Field-induced midgap edge excitations in quantum spin chains

Abstract

A recent study [J. Lou, , Phys. Rev. Lett. 94, 217207 (2005)] reveals that topological edge effects induce bound midgap states in antiferromagnetic Heisenberg chains with a staggered magnetic field. We report density matrix renormalization group calculations that examine the effects of high magnetic field on the gap and midgap states and the range of validity of the field theory prediction on their scaling behavior. We also investigate the effects of the site parity (i.e., even or odd number of sites in the chain), the dilution of the spin chain by dopant atoms, and the uniform magnetic field on the emergence and evolution of the midgap states. The results provide useful insights into the behavior of the edge excitations in spin chain materials, which should facilitate the comparison between theory and experiment and stimulate further investigations.