The phase diagrams of iron-based superconductors: theory and experiments

TL;DR

This review discusses the complex phase diagrams of iron-based superconductors, emphasizing the importance of accurately defining structural, magnetic, and electronic phase boundaries to understand their properties and the interplay of magnetism and superconductivity.

Contribution

It provides a comprehensive summary of experimental and theoretical results for key iron-based superconductor families, highlighting the challenges in defining their phase diagrams.

Findings

Accurate phase boundaries are crucial for understanding Fe-SC properties.

Structural transformations are closely linked to magnetic behavior.

Micro-structural features significantly influence physical properties.

Abstract

Phase diagrams play a primary role in the understanding of materials properties. For iron-based superconductors (Fe-SC), the correct definition of their phase diagrams is crucial because of the close interplay between their crystallo-chemical and magnetic properties, on one side, and the possible coexistence of magnetism and superconductivity, on the other. The two most difficult issues for understanding the Fe-SC phase diagrams are: 1) the origin of the structural transformation taking place during cooling and its relationship with magnetism; 2) the correct description of the region where a crossover between the magnetic and superconducting electronic ground states takes place. Hence a proper and accurate definition of the structural, magnetic and electronic phase boundaries provides an extremely powerful tool for material scientists. For this reason, an exact definition of the thermodynamic phase fields characterizing the different structural and physical properties involved is needed, although it is not easy to obtain in many cases. Moreover, physical properties can often be strongly dependent on the occurrence of micro-structural and other local-scale features (lattice micro-strain, chemical fluctuations, domain walls, grain boundaries, defects), which, as a rule, are not described in a structural phase diagram. In this review, we critically summarize the results for the most studied 11-, 122- and 1111-type compound systems, providing a correlation between experimental evidence and theory.