Abstract
By means of the density matrix renormalization group (DMRG) method, the magnetic properties of theJ–J–J ′ quantum Heisenbergchains with spin S = 1/2, 1, 3/2 and 2 in the ground states are investigated in the presence ofa magnetic field. Two different cases are considered. (a) WhenJ is antiferromagneticand J ′ is ferromagnetic (i.e. the AF–AF–F chain), the system is a ferrimagnet. Theplateaus of the magnetization are observed. It is found that the width of theplateaus decreases with increasing ferromagnetic coupling, and disappears whenJ ′/J passesa critical value. The saturated field is observed to be independent of the ferromagnetic coupling. (b) WhenJ is ferromagneticand J ′ is antiferromagnetic (i.e. the F–F–AF chain), the system becomes an antiferromagnet.The plateaus of the magnetization are also seen. The width of the plateausdecreases with decreasing antiferromagnetic coupling, and disappears whenJ ′/J passes a critical value. Though the ground state properties are quite different, themagnetization plateaus in both cases tend to disappear when the ferromagnetic couplingbecomes more dominant. Besides, no fundamental difference between the systems with spinhalf-integer and integer has been found.
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