Abstract Body

Vapor-Liquid-Solid Growth technique (VLS) enables rapid film growth. It was confirmed that the deposition rate could reach up to a maximum in our experiments of 26 nm/sec using a 100 Hz excimer laser ^(1,2). Moreover, while an a-axis oriented layer can be formed on films with a thickness of several micrometers fabricated by conventional PLD (Pulsed Laser Deposition), VLS films were observed to maintain a c-axis orientation even in the thicknesses of 10 μm. Furthermore, cross-sectional TEM observations revealed a single-crystal-like structure with no defects compared to PLD films. The critical current density (J_c) at 77K of VLS films is lower than that of PLD films. This is due to the lack of pinning centers such as stacking faults.

In the case of a VLS-REBCO+BaHfO₃ (BHO-VLS) coated conductor (c.c.) deposited with rapid deposition rates, TEM observations confirm no nanorod alignment parallel to the c-axis. The Jc-B-θ at 77K also showed that the ratio of J_c at B//c to J_c min was ~1.1, suggesting an absence of c-axis correlated pinning effect due to the no aligned nanorods.

In this study we investigate a suitable APC material for improvement of superconducting properties in REBCO thick c.c. prepared by the VLS technique. Based on extrapolations regarding the solubility of BMO materials into a liquid layer consisting of Ba-Cu-O in the VLS technique, it was anticipated that BHO is less soluble in the liquid layer, whereas BaSnO₃ (BSO) could be more soluble. Therefore, from the viewpoint of the solubility of BMO materials, through Monte Carlo simulations on REBCO+BMO growth by VLS technique were performed. As a result, it was found that when the solvation value is small (e.g., BHO), the BMO precipitate tends to segregate into a plate-like crystal along the ab-plane of REBCO. Conversely, when the solvation value is large (e.g., BSO), the precipitate tends to segregate in a block form ⁽³⁾. Consequently, BSO material is expected more suitable material for artificial pinning centers for REBCO c.c. prepared by the VLS technique.

We compared the superconducting properties and microstructure of REBCO+3vol% BSO c.c. (VLS-BSO) and REBCO+3vol% BHO c.c. (VLS-BHO) prepared by the VLS technique on IBAD tapes using a 100Hz excimer laser. Thickness of samples was 1.4 μm. As a result of J_c-B in the samples at 77K and 65K, VLS-BSO c.c. showed higher properties at both 77K and 65K (B//c) in the magnetic field to 9T. Figure 1 shows the J_c-B-θat 77K and 1T for the samples, as well as for PLD-3vol% BHO using a conventional PLD method. The ratio of Jc at B//c to Jc min of the VLS- BSO c.c. is 1.3, and a peak was confirmed in the c-axis direction, confirming the nanorod pinning effect compared to the VLS-BHO and the PLD-BHO.

Figure 2(a) shows a cross-sectional TEM image, and (b) shows a histogram of the tilt angle of the nanorod from the c-axis direction. It was confirmed that the nanorods grew parallel to the c-axis within 10 degrees, and the tendency was similar to the results of the MC simulation. Furthermore, it was confirmed from the planar TEM (Fig. 2(c)) that the diameter of the nanorods was 5~10 nm. From the MC simulation (4, 5), it was inferred that there is a correlation between the growth rate and the nanorods, and this study also demonstrated that the nanorods became thinner by high-speed deposition using the VLS growth technique.

In the future, to examine the demonstration of the high potential of the VLS c.c. , we plan to check the Ic of a 4 μm thick VLS-BSO and superconducting properties in high magnetic field at 20K and 4.2K.

References

(1) T. Ito, et al ., IEEE. Trans. Appl. Supercond., 31, 5, (2021), 6601304.
(2) T. Ito, et al ., IEEE. Trans. Appl. Supercond., 31, 5, (2021), 6601205.
(3) T. Arita, et al. The 36th International Symposium on Superconductivity (2023). (4) Y. Ichino, et al., Jpn. J. Appl. Phys. 56(2017)015601.
(5) Y. Ichino, et al., IEEE Trans. Appl. Supercond. 27(2017)7500304.

Acknowledgment

This work was partly supported by JSPS-KAKENHI(20H02682, 21H01872), NEDO, The Kazuchika Okura Memorial Foundation. The IBAD-MgO tapes were provided by Dr. Izumi and Dr. Ibi of AIST.