  The ability of a body-centered cubic metal to deform plastically is limited
by the thermally activated glide motion of screw dislocations, which are line
defects with a mobility exhibiting complex dependence on temperature, stress,
and dislocation segment length. We derive an analytical expression for the
velocity of dislocation glide, based on a statistical mechanics argument, and
identify an apparent phase transition marked by a critical temperature above
which the activation energy for glide effectively halves, changing from the
formation energy of a double kink to that of a single kink. The analysis is in
quantitative agreement with direct kinetic Monte Carlo simulations.
