Faculty Profiles - KOBAYASHI, Yoshiaki

Papers - KOBAYASHI, Yoshiaki

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Magnetic Excitation Spectra of Superconducting Ca-Fe-Pt-As System Reviewed

M. Sato, T. Kawamata, Y. Kobayashi, Y. Yasui, Takefumi Iida, Kazunori Suzuki, Masayuki Itoh, Taketo Moyoshi, Kiyoichiro Motoya, Ryoichi Kajimoto, Mitsutaka Nakamura, Yasuhiro Inamura, and Masatoshi Arai

Journal of the Physical Society of Japan Vol. 80 page： 093709-(1-4) 2011

Study of Ni-Doping Effect of Specific Heat and Transport Properties for LaFe1-yNiyAsO0.89F0.11 Reviewed

T. Kawamata, E. Satomi, Y. Kobayashi, M. Itoh, and M. Sato

Journal of the Physical Society of Japan Vol. 80 page： 084720-(1-5) 2011

Crystal Structure of New Pt-Doped Fe Pnictide Superconductors Reviewed

Vol. 80 page： 073710-(1-3) 2011

Magnetic Ordering in V-Layers of the Superconducting System of Sr2VFeAsO3 Reviewed

S. Tatematsu, E. Satomi, Y. Kobayashi, and M. Sato

J. Phys. Soc. Jpn. Vol. 79 ( 12 ) page： 123712-(1-4) 2010.12

Distinct physical behaviors of LaFe1-yCoyAsO0.89F0.11 between the superconducting and nonsuperconducting metallic regions of y divided by y similar to 0.05

Sato M., Kobayashi Y., Lee S. C., Takahashi H., Moyoshi T.

PHYSICA C-SUPERCONDUCTIVITY AND ITS APPLICATIONS Vol. 470 page： S475 - S477 2010.12

On the non-superconducting state in the phase diagram of NaxCoO2 center dot yH(2)O

Sato Masatoshi, Kobayashi Yoshiaki, Moyoshi Taketo

PHYSICA C-SUPERCONDUCTIVITY AND ITS APPLICATIONS Vol. 470 page： S752 - S754 2010.12

NMR studies on the superconducting symmetry of iron pnictide systems

Kobayashi Yoshiaki, Lee Sang Chul, Takahashi Hidefumi, Moyoshi Taketo, Sato Masatoshi

PHYSICA C-SUPERCONDUCTIVITY AND ITS APPLICATIONS Vol. 470 page： S472 - S474 2010.12

Magnetic excitations of superconducting LaFeAsO0.89F0.11

Moyoshi T., Lee S. C., Tatematsu S., Yasui Y., Kobayashi Y., Sato M., Kakurai K.

PHYSICA C-SUPERCONDUCTIVITY AND ITS APPLICATIONS Vol. 470 page： S470 - S471 2010.12

Studies on the superconducting state of NaxCoO2 center dot yH(2)O - Overview

Sato Masatoshi, Kobayashi Yoshiaki, Moyoshi Taketo

PHYSICA C-SUPERCONDUCTIVITY AND ITS APPLICATIONS Vol. 470 page： S673 - S677 2010.12

⁷⁵As-NMR Studies of LaFeAsO_1-xF_x for Various x Values Reviewed

Yoshiaki Kobayashi, Erika Satomi, Sang Chul Lee, and Masatoshi Sato

Journal of the Physical Society of Japan Vol. 79 ( 9 ) page： 093709-(1-4) 2010.9

Authorship：Lead author Language：English Publishing type：Research paper (scientific journal)

Results of ⁷⁵As NMR and NQR measurements are reported for superconducting samples of LaFeAsO_1-xF_Rx with various x values. The x dependent widths of the superconducting transition and those of the NQR spectra indicate that the optimally doped LaFeAsO_1-xF_x sample with the superconducting transition temperature T_c=28 K has the smallest width or smallest inhomogeneity of the superconducting order parameter Δ. In the analyses of the temperature (T) dependence of the NMR longitudinal relaxation rate 1/T₁, we have tried to use a simple relation 1/T₁~Tⁿ to see how fast 1/T₁ decreases when the superconducting order parameter grows. The relation holds rather well in the T region of ~0.3T_c<T=<T_c, and n varies from 2.5-6 with varying x, having the maximum value 5-6 at the optimal x. These results indicate that the x dependence of n originates from the spatial distribution of Δ, and that the T^5-6 behavior is considered to be intrinsic to this system.

DOI： 10.1143/JPSJ.79.093709

Superconducting Transition Temperatures and Transport Properties of LaFe1-yRuyAsO0.89F0.11 and LaFeAsO0.89-xF0.11+x Reviewed

Erika Satomi, Sang Chul Lee, Yoshiaki Kobayashi, and Masatoshi Sato

Journal of the Physical Society of Japan Vol. 79 ( 9 ) page： 094702 (6 pages) 2010.9

Language：English Publishing type：Research paper (scientific journal)

The transport behaviors and superconducting transition temperatures Tc of the title systems have been experimentally studied for various x and y values. Together with results reported previously by our group, data are now available for many sets of samples of RFe1-yMyAsO0.89F0.11 with R = La and Nd and M = Co, Mn and Ru and for RFeAsO0.89-xF0.11+x. Dopings of Co and Mn atoms introduce electrons and holes into the FeAs planes, respectively. They also introduce, more or less, randomness effects. Ru atoms introduce scattering centers within the FeAs planes without changing the number of carrier electrons or holes. In contrast, F atoms introduce electron carriers, but not randomness effects or scattering centers into the FeAs planes. From these studies, we can conclude that there are two appreciable mechanisms of Tc suppression by the dopings. The disappearance of or reduction in the area of the hole Fermi surfaces around the Γ point in the reciprocal space has been clearly shown as one of the mechanisms, irrespective of whether or not doped atoms are within the conducting planes. The other mechanism is the electron localization observed for the Mn doping. The rates of the Tc suppression due to the scattering by nonmagnetic Ru and Co atoms have been found to be negligible compared with that expected from the pair breaking theory, indicating that the systems do not have the S± symmetry of the superconducting order parameter.

DOI： 10.1143/JPSJ.79.094702

Web of Science

High-Temperature Multiferroic State of RBaCuFeO5 (R = Y, Lu, and Tm)

Kawamura Yuji, Kai Takahiro, Satomi Erika, Yasui Yukio, Kobayashi Yoshiaki, Sato Masatoshi, Kakurai Kazuhisa

JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN Vol. 79 ( 7 ) 2010.7

Superconducting transition temperature and the thickness of CoO2 planes of NaxCoO2 center dot yH(2)O

Moyoshi Taketo, Kobayashi Yoshiaki, Yasui Yukio, Sato Masatoshi, Kakurai Kazuhisa

SOLID STATE SCIENCES Vol. 12 ( 5 ) page： 656 - 659 2010.5

Effects of Ru Doping on the Transport Behavior and Superconducting Transition Temperature of NdFeAsO_0.89F_0.11 Reviewed

Sang Chul Lee, Erika Satomi, Yoshiaki Kobayashi, and Masatoshi Sato

Journal of the Physical Society of Japan Vol. 79 ( 2 ) page： 023702-(1-4) 2010.2

Language：English Publishing type：Research paper (scientific journal)

The transport behavior and superconducting transition temperature T_c of NdFe_1-yRu_yAsO_0.89F_0.11 have been studied for various y values. Because Ru impurities are isoelectronic to host Fe atoms, we basically expect that the number of electrons does not change with y, at least in the region of small y values. The results indicate that the rate of T_c suppression by Ru atoms is too small to be explained by the pair breaking effect of nonmagnetic impurities expected for the S± symmetry, confirming our previous results for Co doping.

DOI： 10.1143/JPSJ.79.023702

Web of Science

Studies on Effects of Impurity Doping and NMR Measurements of La 1111 and/or Nd 1111 Fe-Pnictide Superconductors Reviewed

Masatoshi Sato, Yoshiaki Kobayashi, Sang Chul Lee, Hidefumi Takahashi, Erika Satomi, and Yoko Miura

Journal of the Physical Society of Japan Vol. 79 ( 1 ) page： 014710-(1-10) 2010.1

NMR studies on the superconducting symmetry of iron pnictide systems Reviewed

Y. Kobayashi, S.C. Lee, H. Takahashi, T. Moyoshi, M. Sato

Physica C: Superconductivity Vol. 470 page： S472-S474 2010

Studies on Effects of Impurity Doping and NMR Measurements of La 1111 and/or Nd 1111 Fe-Pnictide Superconductors Reviewed

Masatoshi Sato, Yoshiaki Kobayashi, Sang Chul Lee, Hidefumi Takahashi, Erika Satomi, and Yoko Miura

Journal of the Physical Society of Japan Vol. 79 ( 1 ) page： 014710 2009.12

Authorship：Lead author Language：English Publishing type：Research paper (scientific journal)

Measurements of the electrical resistivity rho, Hall coefficient RH, thermoelectric power S, and the electronic specific heat coefficient gamma have been carried out for samples of LnFe1-yMyAsO1-xFx (Ln=La, Nd; M=Co, Mn; x=0.11) obtained by doping M atoms into the superconducting LnFeAsO1-xFx (Ln 1111) system. The NMR longitudinal relaxation rate 1/T1 has also been measured for samples of LaFe1-yCoyAsO1-xFx with various x values. Co atoms doped into the superconducting LnFeAsO1-xFx are nonmagnetic, and the rate of Tc-suppression absolute value of dTc/dx by Co atoms has been found to be too small to be explained by the pair-breaking effect expected for the S+- superconducting symmetry proposed as the most probable symmetry for the system. This result throws a serious doubt on whether the symmetry is realized in the system. Instead of the pair breaking, two mechanisms of Tc suppression by the doped impurities have been found: One is the electron localization, which appears when the sheet resistance r/sheet exceeds h/4e2=6.45 k-ohme, and the other is the disappearance (or reduction in the area) of the hole Fermi surfaces around the Gamma point in the reciprocal space. The latter mechanism has been observed when the number of electrons increases with increasing Co doping level and the system changes from an anomalous metal to an ordinary one. Regarding the two distinct T dependences of the NMR longitudinal relaxation rate 1/T₁ of LaFeAsO_1-xF_x, we discuss the origin of such a difference, and show that, at least, the T^2.5-3.0-like dependence of 1/T₁ cannot be considered as the experimental evidence for the S+- symmetry of Delta.

DOI： 10.1143/JPSJ.79.014719/

Studies of Multiferroic System LiCu2O2: I. Sample Characterization and Relationship between Magnetic Properties and Multiferroic Nature Reviewed

Yukio Yasui, Kenji Sato, Yoshiaki Kobayashi, and Masatoshi Sato

J. Phys. Soc. Jpn. Vol. 78 ( 8 ) page： 084720-(1-5) 2009.8

*Studies of Multiferroic System of LiCu2O2: II. Magnetic Structures of Two Ordered Phases with Incommensurate Modulations Reviewed

Yoshiaki Kobayashi, Kenji Sato, Yukio Yasui, Taketo Moyoshi, Masatoshi Sato, and Kazuhisa Kakurai

Journal of the Physical Society of Japan Vol. 78 ( 8 ) page： 084721-(1-5) 2009.8

Authorship：Lead author Language：English Publishing type：Research paper (scientific journal)

Neutron diffraction and 7Li-NMR have been applied to determine the three dimensional magnetic structures of the multiferroic system LiCu2O2, which has four chains (ribbon chains) of edge-sharing CuO4 square planes in a unit cell. First, we have confirmed that there are successive magnetic transitions at TN1 = 24.5 K and TN2 = 22.8 K. In the T region between TN1 and TN2, the quasi one-dimensional spins (S = 1/2) of Cu2+ ions within a chain have a collinear and sinusoidally modulated structure with Cu-moments parallel to the c-axis and with the modulation vector along the b-axis. At T < TN2, an ellipsoidal helical spin structure with the incommensurate modulation has been found. We present detailed parameters, describing the modulation amplitudes, helical axis vectors as well as the relative phases of the modulations of four ribbon chains, which can well reproduce both the NMR and neutron results, in the two magnetically ordered phases. This finding of the rather precise magnetic structures enables us to discuss the relationship between the magnetic structure and the multiferroic nature of the present system in zero magnetic field, as presented in our companion paper (paper I), and open a way how to understand the reported electric polarization under the finite magnetic field.

Studies of Multiferroic System LiCu2O2: I. Sample Characterization and Relationship between Magnetic Properties and Multiferroic Nature

Yasui Yukio, Sato Kenji, Kobayashi Yoshiaki, Sato Masatoshi

JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN Vol. 78 ( 8 ) 2009.8

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Papers - KOBAYASHI, Yoshiaki