Destabilization of Sn2+ 5s2 Lone-Pair States of SnO through Dimensional Crossover.

Materials exhibiting unique electronic properties arising from a characteristic crystal structure have physical properties that are sensitive to structural dimensionality. This study involves the destabilization of Sn 5s2 lone-pair states of SnO films by decreasing their structural dimensionality in the out-of-plane direction. The inherent dispersive band structure of the SnO films remained unchanged between 80 and 11 nm. Below 11 nm, their dispersive band structure disappeared, the O/Sn ratio increased, and the carrier type changed from the p type to the n type, whereas the Sn valency remained constant at +2. These unconventional changes arose from the electronic separation corresponding to the Debye length, which is proportional to permittivity, and were attributed to weakened interactions between Sn 5s2 lone-pair electrons. Therefore, designing low-permittivity materials is beneficial for reducing the crystallite size required for stabilizing lone-pair states. These results are essential for designing emergent p-type oxides and improving their semiconducting properties and performance in transparent or high-power electronics.

Synthesis of rhombohedral LaCuO3 thin films using the oxidation effect of NaClO solution.

LaCuO3 is one of the most fundamental cuprate perovskite blocks and has a similar structure to the cuprate high-temperature superconductor. Therefore, some consider that a study on LaCuO3 is one of the clues to clarify the mechanism of cuprate high-temperature superconductivity. However, since the synthesis of bulk LaCuO3 is difficult due to the need for the high-pressure and high-temperature (HPHT) method, not enough research on LaCuO3 has been done so far. In this paper, I report the successful synthesis of rhombohedral LaCuO3 thin films using the oxidation effect of NaClO solution without using the HPHT method. The nanobeam electron diffraction pattern of the cross-section of LaCuO3-δ thin film oxidized in NaClO solution at 80 °C for 72 hours demonstrates the formation of rhombohedral LaCuO3. The amount of oxygen defects in the rhombohedral LaCuO3 thin films has not been measured directly because it is difficult to do so. However, considering the relationship between the amount of oxygen defects and the symmetry of bulk LaCuO3-δ, these rhombohedral LaCuO3 thin films are considered to be very close to the stoichiometric composition. The temperature dependence of the resistance of the thin films indicates that rhombohedral LaCuO3 is an insulator. Considering the previous studies on the first-principles calculation of stoichiometric LaCuO3 and this study, the rhombohedral LaCuO3 close to the stoichiometric composition is presumed to be a Mott insulator.

Electrolyte-gated SmCoO3 thin-film transistors exhibiting thickness-dependent large switching ratio at room temperature.

A Mott transistor that exhibits a large switching ratio of more than two orders at room temperature is demonstrated by using the electric double layer of an ionic liquid for gating on a strongly correlated electron system SmCoO3. From the thickness dependence of the on-state channel current, we estimate the screening length of the SmCoO3 to be ∼5 nm. The good carrier confinement within the Thomas-Fermi screening length demonstrates that the SmCoO3-channel electric double layer transistor is the first candidate for a two-dimensional Mott transistor.

Strain-mediated phase control and electrolyte-gating of electron-doped manganites.

A prototype Mott transistor, the electric double layer transistor with a strained CaMnO(3) thin film, is fabricated. As predicted by the strain phase diagram of electron-doped manganite films, the device with the compressively strained CaMnO(3) exhibits an immense conductivity modulation upon applying a tiny gate voltage of 2 V.