Spin-orbital entanglement and the breakdown of singlets and triplets in Sr2RuO4 revealed by spin- and angle-resolved photoemission spectroscopy.
Spin-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 dxy and dxz,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 ⟨sk⃗·s-k⃗⟩ 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.
Department of Physics & Astronomy, University of British Columbia, Vancouver, British Columbia V6T 1Z1, Canada.
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Veenstra
Department of Physics & Astronomy, University of British Columbia, Vancouver, British Columbia V6T 1Z1, Canada.
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Zhu
Department of Physics & Astronomy, University of British Columbia, Vancouver, British Columbia V6T 1Z1, Canada.
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Raichle
Department of Physics & Astronomy, University of British Columbia, Vancouver, British Columbia V6T 1Z1, Canada.
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Ludbrook
Department of Physics & Astronomy, University of British Columbia, Vancouver, British Columbia V6T 1Z1, Canada and Quantum Matter Institute, University of British Columbia, Vancouver, British Columbia V6T 1Z4, Canada and Max Planck Institute for Solid State Research, Heisenbergstraße 1, 70569 Stuttgart, Germany.
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Nicolaou
Physik-Institut, Winterthurerstrasse 190, Universitat Zürich-Irchel, CH-8057 Zürich, Switzerland and Swiss Light Source, Paul Scherrer Institut, CH-5232 Villigen PSI, Switzerland.
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Slomski
Physik-Institut, Winterthurerstrasse 190, Universitat Zürich-Irchel, CH-8057 Zürich, Switzerland and Swiss Light Source, Paul Scherrer Institut, CH-5232 Villigen PSI, Switzerland.
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Landolt
Department of Physics, Graduate School of Science, Kyoto University, Kyoto 606-8502, Japan and Institute for Solid State Physics, University of Tokyo, Kashiwa, Chiba 277-8581, Japan.
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Kittaka
Department of Physics, Graduate School of Science, Kyoto University, Kyoto 606-8502, Japan.
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Maeno
Physik-Institut, Winterthurerstrasse 190, Universitat Zürich-Irchel, CH-8057 Zürich, Switzerland and Swiss Light Source, Paul Scherrer Institut, CH-5232 Villigen PSI, Switzerland.
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Dil
Department of Physics & Astronomy, University of British Columbia, Vancouver, British Columbia V6T 1Z1, Canada and Quantum Matter Institute, University of British Columbia, Vancouver, British Columbia V6T 1Z4, Canada.
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Elfimov
Department of Physics & Astronomy, University of British Columbia, Vancouver, British Columbia V6T 1Z1, Canada and Quantum Matter Institute, University of British Columbia, Vancouver, British Columbia V6T 1Z4, Canada and Max Planck Institute for Solid State Research, Heisenbergstraße 1, 70569 Stuttgart, Germany.
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Haverkort
Department of Physics & Astronomy, University of British Columbia, Vancouver, British Columbia V6T 1Z1, Canada and Quantum Matter Institute, University of British Columbia, Vancouver, British Columbia V6T 1Z4, Canada.
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Damascelli