PhD Enhanced superconductivity in granular aluminum and applications for quantum circuits

Bulk aluminum is a superconductor with critical temperature T_c=1.2K. But if one grows granular aluminum thin films, consisting of nanometer-size aluminum grains with a thin oxide shell, then Tc can be enhanced up to a factor of three. Furthermore, T_c depends on the thickness of the oxide shell in a non-monotonic fashion (“superconducting dome”) due to the interplay of different relevant energy scales.

Main goal of this PhD project is to investigate a further enhancement of superconductivity in granular aluminum by growing the films on cryogenically cooled substrates, which should reduce the grain size. The films will be grown in a dedicated deposition system, and extending the accessible substrate temperature range towards lower temperatures will be part of this project. Combination of dc characterization, GHz and THz spectroscopy will allow us to relate the different superconducting energy scales and to evaluate the dependence of the superconducting dome of granular aluminum on the substrate temperature during growth.

Furthermore, we will investigate unconventional electronic states that occur in granular aluminum and that are caused by the peculiar type of superconductivity, in particular a pseudogap regime that might be due to localized, uncondensed Cooper pairs. Using THz spectroscopy, we will also investigate collective modes in the superconducting state. Finally, we will study to which extent cryogenically grown granular aluminum offers material properties that are superior compared to other superconductors for the application in quantum circuits, e.g. for superconducting quantum computation.

Uwe S. Pracht, Nimrod Bachar, Lara Benfatto, Guy Deutscher, Eli Farber, Martin Dressel, Marc Scheffler
Enhanced Cooper pairing versus suppressed phase coherence shaping the superconducting dome in coupled aluminum nanograins
Phys. Rev. B 93, 100503(R) (2016)

Uwe S. Pracht, Tommaso Cea, Nimrod Bachar, Guy Deutscher, Eli Farber, Martin Dressel, Marc Scheffler, Claudio Castellani, Antonio M. García-García, Lara Benfatto
Optical signatures of the superconducting Goldstone mode in granular aluminum: Experiments and theory
Phys. Rev. B 96, 094514 (2017).