ENHANCEMENT OF ANTIFERROMAGNETIC CORRELATIONS INDUCED BY NONMAGNETIC IMPURITIES - ORIGIN AND PREDICTIONS FOR NMR EXPERIMENTS

Spin models that have been proposed to describe dimerized chains, ladd ers, two-dimensional antiferromagnets, and other compounds are studied here when some spins are replaced by spinless vacancies, such as it o ccurs by Zn doping. A small percentage of vacancies rapidly destroys t he spin gap, and their presence induces enhanced antiferromagnetic cor relations near those vacancies. The study is performed with computatio nal techniques which includes Lanczos, world-line Monte Carlo, and the density-matrix renormalization-group methods. Since the phenomenon of enhanced antiferromagnetism is found to occur in several models and c luster geometries, a common simple explanation for its presence may ex ist. It is argued that the resonating-valence-bond character of the sp in correlations at short distances of a large Variety of models is res ponsible for the presence of robust staggered spin correlations near v acancies and lattice edges. The phenomenon takes place regardless of t he long distance properties of the ground state, and it is caused by a ''pruning'' of the available spin singlets in the vicinity of the vac ancies. The effect produces a broadening of the law-temperature NMR si gnal for the compounds analyzed here. This broadening should be experi mentally observable in the structurally dimerized chain systems Cu(NO3 )(2) . 2.5H(2)O, CuWO4, (VO)(2)P2O7, and Sr(14)4Cu(24)O(41), in ladder materials such as SrCu2O3, in the spin-Peierls systems CuGeO3 and NaV 2O5, and in several others since it is a universal effect common to a wide variety of models and compounds. In addition, it is argued that t he Neel order observed in SrCu2O3 upon Zn doping is induced by the loc al antiferromagnetic order discussed in this paper, enhanced by a favo rable ratio between the actual Heisenberg couplings along chains and r ungs, as reported in recent experimental literature. Based on this rea soning it is predicted here that other ladder materials such as Zn-dop ed Cu-2(C5H12N2)(2)Cl-4 will not present Neel order at small Zn concen trations. [S0163-1829(98)03018-5].