Unconventional superconductivity in Sr₂RuO₄ probed under stressed conditions

The stoichiometric transition metal oxide Sr₂RuO₄ is widely considered a model unconventional superconductor, owing to experimental evidence for strong correlations, and to the emergence of an odd-parity superconducting ground state with transition temperature near 1 K. With additional evidence for time-reversal symmetry breaking came the proposal for a two-component chiral p-wave order parameter, for which there is an expectation for a split superconducting transition when subjected to in-plane uniaxial stress. Recent studies instead revealed a factor 2.5 increase in the transition temperature of stressed samples, motivating us to probe the evolution of the normal and superconducting states using ¹⁷O NMR. Under stressed conditions, the normal state Knight shifts are consistent with tuning the Fermi surface through a van Hove singularity, along with an associated Stoner factor enhancement [1]. Over a broad temperature range, the magnetic response implies a dominant influence of the small energy difference between E_F and that of the van Hove singularity, which can be controlled by strain, and independently by the Zeeman interaction. A reduced superconducting state spin polarization for in-plane magnetic elds, for all strain values studied, rules out the long-considered chiral p-wave state [2]. Although the helical state is an unlikely possibility under stressed conditions, further tightening of constraints on the superconducting state is a priority.
[1] Y. Luo, et al., Phys. Rev. X 9, 021044 (2019).
[2] A. Pustogow, Nature 574, 72{75 (2019).