Fig. 2: Structural characterization and electrical transport measurements of epitaxially strained RuO₂ thin films.

a Schematic diagram of the crystal structure and in-plane lattice mismatch of an RuO₂ thin film and the underlying TiO₂(100) substrate. Gray and red spheres represent ruthenium and oxygen atoms, respectively. b AFM image (top panel) and RHEED pattern (bottom panel) with the electron beam incident along the [010] azimuth of the RuO₂ film and TiO₂ substrate of the 9 nm thick RuO₂(100) film. c θ-2θ X-ray diffraction scan acquired with Cu-Kα radiation for the same 9 nm thick RuO₂(100) film. The Bragg peak arising from the TiO₂ substrate is marked with an asterisk, and the peak positions corresponding to unstrained bulk RuO₂ and commensurately strained RuO₂(100) film are indicated by black and green dashed lines, respectively. d Resistivity vs. temperature curves for a bulk RuO₂ single crystal from ref. ²⁵, 18.6 nm thick RuO₂(101) film from ref. ²⁶, 24.2 nm thick RuO₂(110) film from ref. ²⁶, and a 42 nm thick RuO₂(100) film. e Resistance vs. temperature curves of RuO₂(100) films with different film thicknesses, normalized to their values at 4 K. The inset shows the value of the superconducting T_c (purple spheres) as a function of the film thickness (t). The non-superconducting films are presented with green spheres. f Resistivity vs. temperature curves acquired at different applied out-of-plane magnetic field for the 42 nm thick RuO₂(100) sample. g Upper critical magnetic field H_c⊥ vs. superconducting T_cs extracted from measurements in f. The superconducting T_cs are defined as the temperatures at which the resistance crosses 50% of the normal-state resistance at 4 K.