Yasunobu Ando

First-principles study of metal–insulator control by ion adsorption on Ti2C MXene dioxide monolayers

Through first-principles calculations using density functional theory, we investigate the possibility of controlling the metal–insulator properties of the Ti2C MXene dioxide Ti2CO2 by ion adsorption. Our simulation reveals that Ti2CO2 is insulating with an indirect band gap of 0.44 eV. Upon atomic adsorption of H, Li, or Na, Ti2CO2 becomes metallic. This metal–insulator change may be used to produce switching devices with a high on/off ratio and low energy consumption by controlling ionic movement, as in ion batteries.

Scanning tunnelling spectroscopy of superconductivity on surfaces of LiTi 2 O 4 (111) thin films

Unique superconductivity at surfaces/interfaces, as exemplified by LaAlO3/SrTiO3 interfaces, and the high transition temperature in ultrathin FeSe films, have triggered intense debates on how superconductivity is affected in atomic and electronic reconstructions. The surface of superconducting cubic spinel oxide LiTi2O4 is another interesting system because its inherent surface electronic and atomic reconstructions add complexity to superconducting properties. Investigations of such surfaces are hampered by the lack of single crystals or high-quality thin films. Here, using low-temperature scanning tunnelling microscopy and spectroscopy, we report an unexpected small superconducting energy gap and a long coherence length on the surface of LiTi2O4(111) epitaxial thin films. Furthermore, we find that a pseudogap opening at the Fermi energy modifies the surface superconductivity. Our results open an avenue for exploring anomalous superconductivity on the surface of cubic transition-metal oxides, where the electronic states are spontaneously modulated involving rich many-body interactions.

Electric field response in bilayer graphene: Ab initio investigation

Stimulated by quantum capacitance measurements, we have investigated the electric properties of bilayer graphene (BLG) with carrier doping under an external electric field using ab initio calculations. We found that the relative permittivity of BLG depends weakly on the applied electric field, and that the BLG can be regarded as a dielectric material rather than a pair of metallic films. We also found that carrier doping affects the band gap of BLG under electric fields, although carrier doping has a much smaller effect on the band gap and density of states than the application of electric fields.