“Spin‑Orbit Coupling and Electronic Structure of the Locally Noncentrosymmetric Superconductor SrPtAs”

We present an angle‑resolved photoemission spectroscopy (ARPES) study of the electronic structure of the locally noncentrosymmetric superconductor SrPtAs, supported by fully relativistic density functional theory (DFT) calculations. The measured Fermi‑surface topology and band dispersions are in good overall agreement with theory, with systematic shifts of the Fermi wavevectors indicating a rigid chemical‑potential offset. The extracted Fermi velocities show only moderate deviations from DFT, pointing to weak correlation‑induced band renormalisation.

A key result of this work is the direct observation of pronounced spin‑orbit‑coupling‑induced band splitting at the Fermi level. The measured splittings reach several hundred meV, demonstrating that the low‑energy electronic states of SrPtAs are strongly spin–orbit coupled. Despite the dominance of spin‑orbit coupling over the superconducting gap scale, the upper critical fields remain well below the Pauli limit. This indicates that, while spin‑orbit coupling strongly shapes the normal‑state electronic structure, it does not suppress paramagnetic pair breaking and provides no evidence for strong parity mixing in the superconducting state. Our results place important constraints on superconducting pairing scenarios in SrPtAs and related locally noncentrosymmetric materials.