Evidence of quantum spin liquid state in a Cu²⁺-based $S = 1/2$ triangular lattice antiferromagnet

The layered triangular lattice owing to $1:2$ order of B and $B'$ sites in the triple perovskite A₃ B B'₂O₉ family provides an enticing domain for exploring the complex phenomena of quantum spin liquids (QSLs). We report a comprehensive investigation of the ground state properties of Sr₃CuTa₂O₉ that belongs to the above family, by employing magnetization, specific heat, and muon spin relaxation (μSR) experiments down to the lowest temperature of 0.1~K. Analysis of the magnetic susceptibility indicates that the spin-lattice is a nearly isotropic $S = 1/2$ triangular lattice. We illustrate the observation of a gapless QSL, in which conventional spin ordering or freezing effects are absent, even at temperatures more than two orders of magnitude smaller than the exchange energy (JCW/kB -5.04~K). Magnetic specific heat in zero-field follows a power law, Cₘ ~ T^η, below 1.2~K with η ≈ 2/3, which is consistent with a theoretical proposal of the presence of spinon Fermi surface. Below 1.2~K, the μSR relaxation rate shows no temperature dependence, suggesting persistent spin dynamics as expected for a QSL state. Delving deeper, we also analyze longitudinal field μSR spectra revealing strong dynamical correlations in the spin-disordered ground state. All of these highlight the characteristics of spin entanglement in the QSL state.