Possible strongly correlated superconductivity in a copper-based metal-organic framework

Recently the copper(II) benzenehexathiolate coordination polymer [Cu₃(C₆S₆)]_n (Cu-BHT) attracts significant interests as the first superconductor in metal-organic frameworks (MOFs) with a superconducting transition temperature T_c ~ 250 mK. Since Cu-BHT has a perfect 2D Kagome structure formed by S = 1/2 spins of Cu²⁺, unconventional superconductivity related to quantum fluctuations is expected. Although we have already reported that our magnetic penetration depth measurements suggest unconventional superconductivity in Cu-BHT, fundamental superconducting and normal state properties, such as dimensionality, magnetism, and the Fermi energy, which are important to elucidate the superconducting mechanism, are still unrevealed. In this study, therefore, we measured the angle-resolved upper critical field, magnetoresistance, electron spin resonance (ESR), and optical conductivity. By comparing the ratio of T_c to the Fermi temperature with other unconventional superconductors, we revealed that Cu-BHT is possibly a member of strongly correlated electron systems.