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Yamaguchi, Naoya ; Ishii, Fumiyuki ; 山口, 直也 ; 石井, 史之
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金沢大学ナノマテリアル研究所<br />We investigate the effect of tensile strain on spin splitting at the n-type interface of LaAlO3/SrTi
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O3 in terms of spin–orbit coupling coefficient α and spin texture in the momentum space using first-principles density-functional calculations. Our results show that α could be controlled by the tensile strain, and can be enhanced by up to 5 times for a tensile strain of 7%, and the effect of the tensile strain leads to a persistent spin helix, which has a long spin lifetime. The strain effect on the LaAlO3/SrTiO3 interface is important for various applications such as spin field-effect transistors and spin-to-charge conversion.
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Minami, Susumu ; Ishii, Fumiyuki ; Mizuta, Yo Pierre ; Saito, Mineo ; 見波, 将 ; 石井, 史之 ; 斎藤, 峯雄
概要:
金沢大学自然科学研究科数物科学専攻<br />We have performed systematic density functional calculations and evaluated thermoelectric propert
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ies (Seebeck coefficient and anomalous Nernst coefficient) of half-Heusler compounds CoMSb (M = Sc, Ti, V, Cr, and Mn). The carrier concentration dependence of the Seebeck coefficient in nonmagnetic compounds is in good agreement with experiments. We found that the half-metallic ferromagnetic CoMnSb shows a large anomalous Nernst effect originating from Berry curvature near the Brillouin zone boundary. These results help us understand the mechanism of a large anomalous Nernst coefficient and give us a clue to design high performance magnetic thermoelectric materials. © 2018 Author(s).<br />Embargo Period 12 months
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Sawada, K. ; Ishii, Fumiyuki ; Saito, Mineo
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金沢大学理工研究域数物科学系<br />We study magnetism of zigzag graphene nanoribbons (ZGNRs) whose ribbon widths are 1.8-2.2 nm by perf
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orming first-principles density functional theory calculations. In contrast with freestanding ZGNRs, ZGNRs directly adsorbed on Ni(111) do not show flat-band magnetism due to strong orbital hybridization between edge-localized Cp orbitals and Nid orbitals. The flat-band magnetism of the ZGNR is recovered by introduction of a graphene sheet between the ZGNR and Ni(111) as a buffer layer which weakened the orbital hybridization. In this case, a parallel configuration of spin moments at the two edges has lower energy than the antiparallel spin configuration whereas the magnetic ground state of the freestanding ZGNR has an antiparallel spin configuration. We explore the effects of orbital hybridization and charge transfer on the magnetic stability of ZGNRs on graphene/Ni(111). © 2010 The American Physical Society.
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4. Persistent spin helix on a wurtzite ZnO(10¯10) surface: First-principles density-functional study
Absor, Moh Adhib Ulil ; Ishii, Fumiyuki ; Kotaka, Hiroki ; Saito, Mineo
概要:
The persistent spin helix (PSH) that has been widely and exclusively studied in zinc-blende structures is revealed for t
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he first time on the surface of a wurtzite structure. Through first principles calculations of the surface of ZnO(101¯0), a quasi-one-dimensional orientation of the spin textures is identified. Furthermore, the wavelength of this particular PSH is smaller than that observed in various zinc-blende quantum well structures, thus indicating that wurtzite-structured surfaces are suitable for spintronics applications. © 2015 The Japan Society of Applied Physics.
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Kato, Takehiro ; Kotaka, Hiroki ; Ishii, Fumiyuki
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Using first-principles method, we investigated the electronic states of Bi2Te3 and Bi2Se3. We showed that both Bi2Te3 an
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d Bi2Se3 are insulators with a bulk band gap. In contrast, the surface states of Bi2Te3 and Bi2Se3 films have a metallic band connecting the conduction and valence bands. The films have an energy gap at the (Formula presented.) point when the film thickness is less than four quintuple layers (QLs), or about 30 Å. The energy gaps are closed at six QLs and four QLs for Bi2Te3 and Bi2Se3, respectively. We confirmed the metallicity up to nine QLs. Furthermore, we investigated the spin structures of nine-QL films at the Fermi energy in momentum space. We found that both the Bi2Te3 and Bi2Se3 films have Rashba-type spin textures; i.e. the surface states have spin–polarisation. To investigate the spatial distribution of the spin, we decomposed the expected values of the spin for each atom. The expected values of the spin are localised within the third QL from the surface. Our results of nine-QL films clearly show the boundary between the bulk and surface regions.
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Yamaguchi, Naoya ; Kotaka, Hiroki ; Ishii, Fumiyuki
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We performed density functional calculations for ultra-thin bismuth surface alloys: surface alloys of bismuth and face-c
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entered cubic metals Bi/M(111)-(3×3)R30° (M=Cu, Ag, Au, Ni, Co, and Fe). Our calculated Rashba parameters for the Bi/Ag are consistent with the previous experimental and theoretical results. We predicted a trend in the Rashba coefficients α R of bands around the Fermi energy for noble metals as follows: Bi/Ag > Bi/Cu > Bi/Au. As for the transition metals, there is a trend in α R: Bi/Ni > Bi/Co > Bi/Fe. Our finding may lead to design efficient spin-charge conversion materials. © 2016 Elsevier B.V.<br />Embargo Period 12 months
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Sawada, Keisuke ; Ishii, Fumiyuki ; Saito, Mineo
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金沢大学理工研究域数物科学系<br />Although the zigzag graphene nanoribbon attracts scientific interest because of its magnetic propert
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ies, the study of the magnetism of the armchair graphene nanoribbons (AGNRs) is insufficient. By using first-principles calculations, we find that the magnetic state of dehydrogenated AGNRs is stable in a wide range of electron doping. The spin polarization originates from the localized edge state around the Brillouin zone boundary. In the case of field effect transistor doping, some of the electrons injected into indirect-gap AGNRs induce half-metallicity and heavily doped direct-gap AGNRs are antiferromagnetic. We also find that nitrogen doping produces ferromagnetic insulators and half-metals. © 2011 The Physical Society of Japan.
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| 8.
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Sawada, Keisuke ; Ishii, Fumiyuki
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金沢大学理工研究域数物科学系<br />We have performed noncollinear first-principles density-functional calculations of carrier-doped per
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ovskite manganites La1-xSr xMnO3 (0.0≤x≤1.0). In the calculated magnetic phase diagram (T = 0) within the collinear magnetic configurations, ferromagnetic and several antiferromagnetic configurations successively appeared as a ground state with increasing x. The calculated total energies of the ferromagnetic and A-type antiferromagnetic phases are almost degenerate around the phase boundary, x = 0.5. We found that the noncollinear magnetic configurations are stable in a wide range of carrier concentrations 0.3≤x≤0.6. We discuss the effect of lattice distortions on the stability of the noncollinear magnetic phase. © 2009 IOP Publishing Ltd.
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| 9.
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Muhammad, Rifqi Al Fauzan ; Ishii, Fumiyuki ; Saito, Mineo
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Ferroelectric materials are new candidates for photovoltaic materials. Ferroelectrics can produce photocurrents without
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using p-n junction which is necessary for conventional photo-voltaic devices. BiFeO3 is one of the most promising multiferroic materials having band gap of 2.5 eV. This band gap is too wide to be used as photovoltaic devices, therefore lower band gap is necessary. In order to reduce BiFeO3 band gap, we carry out a first-principles calculation. We study Cu substitution impurities. We introduce a single Cu atom in the 2x2x2 supercell and we find that the band gap is reduced by 0.039 eV. We expect the existence of optical transition energy between valence and impurity band with energy value 1.232 eV lower than the band gap of BiFeO3.<br />Selected Papers from the International Symposium on Computational Science - International Symposium on Computational Science Kanazawa University, Japan
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| 10.
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Tanaka, Yuto ; Saito, Mineo ; Ishii, Fumiyuki ; 斎藤, 峯雄 ; 石井, 史之
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金沢大学理工研究域数物科学系<br />By using the nonequilibrium Green's function (NEGF) method, we perform first-principles calculations
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for phosphorene and bismuthene and evaluate their thermoelectric properties on the basis of the Landauer formalism. We find that the figures of merit strongly depend on the transport direction: the figures for the zigzag (ZZ) direction are substantially different from those for the armchair direction. These anisotropies are found to originate from the difference between the transmission functions in the AM and ZZ directions. We find that p-type bismuthene has a large figure of merit when the transport is in the ZZ direction, then we conclude that bismuthene has some advantages over phosphorene as a thermoelectric material. We also clarify that the spin-orbit coupling has a substantial effect on the thermoelectric properties of bismuthene. © 2018 The Japan Society of Applied Physics.<br />Embargo Period 12 months
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