Iron selenide and arsenide superconductors, as the two subgroups of iron-based family, contain similar FeSe₄ and FeAs4 tetrahedron layers as their superconducting blocks, respectively. However, the typical FeSe-based superconductor A_yFe_2-xSe₂ (A=alkali metal ions) exhibits quite different physical properties of its parent and normal state phases from the FeAs-based ones. Thus it raises the question of whether the iron-based family would share a universal underlying physics for its high-T_C superconductivity. To resolve this issue is of fundamental importance for understanding the physical mechanism of all classes of high-T_C superconductors. But in A_yFe_2-xSe₂, the presence of an insulating antiferromagnetic 245 phase obstructs the experimental observation on its intrinsic electronic properties. The appearance of a new FeSe-based superconductor Li_0.8Fe_0.2OHFeSe in 2014, however, marks a turning point in the experimental studies.

Recently, Prof. DONG Xiaoli (from Institute of Physics, Chinese Academy of Sciences) and her collaborators Dr. JIN Kui (IOP-CAS) and Prof. ZHANG Guangming (Tsinghua University) have made a series of progresses in the study of the new high-T_C superconductor system of Li_1-xFexOHFe_1-ySe.

First, based on a series of powder samples prepared by hydrothermal reactions, they have established a complete electronic phase diagram of Li_1-xFexOHFe_1-ySe, which shows that the physical properties in both the antiferromagnetic spin-density-wave and superconducting regimes closely resemble those of FeAs-based one. Their results have revealed that the electronic groud states of FeSe- and FeAs-based high-T_C superconductors are actually similar to each other, thus a commom mechanism for high-T_C superconductivity is expected for the iron-based superconductor family. This work entitled “Phase Diagram of Li_1-xFexOHFe_1-ySe: A Bridge between Iron Selenide and Arsenide Superconductors” has been published in Journal of the American Chemical Society [http://pubs.acs.org/doi/pdf/10.1021/ja511292f].

One step further, by the application of an ionic exchange technique, the researchers have synthesized a large and high-quality superconducting single crystal (Li_0.84Fe_0.16)OHFe_0.98Se, with a T_C = 42 K and an absence of the troublesome 245 phase. This breakthrough in single crystal sample makes it possible to reveal its highly two-dimensional intrinsic electronic properties. They have found a highly two-dimensional charge transport and anomalous linearly temperature dependent susceptibility and resistivity in the normal state below a characteristic temperature T* =120 K, and a strong anisotropy of upper critical field in the superconducting state. Hall coefficient shows that electron carriers dominate the charge transport and the contribution of holes is almost vanishing below this T*. Their results suggest the presence of two-dimensional antiferromagnetic spin fluctuations likely related to the electron pairing for superconductivity, as commonly believed in FeAs-based and cuprate high-T_C materials. Moreover, it is shown that an FeSe-based superconductor of a higher T_C is of an inevitable enhanced two dimensionality of its crystal and electronic structures; in particular, this high-T_C and highly two-dimensional (Li_0.84Fe_0.16)OHFe_0.98Se crystal is a close analogy of an interface superconductor of single unit-cell FeSe film on SrTiO₃ substrate showing a T_C higher than 65 K. Hence the two-dimensional electron-electron interaction is crucial for high-T_C superconductivity in a general sense. This work entitled “(Li_0.84Fe_0.16)OHFe_0.98Se superconductor: Ion-exchange synthesis of large single-crystal and highly two-dimensional electron properties” has been published as Editors’ Suggestion in Physical review B [http://journals.aps.org/prb/pdf/10.1103/PhysRevB.92.064515].

Prof. ZHAO Zhongxian, Prof. ZHOU Fang and Dr. YUAN Jie from IOP-CAS have also made important contributions to the above studies. The Ph.D. candidates, Ms. ZHOU Huaxue, Ms. YUAN Dongna, Mr. HUANG Yulong and Mr. MAO Yiyuan, have been engaged in some related experiments. High-resolution TEM study has been performed by Prof. YANG Huaixin (IOP-CAS), single crystal structural refinement by Prof. SUN Junliang (Peking University), and high temperature magnetism measurement by associate Prof. ZHENG Ping (IOP-CAS).

These studies are supported by projects of Natural Science Foundation of China and the Ministry of Science and Technology of China and by “Strategic Priority Research Program (B)” of Chinese Academy of Sciences.