Calculated cleavage behavior and surface states of LaOFeAs

The layered structure of the iron-based superconductors gives rise to a more or less pronounced two dimensionality of their electronic structure, most pronounced in LaOFeAs. Consequences are distinct surface states to be expected to influence any surface sensitive experimental probe. In this work a detailed density-functional analysis of the cleavage behavior and the surface electronic structure of LaOFeAs is presented. The surface states are obtained to form two-dimensional bands with their own Fermi surfaces markedly different from the bulk electronic structure.

Figure 1

(Color online) The Kohn-Sham GGA band structure of bulk LaOFeAs. The fat bands in red (shadow) weigh the $\text{O-}2p$ and $\text{La-}4f,5d,6s$ orbital contribution to the band state.

Figure 2

(Color online) Slab geometry: unit cells of ${\left(\text{LaO}\right)}_4{\left(\text{FeAs}\right)}_6$ (a) and ${\left(\text{LaO}\right)}_6{\left(\text{FeAs}\right)}_4$ (b) slabs; blue (biggest spheres): La, red (smallest spheres): O, orange (medium, light): Fe, and violet (medium, dark): As.

Figure 3

(Color online) The atom layer distances as functions of the stretched $c$ axis lattice constant. In the upper panel the Fe-O layer distances are shown, the two upper curves across the slit and the two lower curves between the surface and subsurface triple layer on both sides of the slit (FeAs and LaO terminated, respectively). On the lower panel the behavior of layer distances within the triple layers La/O/La and As/Fe/As, respectively, are shown, again for the bulk and for the surface triple layers. Observe the different scales of the ordinates. For more explanation see the text.

Figure 4

(Color online) Band structure of the ${\left(\text{LaO}\right)}_4{\left(\text{FeAs}\right)}_6$ slab. The thickness of the red (shadow) lines represents in turn the orbital weight of (a) surface $\text{As}4p$, (b) subsurface $\text{Fe}3d$, (c) subsurface $\text{As}4p$, and (d) central (bulk) $\text{Fe}3d$.

Figure 5

The 2D FSs of the slab ${\left(\text{LaO}\right)}_4{\left(\text{FeAs}\right)}_6$. The zone center is $\Gamma$ and the corner point is $M$. From $\Gamma$ outward in turn the FSs are two times bulk, three times surface; from $M$ outward: two tiny surface FSs, two bulk FSs. (The small wiggles are due to the resolution of the used $\mathit{k}$ mesh.)

Figure 6

(Color online) Band structure of the ${\left(\text{LaO}\right)}_{10}{\left(\text{FeAs}\right)}_8$ slab. The thickness of the red (shadow) lines represents in turn the orbital weight of (a) surface $\text{La}5d+6s$, (b) subsurface $\text{O}2p$, (c) subsurface $\text{La}5d+6s$, (d) next to surface $\text{Fe}3d$, and (e) inner (bulk) $\text{Fe}3d$.

Figure 7

The 2D FSs of the slab ${\left(\text{LaO}\right)}_{10}{\left(\text{FeAs}\right)}_8$. Compare Fig. 5. The FSs from $\Gamma$ outward are two times bulk $\text{Fe}3d$, the outer one surface $\text{La}6s$; from $M$ outward two times bulk $\text{Fe}3d$, the second one split in the slab, see text for explanation, two outer subsurface $\text{Fe}3d$ FSs.