Dynamical scaling of quantum spin liquid in the triangular lattice $\kappa$-(ET)$_2$Ag$_2$(CN)$_3$*

The static and dynamic spin susceptibilities of quantum spin liquid are investigated in an organic Mott insulator $\kappa$-(ET)$_2$Ag$_2$(CN)$_3$ with a spin-1/2 triangular lattice. The application of negative chemical pressure to $\kappa$-(ET)$_2$Cu$_2$(CN)$_3$ allows for extensive tuning of antiferromagnetic exchange coupling, with $J/k_{\rm B} = 175 - 310$ K, under hydrostatic pressure. Based on $^{13}$C nuclear magnetic resonance measurements under pressure, we uncover universal scaling in the static and dynamic spin susceptibilities down to low temperatures $\sim 0.1k_{\rm B}T/J$. The low-lying spin excitations persists in the nuclear spin-lattice relaxation rate and specific heat coefficient, consistent with the presence of gapless low-lying excitations. The results demonstrate fundamental finite-temperature properties of quantum spin liquid in a wide parameter range.