  We propose a mesoscopic model that explains the factor of two to three
discrepancy between experimentally measured yield stresses of BCC metals at low
temperatures and typical Peierls stresses determined by atomistic simulations
of isolated screw dislocations. The model involves a Frank-Read type source
emitting dislocations that become pure screws at a certain distance from the
source and, owing to their high Peierls stress, control its operation. However,
due to the mutual interaction between emitted dislocations the group consisting
of both non-screw and screw dislocations can move at an applied stress that is
about a factor of two to three lower than the stress needed for the glide of
individual screw dislocations.
