Alle News

16. September 2017; Tobias Wollandt

By: U.S. Pracht, T. Cea, N. Bachar, G. Deutscher, E. Farber, M. Dressel, M. Scheffler, C. Castellani, A.M. García-García, L. Benfatto
Title: Optical signatures of the superconducting Goldstone mode in granular aluminum: experiments and theory
Link: Physical Review B 96, 094514 (2017)


In ordinary superconductors the two collective modes of the complex order parameter are usually strongly resilient to the spectroscopic observation. The amplitude or Higgs mode is weakly coupled to the electromagnetic field and it overlaps with the continuum of single-particle excitations at 2Δ, which buries its signal. The phase or Goldstone mode is optically inert, and it is boosted to the plasma frequency by Coulomb interactions. In the present work, terahertz experiments and theoretical calculations are combined in order to demonstrate the existence of an alternative route for the optical visibility of the Goldstone mode in a nanostructured granular Al thin film. The Coulomb screening due to the grains self-capacitance and the intrinsic inhomogeneity of the local superfluid stiffness due to shell effects lead to finite-frequency subgap absorption of the phase modes, showing that nanograins arrays are a promising setting to control the Goldstone mode via optical means.