Strain-induced symmetry transition and non-Fermi liquid behavior in CaRuO3 thin films

We report the experimental verification of a strain-induced symmetry change associated with octahedral geometry in epitaxial CaRuO ₃ thin films, where the decoupling of octahedral rotation modes dictates both macroscopic elastic and quantum transport properties. High-resolution X-ray diffraction reveals that a 1.43% epitaxial tensile strain from SrTiO ₃ (0 0 1) substrates triggers a structural transition from the bulk orthorhombic ( Pbnm ) to a tetragonal ( P 4/ mbm ) phase. This symmetry transition induces a a ⁰ a ⁰ c ⁺ rotation-angle crossover characterized by the linearization of apical Ru-O-Ru bond angles to 180 ° , and a simultaneous equatorial buckling to 137.5 ° . Such RuO ₆ reconfiguration leads to significant mechanical hardening along the apical axis, manifested by an anomalous effective Poisson ratio (ν _eff ) of 0.13. Furthermore, the anisotropic octahedral distortion, driven by the equatorial buckling and bond elongation (2.095 Å), enhanced electronic correlations, thereby increasing the effective mass and stabilizing a non-Fermi liquid state with a linear temperature dependence of resistivity. By quantifying these structural reconfigurations and their subsequent impact on elastic and electronic functionalities, our findings provide a robust framework for the rational design of ruthenate-based oxide electronics.