Metal-organic frameworks (MOFs), which are self-assemblies of metal ions and organic ligands, provide a tunable platform to search a new state of matter. A two-dimensional (2D) perfect kagome lattice, whose geometrical frustration is a key to realizing quantum spin liquids, has been formed in the π − d conjugated 2D MOF [Cu3(C6S6)]n (Cu-BHT). The recent discovery of its superconductivity with a critical temperature Tc of 0.25 kelvin raises fundamental questions about the nature of electron pairing. Here, we show that Cu-BHT is a strongly correlated unconventional superconductor with extremely low superfluid density. A nonexponential temperature dependence of superfluid density is observed, indicating the possible presence of superconducting gap nodes. The magnitude of superfluid density is much smaller than those in conventional superconductors and follows the Uemura’s relation of strongly correlated superconductors. These results imply that the unconventional superconductivity in Cu-BHT originates from electron correlations related to spin fluctuations of kagome lattice.

The recent discovery of superconductivity in NaSn2As2 with a van der Waals layered structure raises immediate questions on its pairing mechanism and underlying electronic structure. Here, we present measurements of the temperature-dependent magnetic penetration depth λ(T ) in single crystals of NaSn2As2 down to ∼ 40 mK. We find a very long penetration depth λ(0) = 960 nm, which is strongly enhanced from the estimate of first-principles calculations. This enhancement comes from a short mean free path ℓ ≈ 1.7 nm, indicating atomic scale disorder possibly associated with the valence-skipping states of Sn. The temperature dependence of superfluid density is fully consistent with the conventional fully gapped s-wave state in the dirty limit. These results suggest that NaSn2As2 is an ideal material to study quantum phase fluctuations in strongly disordered superconductors with its controllable dimensionality.

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