Roton pair density wave in a strong-coupling kagome superconductor

The transition metal kagome lattice materials host frustrated, correlated and topological quantum states of matter^1-9. Recently, a new family of vanadium-based kagome metals, AV₃Sb₅ (A = K, Rb or Cs), with topological band structures has been discovered^10,11. These layered compounds are nonmagnetic and undergo charge density wave transitions before developing superconductivity at low temperatures^11-19. Here we report the observation of unconventional superconductivity and a pair density wave (PDW) in CsV₃Sb₅ using scanning tunnelling microscope/spectroscopy and Josephson scanning tunnelling spectroscopy. We find that CsV₃Sb₅ exhibits a V-shaped pairing gap Δ ~ 0.5 meV and is a strong-coupling superconductor (2Δ/k_BT_c ~ 5) that coexists with 4a₀ unidirectional and 2a₀ × 2a₀ charge order. Remarkably, we discover a 3Q PDW accompanied by bidirectional 4a₀/3 spatial modulations of the superconducting gap, coherence peak and gap depth in the tunnelling conductance. We term this novel quantum state a roton PDW associated with an underlying vortex-antivortex lattice that can account for the observed conductance modulations. Probing the electronic states in the vortex halo in an applied magnetic field, in strong field that suppresses superconductivity and in zero field above T_c, reveals that the PDW is a primary state responsible for an emergent pseudogap and intertwined electronic order. Our findings show striking analogies and distinctions to the phenomenology of high-T_c cuprate superconductors, and provide groundwork for understanding the microscopic origin of correlated electronic states and superconductivity in vanadium-based kagome metals.