DIAMAGNETIC IMPURITY EFFECTS ON THE NEEL TEMPERATURE IN LA2CUO4 AND RELATED MATERIALS

To investigate why the sensitivity of the Neel temperature T(N) of the antiferromagnetic (AF) layered copper perovskites (typically La2CuO4) to diamagnetic impurities such as Zn is reportedly much larger than i n the AF members of the K2NiF4 family, we first treat the effect of a concentration c of impurities on the uncorrelated electronic states in the coherent potential approximation (CPA). Then we consider the Heis enberg hamiltonian as the large correlation limit of the Hubbard hamil tonian for a single band of impurity-modified electronic states. The c orrelation effects are treated variationally. The model is solved expl icitly by using a rectangular density of states, and we obtain the c-d ependent exchange J, staggered moment S(q) spin wave velocity and tran sverse susceptibility at zero temperature. We take into consideration several recently proposed formulae for T(N) in the clean limit, and in clude the impurity effects by exploiting the results obtained, in orde r to test their predictions against the experimental T(N)(c) data for La2Cu1-cZncO4. Our results suggest that, to explain the difference bet ween the K2NiF4 and the La2CuO4 families, one should consider both the sign and the magnitude of the difference I=epsilon(B)-epsilon(A) betw een impurity (B) and host (A) ionic potentials. The slowly decreasing trend of T(N)(c) in the K2NiF4 family is reproduced if I is negative a nd sizeable, or positive but very small, while the quick decrease typi cal of the copper perovskites requires a positive and rather large L F or reasonable values of the interaction parameters, among the several models we compare, only the model of Chakravarty, Halperin and Nelson is able to semi-quantitatively reproduce the non-linear behaviour of T (N)(c) reported for La2Cu1-cZncO4, provided the spin stiffness is assu med to scale with c as appropriate to Fermi liquids.