Breakdown of singlets and triplets in Sr2RuO4 revealed by spin-resolved ARPES

TL;DR

This study uses spin-resolved ARPES with circularly polarized light to measure the strong spin-orbit coupling in Sr2RuO4, revealing complex spin-orbital entanglement that challenges traditional pairing classifications in its superconducting state.

Contribution

It provides the first direct measurement of the effective spin-orbit coupling in Sr2RuO4 and demonstrates the breakdown of pure singlet or triplet pairing classifications due to spin-orbital entanglement.

Findings

Effective spin-orbit coupling is 130 +/- 30 meV.

Spin and orbital characters are strongly entangled.

Traditional singlet/triplet pairing classification is inadequate.

Abstract

Spin-orbit coupling has been conjectured to play a key role in the low-energy electronic structure of Sr2RuO4. 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. As demonstrated by the spin expectation value obtained 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.