Microscopic modulation of local density of states in superconducting α-(BEDT-TTF)2NH4Hg(SCN)4 studied by site-selective C 13 -NMR spectroscopy

Abstract

The local electronic state at every crystallographically independent molecular site was separately investigated in the organic superconductor α-(BEDT-TTF)2NH4Hg(SCN)4 using site-selective C13-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 α-(BEDT-TTF)2RbHg(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 α-type BEDT-TTF family, and summarized the competition between charge instability and superconductivity. © 2014 American Physical Society.