Elementary excitation in the Haldane state.

A localized two-spin bound state with the resultant spin S=1 is proposed as an elementary excitation of the Haldane state in Ni(${\mathrm{C}}_{2}$${\mathrm{H}}_{8}$${\mathrm{N}}_{2}$)${\mathrm{NO}}_{2}$(${\mathrm{ClO}}_{4}$) (NENP) and Ni(${\mathrm{C}}_{3}$${\mathrm{H}}_{10}$${\mathrm{N}}_{2}$)${\mathrm{NO}}_{2}$(${\mathrm{ClO}}_{4}$) (NINO). The bound state corresponds to the lowest excited triplet and behaves like a soliton or a small exciton. An ESR study has been done, and the resonance of the two-spin bound state is observed. The result is satisfactorily explained by the spin-cluster model and supports the Haldane conjecture. Various magnetic properties of NENP and NINO are well explained by the present model.