The crystal structure and physical properties of multiple components FeSe-based superconductors

Abstract

The sample preparation, crystallographic structure, phase separation and superconductivity of FeSe-based superconductors are discussed here. First of all, we will introduce the ternary high tempeature superconductor K0.8Fe2− y Se2 with superconducting transition tempeature ( T c) at 31 K, which creates the record T c of FeSe-based superconductor under ambient pressure at that time. The single crystals can be grown through the self-flux method at high temperature. The crystal structure at high temperature shows the body-center tetragonal lattice with space group I4/mmm, while the superstructure developes at low tempeature with I4/m symmetry. The Fermi surface measured by angle resolution photoemission spectrum shows that K0.8Fe2− y Se2 has no any hole pockets at Fermi level, which is strikingly different from that of FeAs-based superconducors. This topology of Fermi surface implies that the superconducting mechanism of K0.8Fe2− y Se2 also should be distinguished from that of FeAs-based superconductors. The lack of interaction between electron and hole pockets would induce a possible d -wave superconducting gap. However, the mulitiple superstructure types, such as 5 × 5 , 2 × 2 , 2×2, etc, have been identified by transimission electron microscopy and neutron diffraction measurements, which reveal that K0.8Fe2− y Se2 is a phase-separated compound. Secondly, we develop the low tempeature liquid-NH3 method for synthesizing the pure FeSe-based superconducting compounds without phase separation. It is demonstrated that many alkali-metals, alkali-earth metals and rare-earth metals, such as Li, Na, Ca, Sr, Ba, Eu, and Yb, can be intercalated between FeSe layers and induce superconductivity with T c of 30–46 K, which can further enhance the record T c. In the K x (NH3) y Fe2Se2, two superconducting phases are discovered with different concentration of K ( x ≈0.3 and 0.6). The NH3 molecures play an important role in stabilizing the structures, moreover, the discrete superconducting phases implies that metal-intercalated FeSe superconductors are distinct from other FeAs-based superconductors. It is proposed that the main cause comes from their structures only being stable at particular doping levels and fine-tuning the concentrations of alkali metals may further enhance T c in the FeSe-based superconductors. Thirdly, we discuss the novel FeSe-based superconductors prepared by other metastable methods like the hydrothermal and solvent-thermal methods. The superconducting LiOHFeSe and Na x (En) y Fe2Se2 superconductors with T c over 40 K have been successfully synthesized, in which the former one shows the anti-ferromagentism at 8 K and the latter one exhibits structural tranistion at 350 K. The emergent properties from these novel high tempeature superconductors make the FeSe-based family becomes the hottest field in the community of superconductivity. In addition, the newly discovered monolayer FeSe on SrTiO3 further enhance the T c up to 65–70 K, which is close to the boiling point of liquid nitrogen. Finally, it is remarked that the structural flexibilty and weak bonds between FeSe layers are the most important advantages for exploring new superconductor with higher T c. We believe that more FeSe-based superconductors could be discovered once the new metastable methods are explored.