Signature of Unconventional Superconductivity in a Metal-Organic Framework with a Perfect Kagome Structure

Recently, the superconductivity in a Metal-organic framework (MOF) has been discovered for the first time in copper(II) benzenehexathiolate ([Cu₃(C₆S₆)]_n, Cu-BHT). A theoretical study predicts that the electron-phonon coupling constant of 0.51 for bulk Cu-BHT can lead to superconductivity at T_c ∼ 1.58 K, thus conventional s-wave superconducting state has been proposed as a candidate of the pairing state. Meanwhile, recent measurements revealed strong quantum spin fluctuation possibly related to the 2D Kagome lattice of Cu atoms with S = 1/2 spin. Quantum spin fluctuations can promote unconventional superconducting pairing states, and it is therefore important to experimentally determine whether the superconductivity in Cu-BHT has conventional or unconventional nature.
Here we present measurements of in-plane magnetic penetration depth λ in Cu-BHT films down to 40 mK. The temperature dependence of λ shows a non-exponential, quasi-linear behavior at low temperatures. Our finding suggests that not only unconventional superconductivity with low-energy quasiparticle excitations is realized in this system but also MOFs can provide a flexible platform to investigate the superconducting pairing mechanisms in the presence of spin frustration and strong quantum fluctuations.