Atomic and electronic structures of superconducting ${\mathrm{BaFe}}_2{\mathrm{As}}_2/{\mathrm{SrTiO}}_3$ superlattices

The nanoscale structural, chemical, and electronic properties of artificial engineered superlattice thin films consisting of superconducting Co-doped $\mathrm{BaF}{\mathrm{e}}_2\mathrm{A}{\mathrm{s}}_2\left(\mathrm{Ba}\text{−}122\right)$ and nonsuperconducting $\mathrm{SrTi}{\mathrm{O}}_3\left(\mathrm{STO}\right)$ layers are determined by using atomically resolved scanning transmission electron microscopy and electron energy loss spectroscopy. The bonding of $\mathrm{Ba}\text{−}122/\mathrm{STO}$ occurring between As (Ba) and $\mathrm{SrO}\left(\mathrm{Ti}{\mathrm{O}}_2\right)$ terminated layers has been identified. The thin STO (3 unit cell) insertion layers are a mixture of cations (Ba, Sr, Fe, and Ti) and rich in oxygen vacancies and the Ba-122 layers (10 unit cell) are free of vertical second phases. Our results explain why these superlattices show anisotropic transport response to an external magnetic field, i.e., strong $ab$-axis pinning (enhancing critical current density) and no $c$-axis pinning, which is opposite to single layer Ba-122 thin films. These findings reveal physical and chemical properties of superconducting/nonsuperconducting heterostructures and provide important insights into engineering of superconducting devices.

Figure 1

(a) HAADF image of 24 layers of $\mathrm{Ba}\text{−}122/\mathrm{STO}$ superlattice on LSAT substrate with a 40 nm STO template layer. A schematic is shown in the right of the image. (b) High-resolution HAADF image of the ⟨100⟩ projection of the $\mathrm{Ba}\text{−}122/\mathrm{STO}$ structure with 10 unit cells in each Ba-122 layer and 3 unit cells in each STO layer. A schematic of Ba-122 and STO structures is included to the right of the images. Blue atoms, Ba; purple, Sr; green, As; brown, Fe; Red, oxygen.

Figure 2

(a) HAADF image of the interface between the first Ba-122 layer and the STO template layer. The interface of $\mathrm{Ba}\text{−}122/\mathrm{STO}$ is indicated by the arrow. Near the interface in STO, inhomogeneous intensities in the profile indicate a mixture of Sr and Ba atoms. (b) The $c/a$ ratio across the interface. The calculation is based on the $\mathrm{Ba}/\mathrm{Sr}$ atom positions from the HAADF image which is averaged along the interface (horizontally) to reduce noise. Atom positions were determined by simultaneously fitting two-dimensional Gaussian peaks. The arrows indicate the weak bonding at the interface. (c) HAADF image of interfaces of the Ba-122 layer and the 3 unit cell STO insertion layer. The interfaces of $\mathrm{Ba}\text{−}122/\mathrm{STO}$ are indicated by the arrows. In the insertion layer, inhomogeneous intensities in the profile indicate a mixture of Sr and Ba atoms. (d) The $c/a$ ratio across the insertion layer calculated from $\mathrm{Ba}/\mathrm{Sr}$ positions in the average HAADF image. (e) Enlarged view of the $\mathrm{Ba}\text{−}122/\mathrm{STO}$ interface structure from an averaged image. Left: color scale highlights each atom column. Middle: the atomic schematic structure overlaid on the image. Right: the simulated HAADF image by the mactempas software. (f) The intensity profile from the rectangular region in (e) distinguishes the Fe and As atom columns.

Figure 3

(a) EELS line scan across a STO insertion layer, a Ba-122 layer, and the interface of a $\mathrm{Ba}\text{−}122/\mathrm{STO}$ template layer with steps of $\sim 0.09\mathrm{nm}$. Ba diffusion in the STO template layer is much deeper than that of Fe. In the STO insertion layer, the intensity of Fe decreases, but the intensity of Ba does not show any obvious change. (b) Relative composition extracted from the EELS line scan shows the Fe:Ba ratio is close to 1.91 in the Ba-122 and the Ba:Fe:Ti ratio is close to 1:1:1 in the middle of the insertion STO layer.

Figure 4

(a) Fine structure of O-$K$ edges showing the presence of oxygen vacancies in regions 1 and 2. (b) Fine structure of Ti-$L$ edges shows some reduction of $\mathrm{T}{\mathrm{i}}^{4+}$ to $\mathrm{T}{\mathrm{i}}^{3+}$ in regions 1 and 2. The inset shows the locations where the spectra were acquired. (c) Distribution of $\mathrm{T}{\mathrm{i}}^{3+}$ and $\mathrm{T}{\mathrm{i}}^{4+}$ by fitting Ti-$L$ edges using reference spectra of $\mathrm{T}{\mathrm{i}}^{4+}$ in $\mathrm{SrTi}{\mathrm{O}}_3\text{and}\mathrm{T}{\mathrm{i}}^{3+}$ in $\mathrm{LaTi}{\mathrm{O}}_3$.