Light tunable superconducting devices using transition metal oxides

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Superconductors and superconducting devices have a range of fascinating properties that makes them highly suitable for novel applications. While many metals are superconducting at low temperatures, low density semiconductors typically are not. This limits the device architectures that can be implemented using superconducting materials. The d-electron oxide SrTiO3 is a special material since when electron doped it is superconducting at very low carrier densities, and is simultaneously a high mobility semiconductor. The fundamental limits of superconductivity can therefore be investigated in this material, as well as the possibilities of creating novel devices. A key limitation in chemical doping of SrTiO3, the traditional method of inducing carriers in this system, is that the superconductivity and conductivity collapses in the low density limit, most likely due to inhomogeneous doping. This proposal will develop a light-induced superconducting device architecture to overcome this problem. We will utilize the strong photoconducting response of SrTiO3 at low temperatures as the active element, in combination with the proximity effect with elemental superconductors. It is known that photo-induced electron-hole pairs in SrTiO3 show high electron mobility at low temperatures, with the holes far less mobile. Because of the spatially uniform illumination, it has also been established that we can maintain conductivity to lower densities than for chemical doping. The light wavelength and intensity are powerful tuning parameters: we can continuously tune not only the total number of electrons in the system, but also the local three-dimensional density, exploiting the wavelength dependent absorption co-efficient of SrTiO3 near to the band-gap. Using these techniques this work will investigate the limits of inducing superconducting correlations over a wide range of carrier densities and mobilities. These continuously tunable light-induced Josephson junctions will be used as a foundation for studies of the two-dimensional superconductor-to-insulator transition in artificial systems, allowing fundamental questions about the nature of phase coherence in low density superconductors to be addressed.