Coherent spin dynamics of solitons in the organic spin chain compounds ($o$-DMTTF)$_2X$ ($X$ = Cl, Br)

TL;DR

This study investigates the spin dynamics of solitons in organic spin chain compounds using ESR spectroscopy, revealing defect-induced solitons, spin gaps, and coherent spin behavior deep in the spin-Peierls phase.

Contribution

It provides new insights into the coherent spin dynamics and defect-related solitons in ($o$-DMTTF)$_2X$ compounds through detailed ESR measurements.

Findings

Identification of defect-induced solitons as spin 1/2 quasiparticles.

Spectroscopic estimation of the spin gap in the materials.

Observation of Rabi oscillations indicating coherent spin dynamics.

Abstract

We studied the magnetic properties, in particular dynamics, of the correlated spins associated with natural defects in the organic spin chain compounds ($o$-DMTTF)$_2X$ ($X$ = Br, Cl) by means of electron spin resonance (ESR) spectroscopy. Both materials exhibit spin-Peierls transitions at temperatures around 50 K [P. Foury-Leylekian et al. Phys. Rev. B 84, 195134 (2011)], which allow a separation of the properties of defects inside the chains from the magnetic response of the spin chains. Indeed, continuous wave ESR measurements performed over a wide temperature range evidence the evolution of the spin dynamics from being governed by the spins in the chains at elevated temperatures to a low-temperature regime which is dominated by defects within the spin-dimerized chains. Such defects polarize the antiferromagnetically coupled spins in their vicinity, thereby leading to a finite local alternating magnetization around the defect site which can be described in terms of a soliton, i.e. a spin 1/2 quasiparticle built of many correlated spins, pinned to the defect. In addition, contributions of triplon excitations of the spin-dimerized state to the ESR response below the transition temperature were observed which provides a spectroscopic estimate for the spin-gap of the studied systems. Moreover, details of spin dynamics deep in the spin-Peierls phase were investigated by pulse ESR experiments which revealed Rabi-oscillations as signatures of coherent spin dynamics. From a comparison of the characteristic damping times of the Rabi oscillations with measurements of the spin relaxation times by means of primary-echo decay and CPMG methods it becomes evident that inhomogeneities in local magnetic fields strongly contribute to the soliton decoherence.