A log-motivic cohomology for semistable varieties and its $p$-adic deformation theory

Abstract

We construct log-motivic cohomology groups for semistable varieties and study the $p$-adic deformation theory of log-motivic cohomology classes. Our main result is the deformational part of a $p$-adic variational Hodge conjecture for varieties with semistable reduction: a rational log-motivic cohomology class in bidegree $(2n,n)$ lifts to a continuous pro-class if and only if its Hyodo-Kato class lies in the $n$-th step of the Hodge filtration. This generalises a theorem of Bloch-Esnault-Kerz which treats the good reduction case. In the case $n=1$ the lifting criterion is the one obtained by Yamashita for the logarithmic Picard group. Along the way, we relate log-motivic cohomology to logarithmic Milnor $K$-theory and the logarithmic Hyodo-Kato Hodge-Witt sheaves.