Observation of strong spin-orbital entanglement in Sr2RuO4 by spin-resolved ARPES
After a flurry of experimental activity, Sr2RuO4 has become a hallmark candidate for spin-triplet chiral p-wave superconductivity, the electronic analogue of superfluid 3He. However, despite the apparent existence of such a pairing, some later experiments do not fully support this conclusion, as they cannot be explained within a theoretical model using spin-triplet superconductivity alone. A resolution might come from the inclusion of spin-orbit coupling, which has been conjectured to play a key role in the low energy electronic structure. Using circularly polarized light combined with spin- and angle-resolved photoemission spectroscopy, we directly measure the value of the effective spin-orbit coupling to be 130 +/- 30 meV. This is even larger than theoretically predicted and comparable to the energy splitting of the dxy and dxz,dyz orbitals around the Fermi surface, resulting in a strongly momentum-dependent entanglement of spin and orbital character. As a consequence, the classification of the Cooper pairs in terms of singlets or triplets fundamentally breaks down, necessitating a description of the unconventional superconducting state of Sr2RuO4 beyond these pure spin eigenstates.