We report the epitaxial growth of Ruddlesden–Popper nickelates, Lan+1NinO3n+1, with n up to 5 by reactive molecular beam epitaxy. X-ray diffractions indicate high crystalline quality of these films, and transport measurements show strong dependence on the n values. Angle-resolved photoemission spectroscopy reveals the electronic structure of La5Ni4O13, showing a large hole-like pocket centered around the Brillouin zone corner with a (π, π) back-folded copy.

Lanthanum titanate, LaTiO3, is an antiferromagnetic Mott insulator with a Ti 3d1 electronic configuration and exhibits an intriguing metallic state in its epitaxial film grown on the SrTiO3 substrate. Here, we explore the driving force of the Mott insulator to metal transition in LaTiO3 epitaxial films by a systematic study of the film growth conditions and biaxial strain using reactive molecular beam epitaxy. Within the achievable range (up to −2.4%) of the biaxial compressive strain in our study, we found that the oxygen incorporation plays a more crucial role than the biaxial epitaxial strain in the Mott insulator to metal transition in LaTiO3 films.

High-mobility electron gases in SrTiO3-based heterostructures have been mainly synthesized on single TiO2 terminated SrTiO3 substrates. Here, we show a high-mobility electron gas observed in a Sr2TiO4/SrTiO3 heterostructure based on an untreated SrTiO3 substrate with mixed termination, showing the mobility value up to 28 000 cm2 V−1 s−1 at 2 K. SrO–SrO vertical Ruddlesden–Popper faults in the Sr2TiO4 film provide escape channels for oxygen atoms under high temperatures, enabling the as-grown tuning of oxygen vacancies in the SrTiO3 layer during film deposition and reversible after-growth modulation in a thermal annealing process.