Enhancement of electron correlations and spin density wave fluctuations of the organic superconductor $\lambda \text{−}{\left(\mathrm{B}\mathrm{E}\mathrm{T}\mathrm{S}\right)}_2\mathrm{Ga}{\mathrm{Cl}}_4$ under pressure proved by ${}^{13}\mathrm{C}$ NMR

We performed ${}^{13}\mathrm{C}$ NMR measurements on an organic superconductor $\lambda \text{−}{\left(\mathrm{B}\mathrm{E}\mathrm{T}\mathrm{S}\right)}_2\mathrm{Ga}{\mathrm{Cl}}_4$ [BETS: bis(ethylenedithio)tetraselenafulvalene] to investigate the pressure effect on the spin density wave (SDW) fluctuations at low temperatures, which were observed at ambient pressure. We found that the spin-lattice relaxation rate divided by temperature $(1/T_{1}T)$ is significantly enhanced at low temperatures at 3 kbar, implying enhancement of the SDW fluctuation. When further pressure is applied, increase in $1/T_{1}T$ at low temperatures is suppressed and $1/T_{1}T$ becomes temperature independent, indicating the Fermi liquid state. Moreover, we revealed that the Korringa factor under pressure becomes double compared to its value at ambient pressure, suggesting a significant change in electron correlation. We discuss the effect of the SDW fluctuation and the electron correlation on the unconventional superconductivity.

Figure 1

(a) Molecular structures of the ET, us-STF, and BETS. (b) The $P\text{−}T$ phase diagram of $\lambda \text{−}{\left(D\right)}_2\mathrm{Ga}{\mathrm{Cl}}_4$ for the three compounds, D = ET, us-STF, or BETS [10, 11, 12]. (c) The phase diagram of $\lambda \text{−}{\left(\mathrm{B}\mathrm{E}\mathrm{T}\mathrm{S}\right)}_2\mathrm{Ga}{\mathrm{Br}}_x{\mathrm{Cl}}_{4-x}$ with $x$ ranging from 0 to 2 [10, 13]. The left side of this diagram is akin to a high pressure (large $U/W$) region.

Figure 2

(a) The crystal structure of $\lambda \text{−}{\left(\mathrm{B}\mathrm{E}\mathrm{T}\mathrm{S}\right)}_2\mathrm{Ga}{\mathrm{Cl}}_4$. The applied magnetic field direction is parallel to the long axis of BETS molecule. (b) The BETS molecule was enriched with ${}^{13}\mathrm{C}$ at the central double bond. (c) The schematic of inequivalent ${}^{13}\mathrm{C}$ sites.

Figure 3

Temperature dependence of the local spin susceptibility (left axis) and the difference in NMR shift $\mathrm{\Delta }\delta$ (right axis) at ambient pressure. The blue and red circles indicate the results of the present study and a previous study, respectively [15]. The inset shows the NMR spectra at 100 K and 10 K at ambient pressure. The red and blue arrows indicate Peak 1 and Peak 2, respectively.

Figure 4

(a) NMR spectra under each pressure at 50 K. The red and blue arrows instruct the Peak 1 and Peak 2, respectively. (b) The temperature dependence of local spin susceptibility (left) and the difference of NMR shift (right) under pressure.

Figure 5

The plot of recovery curves at ambient (100 and 30 K) and under each pressure at 30 K. The solid line is the fitting plot with $\beta$ = 0.8.

Figure 6

Temperature dependence of $1/T_{1}T$ at ambient and at pressures of 1, 3, 6, and 11 kbar. The inset shows the results of a previous study at ambient pressure [15]. The dashed lines are a visual aid for the averaged values in the FL state.

Figure 7

Pressure dependence of Korringa factor, spin susceptibility, the ratio of $1/T_{1}T$ at 10 K which is divided by the value at ambient pressure, and superconducting $T_C$ [10]. The dashed lines are a visual aid. The ratio of $1/T_{1}T$ at 10 K corresponds to the strength of the SDW fluctuation.

Figure 8

The schematics of Fermi surface for (a) ${\left(\mathrm{T}\mathrm{M}\mathrm{T}\mathrm{S}\mathrm{F}\right)}_2{\mathrm{PF}}_6$, (b) $\kappa \text{−}{\left(\mathrm{E}\mathrm{T}\right)}_2\mathrm{Cu}{\left(\mathrm{N}\mathrm{C}\mathrm{S}\right)}_2$, and (c) $\lambda \text{−}{\left(\mathrm{B}\mathrm{E}\mathrm{T}\mathrm{S}\right)}_2\mathrm{Ga}{\mathrm{Cl}}_4$ [27, 28, 29].