Symmetry-breaking lattice distortion in Sr_3Ru_2O_7

The electronic nematic phase of Sr${}_3$Ru${}_2$O${}_7$ is investigated by high-resolution in-plane thermal expansion measurements in magnetic fields close to 8 T applied at various angles $\mathrm{\Theta }$ off the c-axis. At $\mathrm{\Theta }<{10}^{\circ }$ we observe a very small (${10}^{-7}$) lattice distortion which breaks the four-fold in-plane symmetry, resulting in nematic domains with interchanged $a$- and b-axis. At $\mathrm{\Theta }\gtrsim {10}^{\circ }$ the domains are almost fully aligned and thermal expansion indicates an area-preserving lattice distortion of order $2\times {10}^{-6}$ which is likely related to orbital ordering. Since the system is located in the immediate vicinity to a metamagnetic quantum critical end point, the results represent the first observation of a structural relaxation driven by quantum criticality.