Critical nucleus charge in a superstrong magnetic field: effect of   screening
  A superstrong magnetic field stimulates the spontaneous production of positrons by naked nuclei by diminishing the value of the critical charge Z_{cr} . The phenomenon of screening of the Coulomb potential by a superstrong magnetic field which has been discovered recently acts in the opposite direction and prevents the nuclei with Z<52 from becoming critical. For Z>52 for a nucleus to become critical stronger B are needed than without taking screening into account. 
S. I. Godunov, B. Machet, M. I. Vysotsky
hep-ph nucl-th