Tate modules of isocrystals and good reduction of Drinfeld modules

Abstract

A Drinfeld module has a $\mathfrak{p}$-adic Tate module not only for every finite place $\mathfrak{p}$ of the coefficient ring but also for $\mathfrak{p} = \infty$. This was discovered by J.-K. Yu in the form of a representation of the Weil group. Following an insight of Taelman we construct the $\infty$-adic Tate module by means of the theory of isocrystals. This applies more generally to pure $A$-motives and to pure $F$-isocrystals of $p$-adic cohomology theory. We demonstrate that a Drinfeld module has good reduction if and only if its $\infty$-adic Tate module is unramified. The key to the proof is the theory of Hartl and Pink which gives an analytic classification of vector bundles on the Fargues-Fontaine curve in equal characteristic.