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

Abstract

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 zerotemperature 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~1 48 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 .

Field-induced quantum spin disordered state in spin-1/2 honeycomb magnet Na2Co2TeO6

“…We, therefore, take 10.5 T as the crossover field from the correlated spindisordered state to the spin-polarized state. The magnetization curve further suggests that the saturation field for the B ⊥ b-axis case is around B S = 12.5 T with saturation magnetization M S = 2.01μ B /Co 2+ obtained by subtracting off the Van-Vleck paramagnetic contribution 49 (Fig. 2c).…”

Survival of itinerant excitations and quantum spin state transitions in YbMgGaO4 with chemical disorder

“…Thermal conductivity was measured by using a “one heater, two thermometers” technique in a 3 He/ 4 He dilution refrigerator at 70 mK–1 K, equipped with a 14 T superconducting magnet 72 , 73 . The sample was cut precisely along the crystallographic axes with the longest and the shortest dimensions are along the a - or the c -axis.…”

Frustration enhanced by Kitaev exchange in a $\boldsymbol{\tilde{j}_{\text{eff}}=\frac12}$ triangular antiferromagnet

“…The first experimental study reported a sizable antiferromagnetic exchange and no long-range ordering down to 50 mK [27]. Very recently, ultralow-temperature specific heat measurements revealed a magnetic ordering transition at T N = 148 mK [28], i.e.,∼ 20 times smaller than the Weiss temperature θ = −2.5 K obtained from the static magnetic susceptibility χ(T ) [28]. Another interesting observation is the linear dependence of the residual thermal conductivity [28], compatible with the spinon Fermi surface scenario advocated for critical spin liquids.…”

“…Usually this signifies proximity to a magnetic phase transition where the spin dynamics experiences a critical slowing down [35]. However, such a phase transition in Na 2 BaCo(PO 4 ) 2 takes place at two orders of magnitude lower temperature of T N = 148 mK [28] which evidences an extended lowtemperature regime of the spin-liquid like, slow spin fluctuations in the electron spin system.…”