From the very beginning K. Alex Müller emphasized that the materials he and George Bednorz discovered in 1986 were hole superconductors. Here I would like to share with Alex and others what I believe to be the key reason for why high Tc cuprates as well as all other superconductors are hole superconductors, which I only came to understand a few months ago.
Hirsch, J. E. (2017). Towards an understanding of hole superconductivity. In Springer Series in Materials Science (Vol. 255, pp. 99–115). Springer Verlag. https://doi.org/10.1007/978-3-319-52675-1_9