Levy Bertrand, F., Klein, T., Grenet, T., Dupre, O., Benoit, A., Bideaud, A., Bourrion, O., Calvo, M., Catalano, A., Gómez, A., Goupy, J., Grunhaupt., Von Leupke, U., Maleeva, N., Valenti, F., Pop, I. M., Monfardini, A. 2019. Electrodynamics of granular aluminum from superconductor to insulator: Observation of collective superconducting modes. Physical Review B 99, 9, DOI: 10.1103/PhysRevB.99.094506
We report on a detailed study of the optical response and T-c-rho phase diagram (T-c being the superconducting critical temperature and rho the normal state resistivity of the film) of granular aluminum, combining transport measurements and a high resolution optical spectroscopy technique. The T-c-rho phase diagram is discussed as resulting from an interplay between the phase stiffness, the Coulomb repulsion, and the superconducting gap Delta. We provide direct evidence for two different types of well resolved subgap absorptions, at omega(1) approximate to Delta and at Delta less than or similar to omega(2) less than or similar to 2 Delta (decreasing with increasing resistivity).