Competing energy scales in granular superconductors: phase-governed superfluid and pseudogap determine the superconducting dome
Final Report Abstract
Granular aluminum, i.e. nm-sized pure aluminum grains closely coupled through aluminum oxide barriers, exhibits peculiar superconducting properties that are not fully understood. In particular, the critical temperature Tc can be enhanced by more than a factor of two compared to pure bulk aluminum (with Tc = 1.2 K) in a phase diagram exhibiting a so-called superconducting dome. In this project we have implemented and optimized a dedicated deposition system using thermal evaporation for the growth of granular aluminum films in controlled oxygen environment and at cryogenic temperatures. We have grown numerous granular aluminum samples at substrate temperatures around 300 K (room temperature), 150 K and 100 K (nitrogen cooling) and 30 K (helium cooling). For these substrate temperatures we find superconducting domes that become higher and more pronounced for reduced substrate temperature. For the case of helium cooling, we find Tc larger than 3 K for the top of the superconducting dome. We have performed broadband microwave spectroscopy studies on these various samples, using Corbino reflectometry for temperatures down to 1.1 K. The oxide covering the granular aluminum films requires detailed modeling of the contact resistance for proper data analysis. Here we developed a new strategy based on spectra extending to very low frequency. This allowed us to determine the kinetic inductance of the Corbino samples, which is directly related to the superfluid density. Furthermore, we fabricated planar GHz resonators from granular aluminum films and studied them down to mK temperatures and in the low-photon regime. This connects more directly our fundamental study of material properties to possible applications of superconductors with high kinetic inductance. For the electrodynamics at somewhat higher frequencies, around the superconducting energy gap, we employ THz spectroscopy. To face the challenge of characteristic energies below the low-frequency and low-temperature limits of conventional THz experimentation, we have designed and installed a dedicated new cryostat for THz studies at temperatures below 1.5 K.
Publications
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Characterizing dielectric properties of ultra-thin films using superconducting coplanar microwave resonators. Review of Scientific Instruments, 90(11).
Ebensperger, Nikolaj G.; Ferdinand, Benedikt; Koelle, Dieter; Kleiner, Reinhold; Dressel, Martin & Scheffler, Marc
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Characterization of harmonic modes and parasitic resonances in multi-mode superconducting coplanar resonators. AIP Advances, 13(10).
Beydeda, Cenk; Nikolaou, Konstantin; Tochtermann, Marius; Ebensperger, Nikolaj G.; Untereiner, Gabriele; Farag, Ahmed; Karl, Philipp; Ubl, Monika; Giessen, Harald; Dressel, Martin & Scheffler, Marc
