  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 $h_s^{2/3}|\log h_s|^{1/6}$ ($h_s$: 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.
