Why the surface of Sr₂RuO₄ is not superconducting

The superconducting order parameter of Sr₂RuO₄ still remains a puzzle after more than twenty years of research, with a wide range of order parameters having been proposed. One of the techniques which can provide direct phase-sensitive information on the superconducting order parameter, and therefore be used for detecting its symmetry, is phase-referenced quasi-particle interference imaging [1,2]. Using quasi-particle interference (QPI), not only the momentum-space structure of the superconducting gap, but also its phase can be detected.

Sr₂RuO₄ is seemingly an ideal material to apply this technique to, with a very two-dimensional electronic structure and cleaving resulting in large, atomically clean terraces[3]. Despite this, evidence for the symmetry of the order parameter from STM has remained inconclusive, and in fact in most measurements, no gap is seen.

In this talk, I will show how we can constrain the electronic structure of Sr₂RuO₄ from quasi-particle interference[3,4,5], and then what we can learn about the superconductivity when calculating the superconducting pairing strength from functional renormalization group calculations[6], demonstrating that indeed from these calculations a complete suppression of superconductivity at the surface is found. I will show how information about the pairing symmetry could be extracted if it was possible to restore the superconductivity in the surface layer using phase referenced quasi-particle interference.

[1] P.J. Hirschfeld et al., Phys. Rev. B 92, 184513 (2015).

[2] S. Chi et al., Determination of the Superconducting Order Parameter from Defect Bound State Quasiparticle Interference, arxiv/1710.09089 (2017).

[3] C.A. Marques et al., Magnetic-Field Tunable Intertwined Checkerboard Charge Order and Nematicity in the Surface Layer of Sr₂RuO₄. Adv. Mat. 33, 2100593 (2021).

[4] A. Kreisel et al., Quasiparticle Interference of the van-Hove singularity in Sr₂RuO₄, npj Quantum Materials 6, 100 (2021).

[5] A. Chandrasekaran et al., Engineering Higher Order Van Hove singularities in two dimensions, accepted in Nat. Commun., arxiv/2310.15331.

[6] J.B. Profe et al., Magic angle of Sr₂RuO₄: Optimizing correlation-driven superconductivity, Phys. Rev. Res. 6, 043057 (2024).