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 in 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 Heisenberg hamiltonian as the large correlation limit of the Hubbard hamiltonian for a single band of impurity-modified electronic states. The correlation effects are treated variationally. The model is solved explicitly by using a rectangular density of states, and we obtain the c-dependent exchange J, staggered moment S(q) spin wave velocity and transverse susceptibility at zero temperature. We take into consideration several recently proposed formulae for T(N) in the clean limit, and include the impurity effects by exploiting the results obtained, in order to test their predictions against the experimental T(N)(c) data for La2Cu1-cZncO4. Our results suggest that, to explain the difference between the K2NiF4 and the La2CuO4 families, one should consider both the sign and the magnitude of the difference I=epsilon(B)-epsilon(A) between 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 and sizeable, or positive but very small, while the quick decrease typical of the copper perovskites requires a positive and rather large L For 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 assumed to scale with c as appropriate to Fermi liquids.