Physics of Mott Semiconductors

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This research project has aimed for the development of semiconductor physics of Mott insulators which are anticipated as key elements of next generation electronics. We have worked on the following subjects in order to clarify experimentally how the principles of semiconductor physics, such as impurity states or interfacial states, are modified in "Mott semiconductors".1.Local spectroscopy on Sr_3Ru_2O_7 by STM/STSThe effect of Mn impurities introduced into Sr_3Ru_2O_7, which is located on the verge of the magnetic and orbital state criticality, was investigated by STM/STS. The length scale of the influence of impurities was found to reach several nm due to the reduced screening of strongly correlated electrons. We also observed the anomalous evolution of electron density of states spectra at the low-energy region, which is related to the metamagnetic instability.2. Metal-insulator transition induced by electrostatic carrier dopingWe have designed oxide field-effect transistors with organic gate insulators. By applying this method to SrTiO_3, the prototype perovskite oxide, we have realized the insulator to metal transition by electrostatic carrier doping. In the metallic phase, the carrier mobility exceeds 1000 cm^2/Vs.3. Resistive switching memory in binary transition-metal oxides Resistive random access memory (RRAM) is expected as an interfacial device of next generation. However, the mechanism of the resistive switching remains to be elucidated. By fabricating planer-type devices with binary transition-metal oxides and directly observing the interfaces, we evidenced the formation of conducting filament induced by pulse electric fields, and also found that the resistive switching occurs at the interface between the filament and metal electrodes.