E. Kojima

The University of Tokyo, Tokyo, Tokyo-to, Japan

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Publications (27)29.22 Total impact

  • Shojiro Takeyama · Eiji Kojima ·
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    ABSTRACT: A copper-lined (CL) primary coil, which is a composite of steel and copper, was devised for the electromagnetic flux compression technique to generate ultrahigh magnetic fields. The newly developed coil was found to be highly efficient for electromagnetic energy transfer and provided stabilization of the liner implosive motion with less influence from the current feeding gap. Dynamical current density distribution of the materials used in a primary coil was evaluated and applied to the design of the CL coil. Fields of up to 730 T were achieved by employing the CL coil with an energy injected from a 4 MJ condenser bank. This value is the highest achieved thus far in an indoor setting. The peak magnetic fields were found to depend significantly on the initial seed magnetic field. The optimum seed fields for obtaining the highest peak magnetic field were determined.
    Journal of Physics D Applied Physics 10/2011; 44(42):425003. DOI:10.1088/0022-3727/44/42/425003 · 2.72 Impact Factor
  • S. Takeyama · H. Sawabe · E. Kojima ·
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    ABSTRACT: We have established a world record for the highest indoor magnetic field by the electro-magnetic flux compression (EMFC) technique; currently the maximum magnetic field is 730 T. The copper lined primary coil, which is a composite of steel and copper was found to be very efficient for generating a higher maximum magnetic field with a stabilized liner implosion. We present some of the recent results of our experiments of the EMFC.
    Journal of Low Temperature Physics 04/2010; 159(1):328-331. DOI:10.1007/s10909-009-0139-x · 1.02 Impact Factor
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    ABSTRACT: Recently, much progress has been made in generation of high magnetic fields in the International MegaGauss Science Laboratory at ISSP of the University of Tokyo. The electro-magnetic flux compression method has been improved to establish the world highest indoor-field of about 730 T. In addition to the destructive methods, a new project aiming at the non-destructive generation of both long pulsed- and 100 T-fields is progressing. The long pulse project is introduced. Not only for the highest field but also for various experiments, many kinds of coils have been developed. A microscope imaging system under pulsed fields, and miniature magnets for the X-ray measurements at the synchrotron radiation facility are also presented. High Field Lab-ISSP
    Journal of Low Temperature Physics 04/2010; 159(1):381-388. DOI:10.1007/s10909-009-0153-z · 1.02 Impact Factor
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    ABSTRACT: Magneto-absorption spectra of SWNTs (Single-Walled Carbon Nanotubes) in the region of near infrared were measured under high magnetic fields up to 54 T. We succeeded to observe clear peak shifts and splittings for four different chiralities of SWNTs, (7,5), (7,6), (8,6), and (8,7). We employed PFO (poly(9,9-dioctylfluorenyl-2,7-diyl))-SWNTs. Absorption peaks are very sharp and we could observe well-defined peaks of each chirality with less mixture of the other peaks of different chirality. Energy positions of band-edge bright and dark excitons were determined distinctly in each chirality.
    Journal of Low Temperature Physics 04/2010; 159(1):267-271. DOI:10.1007/s10909-009-0113-7 · 1.02 Impact Factor
  • E. Kojima · A. Miyata · Y. Motome · H. Ueda · Y. Ueda · S. Takeyama ·
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    ABSTRACT: We investigated the magnetization process of the highly frustrated spinel ZnCr2O4 using the Faraday rotation method at 6 K under the ultra-high magnetic fields up to 400 T by the electro-magnetic flux compression method. We successfully observed the full magnetization process in this material. We found some anomalies which show new phase transitions at 240 T, 290 T, 350 T, and 390 T.
    Journal of Low Temperature Physics 04/2010; 159(1):3-6. DOI:10.1007/s10909-009-0069-7 · 1.02 Impact Factor
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    ABSTRACT: The system was developed for the magnetization measurement in the vertical single-turn coil (V-STC) system at ISSP, which can generate magnetic fields over 100 T in a semi-destructive manner. We have adjusted the electro-magnetic induction method to our V-STC. The new system was applied to the manganite with the perovskite-type structure Bi1/2Ca1/2MnO3. The total magnetization process was obtained up to 105 T in excellent quality comparable to those obtained by the non-destructive long pulse magnet. Single-turn coil-100 tesla-Magnetization-Induction method
    Journal of Low Temperature Physics 04/2010; 159(1):297-301. DOI:10.1007/s10909-009-0126-2 · 1.02 Impact Factor
  • A Miyata · E Kojima · H Ueda · Y Ueda · Y Motome · S Takeyama ·
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    ABSTRACT: Precise magnetization processes of ZnCr2O4 were obtained by means of the Faraday rotation method subjected to ultra high magnetic fields up to 190 T generated by a single turn coil technique. In magnetization processes of ZnCr2O4, there observed a coplanar 2:1:1 canted state (the cant 2:1:1 phase) which is predicted by the theoretical model including a spin-lattice coupling. A magnetic-field width of the 1/2 plateau widens as the temperature increases.
    Journal of Physics Conference Series 02/2010; 200(3):032046. DOI:10.1088/1742-6596/200/3/032046
  • R. Shen · E. Kojima · R. Akimoto · S. Takeyama ·
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    ABSTRACT: The in-plane anisotropy of the spatially indirect (SID) photoluminescence (PL) in an undoped ZnSe/BeTe type-II quantum structure has been systematically investigated and analyzed. With a new method we have succeeded in extracting intrinsic circular polarization degree of this SID PL by a special combination of a 14 wave-plate and a linear polarizer with respect to the in-plane crystal axis, without the influence of the in-plane anisotropy.
    Physica E Low-dimensional Systems and Nanostructures 02/2010; 42(4):1172-1175. DOI:10.1016/j.physe.2009.12.004 · 2.00 Impact Factor
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    ABSTRACT: Development of the first measurement system for near-infrared absorption spectra between 0.9 and 1.7 μm under ultra-high magnetic fields is reported. Spectral signals were integrated for 1 μs with an InGaAs photodiode array around the top of a very short pulsed megagauss field. The magnetic fields were generated using a single-turn coil system. The measurement system was demonstrated in the study of exciton states in single-walled carbon nanotubes up to megagauss fields. A nearly noiseless absorption spectrum with well-resolved absorption peaks was obtained at 105.9 T in the Voigt configuration where the magnetic field was applied parallel to the alignment of the nanotubes.
    Journal of Low Temperature Physics 01/2010; DOI:10.1007/s10909-009-0125-3 · 1.02 Impact Factor
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    ABSTRACT: We studied charged excitons realized in dilute magnetic semiconductor single-quantum wells (Cd,Mn)Te/(Cd,Mg)Te in high magnetic fields by photoluminescence (PL) measurements. We assigned two PL peaks which appeared in high magnetic fields as the singlet and the triplet charged excitons, respectively. It was shown from our PL analysis that the second hidden crossover between the triplet and the singlet charged excitons took place above 60 T. The singlet charged exciton transition was found to be the lowest energy in a higher magnetic side. Linear increase in the conduction-electron g factor with increasing magnetic field was deduced from the intensity ratio of two PL lines in magnetic fields up to 140 T.
    Physical review. B, Condensed matter 03/2009; 79(12). DOI:10.1103/PhysRevB.79.125327 · 3.66 Impact Factor
  • E Kojima · H Ueda · Y Ueda · A Miyata · S Miyabe · S Takeyama ·
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    ABSTRACT: We conducted precise magneto-optical Faraday rotation measurements in CdCr2O4and ZnCr2O4up to ultra-high magnetic fields using methods of the single turn coil(up to 190 T) and also the electro-magnetic flux compression(up to 360 T). In CdCr2O4, we have succeeded in observation of the full magnetization process. In ZnCr2O4, we observed magnetization process up to 2.7 μB(the full moment is 3.0 μB). The magnetization processes of both materials were well described by a 4 sub-lattice Heisenberg-spin model including spin-lattice interactions.
    Journal of Physics Conference Series 02/2009; 145(1):012023. DOI:10.1088/1742-6596/145/1/012023
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    ABSTRACT: Spatially separated indirect photoemission in a non-doped ZnSe/BeTe type II quantum structure is investigated by applying pulse magnetic field up to 50 T. The magnetic field dependences of the dichroic photoluminescence (PL) are well explained by an optical transition model associated with a positively charged exciton composed by an electron and two light holes. The PL is suppressed by magnetic field and also by temperature. Weak PL probably arising from localized excitons at interfaces is observed up to the highest magnetic field, and in high temperatures above 20 K. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
    physica status solidi (c) 01/2009; 6(1):38 - 41. DOI:10.1002/pssc.200879893
  • E. Kojima · A. Miyata · S. Miyabe · S. Takeyama · H. Ueda · Y. Ueda ·
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    ABSTRACT: The total magnetization process of CdCr2O4 , a prototype of the geometrically frustrated Heisenberg spin system, was revealed by Faraday rotation measurements conducted up to 140 T at a wide range of temperatures. We have shown that the Faraday rotation angle is proportional to the magnetization of the system up to the highest magnetic fields. The magnetization processes were well described by a four sublattice Heisenberg spin model [Penc , Phys. Rev. Lett. 93, 197203 (2004); Motome , J. Magn. Magn. Mater. 300, 57 (2006)] including spin-lattice interactions.
    Physical review. B, Condensed matter 06/2008; 77(21). DOI:10.1103/PhysRevB.77.212408 · 3.66 Impact Factor
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    ABSTRACT: We have studied exciton complexes such as charged excitons and biexcitons in dilute magnetic semiconductor Cd1-xZnxTe/Cd1-x-yZnxMnyTe (x=0.07, y=0.43) quantum wells (well widths 4 and 9 nm). Laser excitation power dependence and time-resolved photoluminescence (PL) showed unique features related to excitons, charged excitons, and biexcitons. Four-wave-mixing spectroscopy with different choices of polarized incident light exhibited beating signals arising from exciton-biexciton interaction. Circular polarized PL was measured in magnetic fields of up to 42 T using a pulse magnet. The observed Zeeman splitting and the degree of circular polarization in magnetic fields revealed that the formation of biexcitons is quenched upon applying a magnetic field, and PL from the charged excitons was preserved up to the highest magnetic field.
    Journal of the Physical Society of Japan 04/2008; 77(4). DOI:10.1143/JPSJ.77.044702 · 1.59 Impact Factor
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    ABSTRACT: The magneto-optical properties of a 1-μm-thick Ga0.94Mn0.06As sample having a 110 K Curie temperature are investigated by time-resolved pump-probe and continuous-wave (cw) midinfrared transmittance spectroscopies. The pump pulses are linearly polarized and have a 3.1 eV energy, a 130 fs width, and a 660 μJ∕cm2 fluence. They produce a thermally driven demagnetization process in the hundreds of picosecond time scale. A three-temperature model based on one-dimensional diffusion equations was developed to compute the carrier, lattice, and spin temperatures in the material. We observed that an average photoinduced spin temperature increase near 3 K leads to an augmentation of the Mn acceptor impurity binding energy in the range of 1 meV. cw absorption measurements were performed at temperatures ranging from 4 to 300 K, and a nonmonotonic temperature dependence of the position of the midinfrared absorption peak is found that is consistent with the increased binding energy observed by time-resolved measurements. The variation of the sample resistivity with temperature is phenomenologically related to the optical measurements. Results are compatible with a polaron percolation model of ferromagnetism.
    Physical review. B, Condensed matter 11/2007; 76(19). DOI:10.1103/PhysRevB.76.195323 · 3.66 Impact Factor
  • Shojiro Takeyama · Eiji Kojima · Koichi Kindo · Hiroyuki Yokoi ·
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    ABSTRACT: We review recent activities at the mega-gauss facility at Kashiwa mega-gauss facility. The plan of introducing a giant DC generator is now under process for aiming at 100 T nondestructive magnetic field achievement in near future. Our current progress in the destructive mega-gauss magnetic fields and some of the selected topics focusing on the magneto-optical subjects are described.
    International Journal of Modern Physics B 04/2007; 21(8-9):1238-1246. DOI:10.1142/S0217979207042689 · 0.94 Impact Factor
  • E. Kojima · Y. Natori · S. Takeyama · Y. Arishima · H. Mino ·
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    ABSTRACT: We measured the polarization selective photo-luminescence (PL) and the reflection spectra in the diluted magnetic semiconductor CdZnTe/(Cd,Zn,Mn)Te quantum well under the high magnetic field of up to 40 T. A double peak structure was observed in PL spectra. The higher energy peak could be assigned to the exciton level from the behavior of Zeeman shift or polarization degree. The lower energy peak could not be explained by J=0 biexciton state. We could determined that this peak was due to the charged exciton from the results of the polarization selective reflection measurement.
    International Journal of Modern Physics B 04/2007; 21(8-9):1605-1609. DOI:10.1142/S0217979207043282 · 0.94 Impact Factor
  • E Kojima · S Miyabe · H Mitamura · H Ueda · Y Ueda · H A Katori · H Takagi · S Takeyama ·
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    ABSTRACT: We studied the magnetic properties of ZnCr2O4 under an ultra-high magnetic field by using the Faraday rotation method. We used a single turn coil system to generate a pulsed high field up to 180 T and used a He-Ne laser as a light source. We chose a 45 degree angle between the polarizer and the analyzer and measured a transient Faraday rotation signal by using a photo-diode and a transient recorder. The sample temperature was kept below Neel temperature during the experiment. We observed a discernible discontinuity in a Faraday rotation signal at 120 T. This discontinuity is considered to represent the first-order phase transition which is observed also in CdCr2O4which has the same geometrical structure as ZnCr2O4.
    Journal of Physics Conference Series 12/2006; 51(1):51. DOI:10.1088/1742-6596/51/1/009
  • E Kojima · S Takeyama ·
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    ABSTRACT: Magnetic fields of over 100 T can be generated only through the use of a destructive pulsed magnet. Electro-magnetic flux compression (EMFC) is an efficient method to generate such ultra-high magnetic fields. The EMFC system at Institute for Solid State Physics holds the world record for the highest magnetic field produced in-doors. This system has been used for various sorts of measurements applied to matters under ultrahigh magnetic fields. Recently, we successfully improved the coil system to generate a higher field using less energy injection and more simplified preparation processes. The new system increased the electro-magnetic energy transfer efficiency to at least twice that of the previously employed system. Our new primary coil using a copper current guide has the advantage of shallower high-frequency current skin depth and less contact impedance than previous ones. Therefore the discharge current spark is completely avoided. The improved skin depth resulted in a symmetric implosion of the liner coil with less influence of the feed gap. A high degree of cylindrical liner symmetry was observed during implosion. A fast liner speed of 2.4 km/s was achieved, and 350 T could be generated by 1 MJ and 470 T by 2 MJ.
    Journal of Physics Conference Series 12/2006; 51(1):537. DOI:10.1088/1742-6596/51/1/121
  • E. Kojima · S. Takeyama ·
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    ABSTRACT: Electro-magnetic flux compression (EMFC) is one of the most powerful methods to generate mega-gauss magnetic fields. The EMFC facility at ISSP has attained the world record for indoor generation of magnetic fields. We have recently implemented successful improvements to generate a higher field with less energy injection and with more simplified coil preparation processes. We have changed the structure and the materials of the primary coil. We have tested the new coil system and succeeded in generating fields up to 350 T with 1 MJ, 470 T with 2 MJ and 520 T with 3 MJ. Hence, electro-magnetic energy transfer efficiency has been increased a great deal. The liner implosive motion was much improved with good cylindrical symmetry. A simple calculation has given insight into the relations between the current density distribution of a primary coil and the shape of the liner during the process of the implosion.
    Magagauss Magnetic Field Generation and Related Topics, 2006 IEEE International Conference on; 12/2006