  Unconventional superconductivity in bulk materials under ambient pressure is
extremely rare among the 3d transition metal compounds outside the layered
cuprates and iron-based family. It is predominantly linked to highly
anisotropic electronic properties and quasi-two-dimensional (2D) Fermi
surfaces. To date, the only known example of a Co-based exotic superconductor
is the hydrated layered cobaltate, Na$_{x}$CoO$_{2}\cdot$ yH$_{2}$O, and its
superconductivity is realized in the vicinity of a spin-1/2 Mott state.
However, the nature of the superconductivity in these materials is still a
subject of intense debate, and therefore, finding a new class of
superconductors will help unravel the mysteries of their unconventional
superconductivity. Here we report the discovery of superconductivity at $\sim$
6.3 K in our newly synthesized layered compound Na$_{2}$CoSe$_{2}$O, in which
the edge-shared CoSe$_{6}$ octahedra form [CoSe$_{2}$] layers with a perfect
triangular lattice of Co ions. It is the first 3d transition metal
oxychalcogenide superconductor with distinct structural and chemical
characteristics. Despite its relatively low $T_{c}$, this material exhibits
very high superconducting upper critical fields, $\mu_{0}H_{c2}(0)$, which far
exceeds the Pauli paramagnetic limit by a factor of 3 - 4. First-principles
calculations show that Na$_{2}$CoSe$_{2}$O is a rare example of a negative
charge transfer superconductor. This cobalt oxychalcogenide with a geometrical
frustration among Co spins shows great potential as a highly appealing
candidate for the realization of unconventional and/or high-$T_{c}$
superconductivity beyond the well-established Cu- and Fe-based superconductor
families and opens a new field in the physics and chemistry of low-dimensional
superconductors.
