The recently discovered family of vanadium-based kagome metals with topological band structures offer new opportunities to study frustrated, correlated topological quantum states. These layered compounds are nonmagnetic and undergo charge density wave (CDW) transitions before developing superconductivity at low temperatures. Here we report the observation of unconventional superconductivity and pair density wave (PDW) in the vanadium-based kagome metal CsV3Sb5 using scanning tunneling microscope/spectroscopy (STM/STS) and Josephson STS. The differential conductance exhibits a V-shaped pairing gap Δ~0.5 meV below a transition temperature Tc~2.3 K. Superconducting phase coherence is observed by Josephson effect and Cooper-pair tunneling using a superconducting tip. We find that CsV3Sb5 is a strong-coupling superconductor (2∆/kBTc~5) and coexists with long-range 4a0 unidirectional and 2×2 charge order. Remarkably, we discover a bidirectional PDW accompanied by 4a0/3 spatial modulations of the coherence peak and gap-depth in the tunneling conductance. We term this novel quantum state a roton-PDW that can produce a commensurate vortex-antivortex lattice and account for the observed conductance modulations. Above Tc, we observe a large V-shaped pseudogap in the 4a0 unidirectional and 2a0 bidirectional CDW state. Electron-phonon calculations attribute the 2×2 CDW to phonon softening induced structural reconstruction, but the phonon mediated pairing cannot describe the observed strong-coupling superconductor. Our findings show that electron correlations drive the 4a0 unidirectional CDW, unconventional superconductivity, and roton-PDW with striking analogies and distinctions to the phenomenology of high-Tc cuprate superconductors, and provide groundwork for understanding their microscopic origins in vanadium-based kagome superconductors.

Recently, it has been shown that large stacks of intrinsic Josephson junctions in Bi2Sr2CaCu2O8 emit synchronous THz radiation, the synchronization presumably triggered by a cavity resonance. To investigate this effect we use low temperature scanning laser microscopy to image electric field distributions. We verify the appearance of cavity modes at low bias and in the high input-power regime we find that standing-wave patterns are created through interactions with a hot spot, possibly pointing to a new mode of generating synchronized radiation in intrinsic Josephson junction stacks.