Coexistence of one- and three-dimensional excitations in a quasi-one-dimensional S=1 Heisenberg antiferromagnet

Electron-spin resonance (ESR) measurements at high frequency and high magne tic field are performed on the quasi-one-dimensional (1D) S = I Heisenberg antiferromagnet, Ni(C5H14N2)(2)N-3(PF6) that exhibits a field-induced long- range ordering (LRO) at low temperatures. In the Haldane disordered phase, the ESR signals we observed are well described as originating from the tran sitions within the first excited triplet of a single chain. In the LRO phas e, we observed two ESR branches, one of which is assigned to an antiferroma gnetic resonance mode, and the other to a 1D excitation that persists in th e LRO phase. [S0163-1829(99)02037-8].