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, NaxCoO2·yH2O, 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 ∼6.3 K in our newly synthesized layered
compound Na2CoSe2O, in which the edge-shared
CoSe6 octahedra form [CoSe2] 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 TC, this material exhibits very high superconducting upper critical
fields, μ0HC2(0), which
far exceeds the Pauli paramagnetic limit by a factor of 3–4.
First-principles calculations show that Na2CoSe2O 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-TC 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.