[Paper Review] Evolution of the Fermi surface of the nematic superconductors FeSe1-xSx

A. I. Coldea, SF Blake+14 more|Oxford University Research Archive (ORA) (University of Oxford)|Nov 22, 2016

Iron-based superconductors research8 citations

TL;DR

This study investigates the evolution of Fermi surfaces and quasiparticle properties in FeSe₁₋ₓSₓ using quantum oscillations in high magnetic fields (up to 45 T) at low temperatures. It reveals that chemical pressure from sulfur substitution monotonically increases the Fermi surface size, particularly for outer bands, with orbital-dependent mass renormalization—enhanced for dxy-orbital pockets and suppressed for dxz/yz—while superconductivity diminishes, suggesting nematicity is not essential for high-Tc superconductivity in these systems.

ABSTRACT

We investigate the evolution of the Fermi surfaces and electronic interactions across the nematic phase transition in single crystals of FeSe1-xSx using Shubnikov-de Haas oscillations in high magnetic fields up to 45 tesla in the low temperature regime. The unusually small and strongly elongated Fermi surface of FeSe increases monotonically with chemical pressure, x, due to the suppression of the in-plane anisotropy except for the smallest orbit which suffers a Lifshitz-like transition once nematicity disappears. Even outside the nematic phase the Fermi surface continues to increase, in stark contrast to the reconstructed Fermi surface detected in FeSe under applied external pressure. We detect signatures of orbital-dependent quasiparticle mass renomalization suppressed for those orbits with dominant dxz=yz character, but unusually enhanced for those orbits with dominant dxy character. The lack of enhanced superconductivity outside the nematic phase in FeSe1-xSx suggest that nematicity may not play the essential role in enhancing Tc in these systems.

Motivation & Objective

• To probe the evolution of Fermi surfaces and electronic interactions across the nematic quantum phase transition in FeSe₁₋ₓSₓ using high magnetic fields.
• To determine how chemical pressure from sulfur substitution affects Fermi surface topology and quasiparticle masses.
• To assess the role of nematic order in enhancing superconducting transition temperature (Tc) in iron-based superconductors.
• To compare the effects of chemical pressure in FeSe₁₋ₓSₓ with those of hydrostatic pressure in FeSe, particularly regarding Fermi surface reconstruction and Tc enhancement.
• To investigate whether orbital-selective correlations and Lifshitz transitions play a key role in electronic phase competition.

Proposed method

• Conducted Shubnikov-de Haas quantum oscillation measurements in magnetic fields up to 45 T at temperatures as low as 0.3 K using single crystals of FeSe₁₋ₓSₓ.
• Performed magnetotransport measurements at the National High Magnetic Field Laboratory (NHMFL) and High Field Magnet Laboratory (HFML) with ³He cryogenic systems.
• Used fast Fourier transform (FFT) and maximum entropy method (MEM) to extract frequency spectra from oscillatory resistivity data.
• Mapped Fermi surface extremal areas by shrinking calculated tetragonal-phase Fermi surfaces by over 150 meV, based on prior band structure studies.
• Correlated observed quantum oscillation frequencies with orbital character (dxy, dxz/yz) to infer orbital-dependent quasiparticle mass renormalization.
• Performed linear extrapolation of Fermi surface cross-sections to estimate behavior near x ≈ 1 (FeS), comparing with theoretical predictions.

Experimental results

Research questions

• RQ1 How does chemical pressure from sulfur substitution alter the Fermi surface topology in FeSe₁₋ₓSₓ across the nematic phase transition?
• RQ2 What is the role of orbital-selective quasiparticle mass renormalization in the electronic properties of FeSe₁₋ₓSₓ, and how does it evolve with x?
• RQ3 Does the suppression of nematicity via chemical pressure lead to a Fermi surface reconstruction similar to that seen under hydrostatic pressure in FeSe?
• RQ4 Is nematic order essential for enhancing Tc in FeSe-based systems, as suggested by high-Tc superconductivity under pressure?
• RQ5 How do orbital-dependent correlations and possible Lifshitz transitions influence the electronic phase diagram of FeSe₁₋ₓSₓ?

Key findings

• The Fermi surface of FeSe₁₋ₓSₓ expands monotonically with sulfur content x, even beyond the nematic phase, in contrast to the reconstructed Fermi surface seen under hydrostatic pressure.
• The outer electron band with dominant dxy orbital character exhibits enhanced quasiparticle mass renormalization with increasing x, particularly outside the nematic phase.
• The outer hole band with mixed dxz/yz orbital character shows reduced quasiparticle mass renormalization as x increases, indicating orbital-selective correlations.
• A Lifshitz-like transition is observed in the smallest Fermi surface orbit, associated with the disappearance of nematicity, indicating a topological change in the Fermi surface.
• Despite the continuous increase in Fermi surface size and orbital-dependent correlations, superconducting Tc is suppressed with increasing x, suggesting nematicity is not the primary driver of high-Tc superconductivity.
• The Fermi surface evolution in FeSe₁₋ₓSₓ closely mirrors that of FeSe under low-pressure conditions (below 10 kbar), implying that high-Tc superconductivity under pressure may be decoupled from the nematic state by a new structural or magnetic phase.