Research progress of strongly correlated physics in heavy hole doped iron-based superconductors

Strong correlated physics has been a frontier field in the research of high-temperature superconductors and is considered to be closely related to the microscopic mechanism of high-temperature superconductivity. The discovery of iron-based superconductors in 2008 brought a brand new material platform to the research of high-temperature superconductors. And the research team from the University of Science and Technology of China has made significant contributions to the discovery of iron-based superconductors. With further in-depth study of iron-based superconductors, it has been found that iron-based superconductors are different from copper-based high-temperature superconductors with weaker electronic correlation effects. The picture based on strongly correlated physics for high-temperature superconductivity is greatly challenged. Theoretically, iron-based superconductors are still considered to have the features of the strong correlated physics, but their manifestations are featured by orbital selectivity due to multiple orbitals and the Hund’s coupling. The exploration of strong correlated physics in iron-based superconductors has become a frontier issue in current research of high-temperature superconductors. In this paper, our nuclear magnetic resonance (NMR) studies on the heavily hole-doped iron-based superconductors AFe2As2 (A=K, Rb, Cs) were reviewed. Our results indicate that the electronic correlation effect is very strong in the heavily hole-doped region and many features are consistent with the orbital selective Mott physics. First, the localization and itinerancy of electrons can coexist, and an electronic coherent state similar to the heavy fermion system emerges at the low temperature; secondly, based on site-selective NMR detection, orbital-selective local spin susceptibility measurements were achieved， further confirming the orbital-selective electron correlation effects in iron-based superconductors; finally, for the first time, a new electronic nematic state was observed in heavily hole-doped CsFe2As2 systems. These findings reveal similar strong correlation features between iron-based superconductors and copper-based superconductors and heavy fermion superconductors, which will help to establish a unified high-temperature superconducting mechanism.