Publikationsrepositorium - Helmholtz-Zentrum Dresden-Rossendorf

Low-energy magnetic excitations in the S= 1/2 chain compound [(C₆H₉N₂)CuCl₃]CuCl₃ [known as (6MAP)CuCl₃] are probed by means of tunable-frequency electron spin resonance. Two modes with asymmetric (with respect to the hnu =gµ_BB line) frequency-field dependences are resolved, illuminating the striking incompatibility with a simple uniform S= 1/2 Heisenberg chain model. The unusual ESR spectrum is explained in terms of the recently developed theory for S= 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