Magnetization plateau and cusp in S=1 spin ladder

Abstract

Recently, the organic compound biphenyl (BIP–TENO) was synthesized and found to be the first S=1 spin ladder. The present high-field measurement up to 70 T indicates that a plateau appears in the magnetization curve at M= M s/4, where M s is the saturation magnetization. We propose possible mechanisms of the M= M s/4 plateau in BIP–TENO. The frustration in the first, second and third neighbor antiferromagnetic exchange interactions in the S=1 ladder yields the M= M s/4 plateau, with a spontaneous breaking of the translational symmetry. This can be known by use of the degenerate perturbation theory from the strong rung coupling limit. By use of the numerical diagonalization method, we revealed that the most realistic mechanism of the M= M s/4 plateau in BIP–TENO is due to the third neighbor interaction. Further theoretical analyses suggest the present mechanism should also lead to a cusp in the magnetization curve. The cusp-like anomaly in the experimental magnetization curve is a considerable evidence for our mechanism. We also discuss the M s/2 plateau problem. We show that a small rung coupling cannot yield the M s/2 plateau, which is quite different from the M=0 plateau problem of the S= 1 2 ladder.