Figure 2. Upper critical field as a function of temperature at various doping levels.

Black symbols: SrTi¹⁶O_3−y; red symbols: ¹⁸O-substituted SrTiO_3−y. Panels (a–c) refer to charge carrier densities n ~ 0.004, 0.02, 0.07 nm⁻³, respectively. Open orange triangles and red circles in panel (c) indicate the H_c2 values measured with two mutually perpendicular directions of the applied magnetic field, both being perpendicular to the flowing current. Insets: detail of the conduction bands of SrTiO₃ and Fermi energy at each doping level⁹.