Probing Hidden Crystal Symmetry Breaking via Nonlinear Transport

X-ray and neutron diffraction are foundational tools for determining crystal structures, but their resolution limits can lead to misassignments, particularly in systems with subtle distortions or competing phases. In this talk, we demonstrate nonlinear transport as a complementary probe for hidden crystal symmetries, using the correlated metal Ca₃Ru₂O₇ as a case study. Below 48 K (T_S), where magnetic moments reorient from the a- to the b-axis and a pseudogap opens, our measurements—supported by DFT—reveal a previously overlooked low-symmetry phase. This phase is evidenced by the emergence of longitudinal nonlinear resistance (NLR) along the b-axis, indicating concurrent breaking of translational and time-reversal symmetries. The distortion responsible for this phase lies below the detection limit of conventional diffraction. The NLR is accompanied by a nonlinear Hall effect, both enhanced by a large quantum metric near Weyl chains, establishing nonlinear transport as a sensitive probe of hidden symmetry breaking.