Superconductivity in a Layered Cobalt Oxychalcogenide Na₂CoSe₂O with a Triangular Lattice

Area of Science:

• Condensed Matter Physics
• Materials Science
• Solid-State Chemistry

Background:

• Unconventional superconductivity is rare in 3d transition metals outside cuprates and iron-based families, often linked to 2D electronic properties.
• The only known cobalt-based exotic superconductor is Na2CoO2·yH2O, with its superconductivity mechanism still debated.
• New classes of superconductors are crucial for understanding unconventional superconductivity.

Purpose of the Study:

• To report the discovery of superconductivity in a new layered cobalt oxychalcogenide, Na2CoSe2O.
• To characterize its superconducting properties and electronic structure.
• To explore its potential for unconventional superconductivity.

Main Methods:

• Synthesis of the novel layered compound Na2CoSe2O.
• Experimental measurement of superconductivity transition temperature (Tc) and upper critical fields (μ0Hc2(0)).
• First-principles calculations for electronic structure and charge transfer analysis.

Main Results:

• Superconductivity observed at approximately 6.3 K in Na2CoSe2O.
• The material exhibits very high upper critical fields, exceeding the Pauli limit by 3-4 times.
• First-principles calculations reveal Na2CoSe2O as a negative charge transfer superconductor with geometrical frustration in Co spins.

Conclusions:

• Na2CoSe2O represents the first 3d transition metal oxychalcogenide superconductor.
• Its unique properties, including high critical fields and negative charge transfer, make it a promising candidate for unconventional superconductivity.
• This discovery opens a new research area in low-dimensional superconductors beyond Cu- and Fe-based systems.