Surface nematic order in iron pnictides

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 nematic transition in the compound BaFe2As2−xPx.

Imaging Anomalous Nematic Order and Strain in Optimally Doped BaFe2(As,P)2

“…This suggests that the nematic order develops in a genuine phase transition rather than as a result of local anisotropy amplified by strong nematic susceptibility. Our results are consistent with a surface nematic phase, as has been suggested by calculations incorporating interlayer hopping [32]. The existence of such a phase would relieve the tension between results from bulk and surface probes.…”

“…2(d). This picture is particularly compelling in light of recent work incorporating hopping between Fe−As layers, which has shown that interlayer hopping can produce a surface nematic phase that onsets at significantly higher temperatures than in the bulk [32]. A surface phase, which could also arise due to stabilization of fluctuating order by soft surface phonons [35], would be more susceptible to confinement by boundaries of strain due to the reduced dimensionality and volume of the required region of contiguous deformation, and could be disfavored under transverse compression due to buckling-induced disorder.…”

Nematic transitions in iron pnictide superconductors imaged with a quantum gas

“…By performing the first local measurement of emergent resistivity anisotropy in iron pnictides, we observe a spatially inhomogeneous increase in the temperature at which optical birefringence appears near the surface over that at which anisotropic local transport appears within the bulk. This is consistent with the existence of a higher-temperature so-called 'extraordinary' surface nematic transition [2][3][4], albeit one that emerges inhomogeneously. More broadly, these measurements demonstrate the SQCRAMscope's ability to reveal important insights into the physics of complex quantum materials.…”

“…Together, these measurements reveal that at some, but not all, positions in the Ba(Fe 1−x Co x ) 2 As 2 crystals, twin domains appear in optical birefringence at temperatures higher than in magnetometry. This is consistent with an 'extraordinary' surface nematic transition, at which the surface orders at a higher temperature than the bulk [2][3][4]. Moreover, the transition might be inhomogeneously stabilized by extrinsic strain or disorder.…”

“…However, a combined micro-Laue diffraction and optical pumping study of BaFe 2 (As 1−x P x ) 2 indicates that the nematicity is weakest at the regions of strongest strain, casting doubt on this hypothesis [22]. Song et al have proposed an "extraordinary" nematic surface phase transition [2][3][4], in which the surface breaks C 4 symmetry at a higher temperature than the bulk. Such a scenario would be consistent with the detection of anisotropy above T nem by surface sensitive probes such as ARPES and optical spectroscopy, but not in the bulk specific heat [18].…”