Spin Dynamics in $S=1\mathrm{/}2$ Chains with Next-Nearest-Neighbor Exchange Interactions

Low-energy magnetic excitations in the spin-1/2 chain compound (C${}_6$H${}_9$N${}_2$)CuCl${}_3$ [known as (6MAP)CuCl${}_3$] are probed by means of tunable-frequency electron spin resonance. Two modes with asymmetric (with respect to the $h\nu =g{\mu }_{B}B$ line) frequency-field dependences are resolved, illuminating the striking incompatibility with a simple uniform $S=\frac{1}{2}$ 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)CuCl${}_3$, revealing a good qualitative agreement with experiment.