Direct visualization of soliton stripes in the $\mathrm{Cu}{\mathrm{O}}_2$ plane and oxygen interstitials in $\mathrm{B}{\mathrm{i}}_2\left(\mathrm{S}{\mathrm{r}}_{2-x}\mathrm{L}{\mathrm{a}}_x\right)\mathrm{Cu}{\mathrm{O}}_{6+\delta }$ superconductors

Microstructure features in correlation with the incommensurate modulation and oxygen interstitials in $\mathrm{B}{\mathrm{i}}_2\left(\mathrm{S}{\mathrm{r}}_{2-x}\mathrm{L}{\mathrm{a}}_x\right)\mathrm{Cu}{\mathrm{O}}_{6+\delta }$ superconducting materials were studied by Cs-corrected scanning transmission electron microscopy. Atomic displacements following the modulation wave were well characterized by a sinusoidal wave for each atomic layer, which highlighted clear changes resulting from increases in the La concentration. Careful investigations of the alterations in the local atomic structure revealed that remarkable microstructural features, i.e., notable soliton lines, which arise from the prominent interplay between incommensurate modulation and the basic lattice, appear at the $\mathrm{Cu}{\mathrm{O}}_2$ sheets yielding visible structural anomalies for $x$ ranging from 0.40 to 0.85. The interstitial oxygen atoms between the SrO-BiO layers became clearly visible for $X\ge 0.73$ and showed well-defined ordered states in the $x=1.10$ sample. These structural features, in particular the strong structural effects of the soliton lines on the $\mathrm{Cu}{\mathrm{O}}_2$ sheets, could evidently affect the physical properties of layered La-Bi2201 systems.

Figure 1

(a) High-spatial-resolution ABF STEM image of $\mathrm{B}{\mathrm{i}}_2\left(\mathrm{S}{\mathrm{r}}_{2-x}\mathrm{L}{\mathrm{a}}_x\right)\mathrm{Cu}{\mathrm{O}}_{6+\delta }$ with $x=0.40$. (b) Highly magnified atomic resolution image and schematic diagram showing the appearance of phase slip on the soliton line. (c) Schematic diagram explaining the phase soliton in the $\mathrm{Cu}{\mathrm{O}}_2$ sheets of the $x=0.40$ sample.

Figure 2

(a) Dark-field TEM image showing the configurations of the stripe lines in some regions. (b) Schematic line diagrams illustrating the configurations of the soliton domain patterns observed in our experimental investigations.

Figure 3

(a) High-spatial-resolution ABF STEM image of $\mathrm{B}{\mathrm{i}}_2\left(\mathrm{S}{\mathrm{r}}_{2-x}\mathrm{L}{\mathrm{a}}_x\right)\mathrm{Cu}{\mathrm{O}}_{6+\delta }$ with $x=1.10$, revealing that the interstitial oxygen appears between the SrO and BiO layers. (b) Highly magnified image of the area in (a). (c) A simulated ABF STEM image. (d) A superstructure model based on the experimental images.