X-ray and neutron diffraction are foundational tools for structure determination; however, their resolution limits can lead to misassignments in materials with subtle distortions. Here we demonstrate that nonlinear transport provides a powerful complementary approach to uncover hidden crystal symmetries, using Ca3Ru2O7 as a case study. Below the magnetic transition at TS = 48 K, our experiment reveals a previously overlooked lower-symmetry phase. This is evidenced by the emergence of longitudinal nonlinear resistance (NLR), indicating combined translational and time-reversal symmetry breaking, and thus rendering Ca3Ru2O7 an altermagnetic candidate in terms of symmetry classification. DFT calculation suggests that the lower-symmetry phase arises from an extremely subtle lattice distortion (~0.1 pm) below TS, below the detection limit of conventional diffraction. Moreover, NLR is accompanied by nonlinear Hall effect, both enhanced by the large quantum metric associated with Weyl chains. Our findings establish nonlinear transport as a sensitive probe of hidden symmetry breaking and highlight an alternative route to discovering altermagnetic states. Altermagnets are characterized by fully compensated magnetic moments yet break time-reversal symmetry, leading to symmetry-enforced momentum dependent spin splitting. While demonstrations of almagnetism have typically proceeded via spectroscopic probes like ARPES, here Mali, Zhao and coauthors show that nonlinear transport can serve as a sensitive probe to screen for altermagnetic candidates, using Ca3Ru2O7 as a case study.
Mali et al. (Mon,) studied this question.