Discovery of Orbital-Selective Cooper Pairing in FeSe

Abstract: The superconductor FeSe is of intense interest thanks to its unusual non-magnetic nematic state and potential for high temperature superconductivity. But its Cooper pairing mechanism has not been determined. Here we use Bogoliubov quasiparticle interference imaging to determine the Fermi surface geometry of the bands surrounding the $\Gamma = (0,0)$ and $X=(\pi / a_{Fe}, 0)$ points of FeSe, and to measure the corresponding superconducting energy gaps. We show that both gaps are extremely anisotropic but nodeless, and exhibit gap maxima oriented orthogonally in momentum space. Moreover, by implementing a novel technique we demonstrate that these gaps have opposite sign with respect to each other. This complex gap configuration reveals the existence of orbital-selective Cooper pairing which, in FeSe, is based preferentially on electrons from the $d_{yz}$ orbitals of the iron atoms.

• The paper reveals that orbital-selective Cooper pairing in FeSe leads to anisotropic and sign-changing superconducting gaps observed with precise BQPI imaging.
• Using BQPI, the study quantifies superconducting gaps (≈2.3 meV and ≈1.5 meV) with ratios exceeding 15, underscoring strong orbital differentiation.
• The research highlights the role of dyz orbital electrons and electronic correlations in driving unconventional superconductivity and refining theoretical models.

Orbital-Selective Cooper Pairing in FeSe: Insights and Implications

The paper "Discovery of Orbital-Selective Cooper Pairing in FeSe" makes significant contributions to the understanding of superconductivity in iron-based materials, specifically focusing on the superconductor FeSe. This material is notable for its unusual non-magnetic nematic state and the absence of a magnetically ordered state, which distinguishes it from other iron-based superconductors. The investigation reveals a unique feature in the superconducting pairing mechanism: orbital-selective Cooper pairing.

Key Findings

The study harnesses Bogoliubov quasiparticle interference (BQPI) imaging to map the Fermi surface and superconducting gaps in FeSe with high precision, identifying highly anisotropic, nodeless energy gaps. A standout finding is the revelation that these gaps exhibit opposite signs and orientation, suggesting an underlying mechanism of orbital-selective Cooper pairing driven preferentially by electrons from the dyz orbitals of iron atoms.

Experimental Techniques and Results

1. BQPI Imaging: This technique allowed for the high-precision measurement of the superconducting energy gaps on the Fermi surfaces surrounding the Γ=(0,0) and X=(π/a, 0) points. The resolution and accuracy achieved by BQPI were critical in confirming the anisotropic nature and sign inversion of the gaps.
2. Energy Gap Analysis: The energy gaps on the α- and ε-bands show orthogonal orientations in momentum space, with Δα_max ≈ 2.3 meV and Δε_max ≈ 1.5 meV. The anisotropy was significant, with ratios Δmax/Δmin exceeding 15, yet the system remains fully gapped.
3. Sign Reversal: Through phase-resolved BQPI, the study presents evidence that the superconducting energy gaps have opposite signs (± pairing symmetry), a characteristic atypical for iron-based superconductors with an orthorhombic crystal structure.

Theoretical Insights and Implications

1. Orbital-Selective Pairing: This phenomenon suggests a type of superconductivity where Cooper pairing affects only electrons in specific orbital channels, in this case, primarily those with dyz character. This could explain the unusual anisotropic gap structure observed, as orbital-selective pairing tends to align with the dominant orbital character at specific Fermi surface regions.
2. Influence of Electronic Correlations: The study refers to Hund's coupling and the resulting electronic environments which differ significantly among various orbitals, potentially leading to distinct correlation strengths. Such conditions can give rise to suppressed inter-orbital fluctuations and promote orbital-selective Cooper pairing.
3. Modeling and Simulations: Adjusting correlation and pairing parameters to account for orbital selectivity yielded theoretical models that matched experimental observations. These models highlight the substantial impact of orbital character on the superconductive properties of FeSe, accentuating the role of orbital-selective correlations.

Future Directions

This research opens avenues for developing new theoretical models for Cooper pairing mechanisms in iron-based and possibly other unconventional superconductors. The implications of orbital-selective pairing might extend beyond FeSe, warranting further studies into other materials exhibiting similar orbital-specific interactions. The ability to experimentally discern sign differences in superconducting gaps also provides a novel approach for exploring the pairing symmetries in other complex systems.

In conclusion, this work significantly enriches our understanding of superconductivity in FeSe by not only identifying a highly anisotropic gap structure but also by presenting compelling evidence of orbital-selective Cooper pairing. These insights may pave the way for deeper explorations into the interplay between crystal structure, electronic correlations, and superconductivity, potentially leading to the discovery of novel superconducting phases and higher critical temperatures.