13C NMR study of superconductivity near charge instability realized in beta"-(BEDT-TTF)4[(H3O)Ga(C2O4)3]C6H5NO2

TL;DR

This study uses 13C NMR to explore superconductivity near a charge instability in a molecular superconductor, revealing spin-singlet pairing, strong electron correlations, and potential charge-fluctuation-induced pairing mechanisms.

Contribution

First NMR investigation of superconductivity near charge instability in beta''-(BEDT-TTF) compound, highlighting charge fluctuations' role in pairing.

Findings

Cooper pairs are in spin-singlet state.

Strong electron-electron correlations in the normal state.

Charge fluctuations may induce superconductivity.

Abstract

To investigate the superconducting (SC) state near a charge instability, we performed ^{13}C NMR experiments on the molecular superconductor beta"-(BEDT-TTF)_{4}[(H_{3}O)Ga(C_{2}O_{4})_{3}]C_{6}H_{5}NO_{2}, which exhibits a charge anomaly at 100 K. The Knight shift which we measured in the SC state down to 1.5 K demonstrates that Cooper pairs are in spin-singlet state. Measurements of the nuclear spin-lattice relaxation time reveal strong electron-electron correlations in the normal state. The resistivity increase observed below 10 K indicates that the enhanced fluctuation has an electric origin. We discuss the possibility of charge-fluctuation-induced superconductivity.