Microscopic modulation of local density of states in superconducting $\alpha -\left(\text{BEDT-TTF}{)}_2{\mathrm{NH}}_4\mathrm{Hg}\right(\mathrm{SCN}{)}_4$ studied by site-selective ${}^{13}\mathrm{C}$-NMR spectroscopy

The local electronic state at every crystallographically independent molecular site was separately investigated in the organic superconductor $\alpha -\left(\text{BEDT-TTF}{)}_2{\mathrm{NH}}_4\mathrm{Hg}\right(\mathrm{SCN}{)}_4$ using site-selective ${}^{13}\mathrm{C}$-NMR spectroscopy. The density of states (DOS) at three molecular sites shows independent temperature variation, which is caused by the long-range Coulomb interaction. Comparing the present results with those for a nonsuperconducting counterpart $\alpha -\left(\text{BEDT-TTF}{)}_2\mathrm{Rb}\mathrm{Hg}\right(\mathrm{SCN}{)}_4$, we found that the uniform DOS at two of three independent molecular sites is crucial to induce superconductivity. Taking the site-dependent DOS as a parameter, we constructed a phase diagram for a superconductor with long-range Coulomb interaction, an $\alpha$-type BEDT-TTF family, and summarized the competition between charge instability and superconductivity.

Figure 1

(a) Molecular arrangement of $\alpha$-(BEDT-TTF)${}_2$NH${}_4$Hg(SCN)${}_4$ in the conduction plane. Three crystallographically independent molecules A, B, and C form the A and B-C columns along the $c$ direction. (b) The ${}^{13}\mathrm{C}$-NMR spectrum obtained at room temperature. The observed four separate peaks were assigned to each ${}^{13}\mathrm{C}$ site, as denoted in the figure from the angle-resolved NMR measurement. A(a) and A(b) are two independent ${}^{13}\mathrm{C}$ sites on the A molecule.

Figure 2

(a) Temperature dependence of local spin susceptibility at each site in NH${}_4$ salt (solid symbols) and Rb salt (open symbols [13]). The same symbols for each ${}^{13}\mathrm{C}$ site were used throughout this work. The difference in static susceptibility between NH${}_4$ and Rb salts was observed only below 100 K. (b) The NMR linewidth determined at each peak. The line broadening at A sites below 180 K indicates that the charge instability reported in Rb salt exists also in the SC NH${}_4$ salt.

Figure 3

(a) Temperature dependence of ${\left(T_{1}T\right)}^{-1}$ measured at each ${}^{13}\mathrm{C}$ site. The results for Rb salt (open symbols) are displayed together for comparison [24]. The temperature-independent Fermi liquid behavior was observed between 4 and 100 K. (b) Local DOS for each site estimated using Eq. (1) (see text). The DOS at the A and B sites becomes almost equivalent at low temperatures in NH${}_4$ salt, while in Rb salt the DOS of the B site decreases to the values of the C site. (c) The local DOS for $\alpha$-(BEDT-TTF)${}_2$I${}_3$ at ambient pressure [25]. The DOS at the B site is comparable to that at the C site. The low-temperature data below 135 K were not obtained due to a CO transition.

Figure 4

A phase diagram for $\alpha$-type BEDT-TTF salts. The ratio between local DOS at B and A sites $N_B/N_A$ was used as the horizontal axis. This ratio parametrizes the homogeneity of local DOS along the $a$ direction. In this phase diagram, the charge ordered state in I${}_3$ and Rb salts appears on the same side of the SC state. The crystal structure in the figure denotes a schematic image of the local DOS at each molecular site.