Co valence transformation in isopolar LaCoO₃ / LaTiO₃ perovskite heterostructures via interfacial engineering

Charge transfer up to a single electron per interfacial unit cell across the non-polar heterointerface consisting of the Mott insulator LaTiO₃ and the charge-transfer insulator LaCoO₃ was studied in multi-layered films grown by PLD, using element-specific soft X-ray absorption, magnetic dichroism, TEM and hard X-ray photoemission spectroscopies.

We provide robust evidence of systematic and controllable conversion of trivalent to divalent cobalt that occurs within an interfacial region of three unit cells in LaCoO₃ as a direct result of the ensuing Fermi energy mismatch, without the presence of dopants or structural modification, in agreement with the theoretical prediction. Through the excellent control of the growth process, the average Co valence is tuned at the nanoscale via a proper choice of the number of LaTiO₃|LaCoO₃ interfaces and the thickness of LaCoO₃. Notably, the insertion of an ultrathin LaAlO₃ ‘break’ layer interrupts the charge transfer, highlighting its interfacial nature.

Besides the strong enhancement of the magnetic polarizability due to formation of divalent Co, this technologically-relevant guiding principle illustrates the efficacy of O2p-band alignment for property design in oxide heterointerfaces.