Antiferromagnetic ordering of organic Mott insulator λ−(BEDSe-TTF)2GaCl4

Abstract

The band structure and magnetic properties of organic charge-transfer salt $\ensuremath{\lambda}\text{\ensuremath{-}}{(\text{BEDSe-TTF})}_{2}{\mathrm{GaCl}}_{4}$ [BEDSe-TTF: bis(ethylenediseleno)tetrathiafulvalene; abbreviated as $\ensuremath{\lambda}$-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 $\ensuremath{\lambda}$-BEDSe is larger than one and comparable to that of the isostructural Mott insulator $\ensuremath{\lambda}\ensuremath{-}({\mathrm{ET})}_{2}{\mathrm{GaCl}}_{4}$ (ET: bis(ethylenedithio)tetrathiafulvalene, abbreviated as $\ensuremath{\lambda}$-ET), whereas the $U/W$ of the superconducting salt $\ensuremath{\lambda}\ensuremath{-}({\mathrm{BETS})}_{2}{\mathrm{GaCl}}_{4}$ [BETS: bis(ethylenedithio)tetraselenafulvalene] is smaller than one. $^{13}\mathrm{C}$-NMR and $\ensuremath{\mu}\mathrm{SR}$ measurements revealed that $\ensuremath{\lambda}$-BEDSe undergoes an antiferromagnetic (AF) ordering below ${T}_{\mathrm{N}}=22$ K. In the AF state, discrete $^{13}\mathrm{C}$-NMR spectra with a remaining central peak are observed, indicating the commensurate AF spin structure also observed in $\ensuremath{\lambda}$-ET. The similarity of the structural and magnetic properties between $\ensuremath{\lambda}$-BEDSe and $\ensuremath{\lambda}$-ET suggests that both salts are in the same electronic phase, i.e., the physical properties of $\ensuremath{\lambda}$-BEDSe can be understood by the universal phase diagram of bandwidth-controlled $\ensuremath{\lambda}$-type organic conductors obtained by donor molecule substitution.