Surface Nematic Order in Iron Pnictides

TL;DR

This paper investigates how interlayer tunneling influences nematic order in iron pnictides, revealing conditions for surface nematicity, bulk order configurations, and potential explanations for experimental controversies.

Contribution

It introduces a model accounting for interlayer tunneling effects on nematic order, highlighting the emergence of surface nematicity and different bulk order regimes in layered iron superconductors.

Findings

Interlayer tunneling suppresses bulk nematicity.

Surface nematic order can form above bulk transition temperature.

Coupling with lattice deformation stabilizes uniform bulk nematic order.

Abstract

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