Spin-density wave in the vicinity of superconducting state in λ-(BETS)_{2}GaBr_{x}Cl_{4−x} probed by ^{13}C NMR spectroscopy

Abstract

^{13}C NMR and electrical resistivity of λ-(BETS)_{2}GaBr_{0.75}Cl_{3.25} [BETS: bis(ethylenedithio) tetraselenafulvalene] were simultaneously measured to clarify the electronic state of the insulating phase near the boundary of the superconducting (SC) phase of λ-(BETS)_{2}GaCl_{4}. We found the divergent peak in the nuclear spin-lattice relaxation rate divided by temperature 1/T_{1}T with the metal-insulator transition at 13K, below which the appearance of internal fields in NMR spectra was confirmed. Results of this study indicate that λ-(BETS)_{2}GaBr_{0.75}Cl_{3.25} undergoes spin-density-wave (SDW) ordering. We suggest that the existence of the SDW phase in the vicinity of the SC phase can explain the unexplainable enhancement of 1/T_{1}T at low temperatures observed in λ-(BETS)_{2}GaCl_{4}, and SDW fluctuation plays an important role in the pairing mechanism of λ-type BETS superconductors.