[Paper Review] Iron based superconductors: A brief overview

Haranath Ghosh, Smritijit Sen|arXiv (Cornell University)|Aug 14, 2014

Iron-based superconductors research14 references3 citations

TL;DR

This review provides a comprehensive overview of iron-based superconductors discovered in 2008, highlighting their unconventional superconducting behavior despite lacking Cu-O planes. It emphasizes the role of Fe-3d electrons, unusual Fermi surface topology, and strong correlations between superconductivity, magnetism, and orbital fluctuations, with key evidence from electronic specific heat, isotope effects, and phase diagrams pointing to a non-phonon-mediated pairing mechanism.

ABSTRACT

Fe-based superconductors were discovered in 2008. This discovery with T$_c$ values up to 56 K, generated a new belief in the field of superconductivity. Till its discovery, high temperature superconductivity in cuprates, created a prejudice that Cu-oxides are essential building blocks for a high temperature superconducting material. These Fe based superconductors do not contain Cu-O planes (some of the materials are even O free). It will be argued in this review, that these iron pnictide and chalcogenide (FePn/Ch) superconductors have Fe electrons at the Fermi surface together with an unusual Fermiology that can change rapidly with doping. This may lead to very different normal and superconducting state properties compared to those in standard electron-phonon coupled {\it conventional} superconductors. There are a large number of evidences showing that superconductivity, magnetism, orbital fluctuations are intimately related and coexist in these materials although the mechanism of superconductivity in these compounds is still unknown. The electronic specific heat, $\frac{2Δ}{k_B T_c}$ ratio, phase diagrams, isotope effect, crystal structures and there correlation to T$_c$ from various available experimental data are main inputs of this review to show the above.

Motivation & Objective

• To analyze the emergence of high-Tc superconductivity in iron-based materials independent of Cu-O planes.
• To challenge the prevailing belief that cuprates are the sole path to high-temperature superconductivity.
• To examine the interplay between superconductivity, magnetism, and orbital fluctuations in Fe-based systems.
• To synthesize experimental data on Tc, electronic specific heat, isotope effects, and crystal structures to assess pairing mechanisms.

Proposed method

• Systematic review of experimental data from Fe-pnictide and chalcogenide superconductors across multiple studies.
• Analysis of electronic specific heat data to extract the 2Δ/kBTc ratio as a probe of pairing symmetry.
• Evaluation of isotope effects on Tc to assess the role of electron-phonon coupling.
• Examination of phase diagrams and crystal structures to correlate structural features with superconducting transition temperatures.
• Comparison of Fermi surface topology and doping dependence across different families of iron-based superconductors.
• Use of existing theoretical and experimental results to argue against conventional electron-phonon pairing in favor of unconventional mechanisms.

Experimental results

Research questions

• RQ1 What is the role of Fe-3d electrons and Fermi surface topology in enabling high-Tc superconductivity in iron-based materials?
• RQ2 How do magnetism, orbital fluctuations, and superconductivity coexist and influence one another in Fe-based systems?
• RQ3 To what extent do isotope effects and electronic specific heat data support or contradict conventional electron-phonon coupling in these materials?
• RQ4 Why do some iron-based superconductors exhibit Tc values up to 56 K despite lacking Cu-O planes?
• RQ5 How do structural variations in Fe-pnictides and chalcogenides correlate with their superconducting transition temperatures and electronic properties?

Key findings

• Iron-based superconductors achieve Tc values up to 56 K without Cu-O planes, challenging the notion that cuprates are essential for high-Tc superconductivity.
• The presence of Fe-3d electrons at the Fermi surface and their unusual Fermi surface topology are central to the unconventional electronic behavior observed.
• Strong evidence from electronic specific heat and the 2Δ/kBTc ratio indicates a pairing mechanism distinct from conventional BCS superconductors.
• Isotope effects in these materials are weak, suggesting that electron-phonon coupling is not the dominant pairing mechanism.
• Magnetism, superconductivity, and orbital fluctuations are intimately intertwined and coexist across various doping regimes in iron-based systems.
• Crystal structure variations, including oxygen-free compositions, correlate with Tc values, indicating structural tuning of electronic correlations.