Since its discovery in 2008, at least 20 different structural iron arsenide or iron selenide have been proved to have superconductivity, collectively known as iron-based superconductors. Because iron-based superconductors can also break the 40K mac Milan limit predicted by the BCS strong coupling theory, which is included in the HTS family along with the copper oxide superconductor, the micro-mechanism problem of superconductivity is still the crown jewel in the frontier field of condensed matter physics.
After years of research, it is widely believed that the high temperature superconductivity of copper oxides can be obtained by doping the matrix of the anti-ferromagnetic-Mott insulator, thus there is a phase diagram of the unified description. For iron-based superconductors, the so-called ' matrix ' has the inverse ferromagnetic, but it shows the metal conductivity (with a certain concentration of carriers), through the mother doped electrons, holes and even valence doping can induce the super conductivity. It is even more difficult to understand that a considerable proportion of the not-doped iron-matrix superconducting ' matrix ' itself has a superconductivity, and further doping may inhibit superconducting. Different systems of iron-based superconductor, with strange electronic phase diagram, simply using doping concentration as a variable, has not enough to accurately describe its physical behavior. Therefore, it is one of the key research points to find the uniform variables describing the physical properties of iron-based superconductors and the strict physical meaning of the matrix.
Recently, the Institute of Physics of CAS/Beijing State Laboratory of Condensed State Physics (SC8) The study group studied the fluctuation properties of a large number of iron-based sample matrix and doped samples by measuring the resistance behavior under uniaxial pressure, and found that the inverse ferromagnetic magnetic moment was inversely proportional to the Lieguri constant. This implies that the magnetic ground state of the iron-based superconductor can be obtained by adjusting the intensity of the column fluctuations. Thus, a unified phase diagram of iron base superconductors can be established, in which superconductivity is born in a hypothetical ideal matrix, which has a large order magnetic moment and a weaker fluctuation of the column.
A careful observation of the doping phase diagram of iron-based superconductors in different systems shows that the reverse ferromagnetic order, the superconductivity and the electron-column phase are the most notable characteristics (Fig. 1). In the electron-column phase, the electron state which breaks the inherent rotational symmetry of the lattice is presented in the system, which is the electron state property of the double symmetry in the Crystal ab plane in the iron-based superconductor. It is the key to understand the microscopic mechanism of fe-based superconductivity to find the concrete relation between the inverse ferromagnetic, superconducting and the three-column phase. Although there is no iron-sequence in some iron-based superconductors, the fluctuation of electrons to the column or to the column always exists. The research group based on the independent design of a piezoelectric ceramic sheet based on a single axis pressure measurement device, to achieve a very accurate measurement of the electron-column type fluctuations, and for the first time revealed to the column type quantum critical point and the iron-based superconductivity close connection. By measuring 1111 More extensively, 122, 11, 111, 112 and other iron-based superconductor series of samples, they found that in the presence of the two-stage structure of the sample, or the best doping of the sample, the temperature dependence on the phase of magnetization can be described by the Curie-alien law (Fig. 2). Thus, a Curie constant can be defined to describe the fluctuation intensity of the column phase. The reciprocal of the absolute value of the Curie constant | An|-1, and the static effective magnetic moment m of the iron-based superconductor, becomes a very simple linear scaling relationship, i.e., the stronger the fluctuation of the column, the weaker the inverse ferromagnetic. This is the first time that the magnitude of the reverse ferromagnetic magnetic moment is related to another physical quantity in the experiment. When the reverse ferromagnetic and the column phase disappear, a quantum tipping point is formed that corresponds to the best doping superconductivity (Fig 3). From this, we can define a strictly significant iron matrix superconducting matrix (HPC), which has a large order magnetic moment and a very weak column fluctuation. By increasing the fluctuation of the column, it can inhibit its inverse ferromagnetic and finally obtain the high temperature superconductivity (Fig 4). The establishment of the unified electronic phase diagram opens a new perspective for understanding the complex doping behaviors in different iron-based superconducting systems, and has important implications for the study of the micro-mechanism of fe-based superconductivity. It should be noted that this phase diagram may still be unable to explain some special iron-based superconductor materials, such as Lifeas, the second superconducting region in the ' 1111 ' system, the Fese phase diagram under pressure, and the superconducting Cu, Cr, MN doping system. This work was published in physical Review Letters.
The series of research work has been the strategic pilot science and Technology of CAS (Class B), CAs, the national Key basic research and Development Program (973 plan), the National key Development Program, the National Natural Science Foundation, the National Youth thousand people plan and so on support.
Fig. 1. Conventional doping phase diagram of iron-based superconductors
Fig. 2. Curie-exogenous temperature dependence behavior of the magnetic susceptibility of iron-based superconductors in different systems
Fig. 3. The relationship between the tangent phase Curie constant of iron-based superconductors in different systems and the scaling of the anti-ferromagnetic magnetic moment
Fig. 4. The unified phase diagram of iron base superconductor constructed with the parameters of the Curie constant of the column phase
