SMALL-PARAMETER FOR MIGDAL-TYPE DESCRIPTION OF T-T'-J MODEL

The small parameter, validating Migdal's approximation for the spin-ch arge coupling vertex: in the two-dimensional t-t'-J model close to hal f-filling, is found to be 1/In(t/J), t much greater than J. An effecti ve spin-charge separation, in the vicinity of the Fermi-level, reduces to zero the quasi-particle weight of electron states at half-filling. The calculated optical conductivity sigma(omega) exhibits an ''infrar ed'' 1/omega tail with superimposed, multimagnon resonances.