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Antiferromagnetic ordering of organic Mott insulator $λ$-(BEDSe-TTF)$_2$GaCl$_4$

The band structure and magnetic properties of organic charge-transfer salt $λ$-(BEDSe-TTF)$_2$GaCl$_4$ (BEDSe-TTF: bis(ethylenediseleno)tetrathiafulvalene; abbreviated as $λ$-BEDSe) are investigated. The reported crystal structure is confirmed using X-ray diffraction measurements, and the transfer integrals are calculated. The degree of electron correlation $U/W$ ($U$: on-site Coulomb repulsion, $W$: bandwidth) of $λ$-BEDSe is larger than one and comparable to that of the isostructural Mott insulator $λ$-(ET)$_2$GaCl$_4$ (ET: bis(ethylenedithio)tetrathiafulvalene, abbreviated as $λ$-ET), whereas the $U/W$ of the superconducting salt $λ$-(BETS)$_2$GaCl$_4$ (BETS: bis(ethylenedithio)tetraselenafulvalene) is smaller than one. $^{13}$C-NMR and $μ$SR measurements revealed that $λ$-BEDSe undergoes an antiferromagnetic (AF) ordering below $T_{\rm N} = 22$~K. In the AF state, discrete $^{13}$C-NMR spectra with a remaining central peak are observed, indicating the commensurate AF spin structure also observed in $λ$-ET. The similarity between the structural and magnetic properties of $λ$-BEDSe and $λ$-ET suggests that both salts are in the same electronic phase, i.e., the physical properties of $λ$-BEDSe can be understood by the universal phase diagram of bandwidth-controlled $λ$-type organic conductors obtained by donor molecule substitution.