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Journal Articles

Magnetic excitation in totally symmetric staggered ordered phase of PrFe$$_4$$P$$_{12}$$

Iwasa, Kazuaki*; Hao, L.*; Kogi, Masafumi*; Kuwahara, Keitaro*; Mignot, J.-M.*; Sugawara, Hitoshi*; Aoki, Yuji*; Matsuda, Tatsuma; Sato, Hideyuki*

Journal of the Physical Society of Japan, 81(9), p.094711_1 - 094711_9, 2012/09

 Times Cited Count:8 Percentile:53.03(Physics, Multidisciplinary)

Journal Articles

Pressure-induced antiferromagnetic order in filled skutterudite PrFe$$_{4}$$P$$_{12}$$ studied by single-crystal high-pressure neutron diffraction

Osakabe, Toyotaka; Kuwahara, Keitaro*; Kawana, Daichi*; Iwasa, Kazuaki*; Kikuchi, Daisuke*; Aoki, Yuji*; Kogi, Masafumi*; Sato, Hideyuki*

Journal of the Physical Society of Japan, 79(3), p.034711_1 - 034711_7, 2010/03

 Times Cited Count:14 Percentile:65.65(Physics, Multidisciplinary)

Journal Articles

Visualizing rattling in PrOs$$_4$$Sb$$_{12}$$ by single crystal neutron diffraction and maximum-entropy analysis

Kaneko, Koji; Metoki, Naoto; Kimura, Hiroyuki*; Noda, Yukio*; Matsuda, Tatsuma; Kogi, Masafumi*

Journal of the Physical Society of Japan, 78(7), p.074710_1 - 074710_4, 2009/07

 Times Cited Count:18 Percentile:70.47(Physics, Multidisciplinary)

In order to obtain a detailed picture of rattling in PrOs$$_4$$Sb$$_{12}$$, a single crystal neutron diffraction experiment and a maximum-entropy analysis were carried out. At room temperature, a widely distributed anisotropic Pr nuclear density in the Sb icosahedron cage was revealed. An unusual trapezoidal form in the cross-sectional density profile of Pr clearly demonstrates the presence of strong anharmonicity in the Pr potential, in which the $${langle}1,1,1{rangle}$$ direction has the maximum spread. The highest density is found at an off-center position along the same direction. The Pr density distribution becomes isotropic and sharp by cooling down to 8 K. Moreover, the highest density of Pr at 8 K is found at the center of the cage. The obtained nuclear density distribution and its temperature variation indicate that the rattling in PrOs$$_4$$Sb$$_{12}$$ is a strong anharmonic thermal motion without an off-center potential minimum.

Journal Articles

Strong anharmonicity of Pr thermal vibration in heavy fermion superconductor PrOs$$_4$$Sb$$_{12}$$

Kaneko, Koji; Metoki, Naoto; Kimura, Hiroyuki*; Noda, Yukio*; Matsuda, Tatsuma; Kogi, Masafumi*

Physica B; Condensed Matter, 403(5-9), p.874 - 876, 2008/04

 Times Cited Count:1 Percentile:6.77(Physics, Condensed Matter)

In order to clarify the detailed structure of PrOs$$_{4}$$Sb$$_{12}$$, single crystal neutron diffraction experiments were carried out and obtained data were analyzed by means of the maximum entropy method (MEM). A space- and time-averaged nuclear density distribution reflecting a thermal vibration can be obtained through the MEM analysis. The result unveils a widely spread Pr distribution in the Sb cage at room temperature. The Pr density has a almost flat distribution in the real space centered at the Sb cage, which reaches roughly 0.58${AA}$ in the full width at half maximum (FWHM). These features are far from the simple harmonic model, namely, the present result suggests the strong anharmonicity of the Pr potential in the Sb cage.

Journal Articles

Visualization of anharmonic thermal vibration in filled skutterudite compounds

Kaneko, Koji; Metoki, Naoto; Kimura, Hiroyuki*; Noda, Yukio*; Matsuda, Tatsuma; Kogi, Masafumi*

Journal of the Physical Society of Japan, 77(Suppl.A), p.245 - 247, 2008/00

 Times Cited Count:3 Percentile:28.87(Physics, Multidisciplinary)

In filled skutterudite compounds, the importance of rattling is suggested in connection with the formation of heavy quasiparticles and superconductivity. A combination of the single crystal neutron diffraction and the maximum entropy method (MEM) offers an ability to unveil the thermal motion of Pr in PrOs$$_4$$Sb$$_{12}$$ without any specific model. The result reveals the widely spread Pr nuclear density distribution at room temperature, which has a trapezoidal shape in the real space with the width of roughly 0.6 ${AA}$ in the full width at half maximum. In contrast, the Pr distribution at 8 K has a sharp, isotropic gaussian shape, and the highest density locates at the center of the cage. These features indicate the strong anharmonicity of the Pr potential in the Sb cage.

Journal Articles

Unusual electronic state of the low-carrier system CeSb under high pressure studied by simultaneous measurement of electrical resistivity and lattice parameter

Hannan, A.*; Okayama, Yasushi*; Osakabe, Toyotaka; Kuwahara, Keitaro*; Kogi, Masafumi*

Journal of the Physical Society of Japan, 76(5), p.054706_1 - 054706_6, 2007/05

 Times Cited Count:1 Percentile:11.47(Physics, Multidisciplinary)

We have studied the origin of the anomalous peak structure of resistivity of the low-carrier system CeSb appeared around the antiferromagnetic transition temperature T$$_{N}$$ under high pressures by means of simultaneous measurement of lattice parameter by X-ray diffraction and resistivity together with neutron diffraction technique. It was found that there is a strong relationship between the unusual decrease of lattice parameter and the unusual increase of resistance below T$$_{N}$$ under high pressure. Both of these phenomena can be explained by the appearance of the $$Gamma$$$$_{8}$$ - like magnetic state of Ce$$^{3+}$$ ion below T$$_{N}$$.

Journal Articles

The Unconventional superconductivity of skutterudite PrOs$$_4$$Sb$$_{12}$$; Time-reversal symmetry breaking and adjacent field-induced quadrupole ordering

Aoki, Yuji*; Tayama, Takashi*; Sakakibara, Toshiro*; Kuwahara, Keitaro*; Iwasa, Kazuaki*; Kogi, Masafumi*; Higemoto, Wataru; MacLaughlin, D. E.*; Sugawara, Hitoshi*; Sato, Hideyuki*

Journal of the Physical Society of Japan, 76(5), p.051006_1 - 051006_13, 2007/05

 Times Cited Count:60 Percentile:59.09(Physics, Multidisciplinary)

This review presents a summary and evaluation of the experimental properties of unconventional superconductivity in PrOs$$_4$$Sb$$_{12}$$. After a brief introduction of filled skutterudites, we argue that the normal-state properties of PrOs$$_4$$Sb$$_{12}$$ are quite different from ordinary heavy-fermion superconductors.

Journal Articles

Elastic anomalies of UGe$$_2$$

Kuwahara, Keitaro*; Sakai, Takashi*; Kogi, Masafumi*; Haga, Yoshinori; Onuki, Yoshichika

Journal of Magnetism and Magnetic Materials, 310(2, Part1), p.362 - 364, 2007/03

We have performed the ultrasonic sound velocity measurement on the single crystal of UGe$$_2$$ in the temperature range 4-120 K at ambient pressure, focusing attention on the elastic behavior around the characteristic temperature $$T^{*}$$ = 30 K. In the elastic constants forthree modes $$c$$$$_{11}$$, $$c$$$$_{55}$$ and $$c$$$$_{66}$$, all modes show the hardening with a kink at the Curie temperature as decreasing temperatures. We found the elastic anomaly related to $$T^{*}$$ in the longitudinal $$c$$$$_{11}$$ mode. However, the anomaly around $$T^{*}$$ is very broad. Therefore, this result may suggest that $$T^{*}$$ is not related to a phase boundary of some second-order phase transition. On the other hand, no clear anomaly wasobserved around $$T^{*}$$ in the transverse $$c$$$$_{55}$$ and $$c$$$$_{66}$$ modes.

Journal Articles

Neutron scattering study on the field-induced $$O_{xy}$$-type antiferroquadrupolar ordering of the heavy-fermion superconductor PrOs$$_4$$Sb$$_{12}$$

Kaneko, Koji; Metoki, Naoto; Shiina, Ryosuke*; Matsuda, Tatsuma; Kogi, Masafumi*; Kuwahara, Keitaro*; Bernhoeft, N.*

Physical Review B, 75(9), p.094408_1 - 094408_7, 2007/03

 Times Cited Count:20 Percentile:65.43(Materials Science, Multidisciplinary)

Neutron scattering experiments have revealed a field-induced antiferroquadrupolar order parameter in the Pr-based heavy-fermion superconductor PrOs$$_4$$Sb$$_{12}$$. We observed the field-induced antiferromagnetic dipole moment with the propagation vector $$q$$ = (1 0 0) for the applied field direction both $$H parallel$$ [1 1 0] and [0 0 1]. For $$H parallel$$ [1 1 0] at 8 T, it should be noted that the induced antiferromagnetic moment of 0.16 $$mu_{rm B}$$/ Pr orients parallel to the field within our experimental accuracy. This observation is strong evidence that the $$O_{xy}$$ electric quadrupole to be the primary order parameter for $$H parallel$$ [1 1 0]. A mean-field calculation, based on $$Gamma_1$$ singlet and $${Gamma}_4^{(2)}$$ excited triplet with the $$O_{xy}$$ quadrupolar interaction, reproduces the induced moment direction and its field response. These facts indicate the predominance of $$O_{xy}$$-type antiferroquadrupolar interaction in PrOs$$_4$$Sb$$_{12}$$.

Journal Articles

Structural transformation on the pressure-induced metal-insulator transition in PrFe$$_{4}$$4P$$_{12}$$

Kawana, Daichi*; Kuwahara, Keitaro*; Sato, Masashi*; Takagi, Masatoshi*; Aoki, Yuji*; Kogi, Masafumi*; Sato, Hideyuki*; Sagayama, Hajime*; Osakabe, Toyotaka; Iwasa, Kazuaki*; et al.

Journal of the Physical Society of Japan, 75(11), p.113602_1 - 113602_4, 2006/11

 Times Cited Count:11 Percentile:57.78(Physics, Multidisciplinary)

We report the first X-ray diffraction experiments of PrFe$$_{4}$$P$$_{12}$$ under high pressure. We discovered a symmetry lowering from cubic to orthorhombic or lower at the metal-insulator transition temperature, accompanied by a jump in the lattice constant, which indicates a first-order phase transition. The superlattice reflections at q=(1,0,0) observed in the low-pressure nonmagnetic ordered phase are found to disappear in the insulating phase. The temperature dependence of the lattice constant and the absence of the superlattice reflections are well explained by taking into account an antiferromagnetic ordering.

Journal Articles

Inelastic neutron scattering study on low-energy excitations of the heavy-fermion superconductor PrOs$$_4$$Sb$$_{12}$$

Kuwahara, Keitaro*; Iwasa, Kazuaki*; Kogi, Masafumi*; Kaneko, Koji; Metoki, Naoto; Raymond, S.*; M$'e$asson, M.-A.*; Flouquet, J.*; Sugawara, Hitoshi*; Aoki, Yuji*; et al.

Physica B; Condensed Matter, 385-386(Part 1), p.82 - 84, 2006/11

 Times Cited Count:3 Percentile:18.82(Physics, Condensed Matter)

We report inelastic neutron scattering experiments performed to investigate the low energy magnetic excitations on single crystals of the heavy-fermion superconductor PrOs$$_4$$Sb$$_{12}$$. The observed excitation clearly softens at a wave vector Q=(1,0,0), which is the same as the modulation vector of the field-induced antiferro-quadrupolar ordering, and its intensity at Q=(1,0,0) is smaller than that around the zone center. This result directly evidences that this excitonic behavior is derived mainly from nonmagnetic quadrupolar interactions. Furthermore, the narrowing of the linewidths of the excitations below the superconducting transition temperature indicates the close connection between the superconductivity and the excitons.

Journal Articles

Field-induced antiferroquadrupolar order in the heavy fermion superconductor PrOs$$_4$$Sb$$_{12}$$

Kaneko, Koji; Metoki, Naoto; Matsuda, Tatsuma; Kuwahara, Keitaro*; Kogi, Masafumi*; Shiina, Ryosuke*; Mignot, J.-M.*; Gukasov, A.*; Bernhoeft, N.*

Physica B; Condensed Matter, 378-380, p.189 - 191, 2006/05

 Times Cited Count:1 Percentile:7.01(Physics, Condensed Matter)

Neutron diffraction experiments under magnetic fields were carried out on the heavy fermion superconductor PrOs$$_4$$Sb$$_{12}$$ in order to reveal the origin of the field-induced ordered phase (FIOP) for $$H$$$$Vert$$[1,1,0]. The application of magnetic field induces the weak superlattice peaks with the ordering vector $$q$$=(1,0,0) in FIOP, which is the same as for $$H{parallel}$$[0,0,1]. The observed superlattice reflections for $$H$$$$Vert$$[1,1,0] result from the antiferromagnetic component of $$J_x$$+$$J_y$$; the induced tiny antiferromagnetic moments orient parallel to the applied field. This result cannot be explained by magnetic interactions and strongly evidences the underlying AFQ order with $$O_{xy}$$ as a primary order parameter. These facts clarify the dominant role of $$O_{xy}$$-type antiferroquadrupolar interaction in PrOs$$_4$$Sb$$_{12}$$.

Journal Articles

Rattling of Pr in heavy-fermion superconductor PrOs$$_4$$Sb$$_{12}$$ revealed by neutron powder diffraction

Kaneko, Koji; Metoki, Naoto; Matsuda, Tatsuma; Kogi, Masafumi*

Journal of the Physical Society of Japan, 75(3), p.034701_1 - 034701_5, 2006/03

 Times Cited Count:35 Percentile:80.9(Physics, Multidisciplinary)

Neutron powder diffraction experiments were carried out on the heavy-fermion superconductor PrOs$$_4$$Sb$$_{12}$$ down to 7.7 K in order to reveal the thermal displacement of Pr and investigate the possible existence of off-center potential minima for Pr in the Sb cage. Rietveld analysis with the structure in which Pr is located at the center of the Sb cage well reproduces the observed patterns. A large thermal displacement was obtained for Pr of $$U_{rm Pr}$$ =0.0344 $AA$^2$$ at 295.1 K, roughly corresponding to a mean thermal displacement of 0.18 $AA . This value is 5 times larger than those of other constituents, $U_{rm Os}$$ and $$U_{rm Sb}$$. $$U_{rm Pr}$$ remains large, $$U_{rm Pr}$$ =0.0062 $AA$^2$$, even at 7.7 K, which is twice as large as $$U_{rm Os}$$ and $$U_{rm Sb}$$. Neither structural transition nor significant change of the Sb cage accompanied this change in $$U_{rm Pr}$$. The large $$U_{rm Pr}$$ and its strong temperature dependence indicate the rattling of Pr under a shallow potential in the Sb cage. An equal distribution of Pr at 12$$e$$ sites ($$x$$,0,0) was introduced to approximate the off-center potential minima. The $$R$$ factors were not improved by the introduction of the off-center site.

Journal Articles

Direct observation of quadrupolar excitons in the heavy-fermion superconductor PrOs$$_4$$Sb$$_12$$

Kuwahara, Keitaro*; Iwasa, Kazuaki*; Kogi, Masafumi*; Kaneko, Koji; Metoki, Naoto; Raymond, S.*; M$'e$asson, M.-A.*; Flouquet, J.*; Sugawara, Hitoshi*; Aoki, Yuji*; et al.

Physical Review Letters, 95(10), p.107003_1 - 107003_4, 2005/09

 Times Cited Count:91 Percentile:92.46(Physics, Multidisciplinary)

We report inelastic neutron scattering experiments performed to investigate the low energy magnetic excitations on single crystals of the heavy-fermion superconductor PrOs$$_4$$Sb$$_{12}$$. The observed excitation clearly softens at a wave vector $$Q$$ = (1,0,0), which is the same as the modulation vector of the field-induced antiferro-quadrupolar ordering, and its intensity at $$Q$$ = (1,0,0) is smaller than that around the zone center. This result directly evidences that this excitonic behavior is derived mainly from nonmagnetic quadrupolar interactions. Furthermore, the narrowing of the linewidths of the excitations below the superconducting transition temperature indicates the close connection between the superconductivity and the excitons.

Journal Articles

Magnetic excitations in the heavy-fermion superconductor PrOs$$_4$$Sb$$_{12}$$

Raymond, S.*; Flouquet, J.*; Kuwahara, Keitaro*; Iwasa, Kazuaki*; Kogi, Masafumi*; Kaneko, Koji; Metoki, Naoto; Sugawara, Hitoshi*; Aoki, Yuji*; Sato, Hideyuki*

Physica B; Condensed Matter, 359-361, p.898 - 900, 2005/06

 Times Cited Count:2 Percentile:12.3(Physics, Condensed Matter)

The low lying magnetic excitations of the unconventional heavy fermion superconductor PrOs$$_4$$Sb$$_{12}$$ were studied by inelastic neutron scattering on single crystalline sample. The crystal field excitation first evidenced at 0.7 meV on powder samples is found to have a minimum of energy at the zone boundary corresponding to the wavevector of antiferro-quadrupolar instability. The evolution of the magnetic excitation spectrum under magnetic field confirms the singlet ground state.

Journal Articles

Electrical and magnetic properties of a single crystal UCu$$_2$$Si$$_2$$

Matsuda, Tatsuma; Haga, Yoshinori; Ikeda, Shugo; Galatanu, A.; Yamamoto, Etsuji; Shishido, Hiroaki*; Yamada, Mineko*; Yamaura, Junichi*; Hedo, Masato*; Uwatoko, Yoshiya*; et al.

Journal of the Physical Society of Japan, 74(5), p.1552 - 1556, 2005/05

 Times Cited Count:21 Percentile:70.75(Physics, Multidisciplinary)

We have succeeded in growing a high-quality single crystal of UCu$$_2$$Si$$_2$$ with the tetragonal structure by the Sn-flux method and measured the electrical resistivity, magnetic susceptibility, magnetization and specific heat. UCu$$_2$$Si$$_2$$ is found to order antiferromagnetically below $$T_{rm N}$$ = 106 K, and follows a successive ferromagnetic ordering at $$T_{rm C}$$ = 100 K. The magnetic properties are highly anisotropic, reflecting the crystal structure. An easy-axis of magnetization is found to be the [001] direction ($$c$$-axis) both in the antiferromagnetic and ferromagnetic phases, while the [100] direction ($$a$$-axis) corresponds to the hard-axis in magnetization. The magnetization curve in the antiferromagnetic phase indicates a clear metamagnetic transition at a low field of about 1 kOe and changes smoothly into a ferromagnetic magnetization curve below $$T_{rm C}$$ = 100 K. The saturation moment is determined as 1.75$$mu_{rm B}$$/U at 2 K. The electronic specific heat coefficient is also determined as 20 mJ/K$$^2$$$$cdot$$mol.

Journal Articles

Magnetic excitations in heavy-fermion superconductor PrOs$$_{4}$$Sb$$_{12}$$

Kuwahara, Keitaro*; Iwasa, Kazuaki*; Kogi, Masafumi*; Kaneko, Koji; Araki, Shingo; Metoki, Naoto; Sugawara, Hitoshi*; Aoki, Yuji*; Sato, Hideyuki*

Journal of the Physical Society of Japan, 73(6), p.1438 - 1441, 2004/06

 Times Cited Count:64 Percentile:88.22(Physics, Multidisciplinary)

no abstracts in English

Journal Articles

Neutron scattering study on the field-induced antiferro-quadrupolar ordering in the heavy fermion superconductor PrOs$$_{4}$$Sb$$_{12}$$

Metoki, Naoto; Kaneko, Koji; Araki, Shingo; Kogi, Masafumi*; Iwasa, Kazuaki*; Kuwahara, Keitaro*; Bernhoeft, N.*; Mignot, J.-M.*; Gukasov, A.*; Sato, Hideyuki*; et al.

Journal of Magnetism and Magnetic Materials, 272-276(Suppl.), p.e91 - e92, 2004/05

 Times Cited Count:1 Percentile:7.49(Materials Science, Multidisciplinary)

no abstracts in English

Journal Articles

High-pressure apparatus for magnetic neutron diffraction beyond 3 GPa at low temperature

Osakabe, Toyotaka; Kogi, Masafumi*; Kawana, Daichi*; Hannan, A.*; Soyama, Kazuhiko; Kitazawa, Hideaki*

Journal of Magnetism and Magnetic Materials, 272-276, p.2157 - 2158, 2004/05

 Times Cited Count:3 Percentile:20.74(Materials Science, Multidisciplinary)

no abstracts in English

Journal Articles

Higher-order collinear interaction and magnetic excitation in the 5f localized system U$$_{3}$$Pd$$_{20}$$Si$$_{6}$$

Metoki, Naoto; Koike, Yoshihiro; Haga, Yoshinori; Kaneko, Koji; Araki, Shingo; McEwen, K. A.*; Kogi, Masafumi*; Aso, Naofumi*; Lander, G. H.; Komatsubara, Takemi*; et al.

Journal of Physics; Condensed Matter, 15(28), p.S1957 - S1963, 2003/07

 Times Cited Count:2 Percentile:15.73(Physics, Condensed Matter)

no abstracts in English

39 (Records 1-20 displayed on this page)