The development of a superconducting diode with different critical currents I_cs depending on the current direction has been expected as a novel rectifying element operating at cryogenic conditions [1, 2]. In our previous study, we reported that REBa₂Cu₃O_y films had an asymmetric I_c which was enhanced by reducing the surface roughness [3]. In this study, focusing on the lattice strain at the bottom interface of YBa₂Cu₃O_y (YBCO) films, we fabricated YBCO films grown on PrBa₂Cu₃O_y (PrBCO) buffer layers which have a small lattice mismatch of 0.13 % with YBCO and investigated the asymmetric I_c in the films. Inset of Fig. 1(a) shows a schematic drawing of the films.

 The YBCO/PrBCO films were fabricated on IBAD substrates using the pulsed laser deposition method. Thicknesses of PrBCO andYBCO are defined as d_PrBCO and d_YBCO, respectively. The films were processed into micro bridges. Current-voltage characteristics including the reverse current were measured at 0-0.4 T and 77.3 K by the four-probe method. Inset of Fig. 1(a) shows the definition of current direction and the asymmetricity Asym.. The maximum of the Asym. for the magnetic field is defined as Asym.^MAX.

 Fig. 1(a) shows d_PrBCO dependences of the Asym.^MAX in the YBCO/PrBCO films. The Asym.^MAX inverted from a positive to a negative with the d_PrBCO larger than 12.5 nm. This result indicates that the surface barrier at the bottom interface in the YBCO films is greater than one at the surface. Fig. 1(b) shows d_YBCO dependences of the Asym.^MAX in the films. The Asym.^MAX showed a peak with the maximum value of -103.2 % when the d_YBCO was 450 nm, excluding the results with critical current density J_c less than 1 MA/cm². We will discuss the quantitative analysis of the surface barrier in the films and the structural analysis using a transmission electron microscopy.

This work was partly supported by JSPS-KAKENHI (19K22154, 20K15217, 20H02682), JST-A-STEP, and NEDO. The metal substrates were provided from Dr. Iijima of Fujikura Ltd.

Reference
[1] F. Ichikawa et al.: Physica C 235, 3095-3096 (1994).
[2] S. A. Harrington et al.: Appl. Phys. Lett. 95, 022518 (2009).
[3] Y. Tsuchiya et al.: Abstracts of ISS conference (2019).

 Fig. 1(a) PrBa₂Cu₃O_y layer thickness d_PrBCO and (b) YBa₂Cu₃O_y layer thickness d_YBCO dependences of maximum asymmetricity in YBa₂Cu₃O_y films grown on PrBa₂Cu₃O_y buffer layers.

Keywords: YBa2Cu3Oy films, critical current Ic, asymmetricity, PrBa2Cu3Oy buffer layers