Interfacial Strain Modulated Correlated Plasmons in La _1.85 Sr _0.15 CuO ₄ and Their Role in High‐Temperature Superconductivity

ABSTRACT High‐temperature superconductivity in cuprate materials remains a major challenge in physics due to the complexity of their strongly correlated electronic states. Interfacial strain is a powerful lever for tuning electronic correlations in complex oxides, offering new pathways to control emergent quantum phases. Here, we report the discovery of interfacial strain‐modulated correlated plasmons observed exclusively in superconducting La _1.85 Sr _0.15 CuO ₄ (LSCO) through spectroscopic ellipsometry. This form of plasmons is absent in the non‐superconducting LSCO counterparts. Detailed analysis reveals that these correlated plasmons, arising from the collective excitations within Mott‐correlated bands, are driven by long‐range electronic correlations in the Cu─O planes. Furthermore, long‑range electronic correlations, intricately modulated by interfacial strain, may play a crucial role in the emergence of superconductivity and in tuning the transition temperature. Dynamical cluster approximation (DCA) with quantum Monte Carlo (QMC) calculations of the extended Hubbard model suggest that long‐range Coulomb interactions play an important role in LSCO, showing good agreement with our experimental findings. The collective evidence from both the experimental results and theoretical findings provides new insights into the nature of collective excitations and their pivotal role in the emergence of high‐temperature superconductivity.