The second altermagnet candidate in organic conductors: $\kappa$-(BEDT-TTF)$_2$$m$-HOOCC$_6$H$_4$SO$_3$

Abstract

We have developed a novel BEDT-TTF-based organic conductor, $\kappa$-(BEDT-TTF)$_2 m$-HOOCC$_6$H$_4$SO$_3$ ($\kappa$-$m$-SBA), and propose it as a candidate for altermagnet. Tight-binding band calculations of $\kappa$-$m$-SBA provide a $t'/t$ of 1.01 at 100 K, indicating that the spin structure is closely aligned to an equilateral triangle ($t'/t= 1$). While most $\kappa$-type BEDT-TTF-based salts become spin liquids due to the spin frustration caused by the triangular lattice, $\kappa$-$m$-SBA surprisingly shows a weak ferromagnetic transition at $T_\mathrm{N} = 14$ K due to a canted antiferromagnetic (AFM) spin structure. Until recently, $\kappa$-(BEDT-TTF)$_2$Cu[N(CN)$_2$]Cl ($\kappa$-Cl) was the only $\kappa$-type organic conductor known to exhibit this order, and it is also recognized as the first candidate for altermagnetism in organic conductors. This was theoretically predicted by Naka et al. in 2019, who demonstrated that $\kappa$-type organic conductors can be candidates for altermagnetism if they display such order. Consequently, $\kappa$-$m$-SBA can be considered the second candidate for altermagnetism in organic conductors. Furthermore, numerical calculations demonstrate a characteristic of altermagnets in $\kappa$-$m$-SBA, namely spin splitting of energy bands.