https://research.ibm.com/publications/orientation-of-the-o2p-holes-in-bi2sr2ca1-cu2o8
Orientation of the O2p holes in Bi2Sr2Ca1 Cu2O8 for Physical Review B - IBM Research
Orientation of the O2p holes in Bi2Sr2Ca1 Cu2O8 for Physical Review B by F.J. Himpsel et al.