Nonlinear transport signatures of hidden symmetry breaking in a Weyl altermagnet

Phase transitions in solids are often accompanied by structural changes, but subtle lattice distortions can remain hidden from conventional crystallographic probes, hindering the identification of the correct order parameters. A case in point is ${\mathrm{Ca}}_{3}{\mathrm{Ru}}_{2}{\mathrm{O}}_{7}$, a correlated polar ruthenate with well-characterized phase transitions, whose ground-state structure has recently become a subject of debate. This uncertainty stems from extremely small atomic displacements $(\ensuremath{\sim}0.001\phantom{\rule{0.16em}{0ex}}\AA{})$ between competing phases, beyond the resolution of x-ray diffraction, neutron scattering, or optical second-harmonic generation. In this work, we propose a method to detect hidden symmetry breaking by leveraging nonlinear transport induced by quantum geometry. We show that ${\mathrm{Ca}}_{3}{\mathrm{Ru}}_{2}{\mathrm{O}}_{7}$ is a Weyl chain semimetal in both phases. The low-symmetry phase, classified as an altermagnet by symmetry, features distorted topological surface states that are asymmetric along the polar $(b)$ axis. However, the nonrelativistic spin splitting is too weak $(\ensuremath{\sim}0.1\phantom{\rule{0.16em}{0ex}}\mathrm{meV})$ to be resolved directly, regarding the altermagnetism. In contrast, Weyl chains generate a large quantum metric at the Fermi surface, leading to nonlinear conductivities that are orders of magnitude stronger in the low-symmetry phase. A longitudinal nonlinear conductivity along the polar axis emerges exclusively in this phase, providing a sensitive probe to qualitatively distinguish it from the high-symmetry structure and demonstrate the emergence of altermagnetism, which is confirmed by a recent experiment. Our work establishes a route for identifying hidden symmetry breaking in complex quantum materials through the interplay of crystal symmetry, topology, and nonlinear quantum transport.