The local enhancement of antiferromagnetic correlations near vacancies observed in a variety of spin systems is analyzed in a single framework. Variational calculations suggest that the resonatingvalence-bond character of the spin correlations at short distances is responsible for the enhancement. Numerical results for uniform spin chains, with and without frustration, dimerized chains, ladders, and two dimensional clusters are in agreement with our conjecture. This short distance phenomenon occurs independently of the long distance behavior of the spin correlations in the undoped system. Experimental predictions for a variety of compounds are briefly discussed.PACS numbers: 64.70. Kb,75.10.Jm,75.50.Ee Studies of ladder compounds continue producing fascinating results. In addition to the discovery of a spin gap in undoped even-leg ladders [1], superconductivity at high pressure in Sr 0.4 Ca 13.6 Cu 24 O 41.84 , with 2-leg ladders and chains in its structure, has been recently reported [2]. Both properties, predicted by theoretical arguments, [3] indicate a close interplay between the spin and charge degrees of freedom leading to a rich phase diagram. More recently, the doping of ladders with nonmagnetic impurities (replacing spin 1/2 Cu 2+ by spin 0 Zn 2+ ) has revealed another surprising property: the spin gap is rapidly suppressed as the Zn concentration increases, and an antiferromagnetic (AF) phase is stabilized [4]. A similar behavior has also been observed in spin-Peierls chains [5], which have a spin gap produced by dimerization. The phenomenon is interesting since a spin ordered state is generated by the random replacement of spins by vacancies, an apparently disordering procedure. These results have been recently addressed with one dimensional (1D) spin models using field theory [6] and numerical techniques. Computational studies found that the AF correlations near a vacancy in dimerized chains [7] and 2-leg ladders [7,8] are enhanced with respect to the undoped case. It was conjectured that this local enhancement may trigger the 3D AF order in Zn-doped dimerized chains and ladders. In-gap weakly interacting S = 1/2 localized states were found near Zn [9]. However, the microscopic origin of the local AF enhancement near a vacancy is still not intuitively understood.Independently of these recent developments, related phenomena have been discussed in a variety of contexts: 1. A staggered moment appears near a vacancy for 1D S = 1 Heisenberg systems [10]; 2. The undimerized 1D S = 1/2 Heisenberg model has an enhanced spin structure factor S(π) near vacancies according to boundary conformal field theory and Monte Carlo (MC) simulations [11]; 3. Near a vacancy injected into a 2D Néel ordered state, the staggered moment increases with respect to the undoped system [12].In this paper it is proposed that all these examples of locally enhanced antiferromagnetism near a vacancy, which have been studied independently in the literature, may have a simple common explanation. The unifying picture relies on the res...

Spin models that have been proposed to describe dimerized chains, ladders, two dimensional antiferromagnets, and other compounds are here studied when some spins are replaced by spinless vacancies, such as it occurs by Zn doping. A small percentage of vacancies rapidly destroys the spin gap, and their presence induces enhanced antiferromagnetic correlations near those vacancies. The study is performed with computational 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 cluster geometries, a common simple explanation for its presence may exist. It is argued that the resonating-valence-bond character of the spin correlations at short distances of a large variety of models is responsible for the presence of robust staggered spin correlations near vacancies and lattice edges. The phenomenon takes place regardless of the long distance properties of the ground state, and it is caused by a "pruning" of the available spin singlets in the vicinity of the vacancies. The effect produces a broadening of the low temperature NMR signal for the compounds analyzed here. This broadening should be experimentally observable in the structurally dimerized chain systems Cu(N O3)2 · 2.5H2O, CuW O4, (V O)2P2O7, and Sr14Cu24O41, in ladder materials such as SrCu2O3, in the spin-Peierls systems CuGeO3 and N aV2O5, and in several others since it is a universal effect common to a wide variety of models and compounds.

A quantum mechanical picture is presented to describe the behavior of confined spinons in a variety of S = 1/2 chains. The confinement is due to dimerization and frustration and it manifests itself as a nonlinear potential V (x) ∝ |x| b , centered at chain ends (b ≤ 1) or produced by modulation kinks (b > 1). The calculation extends to weak or zero frustration some previous ideas valid for spinons in strongly frustrated spin chains. The local magnetization patterns of the confined spinons are calculated. A (minimum) enhancement of the local moments of about 11/3 over a single S = 1/2 is found. Estimates for excitation energies and binding lengths are obtained.