Possible itinerant excitations and quantum spin state transitions in the effective spin-1/2 triangular-lattice antiferromagnet Na(2)BaCo(PO(4))(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 (κ(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 Na(2)BaCo(PO(4))(2), which has an antiferromagnetic order at very low temperature (T(N) ~ 148 mK), and observed a finite κ(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 Na(2)BaCo(PO(4))(2) possibly behaves as a gapless QSL with itinerant spin excitations above T(N) and its strong quantum spin fluctuations persist below T(N).