Freezing of a disorder induced quantum spin liquid

Abstract

$\mathrm{S}{\mathrm{r}}_2\mathrm{CuT}{\mathrm{e}}_{0.5}{\mathrm{W}}_{0.5}{\mathrm{O}}_6$ is a square-lattice magnet with super-exchange between S=1/2 $\mathrm{C}{\mathrm{u}}^{2+}$ spins mediated by randomly distributed Te and W ions. Here, using sub-K temperature and 20 $\hat{\mathrm{I}}¼$eV energy resolution neutron scattering experiments we show that this system transits from a gapless disorder-induced quantum spin liquid to a new quantum state below ${\mathrm{T}}_{\mathrm{f}}$ = 1.7(1) K, exhibiting a weak frozen moment of <S>/S ~ 0.1 and low energy dynamic susceptibility,$\ddot{I}{‡}^{\prime \prime }(\hslash \ddot{I}‰)$ linear in energy which is surprising for such a weak freezing in this highly fluctuating quantum regime.