Spin dynamics in theS=12antiferromagnetic chain compoundsδ−(EDT-TTF-CONMe2)2X(X=AsF6,Br): A multifrequency electron spin resonance study

Abstract

We present a multifrequency electron spin resonance study in the range of 4--420 GHz of the quasi-one-dimensional, nondimerized, quarter-filled Mott insulators, $\ensuremath{\delta}\text{\ensuremath{-}}{({\text{EDT-TTF-CONMe}}_{2})}_{2}X$ $(X={\text{AsF}}_{6},\text{Br})$. In the high-temperature orthorhombic phase above $T\ensuremath{\sim}190\text{ }\text{K}$, the magnitude and the temperature dependence of the high-temperature spin susceptibility are described by a $S=\frac{1}{2}$ Heisenberg antiferromagnetic chain with ${J}_{{\text{AsF}}_{6}}=298\text{ }\text{K}$ and ${J}_{\text{Br}}=474\text{ }\text{K}$ coupling constants for $X={\text{AsF}}_{6}$ and Br, respectively. We estimate from the temperature dependence of the linewidth $(\ensuremath{\Delta}H)$ an exchange anisotropy, ${J}^{\ensuremath{'}}/J$ of $\ensuremath{\sim}2\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}$. The frequency dependence of $\ensuremath{\Delta}H$ and the $g$ shift have an unusual quadratic dependence in all crystallographic orientations that we attribute to an antisymmetric exchange (Dzyaloshinskii-Moriya) interaction.