Spin dynamics in the $S=\frac{1}{2}$ antiferromagnetic chain compounds $\delta \text{−}{\left({\text{EDT-TTF-CONMe}}_2\right)}_{2}X$ $\left(X={\text{AsF}}_6,\text{Br}\right)$: A multifrequency electron spin resonance study

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, $\delta \text{−}{\left({\text{EDT-TTF-CONMe}}_2\right)}_{2}X$ $\left(X={\text{AsF}}_6,\text{Br}\right)$. In the high-temperature orthorhombic phase above $T\sim 190\text{K}$, the magnitude and the temperature dependence of the high-temperature spin susceptibility are described by a $S=\frac{1}{2}$ Heisenberg antiferromagnetic chain with $J_{{\text{AsF}}_6}=298\text{K}$ and $J_{\text{Br}}=474\text{K}$ coupling constants for $X={\text{AsF}}_6$ and Br, respectively. We estimate from the temperature dependence of the linewidth $\left(\Delta H\right)$ an exchange anisotropy, $J^{\prime }/J$ of $\sim 2\times {10}^{-3}$. The frequency dependence of $\Delta H$ and the $g$ shift have an unusual quadratic dependence in all crystallographic orientations that we attribute to an antisymmetric exchange (Dzyaloshinskii-Moriya) interaction.

Figure 1

(Color online) Room-temperature $P2nn$ charge ordered structure of $\delta \text{−}{\left({\text{EDT-TTF-CONMe}}_2\right)}_{2}X$. Cations are omitted for clarity. High-charge ${\text{EDT-TTF-CONMe}}_2$ molecules are drawn by red and connected by arrow, low-charge molecules are drawn by blue in between. Antiferromagnetic Heisenberg chains are parallel with $a$. $J_{\parallel }$ is the strong antiferromagnetic coupling between spins along $\left(a\right)$ and $J^{\prime }$ is the small anisotropic exchange interaction.

Figure 2

(Color online) Temperature dependence of the spin-susceptibility measured by ESR at 9.4 GHz for the ${\text{AsF}}_6$ and Br crystals, open red and closed green circles, respectively. Lines are the Bonner-Fisher calculation with $J_{{\text{AsF}}_6}=298\text{K}$ and $J_{\text{Br}}=474\text{K}$.

Figure 3

(Color online) Room-temperature angular dependence of the ESR linewidth ($\Delta H$ filled symbols) and $g$ shift ($\Delta g$ open symbols) of $\delta \text{−}{\left({\text{EDT-TTF-CONMe}}_2\right)}_2{\text{AsF}}_6$ measured at 9.4 GHz in the $\left(a,b\right)$ and $\left(b,c\right)$ planes. $\varphi$ is the angle between the $b$ axis and field ${\mathbf{B}}_0$.

Figure 4

(Color online) Room-temperature angular dependence of the ESR linewidth $\left(\Delta H\right)$ and $g$ shift of $\delta \text{−}{\left({\text{EDT-TTF-CONMe}}_2\right)}_2\text{Br}$ measured at 9.4 GHz in the $\left(b,c\right)$ and $\left(a,c\right)$ planes. $\varphi$ is the angle between the $c$ axis and the external magnetic field ${\mathbf{B}}_0$.

Figure 5

(Color online) Temperature dependence of the ESR linewidth $\left(\Delta H\right)$ of $\delta \text{−}{\left({\text{EDT-TTF-CONMe}}_2\right)}_2{\text{AsF}}_6$ measured at 9.4 GHz in the principle directions. Lines are results of linear fits. Note that the discontinuity of the data at $T=190\text{K}$ is likely to be associated with the orthorhombic to monoclinic phase transition (Ref. 13) and that the topology of the CO structure in the low-temperature monoclinic phase is not known yet.

Figure 6

(Color online) (a) Frequency dependence of ESR parameters in $\delta \text{−}{\left({\text{EDT-TTF-CONMe}}_2\right)}_2\text{Br}$ at $T=300\text{K}$. Measurements in the $a$, $b$, and $c$ orientations are plotted by solid circle, square, and triangle, respectively. Lines show parabolic frequency dependence. (a) Frequency-dependent part of the linewidth. (b) Frequency-dependent part of the $g$ factor $g_{\nu }$.