Competing Orders in FeAs Layers

Using the unrestricted Hartree-Fock approximation and Landau theory we identify possible phases competing with superconductivity in FeAs layers. We find that close to half-filling the transition from the paramagnet to the magnetically ordered phase is first order, making anharmonicities relevant and leading to a rich phase diagram. Between the already known one-dimensionally modulated magnetic stripe phase and the paramagnet we find a new phase which has the same structure factor as the former but in which magnetic moments at nearest-neighbor sites are at right angles making electrons acquire a nontrivial phase when circulating a plaquette at strong coupling. Another competing phase has magnetic and charge order and may be stabilized by charged impurities.

Figure 1

Upper panel: $T=0$ phase diagram of the two-orbital model in the $U$-density plane. The solid circle is the tricritical point. The insets shows the magnetic structure of the ordered phases considered in this study. Lower panels: The local HF DOS for the MS and OM states.

Figure 2

Phase diagram for the Landau toy model (a) and the full Landau model (b). The dashed line is the limit of metastability of the ordered phases. The limit of metastability of the PM phase is given by the line $\alpha =0$. The thick lines represent weakly first-order transitions ending at the tricritical point. The insets show the behavior of the energy for the magnetic phases (full line) and the SCO phase (dashed line) in the regions of metastability.