Novel Fe-Based Superconductor ${\mathrm{LaFe}}_2{\mathrm{As}}_2$ in Comparison with Traditional Pnictides

The recently discovered Fe-based superconductor (FeBS) ${\mathrm{LaFe}}_2{\mathrm{As}}_2$ seems to break away from an established pattern that doping an FeBS beyond $0.2e/\mathrm{Fe}$ destroys superconductivity. ${\mathrm{LaFe}}_2{\mathrm{As}}_2$ has an apparent doping of $0.5e$, yet superconducts at 12.1 K. Its Fermi surface bears no visual resemblance with the canonical FeBS fermiology. It also exhibits two phases, none magnetic and only one superconducting. We show that the difference between them nonetheless has a magnetic origin, the one featuring disordered moments, and the other locally nonmagnetic. We find that La there assumes an unusual valence of $+2.6$ to $+2.7$, so that the effective doping is reduced to $0.30-0.35e$. A closer look reveals the same key elements: hole Fermi surfaces near $\mathrm{\Gamma }-Z$ and electron ones near the $X-P$ lines, with the corresponding peak in susceptibility, and a strong tendency to stripe magnetism. The physics of ${\mathrm{LaFe}}_2{\mathrm{As}}_2$ is thus more similar to the FeBS paradigm than hitherto appreciated.

Figure 1

(a) Relevant exchange paths in ${\mathrm{LaFe}}_2{\mathrm{As}}_2$. (b) Brillouin zone of the $I4/mmm$ space group of ${\mathrm{LaFe}}_2{\mathrm{As}}_2$.

Figure 2

Band structure of UT ${\mathrm{LaFe}}_2{\mathrm{As}}_2$ along the path shown in Fig. 1 with total La $5d$ weights marked. The occupied La weight is mostly of $5d_{xy}$ character.

Figure 3

Band structure and Fermi surfaces of UT ${\mathrm{LaFe}}_2{\mathrm{As}}_2$ calculated within GGA. The high symmetry points are standard body-centered tetragonal points as listed in Ref. [27], except for $Z^{\prime }$ which is half way between $\mathrm{\Gamma }$ and $Z$ [see the path marked in Fig. 1].

Figure 4

Noninteracting susceptibility of UT ${\mathrm{LaFe}}_2{\mathrm{As}}_2$.