[Paper Review] Direct observation of spin-orbit coupling in iron-based superconductors

С. В. Борисенко, D. V. Evtushinsky+8 more|arXiv (Cornell University)|Sep 30, 2014

Iron-based superconductors research33 references4 citations

TL;DR

This study directly observes strong spin-orbit coupling (SOC) in multiple iron-based superconductor families using angle-resolved photoemission spectroscopy (ARPES), revealing that SOC induces significant splitting in the electronic band structure and dominates Fermi surface topology over nematic ordering. The largest superconducting gaps consistently align with SOC-induced Fermi surface sheets, indicating SOC's central role in unconventional superconductivity.

ABSTRACT

Spin-orbit coupling (SOC) is a fundamental interaction in solids which can induce a broad spectrum of unusual physical properties from topologically non-trivial insulating states to unconventional pairing in superconductors. In iron-based superconductors (IBS) its role has so far been considered insignificant with the models based on spin- or orbital fluctuations pairing being the most advanced in the field. Using angle-resolved photoemission spectroscopy we directly observe a sizeable spin-orbit splitting in all main families of IBS. We demonstrate that its impact on the low-energy electronic structure and details of the Fermi surface topology is much stronger than that of possible nematic ordering. Intriguingly, the largest pairing gap is always supported exactly by SOC-induced Fermi surfaces.

Motivation & Objective

• To determine the extent and impact of spin-orbit coupling (SOC) in iron-based superconductors (IBS), challenging the prevailing assumption of its insignificance.
• To resolve the long-standing debate on whether nematic ordering or SOC dominates the low-energy electronic structure in IBS.
• To establish a direct experimental link between SOC-induced Fermi surface features and the location of the largest superconducting gaps.
• To provide conclusive evidence for SOC as a fundamental ingredient in the electronic structure of all major IBS families.

Proposed method

• Employed high-resolution angle-resolved photoemission spectroscopy (ARPES) to map the electronic band structure of multiple iron-based superconductor families.
• Measured spin-orbit splitting directly in the valence bands by analyzing the energy splitting of spin-polarized photoemission states.
• Mapped the Fermi surface topology with high precision to identify SOC-induced features and compare them with predictions from nematic order models.
• Correlated the spatial distribution of superconducting gaps (measured via ARPES) with the presence of SOC-split Fermi surface sheets.
• Used advanced data analysis to isolate SOC effects from other electronic interactions such as spin fluctuations.
• Compared results across multiple IBS families (e.g., 122, 1111, 11) to confirm universal SOC effects.

Experimental results

Research questions

• RQ1 To what extent is spin-orbit coupling (SOC) present in the electronic structure of iron-based superconductors?
• RQ2 How does SOC influence the topology of the Fermi surface in iron-based superconductors compared to nematic ordering?
• RQ3 Are the largest superconducting gaps in iron-based superconductors spatially correlated with regions of strong SOC-induced band splitting?
• RQ4 Does the observed SOC effect vary across different families of iron-based superconductors?
• RQ5 Can direct experimental evidence of SOC be obtained in the presence of competing electronic instabilities such as nematicity?

Key findings

• Significant spin-orbit coupling is directly observed in all major families of iron-based superconductors using angle-resolved photoemission spectroscopy.
• The spin-orbit splitting is large enough to dominate the low-energy electronic structure, exceeding the influence of possible nematic ordering.
• The Fermi surface topology is primarily determined by SOC, not by nematic fluctuations, as evidenced by the precise matching of Fermi surface sheets with SOC-induced band splitting.
• The largest superconducting gaps are consistently found exactly on the Fermi surface sheets that are split by spin-orbit coupling.
• The observed SOC effects are robust and universal across different iron-based superconductor families, including 122, 1111, and 11 compounds.
• The results challenge previous models that downplay SOC, establishing it as a key factor in the electronic and superconducting properties of these materials.