Evaluation of chiral superconductivity in Sr2RuO4

Clifford Hicks

Several previous experimental results give evidence that the superconductivity of Sr2RuO4 is chiral. These include measurements of the Kerr effect, critical currents across junctions between Sr2RuO4 and conventional superconductors, sound velocities, and muon spin relaxation. Through recent NMR Knight shift measurements it is now understood that the pairing is most likely spin-singlet [1,2], and on the tetragonal lattice of Sr2RuO4 the combination of singlet pairing and chirality compels consideration of an unlikely order parameter: dxz ± idyz. It is unlikely because it has a horizontal line noad at kz=0, and Sr2RuO4 has a very low c-axis conductivity. Therefore, a firm determination of whether or not the superconductivity of Sr2RuO4 is chiral is highly important, as it may imply a new form of pairing. Here, I present evidence from experiments under uniaxial stress applied along the [100] direction. Tc rises rapidly [3]. By lifting the tetragonal symmetry of the lattice, uniaxial stress is expected to induce a splitting between Tc and the onset temperature of chirality, TTRSB. In muon spin rotation experiments under uniaxial stress, a large splitting is observed: TTRSB is found to remain low while Tc i ncreases [4]. However, in heat capacity measurements, no second heat capacity anomaly is observed; the upper limit on any heat capacity anomaly at TTRSB is ~5% of that at Tc [5]. In scanning SQUID magnetometry studies [6], a cusp in the strain dependence of Tc, implied by transition splitting, is not observed. In this talk I will discuss possibilities to reconcile these results.

[1] A. Pustogow et al, arXiv 1904.090047 (2019)
[2] K. Ishida, M. Manago, and Y. Maeno, arXiv 1907.12236 (2019)
[3] A. Steppke et al, Science 355, 148 (2017)
[4] V. Grinenko et al, arXiv 2001.08152 (2020)
[5] Y.-S. Li et al, arXiv 1906.07597 (2019)
[6] C. A. Watson, A. S. Gibbs, A. P. Mackenzie, C. W. Hicks, and K. A. Moler, Phys. Rev. B 98, 094521 (2018)