論文 - 椎名　亮輔

Anisotropic Form Factors of Neutron Scattering by Magnetic Octupole in CeB6

Ryousuke Shiina

INTERNATIONAL CONFERENCE ON STRONGLY CORRELATED ELECTRON SYSTEMS (SCES 2011) ( IOP PUBLISHING LTD )  391   012064   2012年 [ 査読有り ]

掲載種別: 研究論文（学術雑誌）

　概要を見る

The form factor of elastic neutron scattering is calculated for the quadrupole order phase of CeB6 in magnetic fields. It is shown that the dependence of the form factor on the direction of neutron momentum transfer is very small for the Bragg reflections, whereas the scattering due to field-induced octupoles gives rise to a significant anisotropy of the form factor for the super-lattice reflections. These results are discussed in quantitative comparison with recent exprimental results. The similarity and dissimilarity with the results for phase IV in Ce1-xLaxB6 is also discussed.

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Hide and Seek in the Shastry-Sutherland Lattice

椎名 亮輔

J. Phys. Soc. Jpn. Online-News and Comments     2012年 [ 査読有り ]

掲載種別: 研究論文（学術雑誌）

Theory of Low-Temperature Magneto-Transport in Unconventional Charge Order Phase of Skutterudite PrRu4P12

Ryousuke Shiina

JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN ( PHYSICAL SOC JAPAN )  80 ( 9 ) 094710   2011年09月 [ 査読有り ]

掲載種別: 研究論文（学術雑誌）

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The skutterudite PrRu4P12 exhibits a metal-insulator transition at 63 K which is known as a consequence of an unconventional charge order (CO) transition. In this paper, the origin of a giant magneto-transport (GMT) in the CO phase at low temperatures is studied theoretically. First, we introduce an effective low-energy model in the well-developed CO phase, in which a small number of conduction electrons in the pseudo-gap state are coupled magnetically with the degenerate f electron triplet states on one of the sublattices. Then, we formulate the transport equation for the conduction electrons by considering inelastic scattering due to the magnetic interaction with the triplet, and solve it within the relaxation time approximation. It is shown that the characteristic temperature and magnetic field dependences of the resistivity and the Hall effect in this material are reproduced well with a reasonable choice of the model parameters. In particular, the strength of the magnetic interaction estimated in this study is quite consistent with the one expected from the microscopic p-f hybridization model. Contributions of the exchange field and the field-dependent relaxation time to the GMT are discussed through quantitative comparisons with the experiments. The effect of the hyperfine coupling between the triplet and the nuclear spin, which has been shown to play a crucial role below 0.3 K, is also discussed.

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Effective Hamiltonians for Unconventional Charge Ordering in Pr-based Filled Skutterudite Compounds

Ryousuke Shiina, Hiroyuki Shiba

JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN ( PHYSICAL SOC JAPAN )  80   SA011   2011年07月 [ 査読有り ]

掲載種別: 研究論文（学術雑誌）

　概要を見る

The origin of unconventional charge ordering in Pr-based filled skutterudite compounds are studied theoretically. We derive an effective interaction by analyzing the hybridization between the main p band composed of pnictogen molecular orbitals and the Pr 4f(2) states, and show that it depends strongly on the occupancy of the f orbitals. Together with the nesting instability of the conduction band, this interaction naturally leads to a predominant charge order with modulated f-electron crystal field levels. It is also shown that the magnetic interaction among residual triplets in the charge order phase is very small. These characteristics account well for the available experimental results of PrRu4P12. In addition, we discuss briefly a charge order in PrFe4P12, focusing on the differences from that of PrRu4P12. The multiband properties in PrFe4P12 and a possible interpretation on recent pressure experiments are discussed.

f-Electron-Nuclear Hyperfine-Coupled Multiplets in the Unconventional Charge Order Phase of Filled Skutterudite PrRu4P12

Yuji Aoki, Takahiro Namiki, Shanta R. Saha, Takashi Tayama, Toshiro Sakakibara, Ryousuke Shiina, Hiroyuki Shiba, Hitoshi Sugawara, Hideyuki Sato

JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN ( PHYSICAL SOC JAPAN )  80 ( 5 ) 054704   2011年05月 [ 査読有り ]

掲載種別: 研究論文（学術雑誌）

　概要を見る

The filled skutterudite PrRu4P12 is known to undergo an unconventional charge order phase transition at 63 K, below which two sublattices with distinct f-electron crystalline-electric-field ground states are formed. In this paper, we study experimentally and theoretically the properties of the charge order phase at very low temperature, particularly focusing on the nature of the degenerate triplet ground state on one of the sublattices. First, we present experimental results of specific heat and magnetization measured with high quality single crystals. In spite of the absence of any symmetry breaking, the specific heat shows a peak structure at T-p = 0.30 K in zero field; it shifts to higher temperatures as the magnetic field is applied. In addition, the magnetization curve has a remarkable rounding below 1 T. Then, we study the origin of these experimental findings by considering the hyperfine interaction between 4f electron and nuclear spin. We demonstrate that the puzzling behaviors at low temperatures can be well accounted for by the formation of 4f-electron-nuclear hyperfine-coupled multiplets, the first thermodynamical observation of its kind.

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Octupole Ordering Phases and Elastic Neutron Scattering in f-Electron Systems

Ryousuke Shiina

JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN ( PHYSICAL SOC JAPAN )  80   SB08   2011年01月 [ 査読有り ]

掲載種別: 研究論文（学術雑誌）

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The possibilities of elastic neutron scattering due to magnetic octupole ordering in f-electron systems are investigated theoretically. Firstly, we discuss the symmetry properties which lead to selection rules for the scatterings depending on the type of octupoles. Then, it is pointed out that small but finite intensity of such octupole scatterings is expected for incident neutrons with short wave-length. Based on these framework, we study the magnetic form factors originating from field-induced and spontaneous octupole orders in phase II and IV of Ce1-xLaxB6. It is shown that the magnitude and the anisotropy of the calculated form factors are quite consistent with recent experimental observations by Kuwahara et al., evidencing the existence of ordered octupoles in these two phases. The scatterings in related materials showing possible multipole orderings are briefly discussed.

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Theory of Unconventional Charge Ordering and Electrical Transport in Skutterudite PrRu(4)P(12)

Ryousuke Shiina

INTERNATIONAL CONFERENCE ON STRONGLY CORRELATED ELECTRON SYSTEMS (SCES 2010) ( IOP PUBLISHING LTD )  273   012141   2011年 [ 査読有り ]

掲載種別: 研究論文（学術雑誌）

　概要を見る

The unconventional properties of charge ordering associated with the metal-insulator transition in skutterudite PrRu(4)P(12) are studied theoretically. It is demonstrated by a simplified model Hamiltonian that the electrical transport in the charge ordered state is governed by a large f-orbital fluctuation. In particular, the mechanism of an anomalous temperature dependence of the Hall conductivity is discussed in detail. The nature of residual magnetic interaction giving magnetoresistance at low temperature is also discussed.

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Effective Hamiltonian for PrRu4P12 Showing Unconventional Charge Ordering

Ryousuke Shiina, Hiroyuki Shiba

JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN ( PHYSICAL SOC JAPAN )  79 ( 4 ) 044704   2010年04月 [ 査読有り ]

掲載種別: 研究論文（学術雑誌）

　概要を見る

The origin and the nature of the charge ordering in PrRu4P12 are studied theoretically by using a simple model. First, the mixing matrix elements between conduction electrons and Pr 4f states are examined to show that the magnitude of the mixing depends sensitively on the crystal-field (CF) levels. Then, taking the simplest case in which Gamma(1) is the CF ground state and Gamma((2))(4) is a low-lying CF excited state in the normal phase, we derive an effective Hamiltonian for both the charge and spin degrees of freedom. As expected, it results in a two-sublattice charge ordering assisted by the nesting of conduction band, where Gamma((2))(4) becomes the ground state on one of the sublattices. The magnitude of the interaction between Gamma((2))(4) triplets can be estimated within the present model; it turns out to be very weak, suggesting that Gamma((2))(4) triplets behave as almost free even at low temperatures.

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Studies on X-ray Thomson Scattering from Antiferroquadrupolar Order in TmTe

Tatsuya Nagao, Ryousuke Shiina

JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN ( PHYSICAL SOC JAPAN )  79 ( 3 ) 034703   2010年03月 [ 査読有り ]

掲載種別: 研究論文（学術雑誌）

　概要を見る

We study Thomson scattering from the antiferroquadrupole ordering phase in TmTe. On the basis of the group theoretical treatment, we classify the selection rules of the scattering intensity governed by the orientation of the scattering vector G. Then, numerical verification is performed by invoking the ground states which are deduced from a J = 7/2 multiplet model. The obtained intensity varies drastically depending on the magnitude and direction of G. We also calculate the scattering intensities under the applied field for H parallel to (001) and (110). Their results behave differently when the orientation of G is changed, which is ascribed to the difference of their primary order parameters; O-2(0) and O-2(2) for H parallel to (001) and (110), respectively. We make critical comparisons between our results for TmTe and the experimental ones for CeB6. First, we assert that the intensities expected from TmTe at several forbidden Bragg spots are sufficient enough to be experimentally detected. Second, their intensities at (7/2, 1/ 2, 1/ 2) differ significantly and may be attributed to the difference of the order parameters between the Gamma(3)-type (O-2(2) and O-2(0)) and Gamma(5)-type (O-yz, O-zx, and O-xy) components, respectively.

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