Superconductivity occurs due to condensation of paired electrons (Cooper pairs) into a superfluid, and it is found in numerous metals that are cooled to low temperatures. If a superconducting material contains strong disorder (e.g. in amorphous metals), then rather unconventional behavior can be found. The most profound new state of matter in this context would be localized Cooper pairs that constitute a so-called Bose insulator: the electrons are bound as bosonic Cooper pairs, but they do not form a superfluid condensate, but instead are electrically insulating.
We want to use low-frequency optics to study the unconventional physics that can occur in strongly disordered are granular superconducting thin films. Broadband microwave spectroscopy is particularly suited to probe fundamental material quantities such as the superfluid stiffness, but also collective excitations of Cooper pairs that can only be probed in superconductors that are characterized by their strong disorder.