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arXiv:cond-mat/0002082 · Physical review. B, Condensed matter

Local magnetic structures induced by inhomogeneities of the lattice in S=12 bond-alternating chains and their response to a time-dependent magnetic field with noise

Masamichi Nishino, Hiroaki Onishi, Kizashi Yamaguchi, Seiji Miyashita

Published Oct 1, 2000• 20 citations
arXivHTMLPDF

Abstract

We study the effect of inhomogeneities of the lattice in the $S=1/2$ bond-alternating chain by using a quantum Monte Carlo method and an exact diagonalization method. We adopt a defect in the alternating order as the inhomogeneity and we call it ``bond impurity.'' Local magnetic structures induced by the bond impurities are investigated both in the ground state and at very low temperatures. The local magnetic structure can be looked on as an effective $S=1/2$ spin and the weakness of the interaction between the local structures causes the quasidegenerate states in the low energy. We also investigate the force acting between bond impurities and find that the force is generally attractive. We also study the dynamical property of the local magnetic structure. While the local magnetic structure behaves as an isolated $S=1/2$ spin in the response to a time-dependent uniform field, it is found to be robust against the effect of noise applied at each site individually in the sweeping field.

DOI
10.1103/physrevb.62.9463

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We study the effect of inhomogeneities of the lattice in the S=1/2S=1/2S=1/2 bond-alternating chain by using a quantum Monte Carlo method and an exact diagonalization method. We adopt a defect in the alternating order as the inhomogeneity and we call it ``bond impurity.'' Local magnetic structures induced by the bond impurities are investigated both in the ground state and at very low temperatures. The local magnetic structure can be looked on as an effective S=1/2S=1/2S=1/2 spin and the weakness of the interaction between the local structures causes the quasidegenerate states in the low energy. We also investigate the force acting between bond impurities and find that the force is generally attractive. We also study the dynamical property of the local magnetic structure. While the local magnetic structure behaves as an isolated S=1/2S=1/2S=1/2 spin in the response to a time-dependent uniform field, it is found to be robust against the effect of noise applied at each site individually in the sweeping field.
We study the magnetic properties of S=1/2S=1/2S=1/2 antiferromagnetic Heisenberg chains with inhomogeneity of interaction. Using a quantum Monte Carlo method and an exact diagonalization method, we study the bond-impurity effect in the uniform S=1/2S=1/2S=1/2 chain. Here ``bond impurity'' means a bond with a strength different from those in the bulk. In particular, by calculating the local magnetization, the local susceptibility, and the local field susceptibility, the temperature dependence of local magnetic structures induced by bond impurities is investigated. We also investigate the force acting between bond impurities and find that the force is attractive.
Low dimensional strongly correlated systems have great potential for new quantum devices. Among them, quantum wires are actively investigated with development of new methods based on self-organized formation. For magnetic wires (chains), impurity doping, which causes inhomogeneity of magnetic interaction, can bring new magnetic properties to the systems. In the present study, the effect of inhomogeneity (defect) on magnetic chains with bond alternation is focused on. Defects induce local moments in the magnetic chains at low temperatures. However, depending on the relation of the distance between the defects, the magnetic profile is found to be drastically changed.
We study properties of localized effective spins induced in gapped quantum spin chains by local inhomogeneities of the lattice. As a prototype, we study effective spins induced in an impunity sites doped AKLT model by constructing the exact ground state in a matrix product state (MPS) form. We characterize their responses to external fields by studying an extended Zeeman interaction. We also study the antiferromagnetic bond-alternating Heisenberg chain with defect structures. For this model, an MPS representation similar to that for the AKLT model, ``a uniform MPS with windows,'' is constructed, and it gives a good approximation of the ground state. We discuss the trade-off relation between the window length and the precision of the MPS ansatz. The effective exchange interaction between the induced spins is also investigated by using this representation.
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The two-dimensional quantum Heisenberg antiferromagnet at zero temperature, with locally frustrating magnetic defects, is studied. We consider two types of defects---an isolated ferromagnetic bond (FMB) and a quantum impurity spin, coupled symmetrically to the two sublattices. A technique is developed to study strong defect-environment interaction. In the case of a FMB we find that, contrary to the previous linear spin-wave result, the local magnetization stays finite even for strong frustration. For an antiferromagnetic coupling of a quantum impurity spin with its environment, we find a local change in the ground state. All our calculations are compared with numerical results from exact diagonalization.
The characteristics of the nuclear magnetic resonance (NMR), the contributions to the spin-lattice relaxation rate, and Knight shift of the resonance position are calculated exactly for a model spin-1/21/21/2 XYXYXY chain with the magnetic impurity. It is shown that the deviations of the impurity's coupling to the chain and the effective impurity's ggg factor from the values in the host spin chain yield special features in the temperature and magnetic field behavior for these NMR characteristics. The role of impurity-induced local levels in the NMR low-energy spin dynamics of spin chains is determined.