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Quantum phase transition between disordered and ordered states in the spin-1/2 kagome lattice antiferromagnet (Rb$_{1-x}$Cs$_{x}$)$_2$Cu$_3$SnF$_{12}$

We have systematically investigated the variation of the exchange parameters and the ground state in the $S = 1/2$ kagome-lattice antiferromagnet (Rb$_{1-x}$Cs$_{x}$)$_2$Cu$_3$SnF$_{12}$, via magnetic measurements using single crystals. One of the parent compounds, Rb$_2$Cu$_3$SnF$_{12}$, which has a distorted kagome lattice accompanied by four sorts of nearest-neighbor exchange interaction, has a disordered ground state described by a pinwheel valence-bond-solid state. The other parent compound, Cs$_2$Cu$_3$SnF$_{12}$, which has a uniform kagome lattice at room temperature, has an ordered ground state with the $q$ = 0 spin structure. The analysis of magnetic susceptibilities shows that with increasing cesium concentration $x$, the exchange parameters increase with the tendency to be uniform. It was found that the ground state is disordered for $x$ < 0.53 and ordered for $x$ > 0.53. The pseudogap observed for $x$ < 0.53 and the Néel temperature for $x$ > 0.53 approach zero at $x_{\rm c}$ $\simeq$ 0.53. This is indicative of the occurrence of a quantum phase transition at $x_{\rm c}$.