  Despite tremendous theoretical efforts to understand subtleties of the
many-body localization (MBL) transition, many questions remain open, in
particular concerning its critical properties. Here we make the key observation
that MBL in one dimension is accompanied by a spin freezing mechanism which
causes chain breakings in the thermodynamic limit. Using analytical and
numerical approaches, we show that such chain breakings directly probe the
typical localization length, and that their scaling properties at the MBL
transition agree with the Kosterlitz-Thouless scenario predicted by
phenomenological renormalization group approaches.
