Unconventional sign-changing superconductivity near quantum criticality in YFe${}_2$Ge${}_2$

I present the results of first principles calculations of the electronic structure and magnetic interactions for the recently discovered superconductor YFe${}_2$Ge${}_2$ and use them to identify the possible nature of superconductivity and quantum criticality in this compound. I find that the Fe $3d$ derived states near the Fermi level show a rich structure with the presence of both linearly dispersive and heavy bands. The Fermi surface exhibits nesting between hole and electron sheets that manifests as a peak in the susceptibility at $(1/2,1/2)$. The antiferromagnetic spin fluctuations associated with this peak may be responsible for mediating the superconductivity in this compound resulting in a $s_{\pm }$ state similar to that of the previously discovered iron-based superconductors. I also find that various magnetic orderings are almost degenerate in energy, which indicates that the proximity to quantum criticality is due to competing magnetic interactions.

Figure 1

Top: LDA non-spin-polarized band structure of YFe${}_2$Ge${}_2$. Bottom: A blowup of the band structure around the Fermi level. The long $\Gamma$-$Z$ direction is from $(0,0,0)$ to $(1,0,0)$ and the short one is from $(0,0,0)$ to $(0,0,1/2)$. The $X$ point is $(1/2,1/2,0)$. The stacking of the Brillouin zone is such that $(1,0,0)=(0,0,1/2)$. See Fig. 1 of Ref. [13] for a particularly illuminating illustration of the reciprocal-space structure.

Figure 2

Electronic density of states of non-spin-polarized YFe${}_2$Ge${}_2$ and projections onto the LAPW spheres per formula unit for both spin basis.

Figure 3

Top: LDA Fermi surface of YFe${}_2$Ge${}_2$. Bottom: The Fermi surface without the large sheet. The shading is by velocity.

Figure 4

Real part of bare susceptibility calculated with the matrix element set to unity.