Digital modulation of the nickel valence state in a cuprate-nickelate heterostructure - Wrobel, F.; Geisler, B.; Wang, Y.; Christiani, G.; Logvenov, G.; Bluschke, M.; Schierle, E.; van Aken, P. A.; Pentcheva, R.; Benckiser, E.; Keimer, B. (2017)