Controlled Frustration Release on the Kagome Lattice by Uniaxial-Strain Tuning

It is predicted that strongly interacting spins on a frustrated lattice may lead to a quantum disordered ground state or even form a quantum spin liquid with exotic low-energy excitations. However, a controlled tuning of the frustration strength, separating its effects from those of disorder and other factors, is pending. Here, we perform comprehensive $^{1}\mathrm{H}$ NMR measurements on ${\mathrm{Y}}_{3}{\mathrm{Cu}}_{9}{(\mathrm{OH})}_{19}{\mathrm{Cl}}_{8}$ single crystals revealing an unusual $\stackrel{\ensuremath{\rightarrow}}{Q}=(1/3\ifmmode\times\else\texttimes\fi{}1/3)$ antiferromagnetic state below ${T}_{N}=2.2\text{ }\text{ }\mathrm{K}$. By applying in situ uniaxial stress, we break the symmetry of this disorder-free, frustrated kagome system in a controlled manner yielding a linear increase of ${T}_{N}$ with strain, in line with theoretical predictions for a distorted kagome lattice. In-plane strain of $\ensuremath{\approx}1%$ triggers a sizable enhancement $\mathrm{\ensuremath{\Delta}}{T}_{N}/{T}_{N}\ensuremath{\approx}10%$ due to a release of frustration, demonstrating its pivotal role for magnetic order.