Abstract Body

Introduction
To increase critical current density(Jc) of YBa₂Cu₃O_7-δ(YBCO) superconducting thin films, artificial pinning center(APC) such as BaMO₃(BMO, M = Zr, Hf, Sn, etc.) are incorporated into YBCO films. BMO nanoparticles and nanorods are effective pinning centers, but the incorporation of second phase materials like BMO degrades the YBCO matrix. The difference of lattice parameter results in distortion at the interface between YBCO and BMO. In addition, the nanorod morphology is not stable, such as uneven distribution or deformation, in the case of large BMO content. The introduction of APCs without using second phase material should be discussed for further J_c improvement. Substrate-induced pinning centers in YBCO thin films has been reported [1,2]. The density of substrate-induced pinning is low in the previous reports, and higher density is needed to improve the J_c in high magnetic field. Therefore, we focus on the structure of buffer layer, which affects the initial stage of YBCO growth. Both SrTiO₃(STO) and CeO₂ are used as the buffer layer for YBCO growth, and it has been reported that nanoscale structure can be fabricated in the nanocomposite of STO and CeO₂ [3]. Therefore, we investigated the pinning centers induced by the nanocomposite buffer layer of STO and CeO₂ to introduce high density of substrate-induced pinning centers.

Experimental method
We prepared YBCO thin films on the STO and CeO₂ nanocomposite buffer layer by using Pulsed Laser Deposition(PLD) method. MgO was used as the substrate. The volume percentage of STO:CeO₂(STO:CeO₂ratio) was changed to control the structure of buffer layer, and the X:100-X film denotes the YBCO film deposited on the buffer layer with the STO:CeO₂ ratio of X:100-X. We used Dynamic Force Mode(DFM) of Atomic Force Microscope(AFM) to observe the surface roughness of the buffer layer. Energy dispersive X-ray spectroscopy(EDX) was used to evaluate the atomic fraction. Using X-Ray Diffraction(XRD), the phase formation and film orientation were evaluated. We also measured critical temperature(T_c) and J_c in the temperatures of 20 K~77 K and magnetic field of 0 T-9 T.

Experimental results and discussion
STO n00 and CeO₂ n00 peaks in XRD 2θ-ω scan were observed in the YBCO/(STO, CeO₂)/MgO thin films. The φ-scan shows that STO grows with the cube-on-cube orientation relationship. On the other hand, CeO₂ has two types of orientation relationships of the cube-on-cube orientation and the 45-degree rotated orientation. The X-ray intensity ratio of the cube-on-cube orientation increases with an increase in the volume fraction of CeO₂. This shows that the crystalline defects such as grain boundaries were successfully controlled by changing the STO:CeO₂ ratio.

Fig.1(a) shows J_c-B characteristics at 65 K and 20 K in 0-9 T. Regardless of temperature, the 74:26 film has the highest J_c,self. However, T_c was 76.9 K for the 37:63 sample, 79.7 K for the 74:26 sample, and 81.3 K for the 47:53 sample. Due to the low T_c, the Jcvalues are not so high. To discuss the J_c behavior, Fig.1(b) shows the result of the normalized J_c (J_c/J_c,self). Interestingly, the J_c variation with temperature and magnetic field depends on the structure of the nanocomposite buffer layer. While the normalized J_c is the highest in the 47:53 film at 65 K, the highest normalized J_c was obtained for the 37:63 film at 20 K. This result cannot be explained by the T_c difference, and the influence of vortex pinning dominated the J_c-B characteristics in the YBCO films on the nanocomposite buffer layer. The nanocomposite buffer layers in this study consist of nanoscale STO and CeO₂, which causes stress and crystalline defects. The crystalline defects induced by the buffer layer work as pinning centers. On the other hand, the Tc degradation may be also due to degraded crystallinity or grain boundaries. Further optimization of the volume fraction of STO and CeO₂ is required for simultaneous achievement of high T_c and controlled defects in the YBCO films on the nanocomposite buffer layer.

References

[1] B. Maiorov, et al; Supercond. Sci. Technol. 19, 891 (2006).
[2] K. Matsumoto, et al; Jpn. J. Appl. Phys. 44, L246 (2005).
[3] Sang Mo Yang, Shinbuham Lee, et al; Nature Communications 6, 8588 (2015).

Acknowledgment

This work was partly supported by JSPS-KAKENHI(20H02682, 21H01872), NEDO, The Kazuchika Okura Memorial Foundation, and The Hibi Science Foundation.