  In rare-earth-based frustrated magnets, the synergistic interplay between
spin correlations, spin-orbit coupling and competing exchange interactions
provide a promising route to realize exotic quantum states with nontrivial
excitations. Here, through thermodynamic and local-probe measurements down to
16 mK, we demonstrate the exotic magnetism and spin dynamics in the nearly
perfect emergent honeycomb lattice antiferromagnet TbBO3. The latter embodies a
frustrated lattice with a superimposed triangular lattice, constituted by
additional Tb3+ ions at the center of each hexagon. Thermodynamic experiments
reveal the presence of dominant antiferromagnetic interactions with no
indications of either long-range order or spin freezing down to 50 mK. Despite
sizable antiferromagnetic exchange interactions between the Tb3+ moments,
muon-spin relaxation does not detect any signatures of long-range magnetic
order or spin-freezing down to 16 mK. This suggests that the spin-orbit-driven
anisotropic exchange interaction engenders a strong frustration, crucial to
induce persistent spin dynamics. The specific-heat data exhibit a T^2.2
power-law behavior at low temperatures, suggesting gapless excitations
consistent with theoretical predictions. The scaling of muon relaxation rate as
a function of the characteristic energy scale for several spin-liquid
candidates, including TbBO3, demonstrates a thermally activated behavior. This
is consistent with NMR results on TbBO3 and reminiscent of a universal QSL
behavior, here attributed to short-range spin correlations. Our experimental
results are supported by density functional theory + Hubbard U and crystal
electric-field calculations, which propose TbBO3 as a promising platform to
realize the theoretically proposed quantum disorder state in an
anisotropy-driven frustrated honeycomb lattice.
