Low-temperature far-infrared absorption in the antiferromagnetic organic superconductor $\kappa \text{−}{\left(\text{BETS}\right)}_2{\text{FeBr}}_4$

Organic salt $\kappa \text{−}{\left(\text{BETS}\right)}_2{\text{FeBr}}_4$ is a unique compound in which local moment antiferromagnetism of ${\text{Fe}}^{3+}$ ions (below Néel temperature $T_N$ of 2.5 K) coexists with bulk superconductivity (below the superconducting transition temperature $T_C=1.1\text{K}$). To probe this unique coexistence we studied the low-temperature far-infrared optical response in a frequency range of $7–40{\text{cm}}^{-1}$, a characteristic energy range for superconducting and magnetic gaps. Measurements were undertaken using a polarizing interferometer and a ${\text{He}}^3$ cryostat in a temperature range 0.5–2.8 K. The spectrum shows a clear change on crossing both $T_N$ and $T_C$. An absorption feature below $T_N$ is interpreted as a signature for the formation of a magnetic pseudogap. The observed increased reflectance relative to the normal state at temperatures below $T_C$ sets a value of the superconducting energy gap ${\Delta }_0$ in the strong coupling regime, $2{\Delta }_0\approx 8k_b{T}_C$.

Figure 1

Temperature dependence of electrical resistivity of $\kappa \text{−}{\left(\text{BETS}\right)}_2{\text{FeBr}}_4$, presented normalized by its room-temperature value. Inset shows zoom of the low-temperature part, showing anomalies in resistivity at $T_N$ and $T_C$.

Figure 2

Thermal-reflectance ratios of $\kappa \text{−}{\left(\text{BETS}\right)}_2{\text{FeBr}}_4$ along the $a$ (left column of panels) and $c$ axis (right column of panels). Panels (a) and (b) show the ratios of the reflectance at 0.5 K in the superconducting state to that at 1.4 K in the AFM normal state. Panels (c) and (d) show the ratio of the reflectance at 0.5 K in the superconducting state to that at 2.8 K in the paramagnetic normal state. Panels (e) and (f) show the ratio of the reflectance at temperatures of 1.4 K in the antiferromagnetic normal state to that at 2.8 K in the paramagnetic normal state, panels (g) and (h) show analogous ratios for measurements at 1.9 K and 2.8 K. Uncertainty in the thermal-reflectance ratio is $\approx \pm 0.015$.