Upper critical field and fluctuation conductivity in Nb-doped strontium titanate thin films

Abstract

We show that the transition temperature ${T}_{c}$ of Nb-doped strontium titanate thin films grown in vacuum by pulsed laser deposition is similar to bulk and that these films follow a similar dependence of ${T}_{c}$ on the carrier density as bulk. We have measured the temperature dependence of the upper critical field ${H}_{c2}$ in thin films. We fit the data to the empirical temperature dependence ${H}_{c2}\ensuremath{\propto}(1\ensuremath{-}{t}^{2}{)/(1+t}^{2}),$ where ${t=T/T}_{c},$ and determined the electronic diffusion constant from the slope of ${\mathrm{dH}}_{c2}/dT$ near ${T}_{c}.$ We further fit the temperature dependence of the fluctuation conductivity above ${T}_{c}$ successfully to the theories of Aslamazov-Larkin and Maki-Thompson.