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Journal Articles

Crystal structure and physical properties of trigonal NpGa$$_3$$ and Np$$_{3}$$Ga$$_{11}$$

Haga, Yoshinori; Aoki, Dai*; Homma, Yoshiya*; Matsuda, Tatsuma; Ikeda, Shugo*; Sakai, Hironori; Yamamoto, Etsuji; Nakamura, Akio; Shiokawa, Yoshinobu*; Onuki, Yoshichika

Journal of the Physical Society of Japan, 80(Suppl.A), p.SA109_1 - SA109_3, 2011/07

 Times Cited Count:0 Percentile:100(Physics, Multidisciplinary)

Journal Articles

La substitution effect and hyperfine-enhanced $$^{141}$$Pr nuclear spin dynamics in PrPb$$_3$$; $$^{139}$$La NMR study in Pr$$_{0.97}$$La$$_{0.03}$$Pb$$_3$$

Tokunaga, Yo; Sakai, Hironori; Chudo, Hiroyuki; Kambe, Shinsaku; Yasuoka, Hiroshi; Suzuki, Hiroyuki*; Walstedt, R. E.*; Homma, Yoshiya*; Aoki, Dai*; Shiokawa, Yoshinobu*

Physical Review B, 82(10), p.104401_1 - 104401_6, 2010/09

 Times Cited Count:5 Percentile:69.3(Materials Science, Multidisciplinary)

$$^{139}$$La NMR studies have been performed on PrPb$$_3$$ substituted with 3% La. The temperature dependence of the local magnetic susceptibility around La ions has been extracted from the $$^{139}$$La Knight shift. These data show that the non-magnetic $$Gamma_3$$ crystalline-electric-field (CEF) ground state (GS) is preserved even at the nearest neighboring Pr ions, although their CEF level scheme is slightly modified due to the La substitution. On the other hand, the temperature dependence of the nuclear spin-lattice relaxation rate $$1/T_1$$ is found to be well reproduced by assuming the same CEF level scheme over a wide temperature range. However, $$1/T_1$$ shows a strong upturn below 10 K, which is not expected from the non-magnetic CEF GS. We show that the low temperature anomaly can be quantitatively understood in terms of a cross-relaxation process between $$^{141}$$Pr and $$^{139}$$La nuclear spins.

Journal Articles

Neutron scattering study on U-dichalcogenides

Metoki, Naoto; Kaneko, Koji; Ikeda, Shugo*; Sakai, Hironori; Yamamoto, Etsuji; Haga, Yoshinori; Homma, Yoshiya*; Shiokawa, Yoshinobu*

IOP Conference Series; Materials Science and Engineering, 9, p.012088_1 - 012088_8, 2010/05

 Times Cited Count:0 Percentile:100

Inelastic neutron scattering study has been carried out on US$$_{2}$$. A clear crystalline electric field (CEF) peak has been observed at 7.4 meV, which is consistent with the level scheme obtained from susceptibility data assuming 5f$$^{2}$$ configuration. The CEF becomes very weak and broad, and a quasi-elastic response emerges above the characteristic temperature $$sim$$100 K. It indicates the crossover of the 5f character between localized and itinerant. A quasi-elastic response can be concluded as the hybridization effect of the 5f electrons with ligand p-electrons. Our study suggests that the crossover of the 5f character is the origin of the metal-insulator transition in US$$_{2}$$.

Journal Articles

$$^{17}$$O NMR study in (Pu$$_{0.91}$$Am$$_{0.09}$$)O$$_2$$

Tokunaga, Yo; Osaka, Masahiko; Kambe, Shinsaku; Miwa, Shuhei; Sakai, Hironori; Chudo, Hiroyuki; Homma, Yoshiya*; Shiokawa, Yoshinobu*

Journal of Nuclear Materials, 396(1), p.107 - 111, 2010/01

 Times Cited Count:3 Percentile:70.6(Materials Science, Multidisciplinary)

In order to investigate the magnetic properties of Am$$^{4+}$$ ions in the cubic fluorite structure, we have performed $$^{17}$$O NMR measurements on (Pu$$_{0.91}$$Am$$_{0.09}$$)O$$_2$$. We have observed a temperature dependent $$^{17}$$O NMR line broadening induced by classical dipolar hyperfine fields from the Am 5$$f$$ moments. From NMR line simulations, the effective moment of the Am moments has been estimated to be $$mu_{rm eff}=1.38 mu_B/{rm Am}$$. This value is comparable with $$mu_{rm eff}= 1.32 $$ or $$1.53 mu_B/{rm Am}$$ reported for AmO$$_2$$. We have also carried out nuclear relaxation measurements for $$^{17}$$O nuclei. The magnetization recovery has been found to exhibit a nonexponential time dependence with an exponent $$betasim0.5$$. This result is well understood in terms of the nuclear relaxation mechanism induced by the Am 5$$f$$ moment fluctuations via dipolar hyperfine fields.

Journal Articles

Possible existence of magnetic polaron in nearly ferromagnetic semiconductor $$beta$$-US$$_2$$

Ikeda, Shugo*; Sakai, Hironori; Tateiwa, Naoyuki; Matsuda, Tatsuma; Aoki, Dai*; Homma, Yoshiya*; Yamamoto, Etsuji; Nakamura, Akio; Shiokawa, Yoshinobu*; Ota, Yuki*; et al.

Journal of the Physical Society of Japan, 78(11), p.114704_1 - 114704_10, 2009/11

 Times Cited Count:10 Percentile:40.69(Physics, Multidisciplinary)

Journal Articles

Element and orbital-specific observation of two-step magnetic transition in NpNiGa$$_5$$; X-ray magnetic circular dichroism study

Okane, Tetsuo; Okochi, Takuo*; Inami, Toshiya; Takeda, Yukiharu; Fujimori, Shinichi; Kawamura, Naomi*; Suzuki, Motohiro*; Tsutsui, Satoshi*; Yamagami, Hiroshi; Fujimori, Atsushi; et al.

Physical Review B, 80(10), p.104419_1 - 104419_7, 2009/09

 Times Cited Count:6 Percentile:66.3(Materials Science, Multidisciplinary)

Journal Articles

Hyperfine interactions in the itinerant system UFeGa$$_{5}$$

Kambe, Shinsaku; Sakai, Hironori; Tokunaga, Yo; Chudo, Hiroyuki; Yasuoka, Hiroshi; Matsuda, Tatsuma; Haga, Yoshinori; Ikeda, Shugo; Nakamura, Akio; Yamamoto, Etsuji; et al.

Journal of Nuclear Materials, 385(1), p.1 - 3, 2009/03

 Times Cited Count:1 Percentile:87.94(Materials Science, Multidisciplinary)

In 5f-electron systems, exotic magnetism and unconventional superconductivity appear at low temperatures owing to strong electron correlations and orbital degeneracy. For example, we have investigated the unconventional superconducting states of recently synthesized NpPd$$_5$$Al$$_2$$ ($$T_c$$=5K) and PuRhGa$$_5$$ ($$T_c$$=9K) in terms of both macroscopic (staticsusceptibility, specific heat, resistivity) and microscopic (NMR) probes. The relation between magnetic fluctuations and unconventional superconductivity in f-electron systems is also discussed. Another exciting topic concerns the series AnO$$_2$$ (An; U, Np, Pu, Am). Besides the importance of AnO$$_2$$ as a nuclear fuel, the magnetic properties of these compounds at low temperatures are particularly interesting. On the basis of $$^{17}$$O-NMR studies, multipolar ordered states have been identified in UO$$_2$$ (dipolar + quadrupolar ordering) and NpO$$_2$$ (quadrupolar + octupolar ordering), in contrast with the non-magnetic ground state of PuO$$_2$$.

Journal Articles

$$^{31}$$P-NMR study of hyperfine interactions and magnetic fluctuations in the neptunium-based filled skutterudite NpFe$$_{4}$$P$$_{12}$$

Tokunaga, Yo; Kambe, Shinsaku; Sakai, Hironori; Chudo, Hiroyuki; Matsuda, Tatsuma; Haga, Yoshinori; Yasuoka, Hiroshi; Aoki, Dai*; Homma, Yoshiya*; Shiokawa, Yoshinobu*; et al.

Physical Review B, 79(5), p.054420_1 - 054420_7, 2009/02

 Times Cited Count:5 Percentile:70.72(Materials Science, Multidisciplinary)

$$^{31}$$P-NMR measurements have been performed on a single crystal of the neptunium-based filled skutterudite NpFe$$_{4}$$P$$_{12}$$. We have observed transferred hyperfine interactions at the P sites, which lead to an estimate of the local spin density in the P 3$$p$$ orbitals. It is shown that a fraction of the Np 5$$f$$ spin moments is transferred mostly into the P 3$$p$$ orbitals extending toward the inside of a P-cage. The weak hybridization between Np 5$$f$$ and P 3$$p$$ orbitals suggests a localized character for Np 5$$f$$ electrons. We have also measured the field and temperature dependences of the nuclear spin-lattice relaxation rate ($$1/T_1$$). The $$1/T_1$$ data reveal that the low-energy spin fluctuations of Np 5$$f$$ spin moments are strongly suppressed by applied fields over a relatively wide temperature range. The present NMR results demonstrate that the negative magnetoresistance comes from a reduction of the magnetic scattering from Np 5$$f$$ spin moments by an applied field.

Journal Articles

La-NMR study on a La dilute system of PrPb$$_3$$

Tokunaga, Yo; Suzuki, Hiroyuki*; Sakai, Hironori; Chudo, Hiroyuki; Kambe, Shinsaku; Yasuoka, Hiroshi; Homma, Yoshiya*; Aoki, Dai*; Shiokawa, Yoshinobu*

Journal of Physics; Conference Series, 150, p.042211_1 - 042211_4, 2009/00

 Times Cited Count:2 Percentile:21.99

The first $$^{139}$$La-NMR measurement has been carried out on a La dilute system of PrPb$$_3$$. We have succeeded to detect $$^{139}$$La-NMR signal on a powder sample of Pr$$_{0.97}$$La$$_{0.03}$$Pb$$_3$$ and extracted the temperature dependence of the Knight shift $$^{139}K(T)$$ in a temperature range between 1.5 and 220 K. $$^{139}K(T)$$ has been found to maintain a linear relation with the bulk-susceptibility $$chi(T)^{rm bulk}$$ in wide temperature range, except for a small deviation in the temperature region below 6 K. The slopes of the $$^{139}K$$ vs $$chi^{rm bulk}$$ plots yield for the hyperfine coupling constants $$A_{rm hf}$$ the value 375 Oe/$$mu _B$$. From the NMR results, we discuss the effect of the La substitutions on microscopic magnetic properties of PrPb$$_3$$.

Journal Articles

Crystal structure and magnetic properties of the new ternary actinide compounds AnPd$$_5$$Al$$_2$$ (An = U, Np)

Haga, Yoshinori; Aoki, Dai*; Homma, Yoshiya*; Ikeda, Shugo; Matsuda, Tatsuma; Yamamoto, Etsuji; Sakai, Hironori; Tateiwa, Naoyuki; Dung, N. D.*; Nakamura, Akio; et al.

Journal of Alloys and Compounds, 464(1-2), p.47 - 50, 2008/09

 Times Cited Count:18 Percentile:26.66(Chemistry, Physical)

Journal Articles

$$^{27}$$Al NMR evidence for the strong-coupling $$d$$-wave superconductivity in NpPd$$_5$$Al$$_2$$

Chudo, Hiroyuki; Sakai, Hironori; Tokunaga, Yo; Kambe, Shinsaku; Aoki, Dai*; Homma, Yoshiya*; Shiokawa, Yoshinobu*; Haga, Yoshinori; Ikeda, Shugo*; Matsuda, Tatsuma; et al.

Journal of the Physical Society of Japan, 77(8), p.083702_1 - 083702_4, 2008/08

 Times Cited Count:23 Percentile:2.92(Physics, Multidisciplinary)

We present $$^{27}$$Al NMR studies for a single crystal of the newly developed superconductor NpPd$$_5$$Al$$_2$$ ($$T_c=4.9$$ K). In the superconducting state, the $$^{27}$$Al NMR spectrum shifts to lower frequencies owing to a decrease of the spin part of the Knight shift, and exhibits broadening characteristic of the local field distribution associated with a vortex lattice. We have also found that the $$^{27}$$Al nuclear spin-lattice relaxation rate ($$1/T_1$$) decreases just below $$T_c$$ with no coherence peak and shows $$T^3$$ behavior at low temperatures. The observed temperature dependences of the Knight shift and $$1/T_1$$ in general accord with those assuming a line-node superconducting gap with the gap value of $$2Delta/k_{rm B}T_c simeq 6.4$$. Therefore, the present NMR data provides evidence for strong-coupling $$d$$-wave superconductivity in NpPd$$_5$$Al$$_2$$.

Journal Articles

NMR investigation of quadrupole order parameter in actinide dioxides

Tokunaga, Yo; Homma, Yoshiya*; Kambe, Shinsaku; Aoki, Dai*; Sakai, Hironori; Chudo, Hiroyuki; Ikushima, Kenji*; Yamamoto, Etsuji; Nakamura, Akio; Shiokawa, Yoshinobu*; et al.

Journal of Optoelectronics and Advanced Materials, 10(7), p.1663 - 1665, 2008/07

Multipolar degrees of freedom characteristic of $$f$$-electrons bring rich and complex physics to rare-earth and actinide compounds. In the present work, we demonstrate that microscopic investigation of multipole order parameters is also possible by means of NMR through the hyperfine interactions. Our $$^{17}$$O NMR study performed on a single crystal of NpO$$_2$$ provide definitive support for the proposed longitudinal triple-$$q$$ type antiferro-octupolar ordering below $$T_0=$$26 K. We shall also provide our $$^{17}$$O NMR results on UO$$_2$$ and PuO$$_2$$. The microscopic NMR clearly indicates the different nature of the magnetic ground states among three actinide dioxides.

Journal Articles

Pressure effect on paramagnet $$beta$$-US$$_2$$

Ikeda, Shugo; Sakai, Hironori; Matsuda, Tatsuma; Tateiwa, Naoyuki; Nakamura, Akio; Yamamoto, Etsuji; Aoki, Dai*; Homma, Yoshiya*; Shiokawa, Yoshinobu*; Hedo, Masato*; et al.

Physica B; Condensed Matter, 403(5-9), p.893 - 894, 2008/04

 Times Cited Count:4 Percentile:74.71(Physics, Condensed Matter)

Single crystals of paramagnetic semiconductor $$beta$$-US$$_2$$ with the orthorhombic ($textit{Pnma}$) crystal structure were grown by chemical transport method. We measured the electrical resistivity $$rho$$ under high pressure, and magnetization. With increasing pressure, the temperature dependence of $$rho$$ approaches to semimetallic behavior and a kink anomaly appears at $$T_{rm a}$$.$$T_{rm a}$$ shifts to higher temperature with increasing pressure. From the comparison with ferromagnetic semimetal UTeS having the same crystal structure, it seems that $$T_{rm a}$$ is a ferromagnetic transition.

Journal Articles

Magnetic structure study of antiferromagnet NpPtGa$$_{5}$$ by neutron diffraction

Jonen, Shintaro; Metoki, Naoto; Yamamoto, Etsuji; Homma, Yoshiya*; Aoki, Dai*; Shiokawa, Yoshinobu*; Onuki, Yoshichika

Journal of Alloys and Compounds, 448(1-2), p.84 - 88, 2008/01

 Times Cited Count:1 Percentile:84.21(Chemistry, Physical)

The magnetic structure of NpPtGa$${rm_5}$$ was investigated by neutron diffraction. NpPtGa$${rm_5}$$ exhibits an A-type antiferromagnetic structure with propagation vector $$vec{q}$$ = (0 0 1/2) below $$T_{rm N}$$ = 26 K. The estimated Np magnetic moment is 0.38(5) $$mu_{rm B}$$, and it is oriented parallel to the $$langle$$100$$rangle$$ direction. This small moment would be consistent with the large $$gamma$$ = 123 mJ/(K$${rm^2}cdot$$mol), indicative of the heavy fermion nature. The field-induced anomaly in the magnetic susceptibility is clarified to be a signature for an antiferromagnetic domain change between multi-domains to single-domain states.

Journal Articles

$$^{31}$$P-NMR study of the neptunium-based filled-skutterudite NpFe$$_4$$P$$_{12}$$

Tokunaga, Yo; Aoki, Dai*; Homma, Yoshiya*; Sakai, Hironori; Chudo, Hiroyuki; Kambe, Shinsaku; Matsuda, Tatsuma; Ikeda, Shugo; Yamamoto, Etsuji; Nakamura, Akio; et al.

Journal of the Physical Society of Japan, 77(Suppl.A), p.211 - 213, 2008/00

 Times Cited Count:3 Percentile:69.06(Physics, Multidisciplinary)

$$^{31}$$P-NMR measurements have been performed on a single crystal of NpFe$$_4$$P$$_{12}$$ which is the first crystallized transuranium filled-skutterudite compound. The field-orientation dependence of the hyperfine interaction has been measured using the single crystal, and the results compared with those obtained previously in isostructural compounds UFe$$_4$$P$$_{12}$$ and PrFe$$_4$$P$$_{12}$$. We have also measured the field and temperature dependence of the nuclear spin-lattice relaxation rate in several fields between 1.8 and 8 T at temperatures between 4.2 and 300 K. The results are discussed on the basis of a localized picture for the 5f electrons.

Journal Articles

Pressure effect on ferromagnet UTeS

Ikeda, Shugo*; Sakai, Hironori; Matsuda, Tatsuma; Tateiwa, Naoyuki; Aoki, Dai*; Homma, Yoshiya*; Nakamura, Akio; Yamamoto, Etsuji; Shiokawa, Yoshinobu*; Hedo, Masato*; et al.

Journal of the Physical Society of Japan, 77(Suppl.A), p.359 - 361, 2008/00

 Times Cited Count:1 Percentile:86.77(Physics, Multidisciplinary)

The ferromagnet UTeS was studied by measurement the resistivity under high pressure. The Curie temperature $$T_{rm C}$$ rapidly increases from 87 K to 166 Kat 8 GPa. The result suggests that there is a correlation between the electrical conductivity and magnetic transition temperature, implying the strong hybridization.

Journal Articles

Heavy fermion superconductivity with the strong Pauli paramagnetic effect on NpPd$$_5$$Al$$_2$$

Aoki, Dai*; Haga, Yoshinori; Matsuda, Tatsuma; Tateiwa, Naoyuki; Ikeda, Shugo*; Homma, Yoshiya*; Sakai, Hironori; Shiokawa, Yoshinobu*; Yamamoto, Etsuji; Nakamura, Akio; et al.

Journal of the Physical Society of Japan, 77(Suppl.A), p.159 - 164, 2008/00

 Times Cited Count:3 Percentile:69.06(Physics, Multidisciplinary)

Superconducting properties are investigated on the new neptunium-based superconductor NpPd$$_5$$Al$$_2$$. The large electronic specific heat, anisotropy of magnetic as well as superconducting properties and strong paramagnetic effect implies spin-singlet pairing with anisotropic superconducting gap.

Journal Articles

Ga NQR relaxation rates in superconductor PuRhGa$$_5$$

Sakai, Hironori; Kambe, Shinsaku; Tokunaga, Yo; Fujimoto, Tatsuya; Walstedt, R. E.*; Yasuoka, Hiroshi; Aoki, Dai*; Homma, Yoshiya*; Yamamoto, Etsuji; Nakamura, Akio; et al.

Journal of Physics and Chemistry of Solids, 68(11), p.2103 - 2106, 2007/11

 Times Cited Count:1 Percentile:91.22(Chemistry, Multidisciplinary)

A lot of attention is currently focused on Pu-based superconductors. These systems have relatively high superconducting (SC) transition temperatures, ${it i.e.,}$ $$T_{rm c}simeq$$ 18 K for PuCoGa$$_{5}$$ and $$T_{rm c}simeq$$ 9 K for PuRhGa$$_{5}$$. Both compounds crystallize in the tetragonal HoCoGa$$_{5}$$ (115) structure, which is well known for the heavy-fermion SC systems Ce$$T$$In$$_{5}$$ ($$T$$=Co, Rh, Ir)with $$T_{rm c} sim$$ 2 K at most. Recently, NMR and NQR experiments have been reported for PuCoGa$$_{5}$$ by Curro et al. and for PuRhGa$$_{5}$$ by our group. In both cases, measurements of $$1/T_{1}$$ using NQR have revealed that the Pu115 superconductors show unconventional superconductivity with an anisotropic gap. Now we have measured the more detailed data of $$1/T_{1}$$ for NQR lines in PuRhGa$$_5$$, especially focusing on the low-lying magnetic excitation in the normal state. From the viewpoint of spin-fluctuations, we will discuss the difference between PuRhGa$$_{5}$$ and PuCoGa$$_{5}$$ and between superconducting Pu115 and non-superconducting An115 series.

Journal Articles

$$^{17}$$O NMR spin-lattice relaxation in NpO$$_2$$; Field-dependent cross-relaxation process driven by $$^{237}$$Np spins

Tokunaga, Yo; Walstedt, R. E.*; Homma, Yoshiya*; Aoki, Dai*; Kambe, Shinsaku; Sakai, Hironori; Fujimoto, Tatsuya; Ikeda, Shugo; Yamamoto, Etsuji; Nakamura, Akio; et al.

Journal of Physics and Chemistry of Solids, 68(11), p.2016 - 2019, 2007/11

 Times Cited Count:2 Percentile:84.86(Chemistry, Multidisciplinary)

In this paper, we report a comprehensive study of $$^{17}$$O nuclear spin-lattice relaxation in NpO$$_2$$ for a wide range of fields and temperatures. In both the paramagnetic and ordered-state regions, a strong, field-dependent $$^{237}$$Np -$$^{17}$$O nuclei cross-relaxation process is observed in addition to the expected background rate. The cross relaxation process, driven by the $$^{237}$$Np spins, is conveyed to the $$^{17}$$O via a greatly enhanced indirect nuclear spin-spin coupling which is described and reported here for the first time. The magnitude and temperature dependence of the indirect coupling are in reasonable accord with a simple theoretical model. The magnitude and temperature dependence of the $$^{237}$$Np $$T_1$$ process are also deduced from the data obtained.

Journal Articles

NMR studies of actinide dioxides

Tokunaga, Yo; Sakai, Hironori; Fujimoto, Tatsuya; Kambe, Shinsaku; Walstedt, R. E.*; Ikushima, Kenji*; Yasuoka, Hiroshi; Aoki, Dai*; Homma, Yoshiya*; Haga, Yoshinori; et al.

Journal of Alloys and Compounds, 444-445, p.241 - 245, 2007/10

 Times Cited Count:13 Percentile:34.03(Chemistry, Physical)

In this paper, we present our recent NMR works on Neptunium and Plutonium compounds. In order to elucidate the nature of the exotic multipolar ordered phase of NpO$$_2$$, we have initiated the first NMR measurements on this system. We have shown that direct observation of multipole order parameters is possible by means of NMR. Our NMR results provide further insight into the microscopic nature of multipole phase transitions in f-electron systems. We have also performed $$^{69,71}$$Ga NMR/NQR studies on a single crystal of PuRhGa$$_5$$. From the temperature dependence of the spin-lattice relaxation rate ($$1/T_1$$), we have found that PuRhGa$$_5$$ is an unconventional superconductor having an anisotropic superconducting (SC) gap 2$$Delta(0)$$=5$$K_BT_c$$. Furthermore, we have confirmed the Knight shift to be decreased below $$T_c$$. This gives strong evidence for spin singlet SC pairing.

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