Quantum spin liquid ground state in the rare-earth triangular antiferromagnet SmTa7 O19

The rare-earth-based geometrically frustrated triangular magnets have attracted considerable attention due to the intricate interplay between strong spin-orbit coupling and the crystal electric field (CEF), which often leads to effective spin-1/2 degrees of freedom and therefore promotes strong quantum fluctuations at low temperatures, thus offering an excellent route to stabilize a quantum spin liquid (QSL) ground state. We have investigated the ground state magnetic properties of a polycrystalline sample of SmTa7O19 which we propose to have a gapless QSL ground state by employing powder x-ray diffraction (XRD), x-ray absorption spectroscopy (XAS), DC and AC-magnetic susceptibility, M versus H isotherm, specific heat, and muon spin rotation/relaxation measurements (μSR) down to 30 mK. The combined structural and electronic studies reveal the formation of an edge-sharing equilateral triangular lattice of Sm3+ ions in ab plane. The DC and AC magnetic susceptibility, and heat capacity measurements reveal that SmTa7O19 does not exhibit any long-range magnetic ordering transition down to 50 mK. The zero-field (ZF)-μSR study strongly refutes the long-range magnetically ordered ground state and/or any partial spin-freezing down to at least 30 mK. The ZF-muon-spin relaxation rate is weakly temperature-dependent between 50 and 20 K, rapidly increases below ∼20K and saturates at low temperatures between 2 K and 30 mK, which has been attributed to a characteristic signature of QSL systems. Further, our longitudinal-field (LF)-μSR measurements at 0.1 K reveal a dynamic nature of the magnetic ground state. In addition, our high-field specific heat data suggest a gapless nature of spin excitations in this compound.