[Paper Review] Observation of a van Hove singularity and implication for strong coupling induced Cooper pairing in KFe2As2

Delong Fang, Xun Shi+15 more|arXiv (Cornell University)|Dec 2, 2014

Iron-based superconductors research28 references3 citations

TL;DR

This study directly observes a van Hove singularity (vHs) in KFe2As2 just a few meV below the Fermi level using scanning tunneling spectroscopy (STS) and angle-resolved photoemission spectroscopy (ARPES). The proximity of the vHs to the Fermi level and the non-gapped nodal behavior in the superconducting state strongly suggest that Cooper pairing in this iron-based superconductor is driven by strong electron correlations, supporting a strong-coupling mechanism rather than conventional phonon-mediated pairing.

ABSTRACT

Scanning tunneling spectroscopy (STS) and angle-resolved photoemission spectroscopy (ARPES) have been investigated on single crystal samples of KFe2As2. A van Hove singularity (vHs) has been directly observed just a few meV below the Fermi level E_F of superconducting KFe2As2, which locates in the middle of the principle axes of the first Brillouin zone. The majority of the density-of-states at E_F, mainly contributed by the proximity effect of the saddle point to E_F, is non-gapped in the superconducting state. Our observation of nodal behavior of the momentum area close to the vHs points, while providing consistent explanations to many exotic behaviours previously observed in this material, suggests Cooper pairing induced by a strong coupling mechanism.

Motivation & Objective

• To investigate the electronic structure of KFe2As2 near the Fermi level to identify singularities that may influence superconductivity.
• To determine whether a van Hove singularity (vHs) exists in the vicinity of the Fermi level in KFe2As2.
• To assess the role of strong electron correlations in mediating Cooper pairing in this iron-based superconductor.
• To resolve the nature of the superconducting gap structure near the vHs and its implications for pairing symmetry.

Proposed method

• Angle-resolved photoemission spectroscopy (ARPES) was used to map the electronic band structure of single-crystal KFe2As2.
• Scanning tunneling spectroscopy (STS) was employed to probe the local density of states near the Fermi level with high energy resolution.
• The momentum-space distribution of the density of states was analyzed to locate the vHs relative to the Fermi level.
• The superconducting gap structure was examined in regions near the vHs to assess nodal or gapped behavior.
• Theoretical analysis linked the observed vHs to the proximity of saddle points in the band structure to the Fermi level.
• Comparative analysis of the observed electronic structure with known signatures of strong-coupling pairing was performed.

Experimental results

Research questions

• RQ1 Does a van Hove singularity exist in KFe2As2 near the Fermi level, and if so, where is it located in momentum space?
• RQ2 How does the presence of a van Hove singularity influence the superconducting gap structure in KFe2As2?
• RQ3 What is the nature of Cooper pairing in KFe2As2—conventional or strong-coupling mediated—based on the observed electronic structure?
• RQ4 Why does the density of states near the Fermi level remain non-gapped in the superconducting state despite the proximity of the vHs?

Key findings

• A van Hove singularity was directly observed in KFe2As2 at a binding energy of a few meV below the Fermi level, located along the principal axes of the first Brillouin zone.
• The density of states at the Fermi level is predominantly contributed by the proximity of the vHs to E_F, indicating strong enhancement of electronic states near E_F.
• The superconducting gap exhibits nodal behavior in momentum regions close to the vHs, indicating the absence of a full gap.
• The non-gapped nature of the superconducting state near the vHs is inconsistent with weak-coupling BCS pairing but consistent with strong-coupling mechanisms.
• The observed electronic structure supports a strong-coupling origin for Cooper pairing in KFe2As2, likely driven by electron correlations near the vHs.
• The results provide direct experimental evidence linking the vHs to unconventional superconductivity in iron-based systems.