Tailoring Materials for Mottronics: Excess Oxygen Doping of a Prototypical Mott Insulator.

The Mott transistor is a paradigm for a new class of electronic devices-often referred to by the term Mottronics-which are based on charge correlations between the electrons. Since correlation-induced insulating phases of most oxide compounds are usually very robust, new methods have to be developed to push such materials right to the boundary to the metallic phase in order to enable the metal-insulator transition to be switched by electric gating. Here, it is demonstrated that thin films of the prototypical Mott insulator LaTiO3 grown by pulsed laser deposition under oxygen atmosphere are readily tuned by excess oxygen doping across the line of the band-filling controlled Mott transition in the electronic phase diagram. The detected insulator to metal transition is characterized by a strong change in resistivity of several orders of magnitude. The use of suitable substrates and capping layers to inhibit oxygen diffusion facilitates full control of the oxygen content and renders the films stable against exposure to ambient conditions. These achievements represent a significant advancement in control and tuning of the electronic properties of LaTiO3+x thin films making it a promising channel material in future Mottronic devices.

A Living-Dead Magnetic Layer at the Surface of Ferrimagnetic DyTiO3 Thin Films.

When ferromagnetic films become ultrathin, key properties such as the Curie temperature and the saturation magnetization are usually depressed. This effect is thoroughly investigated in magnetic oxides such as half-metallic manganites, but much less in ferrimagnetic insulating perovskites such as rare-earth titanates RTiO3 , despite their appeal to design correlated 2D electron gases. Here, the magnetic properties of epitaxial DyTiO3 thin films are reported. While films thicker than about 50 nm show a bulk-like response, at low thickness a surprising increase of the saturation magnetization is observed. This behavior is described using a classical model of a "dead layer" but assuming that this layer is actually "living," that is, it responds to the magnetic field with a strong paramagnetic susceptibility. Through depth-dependent X-ray absorption and photoemission spectroscopy, it is shown that the "living-dead layer" corresponds to surface regions where magnetic (S = 1/2) Ti3+ ions are replaced by nonmagnetic Ti4+ ions. Hysteresis cycles at the Dy M 5 and Ti L 3 edges indicate that the surface Ti4+ ions decouple the Dy3+ ions, thus unleashing their strong paramagnetic response. Finally, it is shown how capping the DyTiO3 film can help increase the Ti3+ content near the surface and thus recover a better ferrimagnetic behavior.

In Situ Control of Separate Electronic Phases on SrTiO3 Surfaces by Oxygen Dosing.

Insulating SrTiO3 (STO) can host 2D electron systems (2DESs) on its surfaces, caused by oxygen defects. This study shows that the STO surface exhibits phase separation once the 2DES is formed and relates this inhomogeneity to recently reported magnetic order at STO surfaces and interfaces. The results open pathways to exploit oxygen defects for engineering the electronic and magnetic properties of oxides.