Low-energy magnetic excitations in the spin-1/2 chain compound (C6H9N2)CuCl3 [known as (6MAP)CuCl3] are probed by means of tunable-frequency electron spin resonance. Two modes with asymmetric (with respect to the hν=gμBB line) frequency-field dependences are resolved, illuminating the striking incompatibility with a simple uniform S=21 Heisenberg chain model. The unusual ESR spectrum is explained in terms of the recently developed theory for spin-1/2 chains, suggesting the important role of next-nearest-neighbor interactions in this compound. Our conclusion is supported by model calculations for the magnetic susceptibility of (6MAP)CuCl3, revealing a good qualitative agreement with experiment.
@article{arxiv.1007.2143,
title = {Spin Dynamics in $S=1/2$ Chains with Next-Nearest-Neighbor Exchange Interactions},
author = {M. Ozerov and A. A. Zvyagin and E. Čižmár and J. Wosnitza and R. Feyerherm and F. Xiao and C. P. Landee and S. A. Zvyagin},
journal= {arXiv preprint arXiv:1007.2143},
year = {2015}
}