  The recently discovered family of vanadium-based kagome metals with
topological band structures offer a new opportunity to study frustrated,
correlated and 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 lattice CsV3Sb5 using scanning tunneling
microscope/spectroscopy (STM/STS) and Josephson STS. The differential
conductance exhibits a V-shaped pairing gap about 0.5 meV below a transition
temperature Tc about 2.3 K. Superconducting phase coherence is observed by
Josephson effect and Cooper-pair tunneling to a superconducting tip. We find
that CsV3Sb5 is a strong-coupling superconductor (2delta/kBTc about 5) and
coexists with 4a0 unidirectional and 2x2 charge order. Remarkably, we discover
a 4a0/3 bidirectional PDW accompanied by spatial modulations of the coherence
peak and gap-depth in the tunneling conductance. We term the latter as a
roton-PDW that can produce a commensurate vortex-antivortex lattice to account
for the observed conductance modulations. Above Tc, we observe long-range
ordered 4a0 unidirectional and 2a0 bidirectional CDW and a large V-shaped
pseudogap in the density of state. Electron-phonon calculations attribute the
2x2 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 in the charge sector can drive the
4a0 unidirectional CDW, unconventional superconductivity, and roton-PDW with
striking analogies to the phenomenology of cuprate high-Tc superconductors, and
provide the groundwork for understanding their microscopic origins in the
vanadium-based kagome superconductors.
