Growth and transport properties of superconducting strontium titanate thin films

We have grown metallic Nb-doped SrTiO3 thin films on LaAlO 3 substrates using pulsed laser deposition (PLD) and made the first observation of superconductivity in SrTiO3 thin films. We then investigated the superconducting state to test theories of the Bose-BCS crossover and two-band superconductivity. Doped SrTiO3 was the first metal oxide discovered to be a superconductor in 1964. High-temperature superconductivity was subsequently found in metal oxides in 1986. In the last ten years many new superconductors have been identified. Phenomenologically the high- Tc superconductors (HTSC) and the other new superconductors have the remarkable property Tc ∝ TF, where TF is the Fermi temperature. They have collectively become known as 'exotic' superconductors. Their very short coherence lengths have revived theories which attempt to describe superconductivity in terms of a Bose-Einstein condensation. Underdoped SrTiO3 may occupy a position near a Bose-BCS crossover, and it was suggested that experiments on SrTiO3 may be elucidating. Also early tunneling experiments on SrTiO3 showed two conductance peaks which were interpreted as evidence for two-band superconductivity. This observation and the Bose-BCS crossover are of fundamental interest. In this thesis I will first describe the growth of metallic Nb-doped SrTiO3 thin films by PLD and measurements of the transport properties of the normal state from 300 to 4 K. Inside our dilution refrigerator we found the superconducting transitions and determined the phase boundary of superconducting SrTiO 3 films. We then measured the fluctuation conductivity in zero field and show excellent fits to theory. We also interpret magnetoconductance data with existing theories. In our films superconductivity appears BCS-like and we find no evidence for two-band superconductivity. We will argue that carrier depletion of thin films may allow new experiments for investigations of the superconducting state and the metal-insulator transition. In addition depletion is important for HTSC device technology, where SrTiO3 has been used extensively as a growth substrate.;This research was supported by the Office of Naval Research.