Hong Sub Lee

A new route to the Mott-Hubbard metal-insulator transition: Strong correlations effects in Pr0.7Ca0.3MnO3

Resistive random access memory based on the resistive switching phenomenon is emerging as a strong candidate for next generation non-volatile memory. So far, the resistive switching effect has been observed in many transition metal oxides, including strongly correlated ones, such as, cuprate superconductors, colossal magnetoresistant manganites and Mott insulators. However, up to now, no clear evidence of the possible relevance of strong correlation effects in the mechanism of resistive switching has been reported. Here, we study Pr0.7Ca0.3MnO3, which shows bipolar resistive switching. Performing micro-spectroscopic studies on its bare surface we are able to track the systematic electronic structure changes in both, the low and high resistance state. We find that a large change in the electronic conductance is due to field-induced oxygen vacancies, which drives a Mott metal-insulator transition at the surface. Our study demonstrates that strong correlation effects may be incorporated to the realm of the emerging oxide electronics.

Piezoelectric Transducers on Curved Dispersive Bending Wave and Poke-Charged Touch Screens

Ferroelectric thin films of lead zirconate titanate Pb(Zr0.52,Ti0.48)O3(PZT) and bismuth titanate Bi4Ti3O12(BiT) were prepared by photochemical metal-organic deposition as electromechanical transducers used in optoelectronic devices. Both solutions were deposited on flat and convex amorphous glass substrates to poke-charge the device or detect the touch point coordinate on new generation of free-shaped screens. The effects of optimization on UV exposure, precursor type and annealing temperature (600–800 °C) were investigated on microstructural, optical and ferroelectric properties of deposited thin films. Suppressing the non-uniformity of the film thicknesses due to repetitive deposition, actually there is no structural difference between the two types of substrates and their effects on the film properties. Polarization-voltage hysteresis loops showed relatively larger remnant polarization of P r = 9.3 µC/cm2 and 6.1 µC/cm2 for optimized UV-irradiated PZT and BiT thin films, respectively, than uncured samples.

The oxygen-deficiency-dependent Seebeck coefficient and electrical properties of mesoporous La0.7Sr0.3MnO3−x films

The thermoelectric power factor of La0.7Sr0.3MnO3 (LSMO) is closely related to its oxygen-deficient nature. In this study, the oxygen content of mesoporous LSMO films was controlled using various annealing atmospheres (oxygen, nitrogen, air, and argon) to investigate the relationship between oxygen deficiency and the power factor. The effect of the mesoporous structure on the power factor of the prepared LSMO films was also studied through analyses of structural and electrical properties. The oxygen-deficient state induces an increase in the lattice parameter of LSMO, which results in an increase in the electrical resistivity and an enhanced Seebeck coefficient. This phenomenon was emphasized upon the introduction of a pore structure in LSMO because of an increase of the unstable surface area. The oxygen-deficient state results in an increase in the amount of manganese in the Mn3+ valence state and in an increase of the lattice parameter; these effects were confirmed through photoemission spectroscopic analysis. As a result, LSMO can be applied as a thermoelectric material because of the successful preparation of LSMO films with a mesoporous structure and because of the enhancement of the power factor as a consequence of increased oxygen deficiency.

Thickness and thermal processing contribution on piezoelectric characteristics of Pb(Zr-Ti)O3 thick films deposited on curved IN738 using sol–gel technique

Lead zirconate-titanate (PZT) thick films of perovskite structure Pb(ZrxTi1-x)O3 were fabricated on the curved surface of IN738 nickel-based supper alloy substrate up to 15u2009µm thickness using sol–gel deposition technique without polyvinylpyrrolidone. The films were heated at 200℃ with 10u2009wt% excess PbO, pyrolyzed at 400℃, and subsequently annealed at 650℃, 700℃, and 800℃. Au and Pt thin films were deposited as bottom and top electrodes, respectively. PZT films of different thicknesses and thermal treatment conditions were characterized to investigate the effect of process on crystalline phase development, orientation, and microstructure morphology. Thereby, formation of fairly smooth, semi-dense, and crack-free random orientated thick films as well as an increase in the average grain size and stress relaxation was observed as the film thickness increased. Having optimized the coating process, intrinsic and extrinsic dielectric, ferroelectric and piezoelectric properties were measured as a function of the film thickness, orientation, grain size, and domain wall motions to evaluate the remnant polarization (Pru2009=u20097.6–17.5u2009µC/cm2), coercive field (Ecu2009=u20092.5–4u2009kV/cm), permittivity (ɛru2009=u2009276–326), dielectric loss (tanδ(%)u2009=u20092.7–3), and piezoelectric charge coefficient (d33u2009=u200971–145u2009pm/V) of the PZT thick films prepared potentially to be used as high bandwidth 1–5u2009MHz structural health monitoring transducers.

Band Structure Analysis of La0.7Sr0.3MnO3 Perovskite Manganite Using a Synchrotron

Oxide semiconductors and their application in next-generation devices have received a great deal of attention due to their various optical, electric, and magnetic properties. For various applications, an understanding of these properties and their mechanisms is also very important. Various characteristics of these oxides originate from the band structure. In this study, we introduce a band structure analysis technique using a soft X-ray energy source to study a (LSMO) oxide semiconductor. The band structure is formed by a valence band, conduction band, band gap, work function, and electron affinity. These can be determined from secondary electron cut-off, valence band spectrum, O 1s core electron, and O K-edge measurements using synchrotron radiation. A detailed analysis of the band structure of the LSMO perovskite manganite oxide semiconductor thin film was established using these techniques.

Evaluation of a ferroelectric tunnel junction by ultraviolet-visible absorption using a removable liquid electrode.

Ferroelectric memristors offer a significant alternative to their redox-based analogs in resistive random access memory because a ferroelectric tunnel junction (FTJ) exhibits a memristive effect that induces resistive switching (RS) regardless of the operating current level. This RS results from a change in the ferroelectric polarization direction, allowing the FTJ to overcome the restriction encountered in redox-based memristors. Herein, the memristive effect of an FTJ was investigated by ultraviolet-visible (UV-Vis) absorption spectroscopy using a removable mercury (Hg) top electrode (TE), BaTiO3 (BTO) ferroelectric tunnel layer, La0.7Sr0.3MnO3 (LSMO) semiconductor bottom electrode, and wide-bandgap quartz (100) single-crystal substrate to determine the low-resistance state (LRS) and high-resistance state (HRS) of the FTJ. A BTO (110)/LSMO (110) polycrystal memristor involving a Hg TE showed a small memristive effect (switching ratio). This effect decreased with increasing read voltage because of a small potential barrier height. The LRS and HRS of the FTJ showed quasi-similar UV-Vis absorption spectra, consistent with the small energy difference between the valence-band maximum of BTO and Fermi level of LSMO near the interface between the LRS and HRS. This energy difference stemmed from the ferroelectric polarization and charge-screening effect of LSMO based on an electrostatic model of the FTJ.

A Study on the Resistive Switching of La0.7Sr0.3MnO3 Film Using Spectromicroscopy

Resistive random access memory (ReRAM) is a promising candidate for next generation nonvolatile memory. La0.7Sr0.3.MnO3 (LSMO) of perovskite manganite family has a great deal of attention for ReRAM material because it makes resistive switching (RS) of interface type without a “forming process”. However, the full understanding of the electronic structure and RS mechanism of LSMO remains a challenging problem. Therefore, this study performed spectromicroscopic analysis to understand the relation between the change of electronic structure and RS characteristic. The results demonstrated the electron occupation by field-induced oxygen vacancies and strong correlation effects.