Ultrasoft colloids typically do not spontaneously crystallize, but rather
vitrify, at high concentrations. Combining in-situ rheo-SANS experiments and
numerical simulations we show that shear facilitates crystallization of
colloidal star polymers in the vicinity of their glass transition. With
increasing shear rate well beyond rheological yielding, a transition is found
from an initial bcc-dominated structure to an fcc-dominated one. This
crystal-to-crystal transition is not accompanied by intermediate melting but
occurs via a sudden reorganization of the crystal structure. Our results
provide a new avenue to tailor colloidal crystallization and crystal-to-crystal
transition at molecular level by coupling softness and shear
