Kyushu University Academic Staff Educational and Research Activities Database
List of Papers
Satoshi Iikubo Last modified date:2021.10.07

Professor / Department of Advanced Materials Science and Engineering / Faculty of Engineering Sciences

1. S Yoshii, S Iikubo, T Kageyama, K Oda, Y Kondo, K Murata, M Sato, Anomalous Hall effect of pyrochlore molybdate Nd2Mo2O7, JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN, 10.1143/JPSJ.69.3777, 69, 12, 3777-3780, 2000.12, Hall resistivity rho (H) has been investigated for single crystals of pyrochlore molyhdate Nd2Mo2 O-7. With decreasing temperature T, the anomalous Hall effect appears at around the Curie temperature T-C (similar to 95 K). In the relatively high temperature region (T > 60 K), rho (H) can be described by the well-known equation rho (H) = R0H + 4 piR(s)M, where R-0 and R-s are the ordinary and anomalous Hall coefficients, respectively, and M is the net magnetization predominantly contributed by the Mo spins. With further decreasing T, the ordering of the nTd moments, which is confirmed by neutron diffraction studies carried out by the present authors' group, induces marked change in the behavior of the anomalous part of pH. The behavior in the low temperature region has been found to be simply described by the equation rho (H) = R0H + 4 piR(s)M(Mo) + 4 piR(s)M(Mo) where M-Mo and M-Nd are the net magnetizations of the Mo- and Nd-moments, respectively. This result and the T-dependence of R-s and R-s' are quite different from those known for ordinary ferromagnets and might be related to the nonzero spin chirality observed in the present system..
2. T Kageyama, S Iikubo, S Yoshii, Y Kondo, M Sato, Y Iye, Study on anomalous Hall resistivity of Nd2Mo2-xTixO7, JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN, 10.1143/JPSJ.70.3006, 70, 10, 3006-3010, 2001.10, Hall resistivity PH has been studied for single crystals of Nd2Mo2O7, which has the ferromagnetic and chiral ordered state. Effects of Ti-doping on PH have also been studied down to 3 K under the magnetic field (H) up to 7 T. The T- and H-regions of the measurements have boon extended for non-doped samples to 50 mK and 15 T, respectively. From the results, it has been confirmed that two components of the anomalous Hall resistivity exist. One. is proportional to tho net magnetization of Mo atoms, M-Mo. It changes its sign with the doping. The other is proportional to the net magnetization of Nd atoms, M-Nd and does not exhibit the sign change. The Hall resistivity has a finite and almost T-independent value at very low temperatures. The,,,(, results on the behavior of rho (H) are discussed by considering the existing theories including the one recently proposed by a mechanism related to the spin chiral order..
3. S Iikubo, S Yoshii, T Kageyama, K Oda, Y Kondo, K Murata, M Sato, Magnetic and transport properties of phyrochlore molybdates, JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN, 70, 1, 212-218, 2001.01, Magnetic and transport properties of the pyrochlore molybdate R2-xRx'Mo2O7 (R, R' = rare earth, Y), which exhibits the metal-insulator (M-I) transition with varying x, have been studied. The transition from an antiferromagnetic spin-glass state to a ferromagnetic one occurs at x far from the M-I phase boundary, indicating that the intimate correlation of the magnetic state with the conducting nature is not so obvious. In Nd2Mo2O7, the ordering of Mo spins takes place at about 95 K, where the anomalous Hall resistivity appears with decreasing temperature T along with the spontaneous magnetization. With the ordering of Nd moments which becomes significant below similar to 20K with decreasing T, the behavior of the anomalous Hall resistivity markedly changes, while the Mo-magnetization is partially cancelled by the Nd magnetization. It is shown that the anomalous Hall resistivity can be described by two components, 4 piR(s)M(Mo), and 4 piR(s)'M-Nd, the contributions from the uniform magnetizations M-Mo and M-Nd of Mo and Nd moments, respectively. The sign of R-s' is opposite to that of R-s. Brief arguments are given if the unusual behavior of the Hall resistivity is related with the nonzero chirality of the magnetic moment system..
4. M Ambai, Y Kobayashi, S Iikubo, M Sato, Transport and NQR studies of Nd1.6-xCexSr0.4CuO4 with T* structure, JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN, 10.1143/JPSJ.71.538, 71, 2, 538-542, 2002.02, Transport and Cu-NQR studies have been carried out for the T*-type high temperature superconducting Cu oxide Nd1.6-xCexSr0.4CuO4 with single layered CuO2 planes formed by CuO5 pyramids. No anomaly related to the static or quasi static "stripe" order ha, been observed in the temperature T and x dependence of the thermoelectric powers S. We have not found such kind of anomaly in the x dependence of the superconducting transition temperature T-c either. Although the decrease of the Cu NQR intensity with decreasing T (wipeout) has been observed in this system, it can be understood by considering the loss of the itinerant nature of the electrons, which takes place in the proximity region of the metal-insulator phase boundary, and cannot be connected With the static or quasi static "stripe" order..
5. M Kanada, Y Yasui, Y Kondo, S Iikubo, M Ito, H Harashina, M Sato, H Okumura, K Kakurai, H Kadowaki, Neutron scattering study of the spin correlation in the spin ice system Ho2Ti2O7, JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN, 10.1143/JPSJ.71.313, 71, 1, 313-318, 2002.01, Intensity distribution of the neutron magnetic scattering has been taken in the reciprocal space of a single crystal of the "spin ice" system Ho2Ti2O7 at the elastic energy position in the temperature (T) region between 0.4 K and 50 K. The short range magnetic correlation becomes appreciable with decreasing T at around 30 K. The freezing of the moment system takes place at around 1.5 K with decreasing T. Based on the mean field treatment of the moment system, the distribution of the observed intensity has been reproduced, where the dipolar interaction is confirmed to be primarily important for the determination of the magnetic correlation. Simple calculations of the scattering intensity carried out by using a cluster of 25 moments of Ho3+ ions indicate that there are many types of the moment arrangement within a cluster which have almost degenerated energies, indicating that the system cannot uniquely select the correlation pattern down to rather low temperature, and before going to the lowest energy state, the system undergoes the gradual slowing down to the glassy state..
6. S Iikubo, Y Yasui, K Oda, Y Ohno, Y Kobayashi, M Sato, K Kakurai, Magnetic structure and the Hall resistivity of Cu1-xZnxCr2Se4, JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN, 10.1143/JPSJ.71.2792, 71, 11, 2792-2799, 2002.11, Neutron diffraction, NMR and other magnetic and transport studies have been carried out on several samples of the ferromagnetic Cu1-xZnxCr2Se4 to extract information on the relationship between the unusual behavior of the Hall resistivity rho(H) and the non-trivial magnetic structure or the spin chiral order. For x = 0.3, 0.5 and 0.7, conical structures have been adopted to explain the observed data of neutron diffraction, and parameters which describe the structures have been determined. Arguments on the differences among the values of the Cr moments observed by various method are presented, which elucidates the characteristics of the magnetic state of the present system. It is confirmed that along with the gradual growth of the conical structure, the anomalous Hall coefficient R-s changes its sign. The correlation between the conical angle theta and the low temperature value of R-s is pointed out..
7. M Ito, H Harashina, Y Yasui, M Kanada, S Iikubo, M Sato, A Kobayashi, K Kakurai, Detailed structure of the magnetic excitation spectra of YBa2Cu3Oy and its implication on the physical characteristics of the electron system, JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN, 10.1143/JPSJ.71.265, 71, 1, 265-270, 2002.01, Detailed structure of the magnetic excitation spectra X"(q, w) of the superconducting oxide YBa2CU3O6.5 (the transition temperature T-c congruent to 52K) in the wave vector (q)- and the energy (w)-space, and its temperature (T) dependence have been studied. By adopting an effective energy dispersion of the quasi particles which can reproduce the shape of the Fermi surface and by introducing the exchange interaction between the Cu spins, a rather satisfactory agreement between the calculation and the experimentally observed data can be obtained. In the study, it has been found that the effects of the quasi particle-energy broadening on the excitation spectra X"(q, w) is important. The sharp resonance peak observed at w similar to 40 meV for the optimally doped system of YBa2CU3Oy can be naturally reproduced by the present model..
8. Y Yasui, S Iikubo, H Harashina, T Kageyama, M Ito, M Sato, K Kakurai, Neutron scattering studies of pyrochlore compound Nd2Mo2O7 in magnetic field, JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN, 10.1143/JPSJ.72.865, 72, 4, 865-872, 2003.04, Neutron diffraction studies have been carried out in the applied magnetic field H(parallel to [0 (1) over bar1]) on a single crystal of pyrochlore ferromagnet Nd2Mo2O7, whose Hall resistivity (rho(H)) has been reported to have quite unusual magnetic field (H)- and temperature (T)-dependences. The intensities of the observed magnetic reflections have been reproduced at 1.6 K as a function of H, by considering the change of the magnetic structure with H, where effects of the exchange fields at the Mo and Nd sites induced by the Mo-Mo and Mo-Nd exchange interactions and the single ion anisotropies of Mo- and Nd-moments are considered. From the H-dependent magnetic structure, the H-dependence Of rho(H) has been calculated by using the chiral order mechanism. By comparing the result with the H-dependence of the observed rho(H), it is found that the chiral order mechanism does not work well in the present system..
9. S Iikubo, Y Ohno, Y Yasui, T Fukamachi, K Oda, M Sato, K Kakurai, Magnetic structure and the anomalous Hall effect Cu1-xZnxCr2Se4, PHYSICA B-CONDENSED MATTER, 10.1016/S0921-4526(02)02018-5, 329, 2, 707-708, 2003.05, To study what roles spin structures of ferromagnets play in determining their Hall resistivity rho(H) or to see if the spin chiral order really contributes to the unusual behavior of anomalous Hall resistivity, we have carried out transport and neutron diffraction studies on the spinel-type system Cu1-xZnxCr2Se4, which is a collinear ferromagnet at xsimilar to0 and in the helimagnetic state at x = 1.0. It is established that the sign change of rho(H) takes place, with decreasing temperature, along with the appearance of the conical spin structure in the region 0 < x < 1.0. (C) 2003 Elsevier Science B.V. All rights reserved..
10. Y Yasui, M Soda, S Iikubo, M Ito, M Sato, N Hamaguchi, T Matsushita, N Wada, T Takeuchi, N Aso, K Kakurai, Ferromagnetic transition of pyrochlore compound Yb2Ti2O7, JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN, 10.1143/JPSJ.72.3014, 72, 11, 3014-3015, 2003.11.
11. M.Ito,Y. Yasui, S. Iikubo, M. Soda, M. Sato, A. Kobayashi, K. Kakurai, Effect of “Stripe” on the Magnetic Excitation Spectra of La1.48Nd0.4Sr0.12CuO4, Journal of the Physical Society of Japan, 10.1143/JPSJ.72.1627, 72, 7, 1627-1630, 2003.07.
12. Y Yasui, T Kageyama, S Iikubo, K Oda, M Sato, K Kakurai, Detailed studies on the anomalous Hall effect of pyrochlore molybdates, PHYSICA B-CONDENSED MATTER, 10.1016/S0921-4526(02)02237-8, 329, 2, 1036-1037, 2003.05, Neutron measurements and transport studies have been carried out on single crystals of the pyrochlore ferromagnet Nd2Mo2O7, which has quite unusual Hall resistivity rho(H). The magnetic structure has been determined at 1.6 K as a function of H, and based on this magnetic structure, the H-dependence Of rho(H) is deduced by using the recently proposed chiral order mechanism. The results have turned out not to describe the observed behavior Of rho(H). (C) 2003 Elsevier Science B.V. All rights reserved..
13. M Sato, M Ito, H Harashima, Y Yasui, S Iikubo, A Kobayashi, K Kakurai, Comparative study on the magnetic excitation spectra of Y123 and La214 high-T-c systems - are the dynamical stripes important?, PHYSICA B-CONDENSED MATTER, 10.1016/S0921-4526(02)01990-7, 329, 2, 683-684, 2003.05, The magnetic spectra x'' of YBa2Cu3O6.5 are compared with calculated results by the expression x(q,omega) = x(0), omega)/{1 + J(q)x(0)(q, omega)} with J(q) = J(cos q(x)a + cosq(y)a). For the proper band parameters and other parameters, we obtained satisfactory agreement between experimental and calculated results without considering dynamical "stripes". The quasi particle broadening has been found to suppress the antiferromagnetic ordering. Effects of the dynamical ''stripes" on x''(q,omega) are clearly visible in La2-xSrxCuO4, which is in clear contrast to the results of YBa2Cu3O6.5. (C) 2002 Elsevier Science B.V. All rights reserved..
14. T Kageyama, N Aito, S Iikubo, M Sato, Anomalous Hall effect of reentrant spin glass system Fe1-xAlx (x similar to 0-3), JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN, 10.1143/JPSJ.72.1491, 72, 6, 1491-1494, 2003.06, The anomalous Hall coefficient R-s has been studied for the reentrant spin glass system Fe0.7Al0.3 by measuring the magnetization M and the Hall resistivity rho(H). We have found that R-s exhibits anomalous change of the temperature dependence at the spin-glass transition temperature T-G, indicating that an additional term appears along with the beginning of the spin glass phase. The result is discussed in relation to the recent proposal of the chirality mechanism of the Hall effect in the spin glass phase..
15. M. Ito, Y. Yasui, S. Iikubo, M. Soda, A. Kobayashi, M. Sato, K. Kakurai, C. H. Lee, K. Yamada, Studies on the (non-) existence of "stripes" in high T-c superconductors, JOURNAL OF MAGNETISM AND MAGNETIC MATERIALS, 10.1016/j.jmmm.2003.12.638, 272, 1, E1051-E1053, 2004.05, Magnetic excitation spectra chi ''(q, omega) of YBa2Cu3Oy, La2-xSrxCuO4 and La2-y-xNdySrxCuO4 are compared with those of model calculations. It is important to consider the experimentally observed quasi-particle broadening (Gamma) to reproduce the observed spectra of YBa2Cu3Oy, while to reproduce the data of other two systems, very small Gamma values, much smaller than those reported in the transport studies have to be used, even in the phases without the static "stripe" order. This may be caused by the existence of the "stripe" correlations or "stripe" fluctuations. (C) 2003 Elsevier B.V. All rights reserved..
16. M Ito, Y Yasui, S Iikubo, M Soda, A Kobayashi, M Sato, K Kakurai, CH Lee, K Yamada, Studies on magnetic excitation spectra of high-T-c superconductors, JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN, 10.1143/JPSJ.73.991, 73, 4, 991-999, 2004.04, Magnetic excitation spectra chi'(q,omega) of YBa2Cu3Oy, La2-xSrxCuO4, La2-y-xNdySrxCuO4 and Nd2-xCexCuO4 are compared with those of model calculations, where q and omega are the wave vector and the energy of the excitations. In the model, the dynamical spin susceptibility is calculated by the expression chi(q, omega) = chi(0)(q, omega)/{1 + J(q)chi(0)(q, omega)}, where chi(0)(q, omega) is obtained by adopting the effective energy dispersion of the quasi particles which can reproduce the shapes of the Fermi surface, and J(q) is the exchange coupling between the Cu-Cu electrons. It is important to consider the experimentally observed quasi particle broadening (Gamma) to reproduce the observed spectra of YBa2Cu3Oy, while to reproduce the data of La2-xSrxCuO4 and La2-y-xNdySrCuO4, very small Gamma values have to be used, even in the phases without the static "stripe" order. This may be caused by the existence of the "stripe" correlations or "stripe" fluctuations..
17. M Soda, Y Yasui, M Ito, S Iikubo, M Sato, K Kakurai, Magnetic structures and spin states of NdBaCo2O5, JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN, 10.1143/JPSJ.73.464, 73, 2, 464-468, 2004.02, Neutron diffraction studies have been carried out on a single crystal of oxygen-deficient perovskite NdBaCo2O5 in the temperature range of 7-370K. There have been observed, with decreasing temperature T, two magnetic transitions accompanied with structural changes, to the antiferromagnetic phase at similar to360K and to the charge ordered antiferromagnetic one at similar to250K. We have studied the magnetic structures at 300 K and 7 K. At these temperatures the Co-moments have the so-called G-type antiferromagnetic structure, and Co ions are considered to be in the high spin state over the whole temperature range below T-N similar to 360K..
18. M Soda, Y Yasui, M Ito, S Iikubo, M Sato, K Kakurai, Magnetic structures and spin states of NdBaCo2O5.5, JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN, 10.1143/JPSJ.73.2857, 73, 10, 2857-2862, 2004.10, Neutron diffraction studies have been carried out on a single crystal of oxygen-deficient perovskite NdBaCo2O5.5 in the temperature (T) range between 7 K and 320 K. We have found a magnetic transition at T-c similar to 170 K, and a rapid change of the intensities of several reflections at T* similar to 30 K. The magnetic structures have been analyzed at 7 K (<T*) and 40 K (T*<T<T-c). Non-collinear magnetic structures, which are based on the G-type antiferromagnetic structure, are proposed at these temperatures. In the T region between T* and T-c Co3+ ions of CoO6 octahedra are in the intermediate spin state, and those of CoO5 pyramids are in the high spin state. In the region of T<T*, the magnitude of the Co3+ spins in the octahedra decreases gradually as T decreases, suggesting that the spin state approaches the low spin state. It is pointed out that the spin state of Co ions depends on the local structure around them..
19. S Iikubo, Y Ohno, Y Kobayashi, Y Yasui, M Ito, M Soda, M Sato, K Kakurai, Magnetic structure of Cu0.3Zn0.7Cr2Se4, JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN, 10.1143/JPSJ.73.1023, 73, 4, 1023-1030, 2004.04, Ferromagnetic spinel system Cu1-xZnxCr2Se4 has been studied by neutron diffraction, NMR and magnetization measurements on both polycrystalline and single crystal samples. The magnetic state are discussed mainly for x = 0.7: At low temperature, the system is essentially in the conically ordered state, and the uniform magnetization M and the modulation vector Q(m) are along <100> in zero magnetic field but not necessarily parallel to each other. All Cr moments within a plane perpendicular to Q(m) are parallel, where the conical angle theta(c) of the Cr moment is similar to50degrees and the pitch deltaphi of the azimuthal angles phi is estimated to be similar to38degrees. The width of the random distribution of deltaphi around the average value increases rather significantly with increasing temperature T, while the correlation length of the ferromagnetic component does not exhibit significant change up to room temperature. When the external magnetic field is applied, the conical axis of each domain rotates toward the field direction without changing the modulation vector Q(m). Brief discussion on the relationship between the non-trivial magnetic structure and the behavior of the anomalous Hall resistivity is also given..
20. S Iikubo, Y Yasui, Y Kobayashi, Y Ohno, M Ito, M Soda, M Sato, K Kakurai, Magnetic structure of a spinel systern Cu0.3Zn0.7Cr2Se4, JOURNAL OF MAGNETISM AND MAGNETIC MATERIALS, 10.1016/j.jmmm.2003.12.443, 272, 1, 473-474, 2004.05, Neutron diffraction and NMR studies have been carried out on single crystal samples of a spinel system Cu0.3Zn0.7Cr2Se4, which exhibits unusual behavior of the anomalous Hall resistivity. It is confirmed that the structure is conical and the detailed values of the parameters to characterize the structure are given. (C) 2003 Elsevier B.V. All rights reserved..
21. S Iikubo, M Ito, A Kobayashi, M Sato, K Kakurai, On the magnetic excitation spectra of high-T-c Cu oxides at energies up to the region far above the resonance energy, JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN, 10.1143/JPSJ.74.275, 74, 1, 275-278, 2005.01, The magnetic excitation spectra chi"(q, omega) of YBa2Cu3Oy and La214 systems have been studied. For a sample of the latter system, La1.88Sr0.12CuO4, chi"(q, omega) has been measured up to similar to30 meV. Existing data of these two kinds of system have been analyzed at energies up to the values far above omega(r), around which chi"(q, omega) exhibits the commensurate peak. In the analyses, we have used the phenomenological expression of the generalized magnetic susceptibility chi(q, omega) = chi(0)(q, omega)/(1 + J(q)chi(0)(q, omega)], where chi(0)(q, omega) is the susceptibility of the electrons without the exchange coupling J(q) among them. In the energy region up to similar toomega(r), it has been reported by the authors' group that the expression can explain the characteristics of the q- and omega-dependences of the spectra of YBa2Cu3Oy (YBCO or YBCOy). Here, it is also pointed out that the expression can reproduce the rotation of the four incommensurate peaks of chi"(q, omega) within the a*-b* plane about (pi/a, pi/a) [or so-called (pi, pi)] point by 45degrees, which occurs as omega goes to the region far above omega(r) from an energy below omega(r). This indicates that for the rotation, "stripe" correlation is not required. For La214, the observed and calculated results of chi"(q, omega) are comparatively discussed up to the omega region far above omega(r). These results indicate that for YBCO, the characteristics of chi"(q, omega) can be explained qualitatively by the above expression throughout the omega region studied, while for the La214 system, effects of "stripes" on chi"(q, omega), which cannot be explained by the expression, are more significant in the low-energy region of omega (< omega(r)) than at high energies..
22. C Kato, S Iikubo, M Soda, M Sato, K Kakurai, S Yoshii, Magnetic structure of Sr2MnO3.5, JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN, 10.1143/JPSJ.74.1026, 74, 3, 1026-1029, 2005.03, Magnetic behavior of Sr2MnO3.5 has been studied by measuring the magnetic susceptibility X, highfield magnetization and specific heat C. The spin structure has also been studied by means of the powder neutron diffraction. The system has the two-dimensional linkage of corner-sharing MnO5 pyramids derived by regularly reducing the oxygen atoms from the MnO2 layers of Sr2MnO4. The oxygen shared by two neighboring pyramids belongs, in one case, to the basal planes of both pyramids, or in another case, to the basal plane of one pyramid and the apex of another one. Due to the existence of these two kinds of the bridging oxygens, it provides a quite characteristic example of moment systems with ferromagnetic (F) clusters of four Mn3+ moments, which are interacting with the neighboring clusters in the antiferromagnetic (AF) way. The results of the studies are as follows. The Neel temperature T-N is 126 K. The exchange interactions corresponding to the two kinds of the bridging oxygens are estimated by the high temperature expansion method to be -50.9 K (AF) and 18.7 K (F), where the signs are in agreement with our expectation. The magnetic structure is consistent with the picture of interacting clusters, though the F-interaction is weaker than the AF one. It is collinear AF one with the moments along the b axis..
23. AA Belik, S Iikubo, K Kodama, N Igawa, S Shamoto, S Niitaka, M Azuma, Y Shimakawa, M Takano, F Izumi, E Takayama-Muromachi, Neutron powder diffraction study on the crystal and magnetic structures of BiCoO3, CHEMISTRY OF MATERIALS, 10.1021/cm052334z, 18, 3, 798-803, 2006.02, The crystal and magnetic structures of polycrystalline BiCoO3 have been determined by the Rietveld method from neutron diffraction data measured at temperatures from 5 to 520 K. BiCoO3 (space group P4mm; Z = 1; a = 3.72937(7) (A) over circle and c = 4.72382(15) (A) over circle at room temperature; tetragonality c/a = 1.267) is isotypic with BaTiO3 and PbTiO3 in the whole temperature range. BiCoO3 is an insulator with a Neel temperature of 470 K. A possible model for antiferromagnetic order is proposed with a propagation vector of k = ((1)/(2), (1)/(2), 0). In this model, magnetic moments of Co3+ ions are parallel to the c direction and align antiferromagnetically in the ab plane. The antiferromagnetic ab layers stack ferromagnetically along the c axis, forming a C-type antiferromagnetic structure. Refined magnetic moments at 5 and 300 K are 3.24(2)mu(B) and 2.93(2)mu(B), respectively. The structure refinements revealed no deviation from stoichiometry in BiCoO3. BiCoO3 decomposed in air above 720 K to give Co3O4 and sillenite-like Bi25CoO39..
24. Shin-ichi Shamoto, Katsuaki Kodama, Satoshi Iikubo, Tomitsugu Taguchi, Noboru Yamada, Thomas Proffen, Local crystal structures of Ge(2)Sb(2)Te5 revealed by the atomic pair distribution function analysis, JAPANESE JOURNAL OF APPLIED PHYSICS PART 1-REGULAR PAPERS BRIEF COMMUNICATIONS & REVIEW PAPERS, 10.1143/JJAP.45.87891, 45, 11, 8789-8794, 2006.11, Two types of optical recording crystalline material Ge2Sb2Te5 have been studied by neutron and X-ray powder diffraction measurements. One is the crystalline cubic-phase Ge2Sb2Te5. The other is nanocrystalline-phase Ge2Sb2Te5. In the former, a large atomic displacement of germanium atoms was found by the atomic pair distribution function (PDF) analysis of neutron powder diffraction data. According to the electronic band structure calculation of isoelectronic GeTe, the electrical conductivity at the valence band with holes is not severely affected by the randomness at the cation site, whereas the thermal conductivity is significantly reduced by the randomness. For the nanocrystalline phase, the local crystal structure and the particle size distribution were obtained simultaneously by PDF analysis using a spherical-particle form factor. The lattice parameter is longer than that of the crystalline phase, suggesting an antimony-richer composition than the matrix. This compositional deviation would form the nanoparticle, resulting in nucleation-dominated crystal growth..
25. D Reznik, L Pintschovius, M Ito, S Iikubo, M Sato, H Goka, M Fujita, K Yamada, GD Gu, JM Tranquada, Electron-phonon coupling reflecting dynamic charge inhomogeneity in copper oxide superconductors, NATURE, 10.1038/nature04704, 440, 7088, 1170-1173, 2006.04, The attempt to understand copper oxide superconductors is complicated by the presence of multiple strong interactions in these systems. Many believe that antiferromagnetism is important for superconductivity, but there has been renewed interest in the possible role of electron-lattice coupling(1-4). The conventional superconductor MgB2 has a very strong electron-lattice coupling, involving a particular vibrational mode (phonon) that was predicted by standard theory and confirmed quantitatively by experiment(5). Here we present inelastic scattering measurements that show a similarly strong anomaly in the Cu-O bond-stretching phonon in the copper oxide superconductors La2-xSrxCuO4 (with x=0.07, 0.15). Conventional theory does not predict such behaviour. The anomaly is strongest in La1.875Ba0.125CuO4 and La1.48Nd0.4Sr0.12CuO4, compounds that exhibit spatially modulated charge and magnetic order, often called stripe order(6); it occurs at a wave vector corresponding to the charge order. These results suggest that this giant electron-phonon anomaly, which is absent in undoped and over-doped non-superconductors, is associated with charge inhomogeneity. It follows that electron-phonon coupling may be important to our understanding of superconductivity, although its contribution is likely to be indirect..
26. AA Belik, S Iikubo, K Kodama, N Igawa, S Shamoto, M Maie, T Nagai, Y Matsui, SY Stefanovich, BI Lazoryak, E Takayama-Muromachi, BiSCO3: Centrosymmetric BiMnO3-type oxide, JOURNAL OF THE AMERICAN CHEMICAL SOCIETY, 10.1021/ja057574u, 128, 3, 706-707, 2006.01.
27. Alexei A. Belik, Satoshi Iikubo, Tadahiro Yokosawa, Katsuaki Kodama, Naoki Igawa, Shinichi Shamoto, Masaki Azuma, Mikio Takano, Koji Kimoto, Yoshio Matsui, Eiji Takayama-Muromachi, Origin of the monoclinic-to-monoclinic phase transition and evidence for the centrosymmetric crystal structure of BiMnO3, JOURNAL OF THE AMERICAN CHEMICAL SOCIETY, 10.1021/ja0664032, 129, 4, 971-977, 2007.01, Structural properties of polycrystalline single-phased BiMnO3 samples prepared at 6 GPa and 1383 K have been studied by selected area electron diffraction (SAED), convergent beam electron diffraction (CBED), and the Rietveld method using neutron diffraction data measured at 300 and 550 K. The SAED and CBED data showed that BiMnO3 crystallizes in the centrosymmetric space group C2/c at 300 K. The crystallographic data are a = 9.5415(2) A, b = 5.61263(8) A, c = 9.8632(2) A, beta = 110.6584(12)degrees at 300 K and a = 9.5866(3) A, b = 5.59903(15) A, c = 9.7427(3) A, beta = 108.601(2)degrees at 550 K, Z = 8, space group C2/c. The analysis of Mn-O bond lengths suggested that the orbital order present in BiMnO3 at 300 K melts above T-OO = 474 K. The phase transition at 474 K is of the first order and accompanied by a jump of magnetization and small changes of the effective magnetic moment and Weiss temperature, mu(eff) = 4.69 mu(B) and theta = 138.0 K at 300-450 K and mu(eff) = 4.79 mu(B) and theta = 132.6 K at 480-600 K..
28. Katsuaki Kodama, Satoshi Iikubo, Shin-Ichi Shamoto, Alexei A. Belik, Eiji Takayama-Muromachi, Local crystal structure of multiferroic system BiMnO3 by atomic pair distribution function analysis, JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN, 10.1143/JPSJ.76.124605, 76, 12, 124605-124605, 2007.12, We have studied the local crystal structure of a multiferroic system BiMnO3 by means of atomic pair distibution function (PDF) analysis on neutron powder diffraction data. The domains with space group P2 or P2(1) whose atomic shifts break a symmetry of C-center exist in the bulk BiMnO3 with a space group C2, which is consistent with recent results of electron diffraction. The domain size is larger than 100 (A) over circle, also estimated by the PDF analysis. Because domains which have atomic shifts with different directions, typically the opposite direction, also exist, the bulk material "apparently" has a space group C2..
29. Neutron powder diffraction study on the crystal and magnetic structures of BiCrO3.
30. Magnetovolume effect in Mn3Cu1-xGexN related to the magnetic structure: Neutron powder diffraction measurements.
31. S. Iikubo, K. Kodama, K. Takenaka, H. Takagi, M. Takigawa, S. Shamoto, Local Lattice Distortion in the Giant Negative Thermal Expansion Material Mn(3)Cu(1-x)Ge(x)N, PHYSICAL REVIEW LETTERS, 10.1103/PhysRevLett.101.205901, 101, 20, 205901-205901, 2008.11, Giant negative thermal expansion is achieved in antiperovskite manganese nitrides when the sharp volume change associated with magnetic ordering is broadened by substitution. In this Letter, we address the unique role of the ''magic" element, Ge, for such broadening in Mn(3)Cu(1-x)Ge(x)N. We present evidence for a local lattice distortion well described by the low-temperature tetragonal (T(4)) structure of Mn(3)GeN for a range of x, where the overall structure remains cubic. This structural instability shows a strong correlation with the broadness of the growth of the ordered magnetic moment and, hence, is considered to trigger the broadening of the volume change..
32. Takafumi Kashi, Yukio Yasui, Taketo Moyoshi, Masatoshi Sato, Kazuhisa Kakurai, Satoshi Iikubo, Naoki Igawa, Crystal structure and magnetic properties of CoZn(TeO3)Br-2, JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN, 10.1143/JPSJ.77.084707, 77, 8, 084707-084707, 2008.08, Crystal and magnetic structures as well as magnetic properties have been studied for a newly synthesized system CoZn(TeO3)Br-2. Although the system is isomorphic to Co-2(TeO3)Br-2 as revealed by neutron diffraction study, magnetic characteristics of the two systems are different, because the Zn atoms in CoZn(TeO3)Br-2 decouple the magnetic interaction among the chains of corner-sharing CoO4Br2 octahedra, inducing the one-dimensionality of Co spins. At temperature T-N similar to 9.0 K, it exhibits a transition to an antiferromagnetic state accompanied by the weak ferromagnetic moment. The Weiss temperature theta(W) is estimated to be -68.5 K from the magnetic susceptibility data. The small value of |T-N/theta(W)| (similar to 0.13) indicates the existence of the effect of magnetic fluctuation. The magnetic structure below T-N has also been determined by a powder neutron diffraction study. where Co spins within a chain are found to order basically in an antiferromagnetic way with the spins along the c direction (chain direction). but they arc canted from the c-axis toward the b-axis by an angle of 17 (+/-3)degrees. Because the weak ferromagnetic components of the chains created by this canting align antiferromagnetically, the spontaneous moments stated above is considered to be due to the additional canting toward the a-axis. The existence of a temperature-linear component in the specific heat at low temperatures suggests that a significant fraction of Co spins remains in the disordered or glassy state..
33. Shin-Ichi Shamoto, Katsuaki Kodama, Satoshi Iikubo, Tomitsugu Taguchi, Total scattering of disordered crystalline functional materials, Nuclear Instruments and Methods in Physics Research, Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, 10.1016/j.nima.2008.11.035, 600, 1, 229-231, 2009.02, There are disorders in some modern functional materials. As an example, the crystalline phase of an optical recording material has low thermal conductivity but high electrical conductivity, simultaneously. This contradiction is a challenge to material scientists in designing good functional materials, which should have at least two types of crystallographic sites. One site limits thermal conductivity while the other site carries electrons or holes with high mobility. This problem exists with not only optical recording materials but also thermoelectric materials. The periodic boundary condition gets lost in the disordered parts. This therefore, makes atomic pair distribution function (PDF) analysis with a wide range of real space suitable for investigating the form and size of crystalline parts as well as disordered parts in the material. Pulsed neutron powder diffraction is one of the best tools for use in this new type of emerging research, together with synchrotron X-ray powder diffraction and electron diffraction. © 2008..
34. Yasushi Idemoto, Satoshi Taniyama, Satoshi Iikubo, Shin-ichi Shamoto, James W. Richardson, Relationship between average and local crystal structure and the ferroelectric properties of a Sr-Bi-Ta-Si-O ferroelectric material, JOURNAL OF PHYSICS AND CHEMISTRY OF SOLIDS, 10.1016/j.jpcs.2009.06.017, 70, 8, 1156-1165, 2009.08, We investigated the relationship between the average and local crystal structures and the ferroelectric properties of Bi2SiO5, Bi4Si3O12, or Bi2O3 added Sr1-xBi2+xTa2O9 (X = 0, 0.2) produced by a solid-state reaction. By measuring the P-E hysteresis, we found that Sr1-xBi2+xTa2O9 (X = 0, 0.2) has higher P-r and E-c than SrBi2Ta2O9 (SBT). P-r increased and E-c decreased by adding Bi2SiO5, Bi4Si3O12, or Bi2O3 to Sr1-xBi2+xTa2O9. The average crystal structures were determined by the Rietveld method. On the other hand, the local structure is important, because the ferroelectric property is related to the distortion, and ferroelectric complex oxides have domains. We also investigated the local crystal structure using atomic pair distribution function (PDF) analysis. Based on the results, the bond angle variance, sigma(2), of each TaO6 octahedron increased by substituting Si for the Ta site. The tilting angle, alpha(a),alpha(b), of each TaO6 octahedron increased relative to that of the average structure, and the symmetry of the TaO6 octahedron in the local structure deteriorated in comparison to that of the average structure. This distortion and symmetry of TaO6 contributes to the remanent polarization. (C) 2009 Elsevier Ltd. All rights reserved..
35. M. Fujita, M. Enoki, S. Iikubo, K. Yamada, Neutron-Scattering Study of Impurity Effect on Stripe Correlations in La-Based 214 High-Tc Cuprate, JOURNAL OF SUPERCONDUCTIVITY AND NOVEL MAGNETISM, 10.1007/s10948-008-0423-z, 22, 3, 243-245, 2009.04, We present results of neutron-scattering study of impurity effects on the static stripe correlations in La-based 214 high-T (c) cuprate. We found that the superlattice peaks from the charge-density-wave (CDW) order was induced by doping magnetic Fe ions in the low-temperature-orthorhombic (LTO) phase of La(2-x) Sr (x) CuO(4) (LSCO). The spin-density-wave (SDW) order is also stabilized by Fe-doping, suggesting the appearance of stripe ordered state in the LTO phase, as is seen it the low-temperature-tetragonal (LTT) phase of La(1.875)Ba(0.125)Cu(4) (LBCO). Therefore, the stripe order can be potentially realized in the La-214 system irrespective of crystal structure. On the other hand, the onset temperature for the appearance of SDW and CDW orders is lowered by Zn-doping in La(1.875)Ba(0.125)CuO(4) system. This result suggests that the existing well-stabilized stripe order is degraded by substituting non-magnetic Zn ions..
36. Hiroshi Yanagi, Takumi Watanabe, Katsuaki Kodama, Satoshi Iikubo, Shin-ichi Shamoto, Toshio Kamiya, Masahiro Hirano, Hideo Hosono, Antiferromagnetic bipolar semiconductor LaMnPO with ZrCuSiAs-type structure, JOURNAL OF APPLIED PHYSICS, 10.1063/1.3124582, 105, 9, 093916-093916, 2009.05, Electronic and magnetic properties of a layered compound LaMnPO are examined in relation to a newly discovered iso-structural superconductor LaFeAs(P)O. Neutron diffraction measurements, together with temperature dependent magnetic susceptibility, clarify that LaMnPO is an antiferromagnet at least up to 375 K. The spin moment of a Mn ion is determined to be 2.26 mu(B) at room temperature, and the spin configuration is antiparallel in the Mn-P plane and parallel between the Mn-P planes, which is rather different from that of LaFeAsO. Optical absorption spectra, photoemission spectra, and temperature dependent electrical conductivity indicate that LaMnPO is a semiconductor. Furthermore, nominally undoped LaMnPO exhibits n-type conduction while the conduction type is changed by doping of Cu or Ca to the La sites, indicating that LaMnPO is a bipolar conductor. Density functional calculation using the GGA+U approximation supports the above conclusions; the electronic band structure has an open band gap and the antiferromagnetic spin configuration is more stable than the ferromagnetic one. (C) 2009 American Institute of Physics. [DOI: 10.1063/1.3124582].
37. Satoshi Iikubo, Masaki Fujita, Seiji Niitaka, Hidenori Takagi, Antiferromagnetic Fluctuations in Fe(Se1-xTex)(0.92) (x=0.75, 1) Observed by Inelastic Neutron Scattering, JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN, 10.1143/JPSJ.78.103704, 78, 10, 103704-103704, 2009.10, Motivated by the discovery of superconductivity in F-doped LaFeAsO, we investigated the magnetic fluctuations in related compounds Fe(Se1-xTex)(0.92) (x = 0.75, 1) using neutron scattering technique. Non-superconducting FeTe0.92 shows it pronounced magnetic fluctuation around vertical bar Q vertical bar approximate to 0.9 angstrom(-1), which is slightly smaller than the magnetic ordering vector vertical bar Q vertical bar approximate to 0.97 angstrom(-1). In a superconductor Fe(Se0.25Te0.75)(0.92), we observed that it magnetic fluctuation is located at vertical bar Q vertical bar approximate to 0.9 angstrom(-1) at low energies and shifts to a higher value of vertical bar Q vertical bar approximate to 1.2 angstrom(-1) at higher energies. The latter value is close to a reciprocal lattice vector Q = (0.5, 0.5, 0.5) or (0.5, 0.5, 0), where the AF fluctuations are observed in other FeAs-based materials. The existence of this common characteristic in different Fe-based superconductors suggests that the AF fluctuations may play an important role in superconductivity..
38. S. Iikubo, M. Fujita, S. Niitaka, H. Takagi, H. Ohtani, M. Hasebe, Neutron scattering study of Fe(Se1-xTex)(0.92) (x=0.75, 1), PHYSICA C-SUPERCONDUCTIVITY AND ITS APPLICATIONS, 10.1016/j.physc.2009.10.135, 470, S401-S403, 2010.12, Magnetic fluctuations of Fe(Se1-xTex)(0.92) (x = 0.75, 1) are investigated by using neutron scattering technique. Non-superconducting FeTe0.92 shows a pronounced magnetic fluctuation around vertical bar Q vertical bar approximate to 0.9 angstrom(-1), which is slightly smaller than the magnetic ordering vector vertical bar Q vertical bar approximate to 0.97 angstrom(-1) (Q = (0.5, 0, 0.5)). On the other hand, superconductor Fe(Se0.25Te0.75)(0.92) shows a magnetic fluctuation at vertical bar Q vertical bar approximate to 1.2 angstrom(-1), which is close to a reciprocal lattice vector Q = (0.5,0.5,0.5) or (0.5,0.5,0), at higher energies. This result suggests that the AF fluctuations characterized by 20 vector Q = (0.5,0.5) may play an important role in superconductivity. (C) 2009 Elsevier B.V. All rights reserved..
39. Satoshi Iikubo, Hideki Koyanaka, Shin-ichi Shamoto, Ken Takeuchi, Shinji Kohara, Katsuaki Kodama, Chun-Keung Loong, Local crystal structure of nano-manganese-oxide gold adsorbent, JOURNAL OF PHYSICS AND CHEMISTRY OF SOLIDS, 10.1016/j.jpcs.2010.08.011, 71, 11, 1603-1608, 2010.11, The local crystal structure of dried and deuterated nano-manganese-oxide powder samples was studied via atomic pair distribution function analysis of X-ray and neutron powder diffraction data. The protonated sample shows ultrahigh efficiency as a gold adsorbent even from ppt-level aqueous solutions such as seawater. We show that the nano-manganese-oxide particles have an R-MnO(2)-type local crystal structure. The possible role of the protons on the surface of the nano-particles is discussed. (C) 2010 Elsevier Ltd. All rights reserved..
40. K. Kodama, S. Iikubo, K. Takenaka, M. Takigawa, H. Takagi, S. Shamoto, Gradual development of Gamma(5g) antiferromagnetic moment in the giant negative thermal expansion material Mn3Cu1-xGexN (x similar to 0.5), PHYSICAL REVIEW B, 10.1103/PhysRevB.81.224419, 81, 22, 224419-224419, 2010.06, The antiperovskite compound Mn3Cu1-xGexN with x similar to 0.5 has a giant negative thermal expansion coefficient due to the magnetovolume effect near room temperature. Competition between the Gamma(5g) and Gamma(4g) antiferromagnetic structures around the magnetic ordering temperature could be the driving force for such a phenomenon. In order to examine this possibility, we performed neutron-diffraction experiments on Mn3Cu0.5Ge0.5N and N-14 nuclear magnetic resonance experiments on Mn3Cu0.6Ge0.4N. As a reference material, we also investigated Mn3NiN, whose spin structure is represented by a linear combination of the Gamma(5g) and Gamma(4g) antiferromagnetic components. We conclude that in Mn3Cu1-xGexN (x similar to 0.5) , the Gamma(4g) antiferromagnetic component is nearly absent and the competition between the Gamma(5g) and Gamma(4g) antiferromagnetic structures is irrelevant to the giant negative thermal expansion coefficient. The Gamma(5g) antiferromagnetic ordered moment gradually develops with decreasing temperature. This provides a thorough justification for the analysis presented in the previous letter [S. Iikubo et al., Phys. Rev. Lett. 101, 205901 (2008)]..
41. Satoshi Iikubo, Hiroshi Ohtani, Mitsuhiro Hasebe, First-Principles Calculations of the Specific Heats of Cubic Carbides and Nitrides, MATERIALS TRANSACTIONS, 10.2320/matertrans.MBW200913, 51, 3, 574-577, 2010.03, Calculated specific heats of several carbides and nitrides with a B1 structure have been compared with those of experimental data up to 3000 K. The specific heats at constant pressure are calculated with the quasiharmonic approximation, using phonon dispersions calculated from direct method, and pseudopotential plane-wave method. The calculated results of HfC, TaC, TiC, ZrC, and ZrN are in excellent agreement with the experimental data up to 3000 K. For NbC, TiN, and VC, the calculated results are also in excellent agreement with the experimental data up to 2000 K. while they show an excessive rise over the experimental data above 2000 K. The deviation of the calculation from the experiment at high temperature is caused by instability of the B1 structure or anharmonic effect. [doi:10.2320/matertrans.MBW200913].
42. Masashi Hino, Satoshi Iikubo, Hiroshi Ohtani, Thermodynamic Analysis of the Cu-Sn-P Ternary System, HIGH TEMPERATURE MATERIALS AND PROCESSES, 10.1515/HTMP.2011.061, 30, 4-5, 387-404, 2011.08, A thermodynamic analysis of the Cu-Sn-P system was performed using the CALPHAD technique. To overcome the lack of experimental information on thermodynamic properties, the formation enthalpies of binary and ternary phosphides were evaluated using a first-principles energetic calculation method; specific heats of these phosphides at constant pressure were also calculated using the quasiharmonic and Debye-Gruneisen approximations. We concluded that the ternary compound Cu4SnP10, detected experimentally using X-ray investigation, was unstable; however because of lack of data on its thermodynamic properties, further experimental and thermodynamic investigations on this ternary phosphide are required..
43. Masanori Enoki, Masaki Fujita, Satoshi Iikubo, John M. Tranquada, Kazuyoshi Yamada, Incommensurate Magnetic Excitation in Spin-Glass Phase of Bi2201 Cuprate, J Phys. Soc. Jpn, 10.1143/JPSJS.80SB.SB026, 80, SB026, 2011.05.
44. Rui-Hua He, M. Fujita, M. Enoki, M. Hashimoto, S. Iikubo, S. -K. Mo, Hong Yao, T. Adachi, Y. Koike, Z. Hussain, Z. -X. Shen, K. Yamada, Hidden Itinerant-Spin Phase in Heavily Overdoped La2-xSrxCuO4 Superconductors Revealed by Dilute Fe Doping: A Combined Neutron Scattering and Angle-Resolved Photoemission Study, PHYSICAL REVIEW LETTERS, 10.1103/PhysRevLett.107.127002, 107, 12, 2011.09, We demonstrated experimentally a direct way to probe a hidden propensity to the formation of a spin-density wave in a nonmagnetic metal with strong Fermi surface nesting. Substituting Fe for a tiny amount of Cu (1%) induced an incommensurate magnetic order below 20 K in heavily overdoped La2-xSrxCuO4. Elastic neutron scattering suggested that this order cannot be ascribed to the localized spins on Cu or doped Fe. Angle-resolved photoemission revealed a strong Fermi surface nesting inherent in the pristine La2-xSrxCuO4 that likely drives this order. Our finding presents the first example of the long-sought "itinerant-spin extreme" of cuprates, where the spins of itinerant doped holes define the magnetic ordering ground state; it complements the current picture of cuprate spin physics that highlights the predominant role of localized spins at lower dopings..
45. Satoshi Iikubo, Kensuke Matsuda, Hiroshi Ohtani, Phase stability of long-period stacking structures in Mg-Y-Zn: A first-principles study, PHYSICAL REVIEW B, 10.1103/PhysRevB.86.054105, 86, 5, 2012.08, The phase stability of the long periodic structures in Mg has been investigated at finite temperature by means of first-principles calculations. Free-energy calculation, including the lattice vibration effect, clearly reveals that 14H and 18R type long periodic structures become more stable than 2H-Mg. Furthermore, the stacking fault energies from a structure of ABA (hcp) to ABC (fcc) were calculated for the isotropic lattice variation. We found that the stacking fault energy decreased by lattice expansion and went to nearly zero upon 10% expansion of the lattice. These two calculated results provide important information about the formation of long periodic stacking "ordered" (LPSO) structures in a Mg-Y-Zn system. It has been suggested that the substituted large atoms and temperature effect cooperatively generate a metastable long periodic stacking faults structure that precede LPSO formation..
46. Tatsuya Tokunaga, Hironobu Kominato, Satoshi Iikubo, Hiroshi Ohtani, Thermodynamic Analysis of Phase Equilibria in the Mg-Al-Ho Ternary System, MATERIALS TRANSACTIONS, 10.2320/matertrans.MI201226, 54, 5, 647-655, 2013.05, A thermodynamic analysis of phase equilibria in the Mg-Al-Ho ternary system has been carried out with special emphasis on the metastable phase separation of the hexagonal close-packed (hcp) phase. In this study, the Gibbs free energy of mixing of the hcp phase was evaluated using first-principles calculations combined with the cluster variation method (CVM), and the obtained results as well as the available experimental data were introduced into the analysis. The calculated results enabled the reproduction of experimental results on both the phase equilibria and the thermodynamic properties obtained from the first-principles calculations and the CVM. In addition, the calculations indicated a tendency for metastable two-phase separation in the hcp phase between the Mg-rich corner and the Al-Ho binary side, which was similar to the Mg-Y-Zn ternary system shown in our previous work..
47. Masaki Fujita, Masanori Enoki, Kenji Tsutsumi, Satoshi Iikubo, Kazuyoshi Yamada, Ho-doping effect on the incommensurate magnetic order in La1.88Sr0.12CuO4, JOURNAL OF THE KOREAN PHYSICAL SOCIETY, 10.3938/jkps.62.1840, 62, 12, 1840-1843, 2013.07, We have performed elastic neutron scattering experiments on the superconductor La1.85Ho0.03Sr0.12CuO4 to study the effect of substitution of a cation with a large magnetic moment at La site on the static spin correlation. In the Ho-free sample with a hole concentration of 0.12, the spatially-modulated magnetic order is known to be stabilized at temperature below T (spin) similar to 30 K. In the present Ho-doped sample, we found the appearance of magnetic peaks at (0.5, 0.5 +/- 0.119, 0) positions below T (spin) similar to 35 K, which is similar to the observation in the pristine sample. Furthermore, the volume-corrected intensities in the Ho-free and Ho-doped samples are comparable, suggesting a negligible effect of Ho-substitution at La sites. These results are quite a contrast to the huge enhancement of the magnetic intensity and the increase of T (spin) by substituting a comparable amounts of magnetic Fe3+ ions onto CuO2 planes. Therefore, the stability of the stripe order induced by the cation substitution is sensitive to the site at which the substitution occurs..
48. Tomitsugu Taguchi, Toshiki Miyazaki, Satoshi Iikubo, Kenji Yamaguchi, Deposition of hydroxyapatite on SiC nanotubes in simulated body fluid, MATERIALS SCIENCE & ENGINEERING C-MATERIALS FOR BIOLOGICAL APPLICATIONS, 10.1016/j.msec.2013.10.007, 34, 1, 29-34, 2014.01, SiC nanotubes can become candidate reinforcement materials for dental and orthopedic implants due to their light weight and excellent mechanical properties. However, the development of bioactive SiC materials has not been reported. In this study, hydroxyapatites were found on SiC nanotubes treated with NaOH and subsequently HCl solution after soaking in simulated body fluid. On the other hand, hydroxyapatites did not deposit on as-received SiC nanotubes, the SiC nanotubes with NH4OH solution treatment and SiC bulk materials with NaOH and subsequently HCl solution treatment. Therefore, we succeeded in the development of bioactive SiC nanotubes by downsizing SiC materials to nanometer size and treating with NaOH and subsequently HCl solutions for the first time. (C) 2013 Elsevier B.V. All rights reserved..
49. Murugan Vigneshwaran, Takeshi Ohta, Satoshi Iikubo, Gaurav Kapil, Teresa S. Ripolles, Yuhei Ogomi, Ting Ma, Shyam S. Pandey, Qing Shen, Taro Toyoda, Kenji Yoshino, Takashi Minemoto, Shuzi Hayase, Facile Synthesis and Characterization of Sulfur Doped Low Bandgap Bismuth Based Perovskites by Soluble Precursor Route, CHEMISTRY OF MATERIALS, 10.1021/acs.chemmater.6b02315, 28, 18, 6436-6440, 2016.09, The bismuth based perovskite with the structure (CH3NH3)(3)Bi2I9 (MBI) is rapidly emerging as eco-friendly and stable semiconducting material as a substitute for the lead halide perovskites. A relatively higher bandgap of MBI (about 2.1 eV) has been found to be a bottleneck in realizing the high photovoltaic performance similar to that of lead halide based perovskites. We demonstrate the bandgap engineering of novel bismuth based perovskites obtained by in situ sulfur doping of MBI via the thermal decomposition of Bi(xt)(3) (xt = ethyl xanthate) precursor. Colors of the obtained films clearly changed from orange to black when annealed from 80 to 120 degrees C. Formation of sulfur doped MA(3)Bi(2)I(9) was confirmed by XRD and the presence of sulfur was confirmed through XPS. In this work, obtained sulfur doped bismuth perovskites exhibited a bandgap of 1.45 eV which is even lower than that of most commonly used lead halide perovskites. Hall-Effect measurements showed that the carrier concentration and mobility are much higher as compared to that of undoped MA(3)Bi(2)I(9)..
50. Hirata K, Iikubo S, Fujimoto H, Ohtani H, Thermodynamic assessment of Fe–Ti–S ternary phase diagram, Calphad: Computer Coupling of Phase Diagrams and Thermochemistry, 10.1016/j.calphad.2017.02.006, 57, 62-77, 2017.06.
51. K. Yamamoto, S. Iikubo, J. Yamasaki, Y. Ogomi, S. Hayase, Structural Stability of Iodide Perovskite: A Combined Cluster Expansion Method and First-Principles Study, Journal of Physical Chemistry C, 10.1021/acs.jpcc.7b07910, 121, 50, 27797-27804, 2017.12, To aid the development of Pb-free perovskite solar cells, the stability of the iodide perovskite structure ABI3 has been investigated by first-principles calculations, Bader charge analysis, and the cluster expansion method. At the A sites, methylammonium (MA, CH3NH3 +), formamidinium (FA, CH(NH2)2 +), and Cs+ were modeled, while at the B sites, one or two elements from Pb, Sn, Ge, In, Ga, Bi, and Sr were examined. For the partially substituted system A(B,B′)I3, we found that the stability strongly depends on the identity of the A-site cation. For example, Cs(B,B′)I3 structures are stabilized by a mixture of divalent cations, such as Pb, Sn, and Ge, at the B site. Concerning the stabilization mechanisms, Coulomb energy gain seems to be the origin of the structural stability in A = Cs structures. From our results, Cs(B,B′)I3, where the B site is occupied by divalent cations, are possible candidates for high stability, lead-free solar cell materials..
52. Hermana G, Huang T, Chen C, Lin C, Iikubo S, Yen Y, Phase equilibria of the Cu-Ni-Zr ternary systems at 800 °C and thermodynamic assessment and metallic glass region prediction for the Cu-Ni-Zr ternary system, Journal of Non-Crystalline Solids, 10.1016/j.jnoncrysol.2017.12.016, 481, 612-621, 2018.02.
53. Shuai Zhao, Kumiko Yamamoto, Satoshi Iikubo, Shuzi Hayase, Tingli Ma, First-principles study of electronic and optical properties of lead-free double perovskites Cs2NaBX6 (B = Sb, Bi; X = Cl, Br, I), Journal of Physics and Chemistry of Solids, 10.1016/j.jpcs.2018.02.032, 117, 117-121, 2018.06, Organolead halide perovskite is regarded as the most promising light-harvesting material for next-generation solar cells
however, the intrinsic instability and toxicity of lead are still of great concern. Bismuth is ecofriendly and has electronic properties similar to those of lead, which has gradually attracted interest for optoelectronic applications. However, the valence state of bismuth is different from that of lead, eliminating the possibility of replacing lead by bismuth in organolead halide perovskites. To address this matter, one feasible strategy is to construct B-site double perovskites by the combination of Bi3+ and B+ in 1:1 ratio. In this work, lead-free halide double perovskites of the form Cs2NaBX6 (B = Sb, Bi
X = Cl, Br, I) were investigated by first-principles calculations. The electronic properties, optical absorption coefficients, and thermodynamic stability of these compounds were investigated to ascertain their potential application in solar energy conversion. The results provide theoretical support for the exploration of lead-free perovskite materials in potential optoelectronic applications..
54. An unconventional hydrogen effect that suppresses thermal formation of the hcp phase in fcc steels
© 2018, The Author(s). Iron and steels are extensively used as structural materials, and have three primary phase structures: Body-centered cubic (bcc), face-centered cubic (fcc), and hexagonal closed-packed (hcp). Controlling phase stabilities, especially by the use of interstitials, is a universal method that provides a diverse variety of functional and mechanical properties in steels. In this context, hydrogen, which can act as an interstitial species in steels, has been recognized to promote phase transformation from fcc to hcp. However, we here report a dramatic effect of interstitial hydrogen that suppresses this hcp phase transformation. More specifically, the fraction of hcp phase that forms during cooling decreases with increasing diffusible hydrogen content. This new finding opens new venues for thermodynamics-based microstructure design and for development of robust, strong, and ductile steels in hydrogen-related infrastructures..
55. Masafumi Matsushita, Takafumi Nagata, Jozef Bednarcik, Norimasa Nishiyama, Shoya Kawano, Satoshi Iikubo, Yuji Kubota, Ryo Morishita, Tetsuo Irifune, Michiaki Yamasaki, Yoshihito Kawamura, Masanori Enoki, Hiroshi Ohtani, Key factor for the transformation from Hcp to 18R-type long-period stacking ordered structure in Mg alloys, Materials Transactions, 10.2320/matertrans.M2018213, 60, 237-245, 2019.01, ©2018 The Japan Institute of Metals and Materials. Cast Mg 85 Y 9 Zn 6 has an 18R-type LPSO structure. However, Mg 85 Y 9 Zn 6 recovered after being subjected to a loading pressure of 7 GPa at 973 K shows a fine dual-phase structure composed of a face-centered cubic (fcc) structure showing a superlattice (D0 3 ), as well as a hexagonal close-packed structure (hcp:2H). The D0 3 /hcp structure transformed to 18R-type LPSO during heating at ambient pressure. In this research, the transformation process from the D0 3 /hcp structure to 18R-type LPSO structure was discussed by means of in situ XRD and first-principles calculation. At first, lattice volume of 2H increased with an increase in the temperature, because of the Zn and Y emitted from the D0 3 phase into the 2H lattice. After the volume expansion of 2H lattice, the structure collapsed due to insert of random stacking faults (SFs). Then, a formation of 18R-type LPSO structure occurred. Based on a first-principles calculation for pure Mg, volume expansion of the 2H lattice causes the transformation to an 18R structure. Furthermore, the results of free energy calculations for the hcp and fcc structures in the MgYZn ternary system show that the segregation of Y and Zn atoms on SFs occurs by the Suzuki effect. These segregated Y and Zn atoms in SF layers, which have a local fcc structure, create a synergy between the stacking and chemical modulations. Present result insists that the volume increase of 2H lattice takes place first, and then the transformation from the hcp structure to 18R stacking occurs. [doi:10.2320/matertrans.M2018213].
56. First-Principles Study of Chemical Driving Force for Face Centered Cubic to Hexagonal Close Packed Martensitic Transformation in Hydrogen-Charged Iron
© 2019, The Minerals, Metals & Materials Society and ASM International. This study uses first-principles calculations to investigate the effect of hydrogen on the chemical driving force of the transformation of iron from the face centered cubic (FCC) to hexagonal close packed (HCP) phase. The minimum energy path from FCC to HCP phases shows that FCC becomes stable with increasing hydrogen content. Furthermore, the energy difference between the FCC and HCP phases is observed to be smaller in Fe2H than in Fe throughout the temperature region. These results clearly explain the observed anomalous suppression of the martensitic transformation in the hydrogen-charged steel..