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 (kappa(0)/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 (T-N similar to 148mK), and observed a finite kappa(0)/T extrapolated from the data above T-N. 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 T-N and its strong quantum spin fluctuations persist below T-N. 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.