On bound states in quantum field theory
  The mechanism of formation of bound states in the relativistic quantum field theory is demonstrated by the Yukawa field model. It is shown that the weak coupling regime leads to the potential picture, i.e. it is equivalent to the nonrelativistic limit in the bound state problem. In the strong coupling regime the potential picture is not valid and the method of bosonisation of fermion currents (so-called $Z_2=0$ method) should be used. Essentially the nonlocal fermion currents are found to be responsible for the origin of bound states and ultraviolet convergence of fermion loops. 
G. V. Efimov