Abstract Body

Ⅰ. Introduction
Due to the high cost of rare-earth based superconducting coated conductors, there is a pressing need to reduce production costs. One potential solution is to simplify the architecture by eliminating the silver stabilization layer. This can be achieved by epitaxially growing a conductive oxide on an oriented nickel substrate, followed by the epitaxial growth of YBCO. By reducing the silver layer, a significant cost reduction is expected. In this study, we focused on LaNiO₃ (LNO) as a conductive intermediate layer and investigated the conditions for epitaxial growth of LNO [1, 2]. Furthermore, we explored the conditions for epitaxial growth of YBCO films on LNO films.

Ⅱ. Experimental Methods
In this study, sintered LNO and YBCO targets were prepared, and LNO intermediate layers were deposited on SrTiO₃(STO) (100) single-crystal substrates using pulsed laser deposition (PLD). LNO intermediate layers were deposited on the STOsubstrates at a fixed substrate heater temperature of 900°C and an oxygen pressure of 50 Pa, with varying deposition times from 15 to 60 minutes (film thickness: 130-430 nm). Subsequently, YBCO films were deposited on these LNO layers at a substrate heater temperature of 900°C, an oxygen pressure of 23 Pa, and a deposition time of 60 minutes (film thickness: 480 nm) to fabricate multilayer films. The prepared thin films were characterized by X-ray diffraction (XRD) and temperature-dependent resistivity (R-T) measurements. Additionally, the film thickness was evaluated by step height measurement using a surface profiler.

Ⅲ. Results and Discussion
The relative intensity of the YBCO 103 peak in the XRD patterns decreased as the thickness of the LNO intermediate layer decreased. The YBCO 103 peak is the strongest diffraction peak for polycrystalline YBCO, indicating the presence of polycrystalline YBCO grains in the samples. Two possible reasons for the formation of polycrystalline YBCO are: (1) when the LNO intermediate layer is thick, the heat energy from the STO substrate is less likely to be transferred to the YBCO film being deposited on the intermediate layer, leading to the growth of polycrystalline YBCO films, and (2) as the LNO intermediate layer becomes thicker, the LNO surface becomes rougher, hindering the epitaxial growth of YBCO. On the other hand, Fig. 2 shows that the resistivity of the LNO intermediate layer at 250 K did not change significantly with decreasing film thickness. From these results, it was found that a thinner LNO intermediate layer is more favorable for the epitaxial growth of YBCO and that even a thin layer exhibits sufficiently low resistivity. Furthermore, a superconducting transition temperature of 82 K was obtained for the YBCO/LNO(130nm)/STO sample. In this presentation, we will also report on our investigation of improving the superconducting properties of YBCO thin films on LNO.

References

[1] M. Satyalakshmi et al., Appl. Phys. Lett. 62 (1993) 1233.
[2] M. S. Hegde et al, J. Mater. Res. 9 (1994) 898.

Acknowledgment

The authors thank to Yoshida laboratory at Nagoya University for providing us with access to their X-ray diffraction equipment.

Figure 1. Relative XRD intensity of YBCO 103 peak to STO 200 vs. LNO layer
Figure 2. Resistivity at 250K vs. LNO layer thickness.

Keywords: YBCO, PLD method, LaNiO3, Epitaxial growth