Nanoscale interlayer defects in iron arsenides

Abstract

Using a local real-space microscopy probe, we discover evidence of nanoscale interlayer defects along the c-crystallographic direction in BaFe2As2 ('122') based iron-arsenide superconductors. We find ordered 122 atomic arrangements within the ab-plane, and within regions of ~10 to 20 nm size perpendicular to this plane. While the FeAs substructure is very rigid, Ba ions are relatively weakly bound and can be displaced from the 122, forming stacking faults resulting in the physical separation of the 122 between adjacent ordered domains. The evidence for interlayer defects between the FeAs superconducting planes gives perspective on the minimal connection between interlayer chemical disorder and high-temperature superconductivity. In particular, the Cooper pairs may be finding a way around such localized interlayer defects through a percolative path of the ordered layered 122 lattice that may not affect Tc.

Structural and chemical properties of superconducting Co-doped BaFe2As2 thin films grown on CaF2

“…Another type of planar defect shown in figure 10(b) consists of a single Ba plane, which suddenly exhibits a lower HAADF-STEM intensity. The distance to the neighboring Ba planes is increased by around 20% (figure 10(c)) in agreement with the results by Zheng et al [16]. In the following, these defects are denoted as 'isolated Ba planes'.…”

“…For sample/slow, BaF 2 precipitates and ab-plane-oriented planar defects are visible in HAADF-STEM images (see also figures 3(b) and (c)) and are also observable in the highresolution STEM image in figure 7(b) (horizontal regions with dark contrast). These defects were also observed by other groups in films [12][13][14], polycrystalline bulk samples [15], and single crystals [16,17]. A BaF 2 precipitate is visible (indicated by an arrow in figure 7(b) and verified by FT analysis not shown here), which has formed at the substrate interface.…”

“…Similar to the oxidation of BaF 2 and CaF 2 , residual O could play an important role for the generation of these defects. Overall, stacking faults and isolated Ba planes show a distinct dark contrast in HAADF-STEM images and were observed in thin films [12][13][14] but also in (poly-and single-crystalline) bulk samples of Ba122 [15][16][17], which highlights the importance of understanding their origin. These defects locally introduce strain in Ba122 [48] due to the observed lattice-parameter changes, which influences the superconducting properties.…”

“…Here the same dark contrast at planar defects is observed. Furthermore, these defects are also observed in films on F-free substrates [12] and in bulk Ba122 [15][16][17], which makes in-situ oxidation of an F-rich phase at stacking faults during TEM measurements unlikely. The latter aspect is in contrast to oxidation of BaF 2 /CaF 2, which was discussed earlier (section 3.1).…”

“…Despite this scientific progress, the mechanisms of Ba122 growth on CaF 2 via pulsed laser deposition (PLD) are still not fully understood due to the influence of various fabrication parameters and possible interactions with the (heated) substrate [9][10][11]. For example, different types of planar defects can form in Ba122 thin films and bulk samples [12][13][14][15][16][17]. Furthermore, F diffusion into IBS is possible when grown on CaF 2 [18][19][20].…”