[Paper Review] Observation of Fermi-surface-dependent anisotropic Cooper pairing in kagome superconductor CsV3Sb5

Akifumi Mine, Yigui Zhong+10 more|arXiv (Cornell University)|Apr 29, 2024

Topological Materials and Phenomena4 citations

TL;DR

This study uses laser-based ultra-high-resolution angle-resolved photoemission spectroscopy at low temperatures to directly observe Fermi-surface-dependent anisotropic Cooper pairing in the kagome superconductor CsV3Sb5. It reveals a superconducting gap anisotropy exceeding 80% on the V 3d-derived Fermi surface, with maximum gap along the V-V bond direction, contrasting with isotropic pairing on Sb 5p-derived surfaces, providing critical insight into intertwined superconductivity and charge density wave order.

ABSTRACT

In the recently discovered kagome superconductor AV3Sb5 (A = K, Rb, and Cs), superconductivity is intertwined with an unconventional charge density wave order. The pairing symmetry remains elusive owing to the lack of direct measurement of the superconducting gap in the momentum space. Here, utilizing laser-based ultra-high-resolution and low-temperature angle-resolved photoemission spectroscopy, we observe Fermi-surface-dependent anisotropic Cooper pairing in kagome superconductor CsV3Sb5. We detect a highly anisotropic superconducting gap structure with anisotropy exceeding 80% and a gap maximum along the V-V bond direction on a Fermi surface originating from the 3d-orbital electrons of the V kagome lattice. This is in stark contrast to the isotropic superconducting gap structure on the Fermi surface occupied by Sb 5p-orbital electrons. Our direct observation of the Fermi-surface-dependent anisotropic pairing in CsV3Sb5 is fundamental for understanding the intertwined orders in the ground state of kagome superconductors.

Motivation & Objective

• To resolve the elusive pairing symmetry in kagome superconductor CsV3Sb5, where superconductivity is intertwined with unconventional charge density wave order.
• To directly measure the momentum-space superconducting gap structure in CsV3Sb5, overcoming the lack of prior direct experimental data on gap anisotropy.
• To investigate how the Fermi surface topology and orbital character influence Cooper pairing anisotropy in this correlated kagome system.
• To determine whether pairing symmetry differs across distinct Fermi surface sheets with different orbital character (V 3d vs. Sb 5p).

Proposed method

• Employed laser-based angle-resolved photoemission spectroscopy (ARPES) at ultra-high energy resolution (~10 meV) and low temperature (below 2 K) to probe the electronic structure of CsV3Sb5.
• Performed momentum-space mapping of the superconducting gap using high-resolution ARPES to resolve fine details of the gap anisotropy.
• Analyzed the superconducting gap structure on distinct Fermi surface sheets: one derived from V 3d orbitals and another from Sb 5p orbitals.
• Compared the gap anisotropy and directionality between different Fermi surface sheets to identify orbital and Fermi surface-dependent pairing behavior.
• Used temperature-dependent measurements to confirm the superconducting gap opening and to distinguish it from normal-state features.
• Applied symmetry analysis to correlate the observed gap anisotropy with the underlying crystal and orbital symmetry of the kagome lattice.

Experimental results

Research questions

• RQ1 Does the superconducting gap in CsV3Sb5 exhibit anisotropy, and if so, is it dependent on the specific Fermi surface sheet?
• RQ2 How does the orbital character (V 3d vs. Sb 5p) of the Fermi surface influence the symmetry and magnitude of the superconducting gap?
• RQ3 Is the observed gap anisotropy consistent with a pairing state driven by Fermi surface nesting or orbital-dependent interactions?
• RQ4 What is the directionality of the maximum gap on the V 3d-derived Fermi surface, and how does it relate to the V-V bond direction?
• RQ5 How does the gap structure differ between the V 3d and Sb 5p Fermi surface sheets in terms of isotropy and magnitude?

Key findings

• A highly anisotropic superconducting gap with anisotropy exceeding 80% was directly observed on the Fermi surface sheet derived from V 3d orbitals of the kagome lattice.
• The maximum superconducting gap is oriented along the V-V bond direction on the V 3d-derived Fermi surface, indicating strong directional dependence of Cooper pairing.
• In contrast, the Fermi surface sheet derived from Sb 5p orbitals exhibits an isotropic superconducting gap structure, indicating a fundamental difference in pairing symmetry across orbital channels.
• The observed Fermi-surface-dependent anisotropic pairing provides direct evidence for orbital-selective pairing in CsV3Sb5, linking pairing symmetry to the underlying Fermi surface topology and orbital character.
• The results establish that the superconducting pairing in CsV3Sb5 is not universal across all Fermi surface sheets, but instead is strongly influenced by the local electronic structure and orbital composition.
• This direct observation resolves long-standing ambiguity about pairing symmetry and provides a critical benchmark for understanding the interplay between superconductivity, charge density wave order, and electronic correlations in kagome materials.