ASYMPTOTICALLY EXACT MEAN-FIELD THEORY FOR THE ANDERSON MODEL INCLUDING DOUBLE OCCUPANCY

The Anderson impurity model for finite values of the Coulomb repulsion U is studied using a slave-boson representation for the empty and dou bly occupied f level. In order to avoid well-known problems with a nai ve mean-field theory for the boson fields, we use the coherent-state-p ath integral representation to first integrate out the double occupanc y slave bosons. The resulting effective action is linearized using two -time auxiliary fields. After integration over the fermionic degrees o f freedom, one obtains an effective action suitable for a 1/N(f) expan sion. The same problem remains concerning the constraint as in the inf inite U case. For T --> 0 and N(f) --> infinity, exact results for the ground-state properties are recovered in the saddle-point approximati on. Numerical solutions of the saddle-point equations show that even i n the spin-degenerate case, N(f) = 2 the results are quite good.