Abstract Body

Introduction
For superconductor application, high critical current density (J_c) is required. YBa₂Cu₃O_y(YBCO) is a promising high-T_c superconductor because it currently has the highest J_c in a magnetic field. Grain boundaries of YBCO exhibit a weak link for the superconducting current due to the short coherence length of a few Å and pair symmetry [1]. Therefore, the J_c degradation due to the weak link problem should be suppressed for power and energy applications.

J_c^GB) depends on the grain boundary tilt angle and decreases exponentially with the tilt angle, high J_c can be obtained by texturing the YBCO films. The Ion Beam Assisted Deposition (IBAD) method is used to fabricate the textured substrates. The half width at full maximum of φ scan (Δφ), which shows the degree of in-plane orientation, is as small as 3° in the currently most optimized process. Highly oriented substrates (Δφ<5°) are used for the coated conductors. The second approach is to improve the J_c^GB by elemental doping such as Ca. If the J_c^GB can be improved by the doping, the moderately oriented substrates with Δφ=7-10° become available. It has been reported that the Ca doping improves the J_c^GB for the tilt angle of 7-10° in the YBCO bicrystalline films [2]. However, there are few reports on the fabrication of Ca-doped YBa₂Cu₃O_y(CYBCO) deposited on the IBAD tapes. Therefore, the J_c property of the CYBCO coated conductors is still unclear. In this study, we fabricate high performance CYBCO films on moderately oriented substrates to demonstrate the J_c improvement by Ca doping even in coated conductors.

Experimental method
YBCO and CYBCO films with a thickness of 200-300 nm were deposited on IBAD-MgO substrate using pulsed laser deposition (PLD) with a KrF excimer laser. Here the Ca doping content was 5%. The top layer of the IBAD substrates is a CeO₂ layer with orientations Δφ = 3, 7, 9.8, and 12.3°. Δφ = 3, 7, 9.8 and 12.3°correspond to highly oriented IBAD(H-IBAD), moderately oriented IBAD(M-IBAD), and weakly oriented IBAD(W-IBAD), respectively. The H-IBAD is used for the coated conductors, and the orientation of the M-IBAD and W-IBAD is insufficient for a substrate for coated conductors. The deposition temperature and oxygen partial pressure during deposition were 830-900°C and 53 Pa, respectively. The crystallographic orientation of the films was evaluated by X-ray diffraction analysis. A bridge with a width of 100 µm and a length of 1 mm was fabricated by laser etching for J_c measurement. The J_c-B characteristics of the film were measured using Physical Properties Measurement System(PPMS) in the magnetic field of 0-9 T and temperatures of 20-77 K.

Experimental results and discussion
Figure 1 shows the J_c - B characteristics of YBCO and CYBCO films deposited on the IBAD (Δφ = 3°) and IBAD (Δφ = 7°) at 20 K and 77 K in a magnetic field aligned with the c-axis. The J_c decreases with Δφ, which is consistent with the result in the bicrystalline films. The J_c of CYBCO/IBAD (Δφ = 7°) (0.94 MA/cm²) is about twice as high as that of YBCO/IBAD (Δφ = 7°) (0.44 MA/cm²) at 77 K and 0 T. However, the CYBCO/IBAD (Δφ = 7°) film does not have high J_c in magnetic field at 77 K due to the low Tc (83-84 K). However, a tenfold improvement in J_c was observed at 20 K and 9 T from 0.28 MA/cm2 (YBCO/IBAD (Δφ = 7°)) to 3.2 MA/cm2 (CYBCO/IBAD (Δφ = 7°)). The J_c^GB is significantly reduced by the grain boundary with a tilt angle higher than 4.5° [3]. The J_c in the low-field region was decreased in the YBCO/IBAD (Δφ = 7°), while the Jc degradation in the low-field region was suppressed in the CYBCO/IBAD (Δφ = 7°). This suggests that Ca doping improves the JcGB for the tilt angle of ~7° in the coated conductor. Furthermore, the in-field J_c in the YBCO/IBAD (Δφ = 7°) is much lower than that in the YBCO/IBAD(Δφ = 3°) at 20 K. However, the J_c-B properties of the CYBCO/IBAD (Δφ = 7°) are comparable to those of YBCO/IBAD(Δφ = 3°) at 20 K. Although the YBCO/H-IBAD is the conventional coated conductor, this study suggests that the CYBCO/M-IBAD is a new class of coated conductors whose substrate selection and process design are expanded.

References

[1] P. Chaudhari, R. H. Koch, R. B. Laibowitz, T. R. McGuire, and R. J. Gambino, Phys. Rev. Lett. 58, 2684 (1987).
[2] Sang-li Kim, David C. Larbalestier, Journal of Applied Physics 128, 103905 (2020).
[3] D. T. Verebelyi, et al. Appl. Phys. Lett. 76, 1755-1757 (2000).

Acknowledgment

Fabrication of IBAD-MgO substrates was supported by Dr. A, Ibi, and Dr. T. Izumi at AIST. This work was partly supported by JSPS-KAKENHI(20H02682, 21H01872), NEDO, The Kazuchika Okura Memorial Foundation, and The Hibi Science Foundation.