Superconductivity and magnetism in YFe${}_2$Ge${}_2$

We report calculations of the electronic structure and magnetic properties of YFe${}_2$Ge${}_2$ and discuss the results in terms of the observed superconductivity near magnetism. We find that YFe${}_2$Ge${}_2$ is a material near a magnetic quantum critical point based on comparison of standard density functional results that predict magnetism with experiment. The band structure and Fermi surfaces are very three dimensional and higher conductivity is predicted in the $c$-axis direction. The magnetism is of Stoner type and is predominately from an in-plane ferromagnetic tendency. The interlayer coupling is weak giving a perhaps two dimensional character to the magnetism, which is in contrast to the conductivity and may be important for suppressing the ordering tendency. This is compatible with a triplet superconducting state mediated by spin fluctuations.

Figure 1

Band structure of YFe${}_2$Ge${}_2$ with the Fermi level at 0 eV. The two deep bands centered at $\sim -9$ eV are from the Ge 4$s$ state, while the higher valence bands arise primarily from hybridized Ge $4p$–Fe $3d$ states.

Figure 2

Density of states and projections onto LAPW spheres.

Figure 3

Band structure of YFe${}_2$Ge${}_2$ around the Fermi energy (0 eV, light dotted line), showing the different orbital characters via so-called fat bands plots, in which the size of the plotting points are proportional to the given orbital characters plus a small constant.

Figure 4

Calculated Fermi surfaces of YFe${}_2$Ge${}_2$, showing the five sheets (1–5). The bottom shows the Fermi surface in an extended zone scheme.

Figure 5

Fixed spin moment total energy as a function of constrained spin magnetization for YFe${}_2$Ge${}_2$ on a per formula unit basis. The symbols are the calculated points and the curve is a spline interpolation. Note the instability of the non-spin-polarized state against ferromagnetism.