It is widely accepted that high-temperature superconductivity in cuprates is achieved by introducing carriers into the parent compound, an antiferromagnetic Mott insulator, which results in a unified phase diagram. As to iron-based superconductors, although the so-called “parent compounds” also show long-range antiferromagnetic orders, they are bad metals and superconductivity can be obtained not just by doping electrons or holes but also by isovalent doping. Moreover, some of the “parent compounds” show superconductivity without the presence of antiferromagnetism. Therefore, it is hard to describe the physical properties of different iron-based superconductors with just carrier doping.
Recently, the SC8 group of the Institute of Physics, Chinese Academy of Sciences systematically studied the nematic fluctuations in many parent compounds and their doped samples by measuring the elastoresistivity under uniaxial pressure. They found that the ordered antiferromagnetic moment is inversely proportional to the nematic Curie constant, suggesting that the magnetic ground states can be tuned by the amplitude of nematic fluctuations. Accordingly, they provide the unified phase diagram for iron-based superconductors, where superconductivity emerges from a hypothetical parent compound with large ordered moment and weak nematic fluctuations.
In the phase diagrams of different iron-based superconductors, the most promising features are the antiferromagnetism, superconductivity and nematicity (Fig. 1). The latter represents an electronic state that breaks the four-fold rotational symmetry of the underlying lattice. Therefore, the relationship between them is crucial to understand the superconducting mechanism. Based on a new uniaxial pressure device designed by themselves (Physical Review Letters 117, 157002 (2016)), the SC8 group can now study the nematic fluctuations in very high resolution. They have measured many samples of iron-based superconductors, including the “1111”, “122”, “11”, “111” and “112” systems, and found that nematic fluctuations present in all of them. In the systems that exhibit nematic transitions, the nematic susceptibility can be described by a simple Curie-Weiss-like function (Fig. 2), which gives us a nematic Curie constant An. Surprisingly, the antiferromagnetic ordered moment in these materials has a linear relationship with |An|-1 (Fig. 3). It suggests that the stronger the nematic fluctuations, the weaker the antiferromagnetic order. It is the first time experimentally that the ordered moment is associated with another physical property. The disappearance of nematic order will result in a nematic quantum critical point. Therefore, we can define a hypothetical parent compound for all iron-based superconductors with large ordered moment and weak nematic fluctuations. The superconductivity is thus achieved by enhancing the nematic fluctuations to suppress the long-range antiferromagnetic order. Fig. 4 shows this unified phase diagram. Our results shed new lights in understanding the complex physical properties in iron-based superconductors. It should be noted that this phase diagram may not explain some particular materials, such as LiFeAs, the second superconducting dome in “1111”, the phase diagram of FeSe under high pressure and non-superconducting Cu, Cr or Mn-doped systems.
This work is supported by the Ministry of Science and Technology of China, the National Natural Science Foundation of China, the Strategic Priority Research Program (B) of the Chinese Academy of Sciences, the Fundamental Research Funds for the Central Universities, the Youth Innovation Promotion Association of CAS and the National Thousand-Young-Talents Program of China.