The layered triangular lattice owing to $1:2$ order of $B$ and $B^{\prime }$ sites in the triple perovskite $A_{3}B{B}_2^{\prime }{\mathrm{O}}_9$ 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 ${\mathrm{Sr}}_3{\mathrm{CuTa}}_2{\mathrm{O}}_9$ that belongs to the above family by employing magnetization, specific heat, and muon spin relaxation $\left(\mu \mathrm{SR}\right)$ 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 $(J_{\mathrm{CW}}/k_{\mathrm{B}}\simeq -5.04 \mathrm{K})$. Magnetic specific heat in zero field follows a power law, $C_{\mathrm{m}}\sim T^{\eta }$, below 1.2 K with $\eta \approx 2/3$, which is consistent with a theoretical proposal of the presence of a spinon Fermi surface. Below 1.2 K, the $\mu \mathrm{SR}$ relaxation rate shows no temperature dependence, suggesting persistent spin dynamics, as expected for a QSL state. Delving deeper, we also analyze longitudinal field $\mu \mathrm{SR}$ spectra, revealing strong dynamical correlations in the spin-disordered ground state. All of these highlight the characteristics of spin entanglement in the QSL state.

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• Frustrated magnetism
• Quantum spin liquid
• Triangular lattice
• Muon spin relaxation & rotation