Papers - KOBORI Hiromi
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Magneto-Transport Properties and Hole Self-Doping due to Excess Oxygen Addition in Polycrystalline LaMnO3 Reviewed
H. Kobori,M. Sogabe, A. Hoshino, T. Taniguchi and T. Shimizu
Materials Science Forum 1053 55 - 60 2022.2
Authorship：Lead author
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多結晶LaMnO3中への余剰酸素付加によるホール・セルフドーピングの証拠 Reviewed
1023 9 - 13 2021.3
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Magneto-Transport Properties in LaMnO3 Thin Films on a-SiO2 Substrates Produced by Metal Organic Decomposition Method Reviewed
H. Kobori,a, T. Kitamura, A. Yamasaki, T. Taniguchi and T. Shimizu
Key Engineering Materials 853 63 - 67 2020
Joint Work
Authorship：Lead author
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Strong Hole Self-Doping in LaMnO3 Thin Film on a-SiO2 Substrate produced by Metal Organic Decomposition Method Reviewed
H. Kobori, T. Kitamura, T. Taniguchi and T. Shimizu
Materials Science Forum 2019
Joint Work
Authorship：Lead author
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Enhanced Magnetoresistance in Fe3O4/Si Thin Films Prepared by Rapid Thermal Deoxidation Method Reviewed
H. Kobori, N. Takata, A. Yamasaki, T. Taniguchi and T. Shimizu
Proceedings of Asia-Pacific Engineering and Technology Conference 1012 - 1018 2017
Joint Work
Authorship：Lead author
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Two-Current Spin-Dependent Conduction in Polycrystalline LaMnO3 Produced under Oxygen Gas Flow Reviewed
H. Kobori, A. Hoshino, T. Taniguchi, T. Horie, Y. Naitoh, and T. Shimizu
AIP Conference Proceedings 1566 333 - 334 2014
Joint Work
Authorship：Lead author
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Magneto-transport study of magnetite (Fe3O4) nanoparticles between Au nanogap electrodes on surface-oxidized Si substrate Reviewed
H. Kobori, N. Takata, N. Fukutome and T. Shimizu
Journal of Magnetism and Magnetic Materials 331 88 - 91 2013
Joint Work
Authorship：Lead author
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Magnetoresistance Intensification of Fe3O4/BaTiO3 Nanoparticle-Composite-Sinter Produced by Low Temperature Heat Treatment Reviewed
H. Kobori, K. Uzimoto, A. Hoshino, A. Yamasak, A. Sugimura, T. Taniguchi, T. Horie, Y. Naitoh and T. Shimizu
The Journal of Superconductivity and Novel Magnetism 25 2809 - 2812 2012
Joint Work
Authorship：Lead author
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Intensified magneto-resistance by rapid thermal annealing in magnetite (Fe3O4) thin film on SiO2 glass substrate Reviewed
H. Kobori, K. Morii, A. Yamasaki, A. Sugimura, T. Taniguchi, T. Horie, Y. Naitoh, and T. Shimizu
Journal of Physics: Conference Series 400 1689 - 1692 2012
Joint Work
Authorship：Lead author
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Magneto-resistance enhancement due to self-hole-doping in LaMnO3 produced by low temperature heat treatment Reviewed
H. Kobori, A. Hoshino, A. Yamasaki, A. Sugimura, T. Taniguchi, T. Horie, Y. Naitoh, and T. Shimizu
Journal of Physics: Conference Series 400 1685 - 1688 2012
Joint Work
Authorship：Lead author
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Enhancement of negative magnetoresistance due to weak localization in In2O3 thin films on Si substrate Reviewed
A. Fujimoto, M. Kitamura, H. Kobori, A. Yamasaki, A. Sugimura, A. Ando, H. Kawanaka, Y. Naitoh, T. Shimizu
Physica E 42 1134 - 1137 2010
Joint Work
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Nanomanipulation of single nanoparticle using a carbon nanotube probe in a scanning electron microscope Reviewed
Hiroshi Suga, Yasuhisa Naitoh, Miyuki Tanaka, Masayo Horikawa, Hiromi Kobori, Tetsuo Shimizu
Applied Physics Express 2 ( 5 ) 2009.5
Joint Work
Publisher：IOP PUBLISHING LTD
A novel method is presented for manipulation of a single nanoparticle using a carbon nanotube probe in a scanning electron microscope chamber. Nanomanipulation was achieved without dependence on the conductivity of the nanoparticle. In order to demonstrate the effectiveness of this technique, insulative Fe2O3 and conductive Fe3O 4 were arranged on a nanogap junction for measurement of the electrical conductivity. The result clearly showed the difference between the resistance of the two iron oxides. It is considered that this technique could be a milestone in the measurement of the physical properties of nanomaterials. © 2009 The Japan Society of Applied Physics.
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Appearance of Variable-Range-Hopping Conduction and Enhanced Spin Dependent Transport by Low Temperature Heat Treatment for Magnetite Nanoparticle Sinter（共著）
H. Kobori, K. Ohnishi, A. Sugimura and T. Taniguchi
Proceedings of the International Conference on Solid State Devices and Materials 2007 686 - 687 2007
Joint Work
Authorship：Lead author
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Localization and transport of type-ii excitons in spatially enhanced random potential for highly Si-doped GaAs/AlAs short-period-superlattices Reviewed
H. Kobori, A. Shigetani, I. Umezu, A. Sugimura
AIP Conference Proceedings 850 1520 - 1521 2006
Joint Work
Through the time-resolved photoluminescence measurements, we have studied the localization and transport of the type-II excitons in spatially random potential for highly Si-doped GaAs/AlAs short-period-superlattices (SPS's). The exciton transport analysis has been carried out according to Krivorotov et al. We have found that Si highly-doped into GaAs/AlAs SPS's causes the enhancement of the spatial fluctuation for type-II exciton. © 2006 American Institute of Physics.
DOI： 10.1063/1.2355281
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Unusual behavior on line-broadening of photoluminescence spectrum for type-ii excitons in Highly Si-doped GaAs/AlAs short-period-superlattices Reviewed
H. Kobori, A. Shigetani, I. Umezu, A. Sugimura
AIP Conference Proceedings 850 1522 - 1523 2006
Joint Work
Through the photoluminescence (PL) measurements, we have studied the temperature dependence of the linewidth of PL spectra for type-II excitons in highly Si-doped GaAs/AlAs short-period-superlattices (SPS's). In low temperature regions, we have observed the strong increase of the linewidth of PL spectra with the temperature, different from the case of undoped GaAs/AlAs SPS's. © 2006 American Institute of Physics.
DOI： 10.1063/1.2355282
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Magnetic-Field-induced Two-toThree-Dimensional Transition in Weak Localization and Weak Anti-Localization regimes for In2O3-X Thin Films:（共著） Reviewed
H. Kobori, N. Hatta, M. Kawaguchi and T. Ohyama
Physica E 12 641 - 645 2002
Joint Work
Authorship：Lead author
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Effects of resonant polarons and band nonparabolicity on cyclotron resonance in n−InP Reviewed
Hiromi Kobori, Tomoyuki Nomura, Tyuzi Ohyama
Physical Review B - Condensed Matter and Materials Physics 63 ( 11 ) 2001
Joint Work
We study the effects of resonant polarons and band nonparabolicity on the cyclotron resonance of conduction electrons in n−InP at 4.2 K in the far-infrared regions. By combining photo- and electric field-excitation techniques, we successfully separate and estimate the individual contributions of resonant polarons and band nonparabolicity to the resonance magnetic field of the cyclotron resonance. In analyses of the experimental results obtained by use of the electric field excitation technique, we take into consideration the energy redistribution of conduction electrons by the electric field excitation. Those resonance magnetic fields are revised to the values for conduction electrons populating the bottoms of Landau levels, on the basis of the experimental results obtained by use of the photoexcitation technique. The dispersion of the resonance magnetic field is fitted by theoretical calculations using the improved Wigner-Brillouin perturbation theories by Lindemann et al. and the three-band k·p method by Lax. The conduction-band-edge mass me0 and the polaron coupling constant α are used as fitting parameters. Excellent agreement between the experimental and theoretical results is obtained. We determine me0/mfe = 0.0788 (is the free-electron mass) and α = 0.12 for InP. © 2001 The American Physical Society.
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Linewidth study of electric dipole induced spin resonance in uniaxially stressed n-InSb for far-infrared region: Theoretical Reviewed
Ko Sugihara, Hiromi Kobori, Nobuteru Tsubouchi, Akira Sugio, Kenichi Fujii, Tyuzi Ohyama
Journal of the Physical Society of Japan 69 ( 9 ) 3084 - 3089 2000.9
Joint Work
Experimental investigation on electric dipole induced ESR in n-InSb was reported in the preceding paper (Experimental). Far-infrared laser with λ = 220 μm ∼ 84 μm and the samples with ND - NA = 1015 cm-3 were employed in the experiment. In this article we mainly focus our attentions to the origin of the observed linewidth. Linewidth is nearly constant at low temperature and increases with T above ∼20 K: 0.02 Tesla at 4.2 K and 0.1 Tesla at 60 K. From the relation of |g*|μBΔH = ℏ/T1, we obtain T1 ∼ 10-12 sec at T = 60 K, where |g*| = 44 is obtained from the absorption data for λ = 119 λm. Such a short relaxation time T1 cannot be expected from any spin-lattice relaxation process. Rashba-Sheka's Hamiltonian: H′ = δ0(E)σ · κ, σ: Pauli spin matrix, κx = kykxky - kzkxkz etc., is responsible for the linewidth broadening. This interaction stems from the lack of inversion symmetry in InSb. Linewidth calculation based on the Kubo-Tomita's theory qualitatively explains the temperature-dependence, the order of magnitude of the linewidth and the field dependence.
DOI： 10.1143/JPSJ.69.3084
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Linewidth Study of Electric Dipole Induced Spin Resonance in Uniaxially Stressed n-InSb in Far Infrared Regions: Experimental Reviewed
Hiromi Kobori, Akira Sugio, Nobuteru Tsubouchi, Kenichi Fujii, Tyuzi Ohyama, Ko Sugihara
Journal of the Physical Society of Japan 69 ( 7 ) 2315 - 2323 2000.7
Joint Work
We have carried out the linewidth measurements of the electric dipole induced spin resonance (EDSR) of conduction electrons in uniaxially stressed n-InSb in the far-infrared (FIR) regions. Our employed samples of n-InSb are highly doped with donors (6.0 × 1015 cm-3 and 8.5 × 1015 cm-3) and thus have metallic properties due to degenerate electrons. The linewiclths of the EDSR have been obtained as the functions of the magnetic field (2.10∼6.39 T), the temperature (4.2∼70 K) and the donor concentration. The main features of the experimental results are the followings: (1) It is found that the minimum of the linewidth appears on the magnetic field dependence at 4.2 K. (2) In the lower resonance magnetic field (2.10 T and 2.74 T) and lower temperature regions (T <∼ 20 K), we have recognized a slight reduction of the linewidth with the increasing of temperature. (3) For moderately high temperatures (T >∼ 20 K), the increment of the linewidth is found on our employed experimental conditions for all samples. (4) For the sample with higher donor concentration, the enhancement of the linewidth is also found in comparison with the sample with lower donor concentration. In this paper, the linewidth of the EDSR spectrum is mainly discussed in respect of the experimental results. The detailed theoretical discussion will be presented in the following paper by Sugihara et al.
DOI： 10.1143/JPSJ.69.2315
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Line-Width of Quantum Limit Cyclotron Resonance. I. Phonon Scatterings in Ge, Si, CdS and InSb Reviewed
Hiromi Kobori, Tyuji Ohyama, Eizo Otsuka
journal of the physical society of japan 59 ( 6 ) 2141 - 2163 1990
Joint Work
Studies of the Cyclotron Resonance Line-Width (CRLW) in the quantum limit have been made for phonon scatterings in basic semiconductors (Ge, Si, CdS and InSb). The acoustic deformation potential scattering has been examined for Ge and Si, while the acoustic piezo-electric scattering for CdS. The effect of the inelastic scattering is enhanced in the quantum limit. Furthermore, the spontaneous acoustic phonon emission is clearly affecting CRLW. Temperature dependences of CRLW for deformation potential and piezo-electric scatterings are similar to each other. Difference, however, arises in the magnetic field dependence. With regard to polar optical phonon scatterings, it has been confirmed for InSb that CRLW is dominated by the absorption part. © 1990, THE PHYSICAL SOCIETY OF JAPAN. All rights reserved.
DOI： 10.1143/JPSJ.59.2141
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Line-Width of Quantum Limit Cyclotron Resonance. II. Impurity and Carrier-Carrier Scatterings in Ge, InSb and GaAs Reviewed
Hiromi Kobori, Tyuji Ohyama, Eizo Otsuka
journal of the physical society of japan 59 ( 6 ) 2164 - 2178 1990
Joint Work
Cyclotron Resonance Line-Width (CRLW) has been studied in the quantum limit for impurity and carrier-carrier scatterings in Ge, InSb and GaAs. Ionized impurity and neutral impurity (electron-neutral donor and electron-neutral acceptor) scatterings have been examined with impurity scatterings. The dependences on temperature and magnetic field of CRLW for these two scatterings are similar to each other. From our experimental results it becomes clear that CRLW is approximately (1) independent of temperature and (2) proportional to the inverse of the square root of magnetic field. Electron momentum cross section by neutral acceptor is noted to be some orders of magnitude less as compared with that by neutral donor even in the quantum limit. © 1990, THE PHYSICAL SOCIETY OF JAPAN. All rights reserved.
DOI： 10.1143/JPSJ.59.2164
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Quantum limit cyclotron resonance in p-GaAs Reviewed
H. Kobori, T. Ohyama, E. Otsuka
Solid State Communications 63 ( 2 ) 123 - 126 1987.7
Joint Work
Electron scattering by neutral acceptors has been studied through far infrared (λ=172 μm) cyclotron resonance on p-GaAs at 4.2 K. It is found that (1) The inverse relaxation time is considerably smaller in the quantum limit than that predicted by D.C. transport theory in the absence of a magnetic field. (2) The same increase linearly with neutral acceptor concentration. (3) The electron momentum transfer cross section by neutral acceptor is by more than an order of magnitude smaller than that by neutral donor even in the quantum limit. © 1987.
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Bulk-sensitive photoemission spectroscopy of TlFe<inf>2</inf>Se<inf>2</inf>
Y. Nishitani, A. Okada, A. Higashiya, A. Irizawa, T. Muro, S. Horiguchi, H. Sato, N. Umeyama, N. Miyakawa, S. I. Ikeda, H. Kobori, A. Yamasaki
Journal of Physics: Conference Series 391 ( 1 ) 2012
Joint Work
We have investigated the electronic structures of the parent compound of an iron chalcogenide superconductor TlFe2Se2 by bulk-sensitive photoemission spectroscopy (PES). Valence-band PES demonstrates the energy gap opening at the Fermi level due to the shift of Fe 3d state to the higher binding energy side as compared to the calculated density of states, being consistent with the insulating behaviour in transport measurements. © Published under licence by IOP Publishing Ltd.
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Electronic structures of the FeSe superconductor studied by high-energy photoelectron spectroscopy
A. Yamasaki, Y. Matsui, S. Imada, K. Takase, H. Azuma, T. Muro, Y. Kato, A. Sekiyama, S. Suga, A. Higashiya, M. Yabashi, K. Tamasaku, T. Ishikawa, K. Terashima, H. Kobori, A. Sugimura, N. Umeyama, H. Sato, Y. Hara, N. Miyakawa, S. I. Ikeda
Journal of Physics: Conference Series 391 ( 1 ) 2012
Joint Work
The bulk electronic structures of an iron-based superconductor FeSe have been investigated by photoelectron and photoabsorption spectroscopies (PES and XAS) with use of high-energy photons (hν 0.7-0.9 and 8 keV). The core-level PES and XAS combined with the configuration interaction cluster model calculation reveal the highly coherent nature of the Fe 3d electrons which is realized by the hybridization between Fe 3d and Se 4p states. © Published under licence by IOP Publishing Ltd.
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Electrical- and magneto-resistance control for magnetite nanoparticle sinter by regulation of heat treatment temperature
H. Kobori, T. Asahi, A. Yamasaki, A. Sugimura, T. Taniguchi, A. Ando, H. Kawanaka, Y. Naitoh and T.Shimizu
Journal of Magnetism and Magnetic Materials 323 686 - 689 2011
Joint Work
Authorship：Lead author
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Spin-dependent-magnetoresistance control by regulation of heat treatment temperature for magnetite nano-particle sinter Reviewed
H. Kobori, T. Asahi, A. Yamasaki, A. Sugimura, T. Taniguchi, A. Ando, H. Kawanaka, Y. Naitoh, T. Shimizu
Annalen der Physik (Leipzig) 18 ( 12 ) 935 - 938 2010.12
Joint Work
The control of spin-dependent-magnetoresistance by regulation of the heat treatment (HT) temperature for magnetite (Fe3O3) nano-particle sinter (MNPS) has been studied. The average nano-particle size in the MNPS is 30nm and the HT was carried out from 400° C to 800°C. The HT of the MNPS varies the coupling form, between adjacent magnetite nano-particles and the crystallinity of that. The measurements on electrical resistance (ER), magnetoresistance (MR) and magnetization were performed between 4K and 300K. The behavior of the ER and MR considerably changes at the HT temperature of ∼600°C. Below ∼600°C the ER indicates the variable-range-hopping conduction behavior and the MR shows the large intensity in a wide temperature region. Above ∼600°C the ER shows the indication of the Verwey transition near 110K like a bulk single crystal and the MR designates the smaller intensity. We consider that below ∼600°C the ER and MR are dominated by the grain-boundary conduction and above ∼600° C those are determined by the inter-grain conduction. The magnetic field application to the grain-boundary region is inferred to cause the large enhancement of the MR. © 2009 WILEY-VCH Verlag GmbH & Co. KGaA,.
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Electron correlation in the FeSe superconductor studied by bulk-sensitive photoemission spectroscopy
A. Yamasaki, Y. Matsui, S. Imada, K. Takase, H. Azuma, T. Muro, Y. Kato, A. Higashiya, A. Sekiyama, S. Suga, M. Yabashi, K. Tamasaku, T. Ishikawa, K. Terashima, H. Kobori, A. Sugimura, N. Umeyama, H. Sato, Y. Hara, N. Miyagawa, S. I. Ikeda
Physical Review B - Condensed Matter and Materials Physics 82 ( 18 ) 2010.11
Joint Work
We have investigated the electronic structures of recently discovered superconductor FeSe by soft-x-ray and hard-x-ray photoemission spectroscopy with high-bulk sensitivity. The large Fe3d spectral weight is located in the vicinity of the Fermi level (EF), which is demonstrated to be a coherent quasiparticle peak. Compared with the results of the band-structure calculation with local-density approximation, Fe3d band narrowing and the energy shift of the band toward EF are found, suggesting an importance of the electron correlation effect in FeSe. The self-energy correction provides the larger mass enhancement value (Z-1 ≃3.6) than in Fe-As superconductors and enables us to separate an incoherent part from the spectrum. These features are quite consistent with the results of recent dynamical mean-field calculations, in which the incoherent part is attributed to the lower Hubbard band. © 2010 The American Physical Society.
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Structure-Disorder induced Magneto-Resistance Intensification on Spin-Dependent-Conduction in Magnetite (Fe3O4) Thin Film produced by RF Magnetron Sputtering Method（共著） Reviewed
H. Kobori, D. Shimizu, A. Sugimura, T. Taniguchi, A. Ando, H. Kawanaka, Y. Naitoh and T. Shimizu
J. Phys.: Conf. Ser. 150 022042-1 - 022042-4 2009
Joint Work
Authorship：Lead author
J. Phys.: Conf. Ser. 150 Vol. 150 (2009) pp.022042
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Nanomanipulation of Single Nanoparticle Using a Carbon Nanotube Probe in a Scanning Electron Microscope Reviewed
H. Suga, Y. Naitoh, M. Tanaka, M. Horikawa, H. Kobori and T. Shimizu
Applied Physics Express 2 ( 5 ) 055004-1 - 055004-4 2009
Joint Work
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Magneto-Resistance Control by use of Interparticle-Coupling- Form Variation in Half-Metallic Magnetite Nano-Particle Sinter Reviewed
T. Asahi, H. Kobori, A. Yamasaki, A. Sugimura, T. Taniguchi, A. Ando, H. Kawanaka, Y. Naitoh and T. Shimizu
Journal of Physics: Conference Series 150 022004-1 - 022004-4 2009
Joint Work
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Spin-dependent-magnetoresistance control by regulation of heat treatment temperature for magnetite nano-particle sinter Reviewed
H. Kobori, T. Asahi, A. Yamasaki, A. Sugimura, T. Taniguchi, A. Ando, H. Kawanaka, Y.Naitoh, and T. Shimizu
Annals of Physics 18 ( 12 ) 935 - 938 2009
Joint Work
Authorship：Lead author
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Magneto-resistance control by use of interparticle-coupling-form variation in half-metallic magnetite nano-particle sinter Reviewed
T. Asahi, H. Kobori, A. Yamasaki, A. Sugimura, T. Taniguchi, A. Ando, H. Kawanaka, Y. Naitoh, T. Shimizu
Journal of Physics: Conference Series 150 ( 2 ) 2009
Joint Work
We have studied the magneto-resistance (MR) control by use of a variation of the interparticle-coupling-form (IPCF) in half-metallic magnetite (Fe 3O4) nano-particle sinter (MNPS). The average nano-particle size of the MNPS is 30nm and the measurements were carried out between 4K and 300K. The variation of the IPCF in the MNPS was performed by the heat treatment (HT) between 400°C and 800°C. The behavior of the MR and electrical resistance (ER) considerably changes at the HT temperature of around 600°C. Below around 600°C the ER indicates the variable-range-hopping conduction behavior in a wide temperature region, whereas above around 600°C the ER shows the indication of the Verwey transition near 110K like a bulk single crystal. We have considered that the electrical conduction switching in the MNPS occurs at the HT temperature of around 600°C. Namely, below around 600°C the ER is dominated by the grain-boundary conduction and above around 600°C the ER is determined by the inter-grain conduction. We have found that at room temperature the MR for the MNPS made at the HT temperature of 400°C shows approximately 20 times that of a bulk single crystal. The application of the magnetic field to grain-boundary conduction is considered to cause the large enhancement of the MR. © 2009 IOP Publishing Ltd.
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Structure-Disorder induced Magneto-Resistance intensification on spin-dependent-conduction in magnetite (Fe<inf>3</inf>O<inf>4</inf>) thin film produced by RF magnetron sputtering method Reviewed
H. Kobori, D. Shimizu, A. Sugimura, T. Taniguchi, A. Ando, H. Kawanaka, Y. Naitoh, T. Shimizu
Journal of Physics: Conference Series 150 ( 2 ) 2009
Joint Work
The intensification of magneto-resistance (MR) caused by structure-disorder has been observed in magnetite (Fe3O4) thin film (MTF) produced by the rf magnetron sputtering method. The MTFs with the thickness of 20nm, 50nm and 100nm were deposited on the substrates of SiO2-glass, MgAl2O4 (100) and MgO (100) single crystals. We have observed that the MR of the MTF on SiO2-glass substrate changes 3-5 times larger than that of the MTFs on MgAl2O4 and MgO substrates. From the AFM, SEM and XRD measurements, we have found that the MTFs are composed of magnetite nano-particles (MNPs) and the crystal-axis directions of the MNPs in the MTF on SiO2-glass are mutually random and those of the MTFs on MgAl2O4 and MgO are almost aligned along (100) direction. In addition, from the electrical and magnetization measurements, we have found that the MNPs in the MTF on SiO2-glass show the amorphous-like behavior and those of the MTFs on MgAl2O 4 and MgO indicate the crystal-like behavior. © 2009 IOP Publishing Ltd.
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Soft x-ray magnetic circular dichroism study of magnetite nanoparticles
A. Yamasaki, H. Kobori, H. Osawa, T. Nakamura, A. Sugimura
Journal of Physics: Conference Series 150 ( 4 ) 2009
Joint Work
The magnetic circular dichroism in the soft-x-ray core-level absorption (XMCD) of magnetite nanoparticles has been studied. The XMCD spectrum at the Fe L2,3 edge closely resembles the reported one of bulk single crystals in shape. This suggests that the derived amorphous-like grain boundary and the size effect of the magnetite crystals do not significantly affect the electronic structures and the local magnetism of Fe ions. It is also found that the orbital magnetic moment on Fe ions is rather small or quenched. © 2009 IOP Publishing Ltd.
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Stability of electronic states across the metal-insulator transition in Pr Ru4 P12
A. Yamasaki, S. Imada, A. Sekiyama, H. Fujiwara, M. Yano, J. Yamaguchi, G. Funahashi, H. Sugawara, D. Kikuchi, H. Sato, T. Muro, A. Higashiya, K. Tamasaku, M. Yabashi, H. Kobori, A. Sugimura, T. Ishikawa, H. Harima, S. Suga
Physical Review B - Condensed Matter and Materials Physics 77 ( 16 ) 2008.4
Joint Work
We have investigated the exotic metal-insulator (M-I) transition in Pr Ru4 P12 by using hard-x-ray and soft-x-ray photoemission spectroscopies. The significantly large hybridization strength between conduction and Pr 4f electrons (c-f) is found not to change much between the metal and insulating phases. These results suggest that the M-I transition does not noticeably affect the c-f hybridization nature and P 3p-Pr 4f mixed states are highly stable. © 2008 The American Physical Society.
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Spin-Dependent-Transport for Magnetite (Fe3O4) Thin Films on SiO2-Glass, MgO and MgAl2O4 Substrates prepared by RF-Magnetoron-Sputtering（共著） Reviewed
H. Kobori, D. Shimizu, A. Yamasaki, A. Sugimura, T. Taniguchi, H. Kawanaka, A. Ando and T. Shimizu
Proceedings of International Conference on Magnetic Materials 2007 22 - 24 2008
Joint Work
Authorship：Lead author
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Magnetoresistance Enhancement of Halfmetal Magnetite (Fe3O4) Thin Film on SiO2-Glass Substrate prepared by RF-Magnetoron-Sputtering Reviewed
D. Shimizu, H. Kobori, A. Yamasaki, A. Sugimura, T. Taniguchi, H. Kawanaka, A. Ando and T. Shimizu
Proceedings of International Conference on Magnetic Materials 2007 19 - 21 2008
Joint Work
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Drastic magnetoresistance enhancement on spin-dependent-transport and appearance of spin-glass-like behavior for magnetite nanoparticle sinter calcined at low temperature Reviewed
H. Kobori, K. Ohnishi, A. Sugimura, T. Taniguchi
AIP Conference Proceedings 893 1261 - 1262 2007
Joint Work
We have studied the spin-dependent-transport and magnetization for magnetite nanoparticle sinter (MNPS) calcined at low temperature. As compared with a bulk crystal, the drastic enhancement of negative differential magnetoresistance has been obtained for the MNPS. Below the Verwey transition temperature, we have observed the difference of magnetization between zero-field-cooling and field-cooling. This phenomenon indicates that localized spins in amorphous-like grain-boundary region in the MNPS are spin-glass-like. © 2007 American Institute of Physics.
DOI： 10.1063/1.2730359
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Spatial fluctuation enhancement and nonradiative-recombination-center generation due to high Si-doping into GaAs/AlAs short-period-superlattices Reviewed
H. Kobori, A. Shigetani, I. Umezu, A. Sugimura
Physica B: Condensed Matter 376-377 ( 1 ) 861 - 863 2006.4
Joint Work
Through the time-resolved photoluminescence (TR-PL) measurement for excitons, we have studied the enhancement of spatial fluctuation (SF) and the generation of nonradiative-recombination-centers (NRC) due to high Si-doping into GaAs/AlAs short-period-superlattices (SPS's). We have carried out the exciton transport analysis according to Krivorotov et al. [I.N. Krivorotov, T. Chang, G.D. Gilliland, L.P. Fu, K.K. Bajaj, Phys. Rev. B 58 (1998) 10687]. From this analysis, we have obtained the temperature dependence of the exciton diffusivity, the concentration of the NRC and the average distant between adjacent localized states of excitons. The temperature dependence of the exciton diffusivity is found to be given by the sum of the temperature-independent contribution and the activation-type contribution. For the exciton diffusivity in undoped GaAs/AlAs SPS's, only the activation-type contribution has been observed. Therefore, we point out the possibility that the temperature-independent contribution comes from the tunneling through the impurities. In this experiment, the activation energy and the concentration of the NRC are found to be larger than those of undoped GaAs/AlAs SPS's. We infer that high Si-doping into GaAs/AlAs SPS causes the enhancement of the SF and the generation of NRC. © 2006 Elsevier B.V. All rights reserved.
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Reversal magnetoresistance and unusual localized-spin freezing in magnetite sinter made from low size-dispersion hematite nano-particles with high temperature calcination（共著）
H. Kobori, K. Ohnishi, A. Sugimura and T. Taniguchi
Physica Status Solidi (c) 12 4204 - 4207 2006
Joint Work
Authorship：Lead author
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Spatial fluctuation enhancement and nonradiative-recombination-center generation due to high Si-doping into GaAs/AlAs short-period-superlattices（共著）
H. Kobori, A. Shigetani, I. Umezu and A. Sugimura
Physica B 376-377 861 - 863 2006
Joint Work
Authorship：Lead author
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Intensified spin-dependent-transport and localized-spin freezing in magnetite sinter made from low size-dispersion hematite nanoparticles with low temperature calcination（共著） Reviewed
H. Kobori, K. Ohnishi, A. Sugimura and T. Taniguchi
Physica Status Solidi (c) 12 4200 - 4203 2006
Joint Work
Authorship：Lead author
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Intensified spin-dependent-transport and localized-spin freezing in magnetite sinter made from low size-dispersion hematite nanoparticles with low temperature calcination Reviewed
H. Kobori, K. Ohnishi, A. Sugimura, T. Taniguchi
Physica Status Solidi (C) Current Topics in Solid State Physics 3 ( 12 ) 4200 - 4203 2006
Joint Work
The spin-dependent-transport (SDT) has been studied for magnetite (Fe 3O4) nano-particle sinter (MNPS) made from low size-dispersion hematite (α-Fe2O3) nanoparticles (LSDHN's) with low temperature calcination. Two kinds of LSDHN's are grown by the hydrothermal synthesis. The average sizes of them are 30 nm and 60 nm. The MNPS is produced by calcining the LSDHN's at 500 °C for 5 hours in the atmos phere of Ar(90%)/H2(10%) mixed gases. As compared with a bulk single crystal, the considerable intensification of negative-differential- magnetoresistance (ND-MR) has been observed for the MNPS. We have: not observed abrupt change of the electrical resistivity in the vicinity of the temperature of the Verwey transition (which is the metal-insulator transition) appeared for a bulk single crystal. The ND-MR for 30 nm shows larger values than that of 60 nm on the temperature dependence. From the X-ray diffraction experiment, the MNPS is found to include crystalline magnetite regions. We consider that the MNPS is composed of large amorphous-like grain-boundaries and small crystalline grains. The electrical current is inferred to flow in grain-boundary regions. In grain-boundary regions, since the localized spins are relatively random distributed, the spin-polarized conduction electrons show the SDT. Below the Verwey temperature, we have observed the magnetization difference between zero-field cooling (ZFC) and field cooling (FC). This phenomenon indicates that the localized spins in the amorphous-like grain-boundaries; are frozen in some degree. Above the Verwey temperature, the magnetoresistance is well fitted by the square of the Langevin function. We consider that the localized spins in the amorphous-like grainboundaries do not form perfectly random configuration and are somewhat ordered in a short range region. © 2006 WILEY-VCH Verlag GmbH & Co. KGaA.
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Reversal magnetoresistance and unusual localized-spin freezing in magnetite sinter made from low size-dispersion hematite nano-particles with high temperature calcination Reviewed
H. Kobori, K. Ohnishi, A. Sugimura, T. Taniguchi
Physica Status Solidi (C) Current Topics in Solid State Physics 3 ( 12 ) 4204 - 4207 2006
Joint Work
The magneto-resistance and magnetization have been studied for magnetite (Fe3O4) sinter made from low size-dispersion hematite (α-Fe2O3) nanoparticles (LSDHN's) with high temperature calcination. Two kinds of LSDHN's were grown by the hydrothermal synthesis. The average particle sizes of them are 30 nm and 60 nm. The magnetite sinter was produced by calcining the LSDHN's at 800 °C for 5 hours in the atmosphere of Ar(90%)/H2(10%) mixed gases. We have observed an abrupt change of the electrical resistivity by one order of magnitude in the vicinity of the Verwey temperature (123 K) of a bulk single crystal. From the X-ray diffraction experiment, we have found that the magnetite sinter includes crystalline region. The magnetite sinter is considered to be composed of relatively narrow grain-boundary regions of amorphous-like magnetite and large grain regions of crystalline magnetite. It is regarded that the grainboundary-conduction is dominant below the Verwey temperature and the inter-grain-conduction is dominant above the Verwey temperature. We have observed the positive differential magnetoresistance (PD-MR) in low temperature regions and the negative differential magnetoresistance (ND-MR) in high temperature regions. The ND-MR is an ordinary phenomenon for magnetite, but the PD-MR is a peculiar one In addition, an unusual localized-spin-freezing phenomenon has been also observed. Below the Verwey temperature, the magnetization difference between zero-field-cooling and field-cooling has been observed. The magnetization difference shows a sudden change in the vicinity of the Verwey temperature. © 2006 WILEY-VCH Verlag GmbH & Co. KGaA.
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Considerable enhancement of PL intensity for free exciton in ZnO ultra-fine-particles and oscillatory green band emergence in PL spectrum for Cu-doped ZnO ultra-fine-particles with heat treatment Reviewed
H. Kobori, T. Nanao, S. Kawaguchi, I. Umezu, A. Sugimura
AIP Conference Proceedings 772 867 - 868 2005.6
Joint Work
For the ultra-fine-particles of ZnO with the heat treatment and Cu-doping, we have observed the considerable enhancement of the PL intensity for free excitons and the oscillatory structure of the green band. These experimental results are explained by the localization model of excitons and the donor to Cu-acceptor recombination, respectively. © 2005 American Institute of Physics.
DOI： 10.1063/1.1994381
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Unusual behavior of far-infrared magneto-photoconduction in electric field for n-GaAs near metal-insulator-transition Reviewed
H. Kobori, M. Inoue, K. Fujii, T. Ohyama
AIP Conference Proceedings 772 127 - 128 2005.6
Joint Work
To study the influence of the electric field on the magnetic-field induced metal-insulator-transition, we have carried out far-infrared magneto-photoconductivity measurements in various electric fields for n-GaAs near the metal-insulator transition by use of the far-infrared laser with 119μm between 2.0K and 4.2K. © 2005 American Institute of Physics.
DOI： 10.1063/1.1994026
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Unusual Behavior on Line-Broadening of Photoluminescence Spectrum for Type-II Excitons in Highly Si-Doped GaAs/AlAs Short-Period-Superlattices（共著） Reviewed
H. Kobori, A. Shigetani, I. Umezu and A. Sugimura
Proceedings of 24th International Conference on Low Temperature Physics PD-Tu-228 (Orland, USA 2005) 1522 - 1523 2005
Joint Work
Authorship：Lead author
Proceedings of 24th International Conference on Low Temperature Physics
PD-Tu-229 (Orland, USA 2005) -
Localization and Transport of Type-II Excitons in Spatially Enhanced Random Potential for Highly Si-Doped GaAs/AlAs Short-Period-Superlattices（共著） Reviewed
H. Kobori, A. Shigetani, I. Umezu and A. Sugimura
Proceedings of 24th International Conference on Low Temperature Physics PD-Tu-228 (Orland, USA 2005) 1524 - 1525 2005
Joint Work
Authorship：Lead author
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Estimation of conduction as well as donor electron density through far-infrared resonant faraday effect Reviewed
Masato Suzuki, Ken Ichi Fujii, Tyuzi Ohyama, Hiromi Kobori
International Journal of Infrared and Millimeter Waves 24 ( 12 ) 2043 - 2050 2003.12
Joint Work
We have investigated resonant Faraday effect (RFE), so-called nonlinear Faraday effect, caused by conduction and donor electrons in n-InSb and n-GaAs. Experiments were carried out with using the magneto-optical spectroscopic system combined with two linear polarizers put in before and behind a sample. The rotation angle and the ellipticity for the far-infrared light with elliptical polarization transmitted through the sample were analyzed on the basis of spectra obtained under different experimental configuration for angles between the axes of the polarizer and the analyzer. We have observed some characteristic features associated with RFE in spectra of two samples. Tapping into these features, we could successfully determine the relaxation times and the densities of conduction and donor electrons in the semiconductors.
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Far-infrared resonant Faraday effect in semiconductors Reviewed
Masato Suzuki, Ken Ichi Fujii, Tyuzi Ohyama, Hiromi Kobori, Nobuo Kotera
Journal of the Physical Society of Japan 72 ( 12 ) 3276 - 3285 2003.12
Joint Work
We have studied resonant Faraday effect (RFE) related to the intraband transitions in various semiconductors at low temperature through direct measurements for the Faraday rotation angle and ellipticity of the probe far-infrared (FIR) light. Through cyclotron resonance due to electrons in a thin n-InSb sample and two-dimensional electrons in the interface of GaAs/AlGaAs as well as the impurity absorption by donors in the n-InSb, we have made clear that RFE is dominated by the off-diagonal components of dielectric tensor. Experimental results for bulk n-InSb samples with various thicknesses have revealed a lot of properties of RFE as saturation effect on absorption. The oscillatory spectra induced by several twirls of the polarization plane have been observed in the extremely thick InSb sample. Spectra of the thick n-InSb also show high sensitivity of ellipticity for interference of circularly polarized lights. Besides, making use of the characteristic behavior on RFE, we can classify absorption lines due to impurities in Ge, i.e., the electron-type absorption or the hole-type one. By referring the experimental results to the theory based on Lorentz-oscillator model, we have made clear some characteristic features on RFE. © 2003 The Physical Society of Japan.
DOI： 10.1143/JPSJ.72.3276
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Far-infrared transient-resonant Faraday rotation induced by non-equilibrium electrons in compensated p-InSb under pulsed-photoexcitation Reviewed
Masato Suzuki, Hiromi Kobori, Nobuo Kotera, Ken Ichi Fujii, Tyuzi Ohyama
Physica B: Condensed Matter 329-333 ( II ) 1096 - 1098 2003.5
Joint Work
We have made our first attempt to investigate the resonant-transient Faraday effect induced by conduction and bound electrons in compensated p-InSb under pulsed-photoexcitation at 4.2 K with use of a far-infrared laser. The Faraday rotation angle is extremely sensitive to the refractive index for the left-hand and right-hand circularly polarized waves. Meanwhile ionized donors in compensated semiconductors are neutralized by photoexcitation and the resonant phenomena by these neutralized impurities bring a big change of refractive index in semiconductors. As donor electrons excited by the photo-pulse relax with a finite lifetime, the refractive index dominated by these donor electrons has time-dependent nature, and thus we are able to observe the transient-Faraday effect through the time-resolved experiments. © 2003 Elsevier Science B.V. All rights reserved.
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Studies of excited states for shallow donors in magnetic field on bulk n-InP Reviewed
Masato Suzuki, Hiromi Kobori, Tyuzi Ohyama
Journal of the Physical Society of Japan 72 ( 5 ) 1127 - 1130 2003.5
Joint Work
Through far-infrared (FIR) magneto-optical absorption measurements we have investigated excited states of shallow donors in bulk n-InP at 4.2 K. By utilizing advantage of thick bulk samples we have observed transitions from Is ground state of donors to various excited states as 2p+1 3p +1 and 3d+1 and a series of oscillatory absorptions caused by transitions to magnetically-induced states. By the optical saturation effect it is found that comparatively weak absorptions as the oscillatory ones become prominent due to suppression of strong absorptions. In order to make clear the excited states of donors under a magnetic field signals due to the 1s-3p +1 and 1s-3d+1 transitions are extremely helpful, since the resonance positions and the peak heights include various informations for excited states. Based on variation of the oscillation peaks with increases in donor concentration, we conclude that the oscillatory absorptions originate in transitions from the Is ground state to impurity bands. © 2003 The Physical Society of Japan.
DOI： 10.1143/JPSJ.72.1127
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Far-infrared resonant Faraday effect in Semiconductors（共著） Reviewed
M. Suzuki, K. Fujii, T. Ohyama, H. Kobori and N.Kotera
Journal of the Physical Society of Japan 72 3276 - 3285 2003
Joint Work
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Sizable Enhancement of Anti-Weak Localization Effect in In2O3-X Thin Film caused by H2 Gas Mixing in N2 Gas Atmosphere on Heat Treatment（共著） Reviewed
H. Kobori, M. Kawaguchi, N. Hatta and T. Ohyama
Physica E 18 296 - 297 2003
Joint Work
Authorship：Lead author
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Far-infrared transient-resonant Faraday rotation induced by non-equilibrium electrons in compensated p-InSb under pulsed-photoexcitation（共著） Reviewed
M. Suzuki, H. Kobori, N.Kotera, K. Fujii and T. Ohyama
Physica B 239-333 1096 - 1098 2003
Joint Work
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Crossover from positive to negative magnetoresistance by the rise of electron temperature for Si:Sb in the variable-range hopping regime Reviewed
A. Fujimoto, H. Kobori, T. Ohyama, S. Ishida, K. Satoh, T. Kusaka, Y. Kakehi
Physica B: Condensed Matter 324 ( 1-4 ) 1 - 8 2002.11
Joint Work
Magnetoresistance (MR) measurements in Si:Sb with the Sb concentration just below the critical concentration for metal-insulator transition have been carried out by stepping into the non-ohmic region of the electric current density. We have found that the MR sensitively depends on the electric current density. For the insulating sample positive MR is observed in the low current density region. A crossover from positive to negative MR is found as the current density increases. It is considered that these phenomena originate from the rise of the electron temperature and are related to the transport in the upper-Hubbard band. © 2002 Elsevier Science B.V. All rights reserved.
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Weak Localization and Weak Anti-localization in Polycrystalline In2O3-X Thin Films grown by Sputtering Method:（共著） Reviewed
H. Kobori, M. Kawaguchi, N. Hatta, T. Ohyama and S. Ishida
physica status solidi (b) 230 277 - 280 2002
Joint Work
Authorship：Lead author
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Weak anti-localization in δ-doped Sb layer of Si Reviewed
A. Fujimoto, H. Kobori, T. Ohyama, S. Ishida
Physica Status Solidi (B) Basic Research 230 ( 1 ) 273 - 276 2002
Joint Work
The spin-orbit (SO) interaction in a δ-doped Si: Sb with 0.11 monolayer of Sb atoms is studied, based on the first observation of the positive magnetoresistance in weak perpendicular magnetic fields due to the two-dimensional weak anti-localization in this system. From the fits of the magnetoconductance data in fields perpendicular to the layer, the SO scattering time τSO as well as the inelastic scattering time τin have been extracted as τSO ≈ 3 × 10-11 s and τin ∝ T-1, respectively. The ratio is τSO/4τin ≈ 0.60 at T = 3 K, which is larger than any other value reported, indicating that the strength of the SO interaction in Si:Sb with 0.1 monolayer of Sb is very small compared with the ones found in the thin metal films.
DOI： 10.1002/1521-3951(200203)230:1<273::AID-PSSB273>3.0.CO;2-S
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Quantum magneto-optical oscillation on far-infrared Zeeman spectrum of n-InP Reviewed
M. Suzuki, H. Kobori, T. Ohyama
Physica B: Condensed Matter 298 ( 1-4 ) 203 - 206 2001.4
Joint Work
An oscillatory absorption spectrum which could not be detected in thin samples was observed for FIR magneto-optical absorption measurements in bulk n-InP at 4.2 K. It was found that the peak positions of the oscillation did not correspond to a series of direct transitions from the 1s ground state of donors into the Landau levels, and these peak positions considerably changed with the donor concentration. We have concluded that those oscillations are associated with a series of transitions from the 1s ground state into donor bands formed by the excited states and their accompanying the Landau levels. © 2001 Elsevier Science B.V.
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Two-to-three dimensional transition by magnetic field on weak localization phenomena for tilted magneto-conductance in In2O3-x films Reviewed
N. Hatta, H. Kobori, T. Ohyama
Physica B: Condensed Matter 298 ( 1-4 ) 482 - 485 2001.4
Joint Work
To study the two-to-three dimensional transition on the weak localization phenomena in thin metallic In2O3-x films, we have carried out the magneto-conductance measurements for those films with various thickness up to the magnetic field of 9 T. We have measured the magneto-conductance as functions of the film thickness and the angle between the directions of the electric current and the magnetic field. From the magnitude correlation between the film thickness and the cyclotron diameter, we found that the anisotropic behavior of the magneto-conductance observed in the low magnetic field has been disappeared as the magnetic field increases. As a result we have clearly observed the two-to-three dimensional transition of the electronic system and the crossover effect on the positive magneto-conductance in the weak localization regime. © 2001 Published by Elsevier Science B.V.
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Electromotive Force Generation due to Electron and Impurity Cyclotron Resonances in Compound Semiconductors:（共著） Reviewed
T. Okashita, H. Kobori and T. Ohyama
Physica B 302-303 369 - 373 2001
Joint Work
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Effects of Resonant-Polaron and Band-Nonparabolicity on Cyclotron Resonance in （共著） Reviewed
H. Kobori, T. Nomura and T. Ohyama
Physical Review B 69 2315 - 2323 2001
Joint Work
Authorship：Lead author
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Intensive Far-Infrared Optical Study on the Donor Impurity Band in the presence of a Magnetic Field for n-GaAs:（共著） Reviewed
H. Kobori, M. Inoue and T. Ohyama
Physica B 302-303 17 - 22 2001
Joint Work
Authorship：Lead author
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Comparison of for δ-doped Layer and Bulk Crystals of Si:Sb in the Weak Localization Region:（共著） Reviewed
A. Fujimoto, H. Kobori, T. Ohyama and S. Ishida
Physica B 302-303 7 - 11 2001
Joint Work
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Resonant-Photoelectromagnetic Effect in Semiconductors:（共著） Reviewed
T. Okashita, H. Kobori and T. Ohyama
Physical Review B 64 195333 - 195342 2001
Joint Work
Phys. Rev. B64 pp.195333-195342, (2001)
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Comparison of magnetoconductance of the δ-doped layer and bulk crystal of Si : Sb in the weak localization regime Reviewed
Akira Fujimoto, Hiromi Kobori, Tyuji Ohyama, Shuichi Ishida, Kazuo Satoh, Tadaoki Kusaka, Yoshiharu Kakehi
Physica B: Condensed Matter 302-303 7 - 11 2001
Joint Work
The magnetoresistance of a Si : Sb δ-doped layer has been measured in magnetic fields up to 3.4 T. The results have been compared with those for a heavily doped bulk sample. The portion of the occupation of Sb in δ-doped layer is 0.11 monolayer. We have observed the positive magnetoresistance in the perpendicular weak magnetic fields arising from the weak anti-localization due to spin-orbit interaction for the first time in this system. The ratio of spin-orbit scattering time to inelastic scattering time is fairly large compared with that of the metallic thin films which include heavy elements. When the moderate magnetic field is applied perpendicularly to the layer, the negative magnetoresistance appears. On the other hand, the positive one is observed in parallel fields. The comparative analyses are performed between the two kinds of samples within the framework of weak localization theories. © 2001 Elsevier Science B.V.
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Electromotive force generation due to electron and impurity cyclotron resonances in compound semiconductors Reviewed
Tomonori Okashita, Hiromi Kobori, Tyuzi Ohyama
Physica B: Condensed Matter 302-303 369 - 373 2001
Joint Work
We report on the resonant-photoelectromagnetic (R-PEM) effect in semiconductor, that is the electromotive force generation associated with electron cyclotron resonance (ECR) and impurity cyclotron resonance (ICR). This can be explained with a model that the carriers excited by the resonant absorption of the radiation induce the DC voltage through the thermomagnetic process. In this study, we have applied this technique to the n-InGaAs thin film and observed the negative voltage peak in addition to the usual positive ICR one. From the temperature variation and the resonant magnetic field dependence of these signals, we have concluded that the negative signal is induced by the ECR absorption. In addition, we have confirmed that the inversion of polarity for these signals originates in the inhomogeneous distribution of conduction electrons inside the film, and electrons localized at a fraction of the sample mainly cause the resonant absorption. Through the detailed observation and analysis of the generated DC voltage, we can investigate the dynamics of hot carriers inhomogeneously excited in the sample by the ECR as well as the ICR. Moreover, if we pay special attention to the polarity of the generated voltage, we can determine the distribution of electrons and impurities inside the film. © 2001 Elsevier Science B.V.
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Resonant photoelectromagnetic effect in semiconductors Reviewed
Tomonori Okashita, Hiromi Kobori, Tyuzi Ohyama
Physical Review B - Condensed Matter and Materials Physics 64 ( 19 ) 2001
Joint Work
Extensive studies of the electromotive force (EMF) generation induced by the resonant magnetoabsorption with the far-infrared excitation have been performed. As for the resonant photoelectromagnetic (RPEM) effect whose origins had been obscure, we obtained various information on fundamental properties of the generated EMF and gave the reasonable understandings for the mechanism of the generation of the RPEM voltage. In addition, we propose a simple model consistent with our experimental results. All of the experimental results we have obtained here support the thermomagnetic model where the RPEM voltage is generated through thermomagnetic process due to the temperature gradient induced inside the samples. Accordingly, we concluded that the observed RPEM voltage is generated by the optically induced Nernst-Ettingshausen effect. This is a conventional method to investigate some nonequilibrium electronic systems and their relaxation mechanism complementary to the transmission and photoconductivity measurements, and we can apply the results obtained by this study for the investigation in dynamical properties of electronic system. © 2001 The American Physical Society.
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Intensive far-infrared optical study on the donor impurity band in the presence of a magnetic field for n-GaAs Reviewed
H. Kobori, M. Inoue, T. Ohyama
Physica B: Condensed Matter 302-303 17 - 22 2001
Joint Work
We have intensively studied donor impurity bands formed by the ground and excited states in the presence of a magnetic field by means of the far-infrared optical technique. With this view, we have carried out measurements of the impurity cyclotron resonance absorption (ICRA) for hydrogenic donors in n-GaAs. Concerning the optical transition between the 1s and 2p+1 states, the variation of the full-width at half-maximum (FWHM) for the ICRA has been investigated as functions of the donor concentration and the magnetic field. Those dependencies of the FWHM have been discussed in consideration of energy bandwidths of the 1s and 2p+1 states for donors in the presence of a magnetic field. The energy bandwidths have been estimated by use of the tight binding approximation for the linear combinations of atomic orbitals, assuming the simple cubic configuration of donors. The qualitative agreement between experimental results and numerical estimation has been obtained. © 2001 Elsevier Science B.V.
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Weak-localization analysis for the Upper-Hubbard transport in Si:Sb Reviewed
A. Fujimoto, H. Kobori, K. Fujii, T. Ohyama, S. Ishida
Physica Status Solidi (B) Basic Research 218 ( 1 ) 177 - 180 2000.3
Joint Work
Magnetoresistance (MR) in Si:Sb just below the critical concentration of metal-insulator transition has been carried out as stepping foot into the non-ohmic region of the electric current density. In the ohmic region, the resistivity at low temperatures obeys Efros-Shklovskii type variable-range hopping and the positive MR is observed. As the electric current density increases, the positive MR is depressed and then the negative one appears. This behavior has been explained by the transfer of electrons, due to the rise of the electron temperature, from the strongly localized states to the upper Hubbard band whose state is assumed to be the weakly localized one.
DOI： 10.1002/(SICI)1521-3951(200003)218:1<177::AID-PSSB177>3.0.CO;2-G
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The drastic change of magnetoresistance by the rise of electron temperature in Sb doped Si Reviewed
A. Fujimoto, H. Kobori, T. Ohyama, S. Ishida
Physica Status Solidi (B) Basic Research 210 ( 2 ) 263 - 268 1998.12
Joint Work
Temperature, electric current and magnetic field dependences of dc resistivity and Hall coefficient have been measured for the Si:Sb sample whose donor concentration is 0.80Nc, where Nc is the critical concentration of the metal-insulator transition. From the observation of ∈2 conduction the hard Coulomb gap seems to open at low temperatures, and we have found that the resistivity, Hall coefficient and magnetoresistance sensitively depend on the electric current density. These data have been analyzed using a two-band model including the upper and lower Hubbard bands. In particular, below Nc positive magnetoresistance is observed in the low electric current density regime. As the electric current density increases, a crossover to negative magnetoresistance is observed. It is concluded that these phenomena originate from the rise of the electron temperature and the transfer of electrons from strongly localized to weakly localized states in the upper Hubbard band.
DOI： 10.1002/(SICI)1521-3951(199812)210:2<263::AID-PSSB263>3.0.CO;2-X
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Resonant-Photoelectromagnetic Effects in Far-Infrared Regions for Compound Semiconductors Reviewed
Tomonori Okashita, Hiromi Kobori, Tyuzi Ohyama
Journal of the Physical Society of Japan 67 ( 2 ) 675 - 678 1998.2
Joint Work
The dc voltage generation associated with cyclotron resonance (CR) and impurity cyclotron resonance (ICR) in n-InGaAs and n-InSb has been studied. In both materials, it is found that the observed voltage decreases rapidly with increasing temperature and the line shape has similar structure to the absorption coefficient. Using a thermomagnetic model where a heat current induces a voltage through the Nernst-Ettingshausen effect, we have explained the experimental results. This technique allows us to investigate resonant absorption for a sample in which transmission measurements are difficult to perform, as well as to study the mechanism of phonon transport through the sample.
DOI： 10.1143/JPSJ.67.675
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Electromotive force generation induced by cyclotron resonance in semiconductors Reviewed
T. Okashita, H. Kobori, T. Ohyama
ICMMT 1998 - 1998 International Conference on Microwave and Millimeter Wave Technology, Proceedings 1998-August 698 - 701 1998
Joint Work
© 1998 IEEE. We report on the electromotive force generation associated with electron cyclotron resonance (ECR) as well as impurity cyclotron resonance (ICR) in n-InGaAs. The observed voltage traces with variation in the magnetic field show similar structure to that of the absorption, and decrease rapidly with increasing temperature. An effect characteristic of the thin films is also observed. Comparing with results for the bulk samples and using thermomagnetic model we demonstrate the mechanism of voltage generation in films, and show the usefulness of this method to study the phonon transportation inside the sample and/or at interfaces between thin film and its substrate, and, what is more, to study resonant absorption.
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Bound to unbound state transition of hydrogenic donors in semiconductors with a static screened Coulomb potential - A trial on variational calculation Reviewed
H. Kobori, T. Ohyama
Journal of Physics and Chemistry of Solids 58 ( 12 ) 2065 - 2068 1997.12
Joint Work
The ground state energy of a hydrogenic donor in the static screened Coulomb potential for single-valley semiconductors is calculated as a function of the screening wavenumber qs by use of the variational principle. We employ a more realistic variational wavefunction rather than a hydrogenic one in consideration of the gentle extinction of the true wavefunction in the radial direction due to the strong screening. The effective Bohr radius aD of a screened hydrogenic donor and the average distance 〈r〉 of the bound electron from the positive donor atom are also obtained as a function of qs. We obtained the result that the critical value of qsaB for the unbound state was 1.13, compared with 1 for a hydrogenic type, with an effective Bohr radius aB of an isolated hydrogenic donor. It is found that the Mott concentration is given by N1/3c = 0.325 at qsaB = 1.13 for our calculation (N1/3c aB = 0.254 at qsaB = 1 for the hydrogenic type) if we assume Thomas-Fermi screening and the equal concentration of free electrons and donors at the nonmetal-metal transition (Mott Transition) point. © 1997 Elsevier Science Ltd. All rights reserved.
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Variational calculations of energy levels associated with 1s, 2p and 3d states of hydrogenic donors for semiconductors in arbitrary magnetic field Reviewed
H. Kobori, T. Ohyama
Journal of Physics and Chemistry of Solids 58 ( 12 ) 2057 - 2064 1997.12
Joint Work
The energies of 1s, 2p-1, 2p0, 2p+1, 3d-2, 3d-1, 3d+1 and 3d+2 states for an isolated hydrogenic donor in single-valley semiconductors are calculated as functions of magnetic field by use of the variational principle. We employ the variational wave functions which are applicable to an arbitrary magnetic field strength. At the low and high magnetic field limits, these variational wave functions become equal to the hydrogen and harmonic-oscillator types. Analytic expressions for the expectation values of the energies are presented according to the variational wave functions which we employ. The binding energy is also calculated as a function of magnetic field. It is found that our present calculations show reliable results in an arbitrary magnetic field. © 1997 Elsevier Science Ltd. All rights reserved.
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Magnetic-field-induced narrowing of far-infrared magneto-optical absorption of neutral donors in GaAs Reviewed
Hiromi Kobori, Masutaka Inoue, Tyuzi Ohyama
Solid State Communications 93 ( 5 ) 363 - 366 1995.2
Joint Work
We have studied the magnetic-field-induced narrowing of far-infrared (FIR) magneto optical absorption line for is-2p+1 transition of hydrogenic shallow donors in n-GaAs, of which line-width is dominated by the concentration broadening. The concentration range of donor is 1.5 × 1015 cm-3 to 6.0 × 1016cm-3, which extends over the concentration for occurrence of metal-insulator transition (Molt transition), i.e., 1.6 × 1016cm-3. The linewidth narrowing due to magnetic field is considered to arise from the strong shrinkage of 2p+1 state. © 1995.
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Direct evidence for a charge-controlled optical quenching of el2 centers in semi-insulating gaas Reviewed
Tyuzi Ohyama, Tetsuo Shimizu, Hiromi Kobori, Eizo Otsuka
Japanese Journal of Applied Physics 32 ( 5 R ) 1889 - 1897 1993.5
Joint Work
We present results of microwave photoconductivity as well as far-infrared laser magnetooptical absorption measurements obtained for LEC (liquid encapsulated Czochralski)-grown semi-insulating GaAs crystals. At low temperature we have observed a “photo-quenching effect” both for signals of microwave photoconductivity and for the shallow donor Zeeman absorption measured using a far-infrared laser, under the condition of BBG (below- band-gap) photoexcitation. Through these experimental observations, we conclude that the shallow donor is associated with the metastable state of the EL2 center in LEC-grown semi-insulating GaAs. From the temperature dependence of microwave photoconductivity decay with BBG pulsed photoexcitation, we can estimate the characteristic activation energy, AEk = 2AQ meV, which originates from a barrier in the conduction band related to the Franck-Con- don shift of the EL2 metastable state. © 1993 The Japan Society of Applied Physics.
DOI： 10.1143/JJAP.32.1889
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High magnetic field effects on electron-bound hole recombination in InSb Reviewed
H. Kobori, T. Ohyama, E. Otsuka
Physica B: Physics of Condensed Matter 184 ( 1-4 ) 128 - 131 1993.2
Joint Work
The recombination rate between conduction electrons and holes bound to acceptors has been measured in InSb as a function of magnetic field (up to 9 T) at liquid-helium temperature. The abrupt and slow decreases of the recombination rate are observed in low and high magnetic fields, respectively. © 1993.
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Magnetic field dependence of polaron mass in CdTe Reviewed
Hiromi Kobori, Tyuzi Ohyama, Eizo Otsuka
Solid State Communications 84 ( 4 ) 383 - 388 1992.10
Joint Work
Polaron cyclotron resonance experiments are performed for high-quality CdTe samples at 4.2 K by use of far-infrared lasers. To study the polaron effect in a magnetic field precisely, various far-infrared wavelengths (513, 419, 394, 295, 220 and 172 microm) are employed on the conditions of photo- and electric field excitations. The polaron mass is obtained as a function of magnetic field for the n=0→n=1 ( n: Landau quantum number) and n=1→n=2 cyclotron resonance transitions. Regarding the n=0→n=1 transition, the experimental results are in good agreement with the theoretical prediction of F. M. Peeters and J.T. Devreese. © 1992.
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Diffusion and recombination of photocarriers in Ge and GaAs
H. Nakata, H. Kobori, A. Uddin, T. Ohyama, E. Otsuka
Semiconductor Science and Technology 7 ( 3 B ) 1992
Joint Work
The dynamical behaviour of carriers in highly photo-excited Ge and GaAs is investigated by far-infrared magneto-absorption and photoconductivity measurement. The authors observe far-infrared absorption due to hot holes in electron-hole gas in Ge and extension of carrier lifetime by impact-ionization of impurities in GaAs.
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Experimental Studies of Cyclotron Resonance Line-width in the Quantum Limit for Semiconductors
Hiromi Kobori
Journal of the Physical Society of Japan 59 2141 - 2178 1989.3
Joint Work
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Far-infrared cyclotron resonance studies of impurity scattering in ge, gaas and insb Reviewed
Hiromi Kobori, Tyuzi Ohyama, Eizo Otsuka
Proceedings of SPIE - The International Society for Optical Engineering 1039 447 - 448 1988.11
Joint Work
Contribution of impurity scattering to the electron cyclotron resonance linewidth (inverse relaxation time) has been studied in the quantum limit, after subtracting the contribution of phonon scattering, for typical semiconductors Ge, GaAs and InSb. To our surprise, dependence of inverse relaxation time on temperature, magnetic field and impurity concentration is the same for neutral and ionized impurities. © 1988 SPIE.
DOI： 10.1117/12.978533
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Quantum limit electron-phonon scattering studies in semiconductors by far-infrared cyclotron resonance Reviewed
H. Kobori, T. Ohyama, E. Otsuka
Proceedings of SPIE - The International Society for Optical Engineering 1039 445 - 446 1988.11
Joint Work
Extensively systematic studies of the cyclotron resonance linewidth in the quantum limit have been performed on electron-phonon scattering in basic semiconductors (Ge, CdS, GaAs). Far-infrared (119 to 513 μm) laser cyclotron resonance experiments have been carried out in photo-excitation by pulsed band gap light or in thermal excitation. We present the temperature and magnetic field dependences of cyclotron resonance linewidth for electron-phonon scattering in the quantum limit with respect to various samples. © 1988 SPIE.
DOI： 10.1117/12.978532
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Quantum limit cyclotron resonance in Ge - acoustic deformation potential scattering - Reviewed
H. Kobori, T. Ohyama, E. Otsuka
Solid State Communications 64 ( 1 ) 35 - 39 1987.10
Joint Work
Electron cyclotron resonance linewidth in pure Ge has been studied for acoustic deformation potential scattering in the quantum limit. The relaxation time derived therefrom is obtained as a function of temperature (10 to 160 K) and magnetic field (17 to 75 kG). The inverse relaxation time for acoustic deformation potential scattering in the quantum limit is found to be proportional to temperature and to the square root of the magnetic field. These experimental results are explained qualitatively by Meyer's prediction. © 1987.
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Electron scattering in gaas at the quantum limit Reviewed
Tyuzi Ohyama, Hiromi Kobori, Eizo Otsuka
Japanese Journal of Applied Physics 25 ( 10 R ) 1518 - 1528 1986.10
Joint Work
A number of new features of electron quantum transport have been disclosed in the systematic time-resolved cyclotron resonance of GaAs in the far-infrared. The temperature dependence of electron-phonon scattering, magnetic field and concentration dependence of the electron-neutral donor scattering and electron-density dependence of carrier-carrier scattering have been investigated. Available theories give no good relationship with our observations regarding electron- phonon type scattering. The electron-neutral donor scattering rate was found to be proportional to the donor concentration and the inverse square root of the magnetic field, the magnitude being much lower than predicted by the Erginsoy relation. The carrier-carrier scattering rate has been the only thing comparable with the existing theoretical predictions. © 1986 The Japan Society of Applied Physics.
DOI： 10.1143/JJAP.25.1518