Finite-Field Ground State of the S=1 Antiferromagnetic-Ferromagnetic Bond-Alternating Chain

TL;DR

This paper studies the ground state of an S=1 antiferromagnetic-ferromagnetic chain under magnetic fields, revealing two types of 1/2-magnetization plateaux and mapping their phase diagram using advanced numerical methods.

Contribution

It identifies two distinct 1/2-magnetization plateaux types and constructs their phase diagram using twisted-boundary-condition level spectroscopy and density-matrix renormalization-group methods.

Findings

Discovery of Haldane and large-D type 1/2-plateaux.

Phase diagram of 1/2-plateaux in D-J parameter space.

Magnetization curves showing plateau regions.

Abstract

We investigate the finite-field ground state of the S=1 antiferromagnetic-ferromagnetic bond-alternating chain described by the Hamiltonian ${\calH}=\sum\nolimits_{\ell}\bigl\{\vecS_{2\ell-1}\cdot\vecS_{2\ell} +J\vecS_{2\ell}\cdot\vecS_{2\ell+1}\bigr\} +D\sum\nolimits_{\ell} \bigl(S_{\ell}^z)^2 -H\textstyle\sum\nolimits_\ell S_\ell^z$, where \hbox{$J\leq0$} and \hbox{$-\infty<D<\infty$}. We find that two kinds of magnetization plateaux at a half of the saturation magnetization, the 1/2-plateaux, appear in the ground-state magnetization curve; one of them is of the Haldane type and the other is of the large-$D$-type. We determine the 1/2-plateau phase diagram on the $D$ versus $J$ plane, applying the twisted-boundary-condition level spectroscopy methods developed by Kitazawa and Nomura. We also calculate the ground-state magnetization curves and the magnetization phase diagrams by means of the density-matrix renormalization-group method.