[Paper Review] Fermi surface shrinking and interband coupling in iron-based pnictides

Luciano Ortenzi, E. Cappelluti+2 more|ArXiv.org|Mar 2, 2009

Iron-based superconductors research3 citations

TL;DR

The paper explains that the observed Fermi surface shrinking in iron-based pnictides—where hole and electron bands shift relative to LDA predictions—is due to strong interband coupling mediated by a retarded bosonic mode, driven by particle-hole asymmetry in multiband systems. This coupling induces band shifts with opposite signs for hole and electron pockets, providing direct evidence for dominant interband scattering, with an estimated interband coupling of V ≈ 0.46 eV.

ABSTRACT

Recent measurements of Fermi surface with de Haas-van Alphen oscillations in LaFePO showed a shrinking of the Fermi pockets with respect to first-principle LDA calculations, suggesting an energy shift of the hole and electrons bands with respect to LDA. We show that these shifts are a natural consequence of the strong particle-hole asymmetry of electronic bands in pnictides, and that they provide an indirect experimental evidence of a dominant interband scattering in these systems.

Motivation & Objective

• To resolve the persistent discrepancy between LDA-calculated and experimentally measured Fermi surface areas in iron-based pnictides.
• To identify the origin of band shifts—downward for hole bands, upward for electron bands—observed in de Haas-van Alphen (dHvA) measurements.
• To determine whether interband or intraband scattering dominates in these multiband superconductors.
• To quantify the role of particle-hole asymmetry in generating finite-band self-energy effects that shift bands without breaking particle-hole symmetry.

Proposed method

• Formalism based on multiband Eliashberg theory with retarded electron-boson coupling, using Matsubara self-energy equations.
• Incorporates a retarded bosonic mode with local propagator D(ωₗ) = ∫dΩ 2ΩB(Ω)/(Ω² + ωₗ²), modeling spin-fluctuation-like interactions.
• Solves for self-energy corrections Σₐ(iωₙ) via T∑ₘ,ᵦ Vₐ,ᵦ D(ωₙ - ωₘ)Gᵦ(iωₘ), accounting for non-half-filled, asymmetric bands.
• Uses analytical continuation (Marsiglio-Schöcksmann-Carbotte) to compute spectral functions and visualize Fermi surface shifts.
• Adjusts LDA band structure by a factor of 2 to match ARPES bandwidth renormalization, then recalculates band shifts and coupling strength.
• Compares theoretical Fermi surface areas and band dispersions with dHvA and ARPES data to validate model predictions.

Experimental results

Research questions

• RQ1 Why do dHvA measurements show a shrinking of Fermi surface pockets compared to LDA predictions in iron-based pnictides?
• RQ2 What is the microscopic origin of the opposite-sign band shifts (downward for hole bands, upward for electron bands) observed in experiments?
• RQ3 Does interband scattering dominate over intraband scattering in determining the Fermi surface topology and band renormalization?
• RQ4 How does particle-hole asymmetry in multiband systems lead to finite-band self-energy effects that shift bands without requiring particle-hole symmetry?

Key findings

• The observed Fermi surface shrinking in iron-based pnictides arises naturally from interband coupling via a retarded bosonic mode, not from inaccuracies in LDA.
• The interband coupling strength is estimated at V ≈ 0.46 eV, consistent with dHvA and specific heat measurements, yielding mass enhancements m* / mₑ ≈ 1.6–3.2.
• The model reproduces the experimental trend of hole and electron bands approaching each other near the Fermi level, consistent with ARPES observations in 122 compounds.
• Finite-band self-energy effects due to particle-hole asymmetry persist well beyond the boson energy scale ω₀, explaining the energy-dependent band shifts.
• The kink structure in the electronic dispersion, observed in ARPES, is explained by the retarded interaction, which causes a momentum-dependent renormalization near ω₀.
• Bandwidth narrowing from ARPES (factor of 2) is accounted for by rescaling the LDA band structure, leading to consistent coupling and mass parameters.