Demystifying the growth of superconducting Sr2RuO4 thin films*

Sr₂RuO₄ is an unconventional superconductor with potentially a spin-triplet, odd-parity superconducting ground state. There are many reports of high purity single crystals of Sr₂RuO₄ with a T_c onset of up to 1.5 K. Furthermore, recent studies have shown that the T_c can be further increased up to 3.5 K using uniaxial strain. To date, however, there are only two published reports of superconducting Sr₂RuO₄ thin films. This relative paucity of superconducting thin films is likely due to the extreme sensitivity of the odd-parity superconducting ground state in Sr₂RuO₄ to disorder, with the dominant defect in films being out-of-phase boundaries. According to recent theoretical predictions biaxially strained epitaxial thin films with isotropic in-plane strain can potentially maintain the topologically nontrivial p_x ± ip_y superconducting ground state while simultaneously enhancing T_c by tuning the Fermi level towards a van Hove singularity. Thin films also provide a pathway for scalability, which is critical for potential practical applications of spin-triplet superconductors such as qubits for ground-state quantum computing. In this talk I outline and demonstrate a thermodynamic growth window to achieve repeatable growth of superconducting Sr₂RuO₄ thin films using molecular-beam epitaxy. I hope that identifying this growth window and demystifying the growth process will enable superconducting Sr₂RuO₄ thin films to be routinely grown by many groups. This would enable widespread studies into this extremely interesting unconventional superconductor.