Density Functional Study of ${\mathrm{LaFeAsO}}_{1-x}{\mathrm{F}}_x$: A Low Carrier Density Superconductor Near Itinerant Magnetism

Density functional studies of 26 K superconducting LaFeAs(O,F) are reported. We find a low carrier density, high density of states, $N(E_F)$, and modest phonon frequencies relative to $T_c$. The high $N(E_F)$ leads to proximity to itinerant magnetism, with competing ferromagnetic and antiferromagnetic fluctuations and the balance between these controlled by the doping level. Thus LaFeAs(O,F) is in a unique class of high $T_c$ superconductors: high $N(E_F)$ ionic metals near magnetism.

Figure 1

LDA density of states and projections onto the LAPW spheres on a per formula unit both spins basis. Note that much of the As $p$ character will be outside the As sphere, reducing their apparent weight.

Figure 2

Band structure of LaFeAsO around $E_F$ showing the effect of As breathing along $z$ by $\delta z_{\mathrm{As}}=0.04$ (0.035 Å). The unshifted band structure is indicated by the solid black line, while the shift away (towards) the Fe is indicated by the blue dotted lines (green dashed lines).

Figure 3

LDA Fermi surface of LaFeAsO shaded by velocity [darker (blue) is low velocity]. The symmetry points are $\Gamma =(0,0,0)$, $Z=(0,0,1/2)$, $X=(1/2,0,0)$, $R=(1/2,0,1/2)$, $M=(1/2,1/2,0)$, $A=(1/2,1/2,1/2)$.

Figure 4

Induced moments as a function of antiferromagnetic (AFM) and ferromagnetic (FM) field. The field is applied by an extra spin dependent potential $+/-B$ inside the $\mathrm{Fe}/\mathrm{Ni}$ LAPW spheres.

Figure 5

Phonon dispersions and density of states of LaFeAsO.