Quantum phase transition between disordered and ordered states in the spin-12kagome lattice antiferromagnet(Rb1−xCsx)2Cu3SnF12

Abstract

We have systematically investigated the variation of the exchange parameters and the ground state in the $S=\frac{1}{2}$ kagome-lattice antiferromagnet (${\mathrm{Rb}}_{1\ensuremath{-}x}{\mathrm{Cs}}_{x}$)${}_{2}{\mathrm{Cu}}_{3}{\mathrm{SnF}}_{12}$ via magnetic measurements using single crystals. One of the parent compounds, ${\mathrm{Rb}}_{2}{\mathrm{Cu}}_{3}{\mathrm{SnF}}_{12}$, 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, ${\mathrm{Cs}}_{2}{\mathrm{Cu}}_{3}{\mathrm{SnF}}_{12}$, 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 $x\phantom{\rule{0.16em}{0ex}}<\phantom{\rule{0.16em}{0ex}}0.53$ and ordered for $x\phantom{\rule{0.16em}{0ex}}>\phantom{\rule{0.16em}{0ex}}0.53$. The pseudogap observed for $x\phantom{\rule{0.16em}{0ex}}<\phantom{\rule{0.16em}{0ex}}0.53$ and the N\'eel temperature for $x\phantom{\rule{0.16em}{0ex}}>\phantom{\rule{0.16em}{0ex}}0.53$ approach zero at ${x}_{\mathrm{c}}\phantom{\rule{0.16em}{0ex}}\ensuremath{\simeq}\phantom{\rule{0.16em}{0ex}}0.53$. This is indicative of the occurrence of a quantum phase transition at ${x}_{\mathrm{c}}$.