Acoustic Plasmon Resonance: Breaking the Anderson Stiffness Paradigm in Quasi-Two-Dimensional Superconducting Films

Abstract

Recent experiments on superconducting films have revealed an acoustic plasmon mode that depends critically on the superconducting transition, directly challenging the long-standing Anderson-Higgs paradigm regarding the stiffness of the plasma spectrum in superconductors. In this Letter, we provide a microscopic theoretical framework that explains this behavior and establishes the physical conditions under which classical Anderson-Higgs constraints are bypassed. We demonstrate that in films of finite thickness, the transverse redistribution of normal and superfluid charge densities enables a unique coupling mechanism to electromagnetic radiation - a feature fundamentally absent in the conventional Carlson-Goldman scenario. Our theory predicts an acoustic mode whose dispersion, temperature scaling, and dependence on film thickness are in remarkable agreement with recent experimental observations. By delineating the regime of this acoustic response, we reconcile the observed electromagnetic activity of collective excitations with the fundamental principles of superconductivity.