Interacting electron spins in $\kappa \text{−}{\left(\text{BEDT-TTF}\right)}_2\mathrm{Cu}[\mathrm{N}{\left(\mathrm{CN}\right)}_2]\mathrm{I}$ investigated by ESR spectroscopy

We performed angular and temperature-dependent electron-spin-resonance measurements in the quasi-two-dimensional organic conductor $\kappa \text{−}{\left(\text{BEDT-TTF}\right)}_2\mathrm{Cu}[\mathrm{N}{\left(\mathrm{CN}\right)}_2]\mathrm{I}$. The interlayer spin diffusion is much weaker compared to the Cl and Br analogs, which are antiferromagnetic insulator and paramagnetic metal, respectively; $\kappa \text{−}{\left(\text{BEDT-TTF}\right)}_2\mathrm{Cu}[\mathrm{N}{\left(\mathrm{CN}\right)}_2]\mathrm{I}$ behaves insulating when cooled below $T=200\mathrm{K}$. A spin gap ($\mathrm{\Delta }\approx 18\mathrm{K}$) opens at low temperatures leading to a spin-singlet state. Due to intrinsic disorder a substantial number of spins ($\sim 1\%$) remains unpaired. We observe additional signals below $T=4\mathrm{K}$ with a pronounced anisotropy indicating the presence of local magnetic moments coupled to some fraction of those unpaired spins.

Figure 1

Structure of $\kappa \text{−}{\left(\text{BEDT-TTF}\right)}_2\mathrm{Cu}[\mathrm{N}{\left(\mathrm{CN}\right)}_2]\mathrm{I}$ viewed along the $c$ direction. In subsequent layers the long axis of the BEDT-TTF dimers is tilted by an alternating angle of $\pm {32}^{\circ }$ with respect to the $b$ direction. Here the yellow, brown, blue, purple, and gray color denotes sulfur, carbon, copper, iodine, and nitrogen atoms, respectively. (The atomic positions were taken from Ref. [8].)

Figure 2

Temperature-dependent resistivity $\rho \left(T\right)$ measured along the $b$ axis of $\kappa \text{−}{\left(\text{BEDT-TTF}\right)}_2\mathrm{Cu}[\mathrm{N}{\left(\mathrm{CN}\right)}_2]\mathrm{I}$ and normalized to the room temperature value (red line, left axis). The blue line indicates the activated behavior in a restricted temperature range. The green dots correspond to the spin susceptibility $\chi \left(T\right)$ as extracted from the ESR spectra with the magnetic field along $b$; the data are normalized to the molar susceptibility [18] at 200 K (right axis). The low temperature spin susceptibility can be modelled by an activated behavior due to the formation of a singlet state accompanied by the opening of a spin gap in combination with a Curie contribution due to the remaining unpaired spins (black line).

Figure 3

(a) ESR spectra recorded at $T=2\mathrm{K}$ with the magnetic field in the $ab$ plane. For orientations midway between the $a$ and $b$ axes, a splitting of the signal is visible (shown here for ${45}^{\circ }$). The spectra in the right column demonstrates that also with the external magnetic field in the $ab$ and $bc$ planes additional signals can be observed (black arrows): (b) $ab$ plane when rotated ${20}^{\circ }$ with respect to the $a$ axis at $T=4\mathrm{K}$; (c) $bc$ plane at ${16}^{\circ }$ with respect to the $b$ axis.

Figure 4

(a) The anisotropy of the ESR signal of $\kappa \text{−}{\left(\text{BEDT-TTF}\right)}_2\mathrm{Cu}[\mathrm{N}{\left(\mathrm{CN}\right)}_2]\mathrm{I}$ measured in the $ab$ plane at a temperature of $T=2\mathrm{K}$. The positions of the resonance fields are represented by the gray and black squares. (b) When rotating the crystal within the $ac$ plane, the anisotropy of the main signal (indicated by the black squares) is meager compared to the angular dependence observed in the position of the small dips, shown by red dots and blue triangles. (c) A similar observation is made when rotating in the $bc$ plane. Besides the small angular dependence of the main line, we find a strong anisotropy of the two dips. Note the extended scales for panels (b) and (c) compared to panel (a). The background color corresponds to the ESR signal intensity $\mathrm{d}P/\mathrm{d}B$.