Order parameter of the superconductor $\mathrm{Sr_2RuO_4}$

4 problems

• Location of gap nodes on the $\mathrm{Sr_2RuO_4}$ Fermi sheets

  Partially resolved

  Heat-transport data indicate lines on the Fermi surface where the superconducting gap vanishes, and tunneling maps place them along the diagonals on two of the three sheets; whether these are true zeros and what happens on the third sheet is not known.

• What is the superconducting order parameter of $\mathrm{Sr_2RuO_4}$?

  Contested

  The pairing pattern of $\mathrm{Sr_2RuO_4}$ must explain conflicting data: some measurements suggest a two-part order that breaks time-reversal symmetry, while others see only one transition.

• Is time-reversal breaking in $\mathrm{Sr_2RuO_4}$ an intrinsic bulk property?

  Contested

  Muon and optical experiments see signs that the superconducting state of $\mathrm{Sr_2RuO_4}$ picks a sense of rotation, breaking time-reversal symmetry, but the signal may come from defects.

• Is the $\mathrm{Sr_2RuO_4}$ order parameter two-component?

  Contested

  If the order parameter has two parts, squeezing the crystal along one axis should split the superconducting transition into two; one probe sees a split and another does not.