Breit interaction correction to the hyperfine constant of an external   s-electron in many-electron atom
  Correction to the hyperfine constant $A$ of an external s-electron in many-electron atom caused by the Breit interaction is calculated analytically: $\delta A/A =0.68 Z\alpha^2$. Physical mechanism for this correction is polarization of the internal electronic shells (mainly $1s^2$ shell) by the magnetic field of the external electron. This mechanism is similar to the polarization of vacuum considered by Karplus and Klein long time ago. The similarity is the reason why in both cases (Dirac sea polarization and internal atomic shells polarization) the corrections have the same dependence on the nuclear charge and fine structure constant.   In conclusion we also discuss $Z\alpha^2$ corrections to the parity violation effects in atoms. 
O.P. Sushkov