Oxidation-Induced Giant Resistivity Change Associated with Structural and Electronic Reconstruction in Layered Sr₃Cr₂O_7−δ Epitaxial Thin Films

Ruddlesden–Popper-type Sr 3 Cr 2 O 7−δ (001) epitaxial thin films with high crystallinity and flat surfaces are grown by pulsed laser deposition. The as-grown thin films exhibit transparent color and high resistivity, indicating the existence of a number of oxygen vacancies as well as Cr 3+ ions. Air annealing causes a nonmonotonic variation in lattice constants and a color change to brownish due to the compensation of ordered oxygen vacancies as well as oxidation of the Cr 3+ ions to Cr 4+ ions. At the same time, the room temperature resistivity shows a large decrease exceeding 5 orders of magnitude. Synchrotron-radiation photoemission spectroscopy reveals a wider Mott gap for the Cr 3+ ions than that for Cr 4+ ions. Accordingly, the giant resistivity change is attributed to the synergistic effects of dimensionality variation in the electron conduction pathway and band-structure modulation, which depends on the Cr valence within the layered structure.