Hidetoshi Kizaki

First-principles materials design of CuAlO2 based dilute magnetic semiconducting oxide

The electronic structures and the magnetic properties of dilute magnetic semiconductors (DMSs) based on transparent semiconducting oxide CuAlO2 are calculated by using the Korringa–Kohn–Rostoker method combined with the coherent potential approximation within the local density approximation. We see from the results that we can expect to obtain the half-metallic and high-spin ferromagnetic state is expected to be stable in Mn-, Fe-, Co- and Ni-doped CuAlO2.

Materials Design of CuAlO2-Based Dilute Magnetic Semiconductors by First-Principles Calculations and Monte Carlo Simulations

In order to propose a new dilute magnetic semicoductors (DMS) other than II–VI and III–V semiconductor based DMS, magnetism of CuAlO2 based DMS is investigated by the Korringa–Kohn–Rostoker method combined with the coherent potential approximation within the local spin density approximation and Monte Carlo simulations (MCS). We found that effective exchange interactions (Jij) between magnetic ions are short-ranged and Jijs between magnetic ions in the same Cu-plane are ferromagnetic and ones between the different Cu-planes are nearly negligible in CuAlO2-based DMS. In comparison to this, Jijs between magnetic impurities occupied at Al-sites are slightly longer-ranged due to the stronger hybridization effect. According to MCS calculations, it is found that the value of the Curie temperature (TC) exceeds 80 K in (Cu,Fe)AlO2- and (Cu,Co)AlO2-DMS and that the TC is suppressed due to the strong percolation effect. This effect also appears in Cu(Al,TM)O2, where TM denote 3d-transition metal elements.

First-Principles Study on Electronic Structure and Spin State of Rutile (Ti,Co)O2 by Self-Interaction-Corrected Local Density Approximation: Role of Oxygen Vacancy

Electronic structure of rutile-TiO2 based dilute magnetic semiconductors (DMS) are investigated within self-interaction-corrected local density approximation (SIC-LDA). These results are compared with those calculated within standard LDA. We employ the Korringa–Kohn–Rostoker method combined with coherent potential approximation. It is found that high-spin state in (Ti,Co)O2 with O vacancy is predicted in the SIC-LDA. This result is in good agreement with the experimental results. As a result, we find that O vacancy in (Ti,Co)O2 is the origin of Co2+ high-spin state and SIC-LDA is indispensable to describe the correct electronic structure and spin state of TiO2-based DMS.

General Rule and Materials Design of Negative Effective U System for High-Tc Superconductivity

Based on the microscopic mechanisms of (1) charge-excitation-induced negative effective U in s1 or d9 electronic configurations, and (2) exchange-correlation-induced negative effective U in d4 or d6 electronic configurations, we propose a general rule and materials design of negative effective U system in itinerant (ionic and metallic) system for the realization of high-Tc superconductors. We design a Tc-enhancing layer (or clusters) of charge-excitation-induced negative effective U connecting the superconducting layers for the realistic systems.

Chapter 10 Computational Nano‐Materials Design for the Wide Band‐Gap and High‐TC Semiconductor Spintronics

Publisher Summary This chapter presents a unified physical picture of wide band-gap II–VI and III–V DMS—such as ZnO, ZnS, ZnSe, ZnTe, and GaN—on the basis of the state-of-the-art ab initio electronic structure calculation done by using the Korringa–Kohn–Rostoker coherent potential approximation and local density approximation (KKR–CPA–LDA) and the self-interaction corrected LDA (SIC-LDA) to go beyond the LDA. Zeners double-exchange interaction and superexchange interaction mechanisms are dominant in the magnetic magnetism. In the homogeneous system, the calculated Curie temperature (TC) obtained by using a Monte Carlo simulation (MCS) and electronic structure is in good agreement with the experiment (TC and photoemission spectroscopy [PES]). In the inhomogeneous system, the chapter presents a 3D Dairiseki phase and a 1D Konbu phase caused by spinodal nano-decomposition. A design of a growth position and shape-control method for nanomagnets in the superstructure by using self-organization is discussed. Finally, on the basis of the ferromagnetic (FM) mechanism of Zeners double-exchange mechanism, the chapter discusses a design of a new class of half-metallic ferromagnets without 3d transition metal (TM) impurities, such as C- or N-doped CaO, BaO, MgO, SrO, and SiO2.

First-principles investigation on the segregation of Pd at LaFe1-xPdxO3-y surfaces

First-principles calculations were performed to investigate the effect of Pd concentration and oxygen vacancies on the stability of Pd at LaFeO3 surfaces. We found a much stronger tendency of Pd to segregate by taking the aggregation of Pd at LaFe1-xPdxO3-y surfaces into consideration, resulting in a pair of Pd-Pd around a vacancy. Moreover, we predicted that one oxygen-vacancy-containing FeO2-terminated surfaces would be stable at high temperatures by comparing the stability of LaFe1-xPdxO3-y surfaces, which further supports our previous conclusion that a Pd-containing perovskite catalyst should be calcined at 1,073 K or higher temperatures in air to enhance the segregation of Pd in the vicinity of surfaces to rapidly transform the Pd catalyst from oxidized to reduced states on the perovskite support.

Study on the mechanism of platinum-assisted hydrofluoric acid etching of SiC using density functional theory calculations

Hydrofluoric acid (HF) etching of the SiC surface assisted by Pt as a catalyst is investigated using density functional theory. Etching is initiated by the dissociative adsorption of HF on step-edge Si, forming a five-fold coordinated Si moiety as a metastable state. This is followed by breaking of the Si–C back-bond by a H-transfer process. The gross activation barrier strongly correlates with the stability of the metastable state and is reduced by the formation of Pt–O chemical bonds, leading to an enhancement of the etching reaction.

First‐principles calculations under carrier doping treatment in CuAlO2 based dilute magnetic semiconductor

We investigate the electronic structure and magnetic properties under carrier doping treatment in CuAlO2 based dilute magnetic semiconductors ((Cu0.95, Fe0.05)AlO2, (Cu0.95, Co0.05)AlO2 and (Cu0.95, V0.05)AlO2) from first‐principles for semiconductor spintronics. Hole carriers are introduced by substitutional Mg impurities on Al sites in these materials. Electorons are introduced by substituting Si by Al. The Korringa‐Kohn‐Rostoker method combined with the coherent potential approximation within the local density approximation is employed for the present purpose. It is shown that Curie temperature of (Cu0.95, V0.05)(Al1−x, Mgx)O2 and (Cu0.95, Fe0.05)(Al1−x, Xx)O2 (X=Mg, Si) affected by changing the carrier density, in contrast to the case of (Cu0.95, Co0.05)(Al1−x, Xx)O2, where Curie temperature is insensitive to the carrier density.