Akira Ogasahara
University of Tokyo
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Featured researches published by Akira Ogasahara.
Journal of the Physical Society of Japan | 2002
Hitoshi Ohta; Keizo Kirita; Takashi Kunimoto; Susumu Okubo; Yuko Hosokoshi; Keiichi Katoh; Katsuya Inoue; Akira Ogasahara; Seiji Miyashita
X-band ESR measurements of BIP-TENO (3,3 ′ ,5,5 ′ -tetrakis( N -tert-butylaminoxyl)biphenyl) single crystal, which is an S =1 spin ladder model substance, have been performed in the temperature reg...
Journal of the Physical Society of Japan | 2003
Seiji Miyashita; Akira Ogasahara
The field- and frequency-dependence of Electron Spin Resonance (ESR) in the Shastry–Sutherland lattice with small Dzyaloshinsky–Moriya (DM) interaction is studied by the direct numerical estimation of the Kubo formula. Beside the strong paramagnetic resonance, we find the resonance due to the transitions between the triplet and singlet levels which are prohibited in the case without the DM interaction. Furthermore, we find the resonance which is distributed in a wide range of the field for a specific frequency, which corresponds to the transition between the states with the same magnetization.
Journal of the Physical Society of Japan | 2004
Akira Ogasahara; Koichi Kusakabe
Electron transport is investigated theoretically for an electron which is coupled to a localized spin system. The dc electrical conductivity of spin-fermion models in one dimension is studied by the direct numerical estimation of the Kubo formula. The dependence of the conductivity on temperature and external magnetic field is investigated. When the localized spins are antiferromagnetically coupled on a chain, the conductivity shows a monotonic increase with magnetic field, which is ordinary magnetoresistance. However, in a system where the localized spins form an antiferromagnetically coupled two-leg ladder, the conductivity shows an anomalous non-monotonic field dependence. Analysis of the spin configuration of the localized spin system reveals that quantum fluctuation determines the conductivity through a dimerized-rung spin configuration, and the formation of local triplets on the ladder rungs causes the decrease in conductivity.
Progress of Theoretical Physics Supplement | 2002
Akira Ogasahara; Seiji Miyashita
ESR in an S = ½ spin system is studied by a direct numerical method calculating the dynamical susceptibility expressed by the Kubo formula. Frequency-dependence of the resonant field is calculated in a one-dimensional antiferromagnet. We find that the frequency-dependence of the resonant field changes according to a relation among the amplitude of the interaction and other external parameters, e.g., temperature and frequency.
Journal of the Physical Society of Japan | 2003
Akira Ogasahara; Seiji Miyashita
The field and temperature dependence of the line shape of ESR for strongly interacting quantum spin systems would provide various important informations for the magnetic interaction. We have introduced a method of direct numerical estimation of the dynamical susceptibility with the Kubo formula and applied it to spin systems of various lattice structures. There the ESR line shape shows significant dependence on the strength of the field, and also on the geometrical situation among the directions of the fields and lattice axes.
Molecular Crystals and Liquid Crystals | 2002
Akira Ogasahara; Seiji Miyashita
We have developed a numerical method to estimate the behavior of electron spin resonance (ESR) absorption. By this method we investigated the dependence of the ESR line shape in various configurations of the external parameters in quantum spin systems with short range correlation. We found the dependence of the line shape on the temperature and also on the directions of the field. We propose to use ESR as the method to investigate the details of the geometrical and the magnetic structure of material.
EPR in the 21st Century#R##N#Basics and Applications to Material, Life and Earth Sciences | 2002
Akira Ogasahara; Seiji Miyashita
ESR in S = 1/2 spin system is studied by a direct numerical method calculating the dynamical susceptibility expressed by Kubo formula. Frequency-dependence of the resonant field is calculated in one-dimensional spin system In one-dimensional antiferromagnetic Heisenberg chain we find that the frequency-dependence of the resonant field changes according to relation among the amplitude of the interaction and the external parameters, e.g., temperature and field.
Journal of the Physical Society of Japan | 1999
Seiji Miyashita; Taro Yoshino; Akira Ogasahara
Journal of the Physical Society of Japan | 2000
Akira Ogasahara; Seiji Miyashita
Japanese journal of medical science & biology | 1953
Keiji Uchiyama; Tasutaro Kawakami; Atsumi Sakayori; Akira Ogasahara; Shogo Ishii