Veenstra, C. N.Zhu, Z.-H.Raichle, M.Ludbrook, B. M.Nicolaou, A.Slomski, B.Landolt, G.Kittaka, S.Maeno, Y.Dil, J. H.Elfimov, I. S.Haverkort, M. W.Damascelli, A.2015-06-232015-06-232015-06-23201410.1103/PhysRevLett.112.127002https://infoscience.epfl.ch/handle/20.500.14299/115344WOS:000333920000028Spin-orbit coupling has been conjectured to play a key role in the low-energy electronic structure of Sr2RuO4. By 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 d(xy) and d(xz,yz) orbitals around the Fermi surface, resulting in a strongly momentum-dependent entanglement of spin and orbital character in the electronic wavefunction. As demonstrated by the spin expectation value <(S-k) over arrow.(S-k) over arrow > calculated for a pair of electrons with zero total momentum, the classification of the Cooper pairs in terms of pure singlets or triplets fundamentally breaks down, necessitating a description of the unconventional superconducting state of Sr2RuO4 in terms of these newly found spin-orbital entangled eigenstates.text::journal::journal article::research article