arXiv:cond-mat/9704185 · Physical Review Letters

Local Enhancement of Antiferromagnetic Correlations by Nonmagnetic Impurities

George Balster Martins, Markus Laukamp, José A. Riera, Elbio R. Dagotto

Published May 5, 1997• 102 citations

Abstract

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 resonating-valence-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.

DOI: 10.1103/physrevlett.78.3563

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arXiv:cond-mat/9704185 · Physical Review Letters

Local Enhancement of Antiferromagnetic Correlations by Nonmagnetic Impurities

George Balster Martins, Markus Laukamp, José A. Riera, Elbio R. Dagotto

Published May 5, 1997• 102 citations

arXiv HTML PDF

Abstract

DOI: 10.1103/physrevlett.78.3563

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{\mathrm{CuWO}}_{4}

(\mathrm{VO}{)}_{2}{\mathrm{P}}_{2}{\mathrm{O}}_{7}

, and

{\mathrm{Sr}}_{14}{\mathrm{Cu}}_{24}{\mathrm{O}}_{41},

in ladder materials such as

{\mathrm{SrCu}}_{2}{\mathrm{O}}_{3}

, in the spin-Peierls systems

{\mathrm{CuGeO}}_{3}

and

{\mathrm{NaV}}_{2}{\mathrm{O}}_{5}

, 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 the N\'eel order observed in

{\mathrm{SrCu}}_{2}{\mathrm{O}}_{3}

upon Zn doping is induced by the local antiferromagnetic order discussed in this paper, enhanced by a favorable ratio between the actual Heisenberg couplings along chains and rungs, as reported in recent experimental literature. Based on this reasoning it is predicted here that other ladder materials such as Zn-doped

{\mathrm{Cu}}_{2}({\mathrm{C}}_{5}{\mathrm{H}}_{12}{\mathrm{N}}_{2}{)}_{2}{\mathrm{Cl}}_{4}

will not present N\'eel order at small Zn concentrations.

S=1

impurities leads to the suppression of the original spin gap of the dimerized system and to the enhancement of the antiferromagnetic correlations close to the impurity. These features are interpreted in terms of a valence bond picture of the ground state of both the pure and the doped system. The similitudes and differences between the underlying mechanisms that produce these effects in the cases of nonmagnetic and

S=1

impurities are analyzed. It is stressed that a singlet formed between the spin-1 impurity site and the two spin-1/2 neighbors spins behaves as an effective nonmagnetic vacancy which may be partially responsible for the enhancement of the antiferromagnetic correlations on the whole system. Implications of these results for experiment are also discussed.

SrCu_{2}O_{3}

. Temperature dependence of AF susceptibility shows power-law-like but weaker divergence than the single chain AFH in the temperature range studied.