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There are several energy scales that govern the properties of a superconductor and which can be accessed with low-frequency optical spectroscopy. In particular, the superconducting energy gap of superconductors with critical temperature of a few K can be studied with THz spectroscopy, but also the Cooper pair density (penetration depth, superfluid stiffness) and the quasiparticle response can be investigated.
One material class of interest are strongly disordered superconductors, and I will discuss our results on MoN thin films where we could clearly separate coexisting superconducting and metallic charge carriers . The second example is superconducting granular aluminum, which exhibits a dome-shaped phase diagram as a function of grain coupling, including an up to threefold enhancement of the critical temperature compared to bulk aluminum. We found that this “superconducting dome” is caused by a competition of two energy scales that we determined with THz spectroscopy, namely the energy gap of the Cooper pairs and the superfluid stiffness of the superconducting condensate, and our results support the explanation of enhanced superconductivity in terms of the shell effect .
 J. Simmendinger et al., Superconducting energy scales and anomalous dissipative conductivity in thin films of molybdenum nitride, Phys. Rev. B 94, 064506 (2016).
 U. S. Pracht et al., Enhanced Cooper pairing versus suppressed phase coherence shaping the superconducting dome in coupled aluminum nanograins, Phys. Rev. B 93, 100503(R) (2016).