Strong quantum fluctuations in a quantum spin liquid candidate with a Co-based triangular lattice

Abstract

Currently under active study in condensed matter physics, both theoretically and experimentally, are quantum spin liquid (QSL) states, in which no long-range magnetic ordering appears at low temperatures due to strong quantum fluctuations of the magnetic moments. The existing QSL candidates all have their intrinsic disadvantages, however, and solid evidence for quantum fluctuations is scarce. Here we report a new compound, Na 2 BaCo(PO 4 ) 2 , a geometrically frustrated system with effective spin-1/2 local moments for Co 2+ ions on an isotropic two-dimensional triangular lattice. Magnetic susceptibility and neutron scattering experiments show no magnetic ordering down to 0.05 K. Thermodynamic measurements show that there is a tremendous amount of magnetic entropy present below 1 K in zero applied magnetic field. The presence of localized low-energy spin fluctuations is revealed by inelastic neutron measurements. At low applied fields, these spin excitations are confined to low energy and contribute to the anomalously large specific heat. In larger applied fields, the system reverts to normal behavior as evident by both neutron and thermodynamic results. Our experimental characterization thus reveals that this new material is an excellent candidate for the experimental realization of a quantum spin liquid state. 1 arXiv:1905.02115v1 [cond-mat.str-el] 6 May 2019 SIGNIFICANCE STATEMENTThe experimental search and verification of theoretically predicted quantum spin liquids (QSLs) is challenging, especially because there is no universally accepted evidence to support such exotic quantum states. At present, broad continuous magnetic excitations, i.e. a spinon continuum, observed via inelastic neutron scattering, is considered as the most crucial evidence for a QSL, as described in many studies on the existing QSL candidates. However, as another potential origin of such broad spin excitations, disorder effects in those candidates cannot be neglected. Here we present a new compound with a Co-based triangular lattice that is structurally perfect and without intrinsic disorder. We present experimental results that suggest that this new compound is an ideal QSL candidate for future studies.

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

“…By following the recipe in ref. 46 , we grew single crystals of Na 2 BaCo(PO 4 ) 2 . Figure 1c shows the inverse of the DC magnetic susceptibility (1/χ) with applied field B // ab.…”

“…The single crystals were grown by flux method as reported in ref. 46 . One adjustment made is that Platinum crucibles instead of alumina crucibles were used in our growth.…”

“…The powder X-ray diffraction measurement on the ground single crystals confirmed its lattice structure is the same as reported in ref. 46 . Laue back diffraction confirmed the flat surface of the as grown crystals is the ab plane.…”

“…While searching for new spin-1/2 TAFs without chemical disorder to explore the novel physics of QSL and QSSTs, the new Co-based triangular lattice antiferromagnet Na 2 BaCo(PO 4 ) 2 46 caught our attention. This system has a trigonal crystal structure with lattice parameter a = 5.3185 Å and c = 7.0081 Å.…”

Similarity of thermodynamic properties of the Heisenberg model on triangular and kagome lattices

“…The essential hallmark for material candidates for the presented SL scenario should be a substantial entropy s(T ) persisting well below T ≪ J. There are indeed several studies of s(T ) reported for different SL candidates (with some of them revealing transitions to LRO at very low T ), e.g., for KL systems volborthite [10], YCu 3 (OH) 6 Cl 3 [15], and recent TL systems 1T-TaS 2 [21] and Co-based SL materials [61]. Still, our model studies cannot exclude the relevant effect of additional terms, e.g., the Dzaloshinski-Moriya interaction [62,63] and/or 3D coupling, which can reduce s(T ) or even induce LRO at T → 0.…”