Advantageous grain boundaries in iron pnictide superconductors

Abstract: High critical temperature superconductors have zero power consumption and could be used to produce ideal electric power lines. The principal obstacle in fabricating superconducting wires and tapes is grain boundaries-the misalignment of crystalline orientations at grain boundaries, which is unavoidable for polycrystals, largely deteriorates critical current density. Here, we report that High critical temperature iron pnictide superconductors have advantages over cuprates with respect to these grain boundary issues. The transport properties through well-defined bicrystal grain boundary junctions with various misorientation angles (thetaGB) were systematically investigated for cobalt-doped BaFe2As2 (BaFe2As2:Co) epitaxial films fabricated on bicrystal substrates. The critical current density through bicrystal grain boundary (JcBGB) remained high (> 1 MA/cm2) and nearly constant up to a critical angle thetac of ~9o, which is substantially larger than the thetac of ~5o for YBCO. Even at thetaGB > thetac, the decay of JcBGB was much smaller than that of YBCO.

• The paper demonstrates that iron pnictide superconductors, particularly cobalt-doped BaFe2As2 films, maintain high critical current density up to a misorientation angle of about 9°, outperforming cuprates.
• It employs bicrystal grain boundary junctions to systematically analyze the decay of Jc, showing a gradual decline with values remaining above 1 MA/cm² until exceeding the critical angle.
• The research reveals that high-angle junctions transition to SNS-type behavior with non-hysteretic I-V curves, indicating a shift from strong to weak link superconductivity that eases manufacturing constraints.

Advantageous Grain Boundaries in Iron Pnictide Superconductors

The paper by Katase et al. investigates the grain boundary (GB) properties in high critical temperature (T_c) iron pnictide superconductors, specifically focusing on cobalt-doped BaFe$_2$As$_2$ epitaxial films. The principal challenge addressed is the detrimental impact of grain boundary misalignment on the critical current density (J_c) in polycrystalline superconductors. The study systematically analyzes bicrystal grain boundary junctions with varying misorientation angles ($\theta_{GB}$), aiming to assess the potential of iron pnictides in overcoming the limitations exhibited by cuprates in similar contexts.

Key Findings and Results

1. Grain Boundary Sensitivity: Iron pnictides exhibit less sensitivity to grain boundary misorientation than cuprates. The critical angle $\theta_c$ for maintaining high J_c in cobalt-doped BaFe$_2$As$_2$ is approximately 9°, significantly higher than the 3–5° reported for YBa$_2$Cu$_3$O$_7-\delta$ (YBCO). This implies a more tolerant GB interface, facilitating less stringent fabrication requirements for high-performance applications.
2. Current Density Characteristics: The study demonstrates that the critical current density J_c through bicrystal grain boundaries remains consistently high (>1 MA/cm$^2$) until the misorientation angle exceeds $\theta_c$. Beyond this angle, the decay in J_c is more gradual than that observed in YBCO, suggesting that iron pnictide GBs maintain stronger current-carrying capability at higher misorientations.
3. Josephson Junction Behavior: The I-V characteristics of high-angle BGBs show a transition from strong-to-weak link behavior, with significant RSJ-like characteristics observed at $\theta_{GB} = 45°$. These junctions behave as superconductor-normal metal–superconductor (SNS) junctions, evidenced by non-hysteretic I-V curves even at low temperatures, implying metallic nature of the junction barriers.
4. Microstructural Observations: The structural analysis using HR-TEM reveals periodic dislocation arrangements at low misorientation angles, transitioning to blurred lattice fringes at higher angles ($\theta_{GB} = 45°$). This structural shift correlates with the emergence of weak-link behavior.

Implications and Future Directions

This investigation suggests that iron pnictides could effectively replace cuprates in applications requiring resilient superconducting wires and tapes, due to their higher tolerance to GB misalignment. The larger allowed $\theta_c$ reduces the need for complex buffer layers, offering simpler and lower-cost production methods.

From a practical standpoint, these findings significantly ease the engineering requirements for manufacturing high-joint current superconducting tapes, making them more viable for widespread industrial applications. With further enhancements in J_c, iron pnictide materials could meet the demands for high-field applications, promising advances in the field of superconducting technology.

Theoretically, this work bolsters the understanding of GB superconductivity mechanisms in pnicogen-based materials, potentially guiding the design of new superconducting compounds with optimized property sets. Further research should investigate the interplay between grain boundary dislocation structures and superconducting phase coherence, aiming to unveil deeper insights into the weak-link behavior and further improve material performance.

Conclusion

Katase et al.'s research contributes a significant step forward in the understanding of grain boundary characteristics in iron pnictide superconductors. By demonstrating the superior GB tolerance of cobalt-doped BaFe$_2$As$_2$, the study highlights the potential for simpler yet effective fabrication pathways, paving the way for broader implementation of these materials in superconducting applications.