Enhancement of Antiferromagnetic Correlation Induced by Nonmagnetic Impurities: Origin and Prediction for NMR Experiments

M. LAUKAMP, G.B.MARTINS, C.J.GAZZA, A.L.MALVEZZI, E. DAGOTTO, P. HANSEN, A.C.LOPEZ AND J.RIERA.

PHYSICAL REVIEW B

AMER PHYSICAL SOC

Lugar: New York; Año: 1998 vol. 57 p. 1 - 10

1098-0121

Spin models that have been proposed to describe dimerized chains, ladders, two-dimensional antiferromagnets,and other compounds are studied here when some spins are replaced by spinless vacancies, such as itoccurs by Zn doping. A small percentage of vacancies rapidly destroys the spin gap, and their presence inducesenhanced antiferromagnetic correlations near those vacancies. The study is performed with computationaltechniques which includes Lanczos, world-line Monte Carlo, and the density-matrix renormalization-groupmethods. Since the phenomenon of enhanced antiferromagnetism is found to occur in several models andcluster geometries, a common simple explanation for its presence may exist. It is argued that the resonatingvalence-bond character of the spin correlations at short distances of a large variety of models is responsible forthe presence of robust staggered spin correlations near vacancies and lattice edges. The phenomenon takesplace regardless of the long distance properties of the ground state, and it is caused by a ??pruning?? of theavailable spin singlets in the vicinity of the vacancies. The effect produces a broadening of the low-temperatureNMR signal for the compounds analyzed here. This broadening should be experimentally observable in thestructurally dimerized chain systems Cu(NO3)2•2.5H2O, CuWO4, (VO)2P2O7, and Sr14Cu24O41 , in laddermaterials such as SrCu2O3, in the spin-Peierls systems CuGeO3 and NaV2O5, and in several others since it isa universal effect common to a wide variety of models and compounds. In addition, it is argued that the Ne´elorder observed in SrCu2O3 upon Zn doping is induced by the local antiferromagnetic order discussed in thispaper, enhanced by a favorable ratio between the actual Heisenberg couplings along chains and rungs, asreported in recent experimental literature. Based on this reasoning it is predicted here that other ladder materialssuch as Zn-doped Cu2(C5H12N2)2Cl4 will not present Ne´el order at small Zn concentrations.