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 the Co-based exotic superconductor
was the hydrated layered cobaltate, NaxβCoO2ββ yH2βO, 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 an
active subject of debate, and therefore, finding new class of superconductors
will help unravel the mysteries of their unconventional superconductivity. Here
we report the discovery of unconventional superconductivity at βΌ 6.3 K in
our newly synthesized layered compound Na6βCo3βSe6βO3β, in
which the edge-shared CoSe6β octahedra form [CoSe2β] 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 Tcβ, material exhibits
extremely high superconducting upper critical fields, ΞΌ0βHc2β(0), which
far exceeds the Pauli paramagnetic limit by a factor of 3 - 4. First-principles
calculations show that Na6βCo3βSe6βO3β is a rare example of
negative charge transfer superconductor. This new cobalt oxychalcogenide with a
geometrical frustration among Co spins, shows great potential as a highly
appealing candidate for the realization of high-Tcβ and/or unconventional
superconductivity beyond the well-established Cu- and Fe-based superconductor
families, and opened a new field in physics and chemistry of low-dimensional
superconductors