We show, from first-principles calculations, that the hole-doped side of FeAs-based compounds is different from its electron-doped counterparts. The electron side is characterized as Fermi surface nesting, and SDW-to-NM quantum critical point (QCP) i
s realized by doping. For the hole-doped side, on the other hand, orbital-selective partial orbital ordering develops together with checkboard antiferromagnetic (AF) ordering without lattice distortion. A unique SDW-to-AF QCP is achieved, and $J_2$=$J_1/2$ criteria (in the approximate $J_1&J_2$ model) is satisfied. The observed superconductivity is located in the vicinity of QCP for both sides.
By first-principles calculations, we present a doping-dependent phase diagram of LaO{it M}As ({it M}=V--Cu) family. It is characterized as antiferromagnetic semiconductor around LaOMnAs side and ferromagnetic metal around LaOCoAs. Both LaOFeAs and La
ONiAs, where superconductivity were discovered, are located at the borderline of magnetic phases. Extensive Fermi surface analysis suggests that the observed superconductivity is of electron-type in its origin. We discuss possible pairing mechanisms in the context of competing ferromagnetic phases found in this work and the ferromagnetic spin fluctuations.
The Fermi surfaces (FS) of LaOFeAs (in $k_z$=0 plane) consist of two hole-type circles around $Gamma$ point, which do not touch each other, and two electron-type co-centered ellipses around M point, which are degenerate along the M-X line. By first-p
rinciples calculations, here we show that additional degeneracy exists for the two electron-type FS, and the crucial role of F-doping and pressure is to enhance this orbital degeneracy. It is suggested that the inter-orbital fluctuation is the key point to understand the unconventional superconductivity in these materials.
