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

The most fascinating feature of certain two-dimensional (2D) gapless quantum spin liquid (QSL) is that their spinon excitations behave like the fermionic carriers of a paramagnetic metal. The spinon Fermi surface is then expected to produce a linear increase of the thermal conductivity with temperature that should manifest via a residual value (κ0/T) in the zero-temperature limit. However, this linear in T behavior has been reported for very few QSL candidates. Here, we studied the ultralow-temperature thermal conductivity of an effective spin-1/2 triangular QSL candidate Na2BaCo(PO4)2, which has an antiferromagnetic order at very low temperature (TN ~ 148 mK), and observed a finite κ0/T extrapolated from the data above TN. Moreover, while approaching zero temperature, it exhibits series of quantum spin state transitions with applied field along the c axis. These observations indicate that Na2BaCo(PO4)2 possibly behaves as a gapless QSL with itinerant spin excitations above TN and its strong quantum spin fluctuations persist below TN. 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.