Novel spin excitation in the high field phase of an [formula omitted] antiferromagnetic chain

Abstract

We report the results of high-field multi-frequency ESR experiment on the S = 1 Heisenberg antiferromagnetic chain Ni ( C 5 H 14 N 2 ) 2 N 3 ( PF 6 ) for the fields up to about 55 T and the frequencies up to about 2 THz. We have found that excitation branches above the critical field ( H c ) where the energy gap closes change into one branch around 15 T which becomes close to the paramagnetic line at high fields. The branch above 15 T fits well the conventional antiferromagnetic resonance mode with easy planar anisotropy. We compare the results with those in a weakly coupled antiferromagnetic dimer compound KCuCl 3 and discuss the origin of the branches observed above H c .