SEARCH FOR A HOPF TERM IN A DOPED SPIN-FERMION SYSTEM IN 2 DIMENSIONS

It is shown that the long-wavelength limit of a doped spin-fermion model does not contain the Hopf invariant, when expanded around a staggered spin configuration. Furthermore, the resulting effective action that describes the destruction of long-range antiferromagnetic order by doping is an O(3) nonlinear model with coupling constants that depend on the dopant concentration. © 1990 The American Physical Society.