title: Midgap states in antiferromagnetic Heisenberg chains with a staggered field.
abstract: 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(2/3)(s)[log(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 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.
authors.affiliation: Department of Physics, University of Nevada, Las Vegas, Nevada 89154, USA.
authors.forename: Jizhong
authors.initials: J
authors.lastname: Lou
authors.forename: Changfeng
authors.initials: C
authors.lastname: Chen
authors.forename: Jize
authors.initials: J
authors.lastname: Zhao
authors.forename: Xiaoqun
authors.initials: X
authors.lastname: Wang
authors.forename: Tao
authors.initials: T
authors.lastname: Xiang
authors.forename: Zhaobin
authors.initials: Z
authors.lastname: Su
authors.forename: Lu
authors.initials: L
authors.lastname: Yu