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

TL;DR

This study investigates the spin dynamics in a spin-1/2 chain compound using electron spin resonance, revealing the significance of next-nearest-neighbor interactions through experimental observations and theoretical modeling.

Contribution

It provides experimental evidence and theoretical analysis highlighting the role of next-nearest-neighbor interactions in spin-1/2 chains, which deviates from simple Heisenberg models.

Findings

Two asymmetric ESR modes observed, incompatible with simple models.

Theoretical explanation incorporating next-nearest-neighbor interactions.

Qualitative agreement between susceptibility calculations and experiments.

Abstract

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.