T. Yao, Y. Jiang, C. Chen, X. Yan, A. Tao, L. Yang, C. Li, K. Sugo, H. Ohta, H. Ye, Y. Ikuhara, X. Ma, “Ferroelectric oxide thin film with an out-of-plane electrical conductivity”, Nano Lett., 20 (2020) 1047-1053