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Journal of the Physical Society of Japan

Magnetization Process of anS=1 Frustrated Spin Ladder

Kiyomi Okamoto, Kouichi Okunishi, Tôru Sakai

Published Jan 1, 2005• 3 citations
Publisher

Abstract

We investigate, at T =0, the magnetization process of the S =1 frustrated two-leg spin ladder having the leg interaction ( J ), the rung interaction ( J 0 ) and the diagonal interaction ( J d ), especially focusing on the appearance and the disappearance of the magnetization plateaux. We use the degenerate perturbation theory, the level spectroscopy analysis of the numerical diagonalization data and the DMRG. When J d =0, the M s /2 plateau ( M s is the saturation magnetization) appears for J 0 / J > 1.463. Then there is no M s /2 plateau for the so-called isotropic case J = J 0 . In case of J d > 0, J d helps the M s /2 plateau. And also, the M s /4 and (3/4) M s plateaux appear when J d is sufficiently large. These two plateaux are associated with the spontaneous breaking of the translation symmetry along the leg direction. We show the plateau phase diagram on the J d versus J plane, and also the magnetic phase diagram on the H versus J d plane for fixed J .

DOI
10.1143/jpsjs.74s.165

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Translational Symmetry Broken Magnetization Plateau of the S=12S={{1}\over{2}}S=21​ Anisotropic Spin Ladder with Ferromagnetic Rung Interaction
The magnetization process of the S=1/2S=1/2S=1/2 anisotropic spin ladder with the ferromagnetic rung interaction is investigated using the numerical diagonalization of finite-size clusters. It is found that the translational symmetry broken magnetization plateau would appear at half the saturation magnetization, when the competing anisotropies are sufficiently large. The phase diagram with respect to the anisotropies and several magnetization curves are presented.
We explore the ground-state phase diagram of the S = 1/2 two-leg ladder. The isotropic leg interactions J 1, a and J 1, b between nearest neighbor spins in the legs a and b , respectively, are different from each other. The xy and z components of the uniform rung interactions are denoted by J r and Δ J r , respectively, where Δ is the XXZ anisotropy parameter. This system has a frustration when J 1, a J 1, b < 0 irrespective of the sign of J r . The phase diagrams on the Δ (0≤Δ<1) versus J 1, b plane in the cases of J 1, a = − 0.2 and J 1, a = 0.2 with J r = −1 are determined numerically. We employ the physical consideration, the level spectroscopy analysis of the results obtained by the exact diagonalization method and also the density-matrix renormalization-group method. It is found that the non-collinear ferrimagnetic (NCFR) state appears as the ground state in the frustrated region of the parameters. Furthermore, the direct-product triplet-dimer (TD) state in which all rungs form the TD pair is the exact ground state , when J 1, a + J 1, b = 0 and 0≤ Δ ≲ 0.83. The obtained phase diagrams consist of the TD, XY and Haldane phases as well as the NCFR phase.
The magnetization process of the S = 1 spin ladder with two anisotropies competing each other is investigated using the numerical diagonaization of finite-size clusters and the level spectroscopy analysis. As a result, it is found that the translational symmetry broken magnetization plateaux appear at 1/4 and 3/4 of the saturation magnetization. The phase diagrams with respect to the anisotropies at the 1/4, 1/2 and 3/4 magnetizations are obtained. The magnetization curves for several typical parameters are also presented.