Novel spin excitations in the field-induced phase of the haldane magnet Ni(C ₅H ₁₄N ₂) ₂N ₃(PF ₆)

We have performed electron spin resonance measurements on single crystals of the S = 1 one-dimensional Heisenberg antiferromagnet Ni(C ₅H ₁₄N ₂) ₂N ₃(PF ₆) in magnetic fields up to about 55 T and at frequencies up to about 2 THz. For both H ∥ chain and H ⊥ chain, we have found that one of the two or three excitation modes that are observed in magnetic fields between the critical field (H _c) and about 15 T survives in the high-field region. This mode approaches a paramangetic resonance line at high-fields and fits well with the conventional antiferromagnetic resonance mode with easy-plane anisotropy. This result suggests that the spin-excitation modes above H _c change from modes affected by quantum fluctuations to those expected for a conventional antiferromagnet.