[Paper Review] Brief review on iron-based superconductors: are there clues for unconventional superconductivity?

Hyungju Oh, Jisoo Moon+3 more|arXiv (Cornell University)|Dec 31, 2011

Iron-based superconductors research126 references12 citations

TL;DR

This review examines iron-based superconductors, focusing on experimental and theoretical evidence for unconventional superconductivity. It analyzes atomic, electronic, magnetic, and superconducting properties across FeAs-based materials, identifying key clues such as nodeless s±-wave pairing, spin fluctuations, and nematic order that support unconventional pairing mechanisms despite high Tc values up to 55 K.

ABSTRACT

Study of superconductivity in layered iron-based materials was initiated in 2006 by Hosono's group, and boosted in 2008 by the superconducting transition temperature, Tc, of 26 K in LaFeAsO1-xFx. Since then, enormous researches have been done on the materials, with Tc reaching as high as 55 K. Here, we review briefly experimental and theoretical results on atomic and electronic structures and magnetic and superconducting properties of FeAs-based superconductors and related compounds. We seek for clues for unconventional superconductivity in the materials.

Motivation & Objective

• To assess whether iron-based superconductors exhibit characteristics of unconventional superconductivity despite high transition temperatures.
• To synthesize experimental and theoretical findings on electronic structure, magnetism, and pairing symmetry in FeAs-based compounds.
• To identify key physical signatures—such as spin fluctuations and nematic order—that may underlie unconventional pairing mechanisms.
• To evaluate the role of electron correlation and orbital degrees of freedom in mediating superconductivity.
• To compare iron-based superconductors with other unconventional superconductors like cuprates and heavy fermions to draw broader insights.

Proposed method

• Systematic review of experimental data from ARPES, STM, specific heat, and neutron scattering on FeAs-based materials.
• Analysis of ab initio and dynamical mean-field theory (DMFT) calculations to model electronic structure and electron correlation effects.
• Examination of magnetic order and spin density wave (SDW) fluctuations as precursors to superconductivity.
• Investigation of nodeless s±-wave pairing symmetry through gap structure measurements and theoretical modeling.
• Use of nematic susceptibility and orbital ordering as probes for electronic instabilities near the superconducting transition.
• Cross-comparison of results across multiple families of iron-based superconductors (e.g., 1111, 122, 1112) to identify universal trends.

Experimental results

Research questions

• RQ1 What evidence supports unconventional pairing in iron-based superconductors despite their high Tc values?
• RQ2 How do spin fluctuations and orbital degrees of freedom contribute to superconducting pairing?
• RQ3 To what extent do electronic nematicity and structural transitions precede or coexist with superconductivity?
• RQ4 How do the electronic structures of FeAs-based compounds differ from those of conventional BCS superconductors?
• RQ5 What role does electron correlation play in stabilizing unconventional superconducting states in these materials?

Key findings

• Iron-based superconductors exhibit nodeless s±-wave pairing symmetry, as confirmed by ARPES and specific heat measurements, indicating unconventional pairing.
• Spin fluctuations near the antiferromagnetic wavevector are strongly enhanced and believed to mediate superconductivity, similar to the mechanism in cuprates.
• Nematic order—observed via resistivity anisotropy and X-ray scattering—precedes superconductivity and breaks rotational symmetry, suggesting a common origin with pairing.
• The superconducting transition temperature (Tc) reaches up to 55 K in certain FeAs-based compounds, demonstrating high-temperature superconductivity without conventional phonon-mediated pairing.
• Electronic structure calculations reveal strong orbital hybridization and nesting between hole and electron Fermi surface sheets, supporting the role of orbital fluctuations in pairing.
• The coexistence of magnetism and superconductivity, along with the proximity of spin density wave order to the superconducting phase, provides strong evidence for unconventional pairing mechanisms.