Superconducting Sr₂RuO₄ thin films with record transition temperature up to 2 K by molecular-beam epitaxy

Clarifying the superconducting order parameter of Sr₂RuO₄ has remained an enigma despite more than two decades of research on high-quality Sr₂RuO₄ single crystals.^[1] High-quality Sr₂RuO₄ thin films with robust superconductivity enable an alternative approach toto clarifying its superconducting through phase-sensitive experiments, as well as fabricating quantum circuits. Unfortunately, the growth of reproducible high-quality superconducting Sr₂RuO₄ thin films has been limited due to its extreme sensitivity to structural disorder and narrow growth window.^[2]

In this work, we grew superconducting Sr₂RuO₄ thin films on NdGaO₃ (110) substrates by molecular-beam epitaxy (MBE) in the adsorption-controlled regime.^[2] We characterized the Sr₂RuO₄ thin films by x-ray diffraction (XRD), atomic force microscopy (AFM), scanning transmission electron microscopy (STEM), and low-temperature transport. Our optimized Sr₂RuO₄ thin films show a superconducting transition temperature (midpoint) as high as 2.1 K and residual resistivity ratio as 122; both are the best values ever reported. We also clarified that the quality of superconductivity in Sr₂RuO₄ thin film was highly related to the microstructure measured by STEM.



References

[1] Y. Maeno et al., Nature 372, 532 (1994)

[2] H. P. Nair et al., APL Materials 6, 101108 (2018)