Structural Domain Imaging and Direct Determination of Crystallographic Orientation in Noncentrosymmetric Ca_3Ru_2O_7 Using Polarized Light Reflectance

The noncentrosymmetricity of a prototypical correlated electron system Ca_3Ru_2O_7 renders extensive interest in the possible polar metallic state, along with multiple other closely competing interactions. However, the structural domain formation in this material often complicates the study of intrinsic material properties. It is crucial to fully characterize the structural domains for unrevealing underlying physics. Here, we report the domain imaging on Ca_3Ru_2O_7 crystal using the reflection of polarized light at normal incidence. The reflection anisotropy measurement utilizes the relative orientation between electric field component of the incident polarized light and the principal axis of the crystal, and gives rise to a peculiar contrast. The domain walls are found to be the interfaces between 90^\circ rotated twin crystals by complementary magnetization measurements. A distinct contrast in reflectance is also found in the opposite cleavage surfaces, owing to the polar mode of the RuO_6 octahedra. More importantly, the analysis of the contrast between all inequivalent cleavage surfaces enables a direct determination of the crystallographic orientation of each domain. Such an approach provides an efficient yet feasible method for structural domain characterization, which can also find applications in noncentrosymmetric crystals in general.