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

Abstract

The layered triangular lattice owing to $1:2$ order of $B$ and $B^{\prime }$ sites in the triple perovskite $A_{3}B{B}_2^{\prime }$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 Sr${}_3$CuTa${}_2$O${}_9$ that belongs to the above family, by employing magnetization, specific heat, and muon spin relaxation ($\hat{I}¼$SR) 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 ($J_{\mathrm{CW}}/k_{\mathrm{B}}\simeq -5.04$~K). Magnetic specific heat in zero-field follows a power law, $C_{\mathrm{m}}\sim T^{\hat{I}}\cdot$, below 1.2~K with $\hat{I}\cdot \approx 2/3$, which is consistent with a theoretical proposal of the presence of spinon Fermi surface. Below 1.2~K, the $\hat{I}¼$SR relaxation rate shows no temperature dependence, suggesting persistent spin dynamics as expected for a QSL state. Delving deeper, we also analyze longitudinal field $\hat{I}¼$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.

7 pages, 4 figures. Accepted for publication at Physical Review B (Letter)