Orientational enhancement of surface phonon polarity in superconducting KTaO₃ interfaces.

The critical role of symmetry-breaking interfaces in mediating superconductivity has garnered increasing interest in recent years. KTaO3 (KTO)-based heterointerfaces present a striking example, where the superconducting transition temperature (Tc) exhibits an intriguing dependence on KTO crystallographic orientation. Here we identify a highly anisotropic surface phonon resonance near 107 meV at KTO and KTO-based heterointerfaces using surface-specific, symmetry-sensitive nonlinear optical spectroscopy. Polarity of this phonon mode scales with crystallographic orientation as (111) > (110) ≫ (001), reaching a 35-fold enhancement over non-superconducting interfaces and tracking the superconducting Tc. In addition, a softened local mode near 100 meV is observed and attributed to interfacial oxygen vacancies. Our results help build a framework linking the symmetry-breaking lattice phonons, defects, and anisotropic superconductivity for KTO and related heterointerfaces.