Possible itinerant excitations and quantum spin state transitions in the effective spin-1/2 triangular-lattice antiferromagnet Na2BaCo(PO4)2

Thermal conductivity evidence of a spinon Fermi surface has been rare. Here, the authors report a finite linear increase of thermal conductivity with temperature in Na2BaCo(PO4)2 at ultra-low temperature, suggesting possible gapless quantum spin liquid behavior.