A new iron superconductor stabilized by charge transfer between blocks

Iron-based superconductors (IBSCs) have attracted sustained attention from research over the last decade, in part because new IBSCs have been discovered one after the other in previous years . At the present time, IBSC exploration is becoming increasingly difficult. A research team from Zhejiang University developed a structural design strategy for exploration, from which it managed to find a series of hole-doped IBSCs with double layers of FeAs in recent years. Nevertheless, the electron-doped analogue has not been realized so far.

The recently discovered electron-doped IBSC is the BaTh₂Fe₄As₄(NOT_0.7O_0.3)₂, an interposed compound of undoped BaFe₂As₂ and electron-doped ThFeAsN_0.7O_0.3 (see the box in Figure 1). The new superconductor could only be synthesized when the nitrogen is partially replaced by oxygen, as in the case of the BaTH₂Fe₄As₄(NOT_0.7O_0.3)₂.

Namely, the oxygen-free phase, BaTh₂Fe₄As₄NOT₂could not be prepared due to network pairing. The synthesis process carried out is actually a redox reaction, BaFe₂As₂ + 2ThFeAsN_0.7O_0.3 = BaTh₂Fe₄As₄(NOT_0.7O_0.3)₂, which indicates an essential role of charge transfer between blocks in the stabilization of the intergrowth structure. Note that, although the two constituent building blocks share identical iron atoms, they contain crystallographically different arsenic atoms, as a consequence of charge transfer.

Although the new superconductor is isostructural compared to the previous ones, it has contrasting structural and physical properties. First, the structural details in the FeAs layers are different from those of the doped-hole type 12442 IBSCs, but they are similar to those of most electronically doped IBSCs. Secondly, the Hall effect shows a negative Hall coefficient across the entire temperature range, and the Hall coefficient values ​​correspond to the electronic doping level due to oxygen substitution. Third, superconducting properties such as higher critical fields and specific heat jump are close to most electron-doped IBSCs.

The resistive transition temperature at the beginning of the new FeAs double layer IBSC is 30 K and the zero resistance temperature is 22 K. As a result, the magnetic susceptibility and specific heat data suggest two transitions and the overall superconductivity appears at 22 K The result is in contrast with the counterpart to a single layer of FeAs, ThFeAsN_0.85O_0.15, with the same level of doping. The latter does not show superconductivity greater than 1.8 K.

The essential role of demonstrated block charge transfer seems to be insightful, which could be useful for exploring larger layered materials beyond layered IBSCs.