Evidences for d-wave symmetry of c-axis superconducting gap in atomically thin twisted flakes of bismuth-based HTS cuprates

Abstract

c-axis Josephson tunnelling in atomically thin twisted flakes of quasi-two-dimensional cuprate superconductors is one of the experimental techniques which can be used to reveal the pairing symmetry in HTS cuprates. Until very recent experiments reported by Zhao et al. (Science 382, 1422 (2023)), there was a widely accepted consensus that the c-axis Josephson currents, I_c(T), in twisted cuprate junctions manifest the s-wave gap symmetry for c-axis gap component. Here, I highlighted that this consensus was based on the analysis of the I_c(T) data by using the Ambegaokar-Baratoff equation, where the weak-coupling s-wave gap equation was utilized. In this study, I introduced a more accurate gap equation in the AB model and reanalyzed the I_c(T) data in atomically thin twisted Bi₂Sr_2-xLa_xCuO_6+y (Bi-2201) and Bi₂Sr₂CaCu₂O_8+x (Bi-2212) flakes. In the result, the analysis revealed evidences for d-wave pairing symmetry. Beside this, the analysis of the R_nI_c(T) data reported by Zhao et al. (Science 382, 1422 (2023)) by a piecewise AB model, where a linear low-T dependence of the I_c(T) is used, and where the threshold temperature T_M is a free-fitting parameter, confirmed the d-wave symmetry in these twisted Bi-2212 junctions.