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Journal Articles

Neutron scattering of iron-based superconductors

Shamoto, Shinichi; Wakimoto, Shuichi; Kodama, Katsuaki; Ishikado, Motoyuki; Christianson, A. D.*; Lumsden, M. D.*; Kajimoto, Ryoichi; Nakamura, Mitsutaka; Inamura, Yasuhiro; Arai, Masatoshi; et al.

Physica C, 471(21-22), p.639 - 642, 2011/11

 Times Cited Count:0 Percentile:0(Physics, Applied)

We have studied low-energy spin excitations on polycrystalline LaFeAsO$$_{1-x}$$F$$_{x}$$ samples by inelastic neutron scattering. The dynamical spin susceptibility $$chi$$''($$omega$$) of the superconducting samples is found to be comparable to that of the magnetically ordered parent sample. On the other hand, $$chi$$''($$omega$$) almost vanishes at x=0.158, where the superconducting transition temperature T$$_{c}$$ is suppressed to 7 K. This suggests that the low energy spin excitations are closely connected with high-T$$_{c}$$ superconductivity. The disappearance of the spin excitations at low energy would correspond to the disappearance of the hole Fermi surface as the electron concentration increases with doping, as observed in ARPES measurements of BaFe$$_{2-x}$$Co$$_{x}$$As$$_{2}$$ at x=0.15. The relationship between the spin excitation and the Fermi surface will be discussed in comparison with other iron-based superconductors.

Journal Articles

Soft and isotropic phonons in PrFeAsO$$_{1-y}$$

Fukuda, Tatsuo; Baron, A. Q. R.*; Nakamura, Hiroki; Shamoto, Shinichi; Ishikado, Motoyuki; Machida, Masahiko; Uchiyama, Hiroshi*; Iyo, Akira*; Kito, Hijiri*; Mizuki, Junichiro; et al.

Physical Review B, 84(6), p.064504_1 - 064504_13, 2011/08

 Times Cited Count:9 Percentile:41.39(Materials Science, Multidisciplinary)

Journal Articles

Angle-resolved photoemission spectroscopy study of PrFeAsO$$_{0.7}$$; Comparison with LaFePO

Nishi, Ichiro*; Ishikado, Motoyuki; Ideta, Shinichiro*; Malaeb, W.*; Yoshida, Teppei*; Fujimori, Atsushi*; Kotani, Yoshinori*; Kubota, Masato*; Ono, Kanta*; Yi, M.*; et al.

Physical Review B, 84(1), p.014504_1 - 014504_5, 2011/07

 Times Cited Count:22 Percentile:66.92(Materials Science, Multidisciplinary)

We have performed an angle-resolved photoemission spectroscopy (ARPES) study of the iron-based superconductor PrFeAsO$$_{0.7}$$ and examined the Fermi surfaces and band dispersions near the fermi level. Heavily hole-doped electronic states have been observed due to the polar nature of the cleaved surfaces. Nevertheless, we have found that the ARPES spectra basically agree with band dispersions calculated in the local density approximation (LDA) if the bandwidth is reduced by a factor of $$sim$$ 2.5 and then the chemical potential is lowered by $$sim$$ 70 meV. Comparison with previous ARPES results on LaFePO reveals that the energy positions of the $$d$$$$_{3z^{2}-r^{2}}$$- and $$d$$$$_{yz,zx}$$-derived bands are considerably different between the two materials, which we attribute to the different pnictogen height as predicted by the LDA calculation.

Journal Articles

Phonon spectra of PrFeAsO$$_{1-y}$$ via inelastic X-ray scattering

Fukuda, Tatsuo; Baron, A. Q. R.*; Nakamura, Hiroki; Shamoto, Shinichi; Ishikado, Motoyuki; Machida, Masahiko; Uchiyama, Hiroshi*; Iyo, Akira*; Kito, Hijiri*; Mizuki, Junichiro; et al.

Journal of the Physical Society of Japan, 80(Suppl.B), p.SB015_1 - SB015_4, 2011/01

 Times Cited Count:1 Percentile:11.83(Physics, Multidisciplinary)

We have investigated phonons of PrFeAsO$$_{1-y}$$ using high-resolution inelastic X-ray scattering and $$ab-initio$$ calculations. Extensive measurements show that there are some changes in phonon spectra with temperature and/or doping. However, we compare our data with several calculations and find that the experimentally observed changes are much smaller than differences between the experiment. data and the calculations. Agreement is improved if magnetism is included in the calculations. However, still in disagreement even in the parent sample below T$$_N$$. We consider several modifications of the $$ab-initio$$ calculations. The best agreement is obtained from modifying the nonmagnetic calculation. by softening only the in-plane nearest-neighbor Fe-As force constant. Fluctuating magnetism may be a partial explanation for the failure of the calculations, but seems incomplete in the face of the similarity of the measured phonon response in all the systems investigated including those known to have static magnetism.

Journal Articles

Growth of PrFeAsO$$_{1-y}$$ single crystals and its characterization

Ishikado, Motoyuki; Shamoto, Shinichi; Kito, Hijiri*; Iyo, Akira*; Eisaki, Hiroshi*; Ito, Toshimitsu*; Tomioka, Yasuhide*

Physica C, 470(Suppl.1), p.S322 - S323, 2010/12

 Times Cited Count:3 Percentile:18.14(Physics, Applied)

Single crystals of oxygen deficient oxypnictide superconductor PrFeAsO$$_{1-y}$$ have been grown by using belt-type anvil high pressure synthesis apparatus. Superconducting transition temperature ($textit{T$_c$}$) of the obtained single crystal is 45 K, and typical size is about 800 $$times$$ 1000 $$times$$ 30 $$mu m^3$$. In-plane electrical resistivity shows almost proportional to temperature as well as the copper oxide high temperature superconductors, suggesting the existence of effects of strong inelastic scattering like copper oxide high temperature superconductor. In addition, we obtained non-superconducting parent single crystal by annealing the as-grown single crystal under ambient pressure. Resistivity of parent single crystal is characterized by the kink near 150 K corresponding to the structural transition, and the structure near 12 K, which is thought to be originating from the magnetic order of Pr ion.

Journal Articles

Lower critical fields and the anisotropy in PrFeAsO$$_{1-y}$$ single crystals

Okazaki, Ryuji*; Konczykowski, M.*; Van der Beek, C. J.*; Kato, Tomonari*; Hashimoto, Kenichiro*; Shimozawa, Masaaki*; Shishido, Hiroaki*; Yamashita, Minoru*; Ishikado, Motoyuki; Kito, Hijiri*; et al.

Physica C, 470(Suppl.1), p.S485 - S486, 2010/12

 Times Cited Count:0 Percentile:0(Physics, Applied)

We report the lower critical fields $$H_{c1}$$ measurements on Fe-based oxipnictide PrFeAsO$$_{1-y}$$ single crystals for $$H$$ $$parallel$$ $$c$$ and $$H$$ $$parallel$$ $$ab$$-planes. To avoid the difficulty of $$H_{c1}$$ determinations due to the flux pinning, we developed a novel method using a miniature hall-sensor array which can evaluate the local magnetic induction ai each sensor position. The hall sensor which is placed on the edge of the crystal clearly resolves the first flux penetration from the Meissner state. The temperature dependence of $$H_{c1}$$ for $$H$$ $$parallel$$ $$c$$ is well scaled by the in-plane penetration depth and is consistent with a full-gap superconducting state. The anistropy of $$H_{c1}$$ at low temperatures is estimated to be $$sim$$ 3, which is much smaller than of $$H_{c2}$$. This indicates the multiband superconductivity, in which the active bands for superconductivity are more two-dimensional.

Journal Articles

Disorder and flux pinning in superconducting pnictide single crystals

Van der Beek, C. J.*; Rizza, G.*; Konczykowski, M.*; Fertey, P.*; Monnet, I.*; Okazaki, Ryuji*; Kato, Tomonari*; Hashimoto, Kenichiro*; Shimozawa, Masaaki*; Shishido, Hiroaki*; et al.

Physica C, 470(Suppl.1), p.S385 - S386, 2010/12

 Times Cited Count:1 Percentile:6.88(Physics, Applied)

Crystalline disorder in the pnictide superconductor PrFeAsO$$_{1-x}$$ is studied using magneto-optical visualisation of flux penetration, transmission electron microscopy, and X-ray diffraction using synchrotron radiation. Critical-state like flux distributions and the magnitude and temperature dependence of the critical current demonstrate bulk vortex pinning by oxygen vacancies at all temperatures.

Journal Articles

Resonant inelastic X-ray scattering in single-crystal superconducting PrFeAsO$$_{0.7}$$

Jarrige, I.; Ishii, Kenji; Yoshida, Masahiro; Fukuda, Tatsuo; Ikeuchi, Kazuhiko*; Ishikado, Motoyuki*; Hiraoka, Nozomu*; Tsuei, K.-D.*; Kito, Hijiri*; Iyo, Akira*; et al.

Physica C, 470(Suppl.1), p.S377 - S378, 2010/10

 Times Cited Count:4 Percentile:23.05(Physics, Applied)

Journal Articles

Degradation of superconductivity and spin fluctuations by electron overdoping in LaFeAsO$$_{1-x}$$F$$_{x}$$

Wakimoto, Shuichi; Kodama, Katsuaki; Ishikado, Motoyuki; Matsuda, Masaaki; Kajimoto, Ryoichi; Arai, Masatoshi; Kakurai, Kazuhisa; Esaka, Fumitaka; Iyo, Akira*; Kito, Hijiri*; et al.

Journal of the Physical Society of Japan, 79(7), p.074715_1 - 074715_4, 2010/07

 Times Cited Count:50 Percentile:87.26(Physics, Multidisciplinary)

Journal Articles

Flux pinning in PrFeAsO$$_{0.9}$$ and NdFeAsO$$_{0.9}$$F$$_{0.1}$$ superconducting crystals

Van der Beek, C. J.*; Rizza, G.*; Konczykowski, M.*; Fertey, P.*; Monnet, I.*; Klein, T.*; Okazaki, Ryuji*; Ishikado, Motoyuki; Kito, Hijiri*; Iyo, Akira*; et al.

Physical Review B, 81(17), p.174517_1 - 174517_11, 2010/05

 Times Cited Count:93 Percentile:94.11(Materials Science, Multidisciplinary)

Local magnetic measurements are used to quantitatively characterize heterogeneity and flux line pinning in PrFeAsO$$_{1-y}$$ and NdFeAs(O,F)superconducting single crystals. In spite of spatial fluctuations of the critical current density on the macroscopic scale, it is shown that the major contribution comes from collective pinning of vortex lines by microscopic defects by the mean-free path fluctuation mechanism. The defect density extracted from experiment corresponds to the dopant atom density, which means that dopant atoms play an important role both in vortex pinning and in quasi-particle scattering.

Journal Articles

Strong carrier-scattering in iron-pnictide superconductors ${it Ln}$FeAsO$$_{1-y}$$ (${it Ln}$ = La and Nd) obtained from charge transport experiments

Ishida, Shigeyuki*; Nakajima, Masamichi*; Tomioka, Yasuhide*; Ito, Toshimitsu*; Miyazawa, Kiichi*; Kito, Hijiri*; Lee, C.-H.*; Ishikado, Motoyuki; Shamoto, Shinichi; Iyo, Akira*; et al.

Physical Review B, 81(9), p.094515_1 - 094515_6, 2010/03

 Times Cited Count:21 Percentile:66.68(Materials Science, Multidisciplinary)

Characteristic normal-state charge transport is found in the oxygen-deficient iron-arsenides ${it Ln}$FeAsO$$_{1-y}$$ (${it Ln}$: La and Nd) with the highest ${it T}$ $$_{c}$$'s among known Fe-based superconductors. The effect of "doping" in this system is mainly on the carrier scattering, quite distinct from that in high-${it T}$ $$_{c}$$ cuprates. In the superconducting regime of the La system with maximum ${it T}$ $$_{c}$$ = 28 K, the low-temperature resistivity is dominated by a ${it T}$ $$^{2}$$ term. On the other hand, in the Nd system with ${it T}$ $$_{c}$$ higher than 40 K, the carriers are subject to stronger scattering showing ${it T}$ -linear resistivity and small magnetoresistance. Such strong scattering appears crucial for high-${it T}$ $$_{c}$$ superconductivity in the iron-based system.

Journal Articles

Growth of single crystal PrFeAsO$$_{1-y}$$ and its characterization

Ishikado, Motoyuki; Shamoto, Shinichi; Kito, Hijiri*; Iyo, Akira*; Eisaki, Hiroshi*; Ito, Toshimitsu*; Tomioka, Yasuhide*

Physica C, 469(15-20), p.901 - 904, 2009/10

 Times Cited Count:23 Percentile:68.18(Physics, Applied)

We have successfully grown single crystals of oxygen deficient oxypnictide superconductor PrFeAsO$$_{1-y}$$ by belt-type anvil apparatus using high pressure synthesis method.

Journal Articles

High-pressure synthesis and physical properties of new iron (nickel)-based superconductors

Shirage, P. M.*; Miyazawa, Kiichi*; Ishikado, Motoyuki; Kiho, Kunihiro*; Lee, C.-H.*; Takeshita, Nao*; Matsuhata, Hirofumi*; Kumai, Reiji*; Tomioka, Yasuhide*; Ito, Toshimitsu*; et al.

Physica C, 469(9-12), p.355 - 369, 2009/06

 Times Cited Count:38 Percentile:79.73(Physics, Applied)

We have utilized a high-pressure (HP) technique to synthesize a series of newly-discovered iron (nickel)-based superconductors. The effect of (O)-deficiency, variation of $$Ln$$ ions, and the external pressure on $$T$$$$_{rm c}$$ are examined. All the experimental data indicate strong correlation between the crystal structure and the superconductivity of the oxypnictide superconductors. Upper critical field measurement on single crystalline sample of PrFeAsO$$_{1-y}$$ shows the superconducting anisotropy of 5, which is smaller than cuprates. We also demonstrate that HP technique is applicable for the so-called "122" systems.

Journal Articles

Two-dimensional spin density wave state in LaFeAsO

Ishikado, Motoyuki; Kajimoto, Ryoichi; Shamoto, Shinichi; Arai, Masatoshi; Iyo, Akira*; Miyazawa, Kiichi*; Shirage, P. M.*; Kito, Hijiri*; Eisaki, Hiroshi*; Kim, S.-W.*; et al.

Journal of the Physical Society of Japan, 78(4), p.043705_1 - 043705_4, 2009/04

 Times Cited Count:34 Percentile:81.84(Physics, Multidisciplinary)

Journal Articles

Lower critical fields of superconducting PrFeAsO$$_{1-y}$$ single crystals

Okazaki, Ryuji*; Konczykowski, M.*; Van der Beek, C. J.*; Kato, Tomonari*; Hashimoto, Kenichiro*; Shimozawa, Motoyuki*; Shishido, Hiroaki*; Yamashita, Minoru*; Ishikado, Motoyuki; Kito, Hijiri*; et al.

Physical Review B, 79(6), p.064520_1 - 064520_6, 2009/02

 Times Cited Count:56 Percentile:87.82(Materials Science, Multidisciplinary)

We have studied the lower critical fields H$$_{c1}$$ of superconducting iron oxipnictide PrFeAsO$$_{1-y}$$ single crystals for H parallel and perpendicular to the ab-planes.

Journal Articles

Lattice dynamics of LaFeAsO$$_{1-x}$$F$$_x$$ and PrFeAsO$$_{1-y}$$ via inelastic X-ray scattering and first-principles calculation

Fukuda, Tatsuo; Baron, A. Q. R.*; Shamoto, Shinichi; Ishikado, Motoyuki; Nakamura, Hiroki; Machida, Masahiko; Uchiyama, Hiroshi*; Tsutsui, Satoshi*; Iyo, Akira*; Kito, Hijiri*; et al.

Journal of the Physical Society of Japan, 77(10), p.103715_1 - 103715_4, 2008/10

 Times Cited Count:41 Percentile:83.89(Physics, Multidisciplinary)

The lattice dynamics of LaFeAsO$$_{1-x}$$F$$_x$$ ($$x$$=0, 0.1) and PrFeAsO$$_{1-y}$$ ($$ysim$$0.1) are investigated using inelastic X-ray scattering. Measurements of powder samples provide an approximation to the phonon DOS, while dispersion is measured from a single crystal of PrFeAsO$$_{1-y}$$. A model that agrees reasonably well with all of the data at room temperature is built from results of ab-initio calculations by reducing the strength of the Fe-As bond by 30%.

Journal Articles

Ferroelectricity from iron valence ordering in the charge-frustrated system LuFe$$_{2}$$O$$_{4}$$

Ikeda, Naoshi*; Osumi, Hiroyuki*; Owada, Kenji; Ishii, Kenji; Inami, Toshiya; Kakurai, Kazuhisa; Murakami, Yoichi*; Yoshii, Kenji; Mori, Shigeo*; Horibe, Yoichi*; et al.

Nature, 436(7054), p.1136 - 1138, 2005/08

 Times Cited Count:775 Percentile:99.78(Multidisciplinary Sciences)

By means of resonant X-ray scattering experiments using synchrotron radiation of SPring-8, it was found that a mixed-valence system LuFe$$_{2}$$O$$_{4}$$ is a ferroelectric material owing to a real-space charge-ordering of Fe$$^{2+}$$ and Fe$$^{3+}$$. This ordering is rooted in a charge frustration between the Fe ions on a triangular lattice. The new mechanism of ferroelectricity shown in the present paper offers great potential for designing future ferroelectric devices to be coupled or controlled with the degrees of freedom of electron: charge, spin and orbital.

Journal Articles

Superconductivity of M$$_{I}$$(M$$_{II}$$$$_{0.5}$$,Si$$_{0.5}$$)$$_{2}$$ (M$$_{I}$$=Sr and Ba, M$$_{II}$$=Al and Ga), ternary silicides with the AlB$$_{2}$$-type structure

Imai, Motoharu*; Nishida, Kenji*; Kimura, Takashi*; Kitazawa, Hideaki*; Abe, Hideki*; Kito, Hijiri*; Yoshii, Kenji

Physica C, 382(4), p.361 - 366, 2002/11

 Times Cited Count:57 Percentile:88.82(Physics, Applied)

no abstracts in English

Oral presentation

Ordering of frustrated spin systems coupled to lattice and charge degrees of freedom; Neutron scattering investigations

Kakurai, Kazuhisa; Matsuda, Masaaki; Ishii, Yoshinobu; Nagai, Satoshi*; Ikeda, Naoshi*; Kito, Hijiri*; Igawa, Naoki; Soda, Minoru*; Yasui, Yukio*; Moyoshi, Taketo*; et al.

no journal, , 

Two examples of neutron scattering investigations on the magnetic ordering process in frustrated spin system coupled to lattice and charge degrees of freedom are presented, in both cases leading to a short-ranged magnetic ground state. (1) In LuFe$$_{2}$$O$$_{4}$$ the Fe ions are arranged to form hexagonal double layers along the rhombohedral unique axis. Detailed investigation of the ordering process revealed a magnetic ground state with short-range order along the stacking direction. (2) YBaCo$$_{4}$$O$$_{7}$$ has a structure with alternating stacking of a kagome-like and triangular lattice formed of CoO$$_{4}$$ tetrahedra. Although the Co moments do not exhibit a long range ordering down to 10K, strong diffuse scattering due to the short-range ordering of the moments on the kagome-like and triangular lattices are observed.

Oral presentation

Magnetic ordering process in the charge-frustrated ferroelectric system LuFe$$_{2}$$O$$_{4}$$

Kakurai, Kazuhisa; Ikeda, Naoshi*; Nagai, Satoshi*; Matsuda, Masaaki; Ishii, Yoshinobu; Owada, Kenji; Inami, Toshiya; Yoshii, Kenji; Murakami, Yoichi*; Kito, Hijiri*; et al.

no journal, , 

We report in this paper an anomalous magnetic ordering process in the charge ordered LuFe$$_{2}$$O$$_{4}$$. The neutron scattering experiments were performed on TAS-1 and TAS-2 instruments installed at JAEA-JRR-3. The following results are obtained:1) Despite the triangular configuration a strong 2-D antiferromagnetic correlation with ferrimagnetic component in the double layer in the hexagonal plane develops to a 3-D order along the c-axis at T$$_{N}$$=242K.2) At around 177K there is another characteristic temperature T$$_{f}$$, where new type of broad magnetic peaks start to grow and at the same time the magnetic peaks already developed below T$$_{N}$$ also acquire finite line width, including the peaks due to the ferrimagnetic component. A strong hysteresis is observed at T$$_{f}$$. The existence of the anomalous magnetic ground state will be demonstrated and the possible relation to the orbital degree of freedom will be discussed.

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