Symmetry-breaking lattice distortion in Sr₃Ru₂O₇

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The electronic nematic phase of Sr₃Ru₂O₇ is investigated by high-resolution in-plane thermal expansion measurements in magnetic fields close to 8 T applied at various angles Θ off the c-axis. At Θ<10∘ 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 Θ 10∘ the domains are almost fully aligned and thermal expansion indicates an area-preserving lattice distortion of order 2× 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.