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 13 K, 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.

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A family of organic conductors, α-(BEDT-TTF) 2 MHg(SCN) 4 , is known to show the density wave (DW) state for M = Tl, K, and Rb salts, or the superconducting (SC) state for M = NH 4 salt at low temperatures. In contrast, α-(BEDT-TTF) 2 MHg(SeCN) 4 shows no phase transition and retains its metallic characteristics down to low temperatures. Since no α-(BEDT-TTF) 2 MHg(SeCN) 4 salt shows the DW or SC states, it was unclear whether the system of α-(BEDT-TTF) 2 MHg(SeCN) 4 could be understood by the same phase diagram as that of α-(BEDT-TTF) 2 MHg(SCN) 4 . Here, we succeeded in synthesizing a key material, α-(BEDT-TTF) 2 NH 4 Hg(SeCN) 4 , and determined its crystal structure. The temperature dependence of its electric conductivity and spin susceptibility showed its DW state at low temperatures, indicating that the α-(BEDT-TTF) 2 MHg(SeCN) 4 system is linked to the α-(BEDT-TTF) 2 MHg(SCN) 4 system. From the established phase diagram, we found that the dihedral angle between crystallographically independent A and B molecules, Θ B , and that between A and C molecules, Θ C , are good tuning parameters as in θ-type BEDT-TTF salts. In the Θ B −Θ C plot, the SC salt is located in the large Θ B and small Θ C regions, whereas the metallic salts are located in the small Θ B and large Θ C regions. The DW salts are located in the intermediate Θ B and Θ C regions. The relationship between their location in the Θ B −Θ C plot and the ground states supports the prediction that the local density of state between A and B molecules determines the ground states.

For the unconventional organic superconductor λ-(BETS) 2 GaCl 4 [BETS = bis(ethylenedithio) tetraselenafulvalene], the importance of both spin and charge degrees of freedom has been discussed based on the broadening of the NMR linewidth due to charge disproportionation and the occurrence of a spin-density-wave phase near the superconducting phase. NMR with the nuclear spin of 1/2, previously used for studying organic conductors, is an effective method for revealing electronic states microscopically; however, it cannot distinguish between charge and spin anomalies. To resolve this problem, in this paper, we performed 69,71 Ga-NMR measurements, which enabled us to study both charge and spin dynamics using different gyromagnetic ratios and quadrupole moments between two isotopes. The spin-lattice relaxation rate is dominated by electric-field gradient fluctuations originating from molecular dynamics above 150 K and, below this temperature, it is dominated by spin fluctuations derived from the π electrons of BETS layers. This change in the relaxation mechanism is considered to be due to the development of interactions between GaCl 4 ions and BETS layers upon freezing of the molecular motion by cooling. Below 150 K, the contribution of spin fluctuations monotonically increases, and no increase in the charge fluctuation was observed, suggesting that the spin degree of freedom plays a major role in low-temperature physical properties. Our findings will aid theoretical studies on superconducting properties.

Optical studies of the density wave (DW) salt of α-(BEDT-TTF) 2 KHg(SCN) 4 revealed a pseudogap due to a charge fluctuation below T * 200 K. Moreover, 13 C-NMR studies of the DW salt of α-(BEDT-TTF) 2 RbHg(SCN) 4 and of the superconductivity (SC) salt of α-(BEDT-TTF) 2 NH 4 Hg(SCN) 4 suggested that the NMR line width of the A site increased and the charge disproportionation in the A column developed below T *. These phenomena may be due to the development of a horizontal stripe modulation of the local charge density. However, whether the horizontal stripe modulation develops or not and the relationships among T * anomalies, the pseudogap, and line broadening, were unclear. In this study, 13 C-NMR measurements were obtained of the metallic salt of α-(BEDT-TTF) 2 TlHg(SeCN) 4 , and its electronic structure was compared with those of the DW and SC salts. The NMR line width of the A site and χ B /χ C in Rb(SCN) and NH 4 (SCN) increased and deviated from that in Tl(SeCN) below T * , suggesting the development of the instability of horizontal stripe modulation in Rb(SCN) and NH 4 (SCN) and that T * anomalies could be due to the instability.