Inversion symmetry breaking at the heterointerface of SrRuO₃ thin films

We investigate the structural symmetry of SrRuO₃ thin films grown on the SrTiO₃ substrate by exploiting optical second harmonic generation (SHG) technique. We observe that SHG responses are strongly dependent on the film thickness and temperature; the azimuth-dependence becomes more anisotropic as the film becomes thicker, and the SHG response diminishes as temperature increases above 400 K. We analyze the azimuth dependences by considering the electric dipole and electric quadrupole contributions, and find that the tetragonal and orthorhombic phases are stabilized in fully strained and strain-relaxed regions, respectively. In particular, we demonstrate that the inversion-symmetry-broken state emerges as a stable phase at the interface between centrosymmetric tetragonal and orthorhombic phases.