Impurities in $s=1$ Heisenberg Antiferromagnets

Abstract

The $s=1$ Heisenberg Antiferromagnet is studied in the presence of two kinds of local impurities. First, a perturbed antiferromagnetic bond with $J^{\prime }\ne J$ at the center of an even-length open chain is considered. Using the density matrix renormalization group method we find that, for sufficiently strong or weak $J^{\prime }$, a bound state is localized at the impurity site, giving rise to an energy level in the Haldane gap. The energy of the bound state is in agreement with perturbative results, based on $s=1/2$ chain-end excitations, both in the weak and strong coupling limit. In a region around the uniform limit, $J^{\prime }=J$, no states are found with energy below the Haldane gap. Secondly, a $s=1/2$ impurity at the center of an otherwise even-length open chain is considered. The coupling to the $s=1/2$ impurity is varied. Bound states in the Haldane gap are found {\it only} for sufficiently weak (antiferromagnetic) coupling. For a $s=1/2$ impurity coupled with a strong (antiferromagnetic) bond, {\it no} states are found in the Haldane. Our results are in good qualitative agreement with recent experiments on doped NENP and Y${}_2$BaNiO${}_5$.

29 pages, RevTeX 3.0, 12 uuencoded postscript figures included

References in corpus (7)

• Large-Scale Numerical Evidence for Bose Condensation in the S=1 Antiferromagnetic Chain in a Strong Field
• Finite-lattice extrapolations for a Haldane gap antiferromagnet
• Equal Time Correlations in Haldane Gap Antiferromagnets
• Excitation Spectrum of $S=1$ Antiferromagnetic Chains
• Impurity-Bond Effect on the Spin-1 Antiferromagnetic Heisenberg Chain
• Effect of a Spin-1/2 Impurity on the Spin-1 Antiferromagnetic Heisenberg Chain
• Energy Versus Magnetic-Field Diagram of the Spin-1 Haldane System with an Impurity

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