Enabling coherent BaZrO₃nanorods/YBa₂Cu₃O_7−xinterface through dynamic lattice enlargement in vertical epitaxy of BaZrO₃/YBa₂Cu₃O_7−xnanocomposites

Abstract

One-dimensional c-axis-aligned BaZrO3 (BZO) nanorods are regarded as strong one-dimensional artificial pinning centers (1D-APCs) in BZO-doped YaBa2Cu3O7-x (BZO/YBCO) nanocomposite films. However, a microstructure analysis has revealed a defective, oxygen-deficient YBCO column around the BZO 1D-APCs due to the large lattice mismatch of ~7.7% between the BZO (3a=1.26 nm) and YBCO (c=1.17 nm), which has been blamed for the reduced pinning efficiency of BZO 1D-APCs. Herein, we report a dynamic lattice enlargement approach on the tensile strained YBCO lattice during the BZO 1D-APCs growth to induce c-axis elongation of the YBCO lattice up to 1.26 nm near the BZO 1D-APC/YBCO interface via Ca/Cu substitution on single Cu-O planes of YBCO, which prevents the interfacial defect formation by reducing the BZO/YBCO lattice mismatch to ~1.4%. Specifically, this is achieved by inserting thin Ca0.3Y0.7Ba2Cu3O7-x (CaY-123) spacers as the Ca reservoir in 2-6 vol.% BZO/YBCO nanocomposite multilayer (ML) films. A defect-free, coherent BZO 1D-APC/YBCO interface is confirmed in transmission electron microscopy and elemental distribution analyses. Excitingly, up to five-fold enhancement of Jc (B) at magnetic field B=9.0 T//c-axis and 65-77 K was obtained in the ML samples as compared to their BZO/YBCO single-layer (SL) counterpart’s. This has led to a record high pinning force density Fp together with significantly enhanced Bmax at which Fp reaches its maximum value Fp,max for BZO 1D-APCs at B//c-axis. At 65 K, the Fp,max ~158 GN/m3 and Bmax ~ 8.0 T for the 6% BZO/YBCO ML samples represent a significant enhancement over Fp,max ~36.1 GN/m3 and Bmax ~ 5.0 T for the 6% BZO/YBCO SL counterparts. This result not only illustrates the critical importance of a coherent BZO 1D-APC/YBCO interface in the pinning efficiency, but also provides a facile scheme to achieve such an interface to restore the pristine pinning efficiency of the BZO 1D-APCs.

The benefit of Ca in improving pinning of BaZrO3-Y2O3doubly-doped YBa2Cu3O7-x/Ca0.3Y0.7Ba2Cu3O7-xmultilayer nanocomposite films

“…This means the microstructures shown in figure 1 are indicative of both DD-SL and DD-ML samples since the CaY-123 spacers are not expected to alter the morphology, orientation, and concentration of the APCs. This argument has been confirmed in the reported SD-ML nanocomposite films [26,27,30,37], which suggests that a modulated strain field may be maintained in the DD-ML samples with low BZO doping but not necessarily at a higher level of BZO doping.…”

“…Based on the TEM image presented in figure 1(c), it appears that presence of the Y 2 O 3 NPs (black circles) does not significantly affect the morphology and orientation of the BZO 1D-APCs in this scenario. The typical diameter of BZO 1D-APCs in the sample is 5-6 nm which is comparable to that of the BZO 1D-APCs in BZO/YBCO SD (both SL and ML) films [26,27,35]. As shown in figure 1(c), majority of the BZO 1D-APCs are through the film thickness aligned in c-axis (figure 1(c)).…”

“…We recently reported a multilayer (ML) approach to improve the BZO 1D-APC pinning efficiency by diffusing Calcium (Ca) from thin Ca 0.3 Y 0.7 Ba 2 Cu 3 O 7-x (CaY-123) spacers to BZO/YBCO layers stacked alternatively with the spacers [26,27]. The observed pinning enhancement is attributed to modification of the BZO 1D-APC/YBCO interface.…”

“…The observed pinning enhancement is attributed to modification of the BZO 1D-APC/YBCO interface. Specifically, the diffusion of Ca into YBCO at the interface is argued to be driven by the tensile strain on YBCO lattice, resulting in formation of short segments of stacking faults in the YBCO columns near the interface associated to the Ca/Cu substitution on the Cu-O plane of YBCO and hence elongation of the c-axis constant of YBCO for reduced lattice mismatch at the BZO 1D-APC/YBCO interface [27].…”

“…Specifically, the study aims to explore whether the improved BZO pinning efficiency by Ca diffusion could be implemented in DD-ML samples for both enhanced J c and reduced anisotropy. Both DD-ML and DD-SL samples have a fixed 3 vol.% of Y 2 O 3 doping (with −4% lattice mismatch with YBCO), while the BZO (with +7.7% lattice mismatch with YBCO) concentration was varied to 2, 4 and 6 vol.%, while previous studies have only examined the ML approach in BZO-doped YBCO films without Y 2 O 3 [27][28][29][30][31]. Many studies have been conducted on DD-SL YBCO films with the goal of enhancing J c and angular isotropic J c through the use of mixed APCs [4,6,15,20,21,32,33].…”