Microscopic modulation of local density of states in superconductingα−(BEDT-TTF)2NH4Hg(SCN)4studied by site-selectiveC13-NMR spectroscopy

Unconventional superconductivity has been frequently found after the suppression of the order in the spin degrees of freedom. Properties of unconventional superconductivity near magnetic order have been intensively studied since the discovery of heavy fermion and cuprate superconductors. In these systems, superconductivity is induced by magnetic critical fluctuations enhanced near the second-order phase transition. The organic superconductors (TMTSF)2X (TMTSF: tetramethyltetraselenafulvalene) and κ-(BEDTTTF)2X (BEDT-TTF: bisethylenedithio-tetrathiafulvalene) are also categorized by magnetic-fluctuation-induced superconductivity [1,2]. In some BEDT-TTF salts, however, superconductivity was found near the order in the charge degrees of freedom. One of the most intensively studied examples is the α-(BEDT-TTF)2X family, in which superconductivity appears after suppressing charge instability by chemical substitution or application of mechanical pressures [3‐6]. Theoretical studies have suggested that superconducting (SC) pairs can be mediated by critical charge fluctuations near the charge ordering (CO), which is caused by a sizable off-site Coulomb repulsion [7]. The properties of superconductivity near charge instability should be investigated to understand the diversity of SC pairing mechanisms. To reveal the relationship between superconductivity and charge instability, a phase diagram for various α-type BEDTTTF salts has been proposed [5]. In this phase diagram, the ratio of lattice parameters c/a was used as the tuning parameter, which modifies the anisotropy of the transfer integrals in the conduction (ac � ) plane. Among the α-(BEDTTTF)2X salts, a clear CO transition at TCO = 135 K was reported in α-(BEDT-TTF)2I3 (I3 salt) with c/a � 0.85 [8]. The uniaxial strain along the c direction decreases the c/a ratio to induce superconductivity with the transition temperature Tc reaching 7 K [4]. The resistivity in c axial strainshowssemiconductingbehaviorbeforetheSCtransition, while a axial strain induces metallic resistivity down to the lowest temperature without a SC transition. In another α-type salt, α-(BEDT-TTF)2NH4Hg(SCN)4 (NH4 salt) with a