Midgap States in Antiferromagnetic Heisenberg Chains with a Staggered Field

Jizhong Lou, Changfeng Chen, Jize Zhao, Xiaoqun Wang, Tao Xiang, Zhaobin Su, Lu Yu
2005 Physical Review Letters  
We study low-energy excitations in antiferromagnetic Heisenberg chains with a staggered field which splits the spectrum into a longitudinal and a transverse branch. Bound states are found to exist inside the field induced gap in both branches. They originate from the edge effects and are inherent to spin-chain materials. The sine-Gordon scaling h_s^2/3| h_s|^1/6 (h_s: the staggered field) provides an accurate description for the gap and midgap energies in the transverse branch for S=1/2 and the
more » ... midgap energies in both branches for S=3/2 over a wide range of magnetic field; however, it can fit other low-energy excitations only at much lower field. Moreover, the integer-spin S=1 chain displays scaling behavior that does not fit this scaling law. These results reveal intriguing features of magnetic excitations in spin-chain materials that deserve further investigation.
doi:10.1103/physrevlett.94.217207 pmid:16090347 fatcat:sdocvig4tnatlfwkeozbiovlja