Dynamical symmetry restoration for a higher-derivative four-fermion   model in an external electromagnetic field
  A four-fermion model with additional higher-derivative terms is investigated in an external electromagnetic field. The effective potential in the leading order of large-N expansion is calculated in external constant magnetic and electric fields.   It is shown that, in contrast to the former results concerning the universal character of "magnetic catalysis" in dynamical symmetry breaking, in the present higher-derivative model the magnetic field restores chiral symmetry broken initially on the tree level. Numerical results describing a second-order phase transition that accompanies the symmetry restoration at the quantum level are presented. 
E. Elizalde, S. P. Gavrilov, S. D. Odintsov, Yu. I. Shil'nov