Misfit layered superconductor (PbSe)(NbSe) with possible layer-selective FFLO state.

Two-dimensional (2D) superconductors are known for their novel emergent phenomena, however, lack of experimental probes beyond resistivity has hindered further exploration of diverse superconducting states. Bulk 2D superconductors, with superconducting layers separated by non-superconducting layers, offer a unique opportunity to break this limit. Here, we synthesized a single crystal of misfit layered compound (PbSe)(NbSe), composed of alternately stacked tri-layer NbSe and non-superconducting block layers with incompatible unit cells. Due to its unique structure, 2D Ising superconductivity is maintained even in a bulk form. Resistivity and tunnel diode oscillator measurements reveal two distinct superconducting phases in magnetic field vs. temperature phase diagram. A theoretical analysis suggests that a layer-selective Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state is realized at the high-magnetic-field phase, where Ising and finite-q superconductivity are mixed due to the tri-layer structure. Bulk 2D superconductors with misfit structure offer a novel opportunity for understanding of 2D superconductivity through bulk measurements and interlayer engineering.