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Ground-state phase diagram of an anisotropic S=1/2 ladder with alternating rung interactions

Employing mainly numerical methods, we explore the ground-state phase diagram of an anisotropic $S=1/2$ ladder, in which leg interactions are uniform and isotropic, while rung interactions are alternating and have a common Ising-type anisotropy. We determine the phase diagram in the case where $J_{\rm leg}=0.2$ (antiferromagnetic), $J_{\rm rung}=-1.0$ (ferromagnetic) and $|J_{\rm rung}'|\!\leq\!1.0$, the first one being the magnitude of the leg interaction and the second and third ones those of the rung interactions, which are alternating. It is emphasized that the system has a frustration when $J_{\rm rung}'$ is positive. We find that, in the frustrated region, the Haldane state appears as the ground state even when the Ising character of rung interactions is strong. This appearance of the Haldane phase is contrary to the ordinary situation, and it is called the inversion phenomenon concerning the interaction anisotropy. We also find that an incommensurate state becomes the ground state in a portion of the Haldane phase region.