Surface Nematic Order in Iron Pnictides

摘要

Electronic nematicity plays important role in iron-based superconductors.These materials have layered structure and theoretical description of theirmagnetic and nematic transitions has been well established in two-dimensionalapproximation, i.e., when the layers can be treated independently. However, theinteraction between iron layers mediated by electron tunneling may causenon-trivial three-dimensional behavior. Starting from the simplest model fororbital nematic in a single layer, we investigate the influence of interlayertunneling on bulk nematic order and possible preemptive state where this orderis only formed near the surface. We found that the interlayer tunneling suppresses the bulk nematicity whichmakes favorable formation of a surface nematic above the bulk transitiontemperature. The purely electronic tunneling Hamiltonian, however, favorsalternating from layer-to-layer nematic order parameter in the bulk. Theuniform bulk state typically observed experimentally may be stabilized by thecoupling with the elastic lattice deformation. Depending on strength of thiscoupling, we found three regimes: (i) surface nematic and alternating bulkorder, (ii) surface nematic and uniform bulk order, and (iii) uniform bulkorder without the intermediate surface phase. The intermediate surface-nematicstate may resolve the current controversy about the existence of the weak"meta-nematic transition" in the compound BaFe$_2$As$_{2-x}$P$_{x}$.