Striped antiferromagnetism and electronic structures of SrFeAsF and their implications

We investigate structural, magnetic, and electronic properties of SrFeAsF as a new parent for superconductors using a state-of-the-art density-functional theory method. Calculated results show that the striped antiferromagnetic order is the magnetic ground state in the Fe layer and the interlayer magnetic interaction is tiny. Calculated As and Sr positions are in good agreement with experiment. There are only two quasi-two-dimensional bands near the Fermi level. The valence charge is mainly in the Fe and F layers, and the magnetic moment is confined to the Fe atoms. All the spin couplings within the Fe layer are antiferromagnetic due to the superexchange through the nearest As atoms. These results, with the record-equaling phasetransition temperature in the latest Sm-doped SrFeAsF, show that the SrFeAsF, sharing the same structure with LaFeAsO, is promising for achieving better superconductors. Introduction. – Fe-based superconductors attract more and more attention since superconductivity was found in doped LaFePO [1]. The advent of superconducting F-doped LaFeAsO stimulates a world-wide campaign for more and better Fe-based superconductors [2]. Replacing La by other lanthanides or doping with F has yielded more superconductors, and higher phase-transition temperatures (Tc) have been achieved in some of them [3–5]. Furthermore, much more superconducting materials were found by using other dopants and other parent compounds [6–9]. Even α FeSe can be made superconducting by applying high pressure [10, 11]. Now there are three series of FeAs-based superconductors: RFeAsO (R: lanthanide elements), AFe2As2 (A: alkaline-earth elements), and LiFeAs. So far, the highest Tc is 55-56 K in the case of doped SmFeAsO [5]. Their structural, magnetic, electronic properties are intensively investigated and the microscopic mechanism for the superconductivity in these materials has been explored [12–23]. Very recently, superconductivity was found in Co and La doped SrFeAsF materials [24–27]. SrFeAsF has the same crystal structure as RFeAsO and similar magnetic instability, but does not include any lanthanide [28–30]. It has been found that Sm-doped SrFeAsF can become superconducting at 56 K, and higher transition temperatures should be reached with appropriate dopants [25–27]. Because some magnetic fluctuations are believed to mediate the superconductivity in FaAs-based materials, it is highly desirable to investigate the magnetic orders, electronic structures, and magnetic properties of the parent compound SrFeAsF. Here, we use a state-of-the-art density-functional theory (DFT) method to investigate the structural, electronic, and magnetic properties of the SrFeAsF. Our total energy results show that the striped antiferromagnetic (AF) order, the same as that of LaFeAsO, is the magnetic ground state in the Fe layer, and the interlayer magnetic interaction is tiny. Our calculated position parameters of As and Sr are in good agreement with experiment. The electronic band result shows that there are only two quasi-twodimensional (quasi-2D) bands near the Fermi level. Our charge and magnetization density analysis shows that the valence charge is mainly distributed in the Fe and F layers, and the magnetic moment is confined to the Fe layer. The spin couplings within the Fe layer are AF due to superexchange through the nearest As atoms. The real moment should be much smaller than the DFT value because of quantum spin fluctuations. More detailed results will be presented in the following.