Competing energy scales in granular superconductors: phase-governed superfluid and pseudogap determine the superconducting dome

点击翻译按钮获取中文摘要

By exploring a well-controlled mesoscopic system we want to advance our understanding of their correlated quantum states. The superconducting properties of granular aluminum, which is conveniently grown as thin films, differ drastically from those of bulk aluminum. In particular there occurs an enhancement of the critical temperature Tc up to a factor of three, exceeding 3 K. The origin for this enhanced Tc remained unclear for decades, but using optical THz spectroscopy we recently showed that the key ingredient is an increase of the superconducting energy gap when the aluminum grains are more and more decoupled due to a thin oxide layer. For yet more decoupled grains, however, the suppressed superfluid stiffness enters as limiting factor for Tc. This interplay of energy gap and superfluid stiffness leads to the characteristic phase diagram ofgranular aluminum that features a broad maximum, the so-called superconducting dome. We also found first signs of unconventional behavior, such as a pseudogap in the optical spectra, on the high resistivity/insulating side of the phase diagram. Now we want to explore the peculiar superconducting properties of high-resistivity granular aluminum in detail by combining THz and microwave spectroscopy. One question that we want to answer is how the energy gap, which is enhanced due to quantum confinement, evolves on the insulating side of the phase diagram as Tc decreases. This concerns the putative superconductor-to-insulator transition as well as thepseudogap regime, which might be caused by localized Cooper pairs that cannot form a superconducting condensate. This particularly interesting part of the phase diagram we can only access by extending our experiments to much lower energies in spectral range as well as temperature. Our present understanding of the extraordinary phase diagram of granular aluminum has as key element the electron confinement on the nm scale, in our case aluminum grains of 2 nm size. Further reduction of grain size should lead to further enhancement of superconducting energy gap and Tc. Therefore, we want to grow granular aluminum thin films by thermal evaporation of aluminum and deposition onto substrates that are cooled to 4He temperatures. Such thin films with even smaller grains will allow us to extend our understanding of superconducting granular aluminum to yet higher energies, and they will also demonstrate that control of structural properties on the nm scale can lead to a deliberate enhancement of superconductivity including a higher Tc. This can then serve as example for other superconducting materials. Therefore, we also want to grow other elemental superconductors as granular films and characterize their energy scales. The results may also prove important in the understanding of novel electronic states in other mesoscopic systems.