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Ruthenates are an intensively investigated class of materials because of their wide variety of ground states which include superconductivity in Sr₂RuO₄, metamagnetism in Sr₃Ru₂O₇, ferromagnetism in SrRuO₃ and antiferromagnetism in Ca₂RuO₄, etc. [1-2]. Epitaxial strain has always been a key knob to tune these ground states and realize new functionalities in the Ruddlesden-Popper ruthenates when grown in thin-film form [3]. Recently, RuO₂, the parent binary oxide of these ruthenates has attracted attention because of the discoveries of antiferromagnetism [4,5], and superconductivity [6] in this material. Here, we employ molecular-beam epitaxy to grow thin films of RuO₂ on isostructural (100)-oriented TiO₂ substrates and realize superconductivity at about 1 K. We show that the superconducting transition temperature (T_c) and the nature of the superconducting dip changes as a function of the thickness of the films. In contrast to the strain-induced superconductivity of RuO₂ grown on (110)-oriented TiO₂ [6], our relaxed, thicker films of RuO₂/(100) TiO₂ have higher T_c than the coherently strained thinner RuO₂/(100) TiO₂ films. 

 

[1] Y. Maeno, et al., J. Phys. Soc. Jpn. 81, 011009 (2012).

[2] J. M. Longo, et al., J. Appl. Phys. 39, 1327 (1968).

[3] B. Burganov, et al., Phys. Rev. Lett. 116, 197003 (2016).

[4] Berlijn T. et al, Phys. Rev. Lett. 118, 2 (2017).

[5] Zhu, Z. H. et al , Phys. Rev. Lett. 122, 17202 (2019).

[6] Ruf, J. P. et al, Nat. Commun. 12, 59 (2021).