equivalent respectively, under some lifting assumption, to modules with Higgs fields and integrable connections, both satisfying certain nilpotence conditions, and a morphism of topoi $\underset{‾}{\mathcal{E}}\to {\mathcal{E}}^{\mathrm{\prime }}\mathrm{.}$ We then prove that the pullback functor of this morphism of topoi preserves quasi-coherent crystals and hence induces a functor ${\mathcal{C}}^{\mathrm{\prime }}\to \underset{‾}{\mathcal{C}},$ generalizing the Cartier transform. We finally use a log flat descent theorem for morphisms, that we proved in the first article, to prove that this functor is fully faithful.