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Mechanism of Non-Coplanar Magnetic Ordering Assisted by the Jahn-Teller Effect in UO2

R. Shiina

Journal of the Physical Society of Japan 91 023704-1 - 023704-5 2022年01月 [ 査読有り ]

掲載種別: 研究論文（学術雑誌）

Heavy Fermion Emerging from the Singular Fermi Liquid in Underscreened Kondo Model for Eu Compounds

S Kuniyoshi, R Shiina

Journal of the Physical Society of Japan 90 123702-1 - 123702-4. 2021年11月 [ 査読有り ]

掲載種別: 研究論文（学術雑誌）

Effect of Magnetic Field on Quantum Phase Transition and Heavy Fermion State in a Two-Orbital Impurity Anderson Model

Ryousuke Shiina, Shingo Kuniyoshi

J. Phys. Soc. Jpn. 90 074708-1 - 074708-6 2021年07月 [ 査読有り ]

掲載種別: 研究論文（学術雑誌）

Valence Fluctuation, Spin-Orbit Coupling, and Magnetic-Nonmagnetic Crossover in Eu Compounds

Shingo Kuniyoshi, Ryousuke Shiina

J. Phys. Soc. Jpn 89 124711-1 - 124711-9. 2020年11月 [ 査読有り ]

掲載種別: 研究論文（学術雑誌）

Theory of Magnetic Ordering and Valence Fluctuation in Nearly Trivalent Eu Compounds

Ryousuke Shiina

J. Phys. Soc. Jpn. 88 113705 2019年 [ 査読有り ]

掲載種別: 研究論文（学術雑誌）

Theory of Field-Insensitive Heavy Fermion in Sm compounds,

椎名亮輔

AIP Advances 8 101301 2018年 [ 査読有り ]

掲載種別: 研究論文（学術雑誌）

Stability of Spin-Charge Composite Order Phase in an Extended Kondo Lattice Model with General Band Filling

椎名亮輔

J. Phys. Soc. Jpn. 87 084704 2018年 [ 査読有り ]

掲載種別: 研究論文（学術雑誌）

Properties of heavy fermion in an impurity anderson model with f2 local singlet ground state

Ryousuke Shiina

Journal of the Physical Society of Japan ( Physical Society of Japan ) 87 ( 1 ) 014702 2018年 [ 査読有り ]

掲載種別: 研究論文（学術雑誌）

The Fermi-liquid properties of a two-orbital impurity Anderson model with an f 2 local singlet ground state is studied by the numerical renormalization group method. It is shown that a heavy-fermion (HF) state with a vanishingly small Wilson ratio is realized in the vicinity of the quantum critical point (QCP) between the local-singlet and Kondo-singlet phases. The parameter area in which the HF behavior appears is remarkably enlarged with increasing the excitation energy from the local singlet to triplet states. It is also clarified that the HF state takes place with an intermediate average occupancy of f electrons, reflecting the existence of the QCP under strong valence fluctuation. The possible application of these theoretical results to an unconventional HF state in Sm compounds is discussed.

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Studies on Valence Fluctuation and Orbital Occupancy in an Impurity Anderson Model with f2 Local-Singlet Ground State

R. Shiina

Journal of the Physical Society of Japan 86 034705 2017年03月 [ 査読有り ]

掲載種別: 研究論文（学術雑誌）

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Theory of magneto-transport in spin-charge composite order phase of Sm skutterudite

Shiina R.

Journal of the Physical Society of Japan ( Journal of the Physical Society of Japan ) 85 ( 12 ) 2016年12月 [ 査読有り ]

掲載種別: 研究論文（学術雑誌）

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Continuous control of local magnetic moment by applied electric field in multiferroics Ba2CoGe2O7

Soda M.

Physical Review B ( Physical Review B ) 94 ( 9 ) 2016年09月 [ 査読有り ]

掲載種別: 研究論文（学術雑誌）

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Acoustic Emission Measurements on Metal-Hydrogenation Process by Using Electrochemical Charging Cell

Jiaxiang Piao, Hiroya Shoji, Takahiro Murakami, Ryousuke Shiina, Shuji Harada

MATERIALS TRANSACTIONS ( JAPAN INST METALS ) 56 ( 9 ) 1496 - 1500 2015年09月 [ 査読有り ]

掲載種別: 研究論文（学術雑誌）

In a recent work, a hydrogenation process of palladium has been studied by the acoustic emission (AE) method with a new gas pressure cell. The high H-2 gas pressure condition can, however, be easily provided by an electrochemical hydrogen charging technique. In this paper, characteristic AE power spectra caused by cavitation and/or bubble burst have been appraised by using an electrochemical separated AE cell. A typical AE signal induced by Pd hydride formation has been observed. The fundamental AE mode was 250 kHz in the Pd hydride formation stage, which was equal to the frequency measured by using the gas pressure cell as a standard hydrogenation technique.

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Effect of Magnetic Field on Spin Excitations in the Kondo Lattice Antiferromagnet

Ryousuke Shiina

JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN ( PHYSICAL SOC JAPAN ) 83 ( 12 ) 124705 2014年12月 [ 査読有り ]

掲載種別: 研究論文（学術雑誌）

The properties of spin excitations in the antiferromagnetic ordered phase of the Kondo lattice model in a magnetic field are investigated by the S-1 expansion method with S being the magnitude of localized spins. It is shown that the field dependence of magnon dispersion is different from that in the Heisenberg antiferromagnet, showing an enhancement of excitation energy in the entire Brillouin zone. As a result, the magnon states are found to be unstable and to decay into particle-hole excitations in a high magnetic field.

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Coexistence and Competition of Magnetic and Charge Orders in an Extended Kondo Lattice Model for Sm Skutterudite

Ryousuke Shiina

JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN ( PHYSICAL SOC JAPAN ) 83 ( 9 ) 094706 2014年09月 [ 査読有り ]

掲載種別: 研究論文（学術雑誌）

The properties of possible ordered phases in Sm skutterudite are studied in the framework of an effective Kondo lattice model derived from the p-f hybridization. In particular, the stable region of coexisting magnetic and charge orders is investigated in detail by changing the strength of crystal and magnetic fields in an Sm J = 5/2 multiplet. It is demonstrated that the coexisting phase does take place when the crystal field splitting is small, whereas it does not for an isolated Gamma(7) or Gamma(8) crystal field ground state. Although the phase usually appears under a finite magnetic field, the mean field theory predicts its existence at zero field in a case of the Gamma(8) ground state. The relationship of these theoretical results with experimental results of SmRu4P12 is discussed.

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Spin-Nematic Interaction in the Multiferroic Compound Ba2CoGe2O7

M. Soda, M. Matsumoto, M. Mansson, S. Ohira-Kawamura, K. Nakajima, R. Shiina, T. Masuda

PHYSICAL REVIEW LETTERS ( AMER PHYSICAL SOC ) 112 ( 12 ) 127205 2014年03月 [ 査読有り ]

掲載種別: 研究論文（学術雑誌）

We demonstrate the existence of the spin-nematic interactions in an easy-plane type antiferromagnet Ba2CoGe2O7 by exploring the magnetic anisotropy and spin dynamics. The combination of neutron scattering and magnetic susceptibility measurements reveals that the origin of the in-plane anisotropy is an antiferro-type interaction of the spin-nematic operator. The relation between the nematic operator and the electric polarization in the ligand symmetry of this compound is presented. The introduction of the spin-nematic interaction is useful to understand the physics of spin and electric dipole in multiferroic compounds.

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Order Having Disorder in a New Samarium Compound

椎名亮輔

J. Phys. Soc. Jpn. Online-News and Comments 2014年 [ 査読有り ]

掲載種別: 研究論文（学術雑誌）

p-f Mixing, Competing Interactions, and Unconventional Magnetism in SmRu4P12,

椎名亮輔

JPS Conf. Proc. 3 011006 2014年 [ 査読有り ]

掲載種別: 研究論文（学術雑誌）

Theory of Metal-Insulator Transition and Unconventional Magnetic Ordering in SmRu4P12

Ryousuke Shiina

JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN ( PHYSICAL SOC JAPAN ) 82 ( 8 ) 083713 2013年08月 [ 査読有り ]

掲載種別: 研究論文（学術雑誌）

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Stability of Charge and Magnetic Orderings Due to Competing Interactions in PrFe4P12

Ryousuke Shiina

JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN ( PHYSICAL SOC JAPAN ) 81 ( 10 ) 105001 2012年10月 [ 査読有り ]

掲載種別: 研究論文（学術雑誌）

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Multipolar Interaction Derived from d-f Hybridization in Pr-Based Filled Skutterudite Compounds

Ryousuke Shiina

JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN ( PHYSICAL SOC JAPAN ) 81 ( 2 ) 024706 2012年02月 [ 査読有り ]

掲載種別: 研究論文（学術雑誌）

The conduction bands of the skutterudite compounds are composed of p orbitals of the pnictogen cages and d orbitals of the transition metal ions. In this paper, we address the effect of hybridization between the d bands and localized 4f states in relation with ordering and fluctuation of multipole moments in Pr-based filled skutterudites. First, we study the properties of a non-degenerate d band around the zone center in the framework of a tight-binding band model. Introducing a realistic form of the d-f hybridization, we show that the a(g) band can be hybridized only with the Gamma(8) orbitals of f electrons. Then, we derive d-f multipolar interactions from the hybridization in the localized limit of f electrons in which the basis states are restricted to the singlet-triplet crystal field states. It is found that the quadrupolar interaction of the O-xy-type becomes predominant in the case of the singlet ground state, whereas the triplet ground state apparently favors the magnetic interaction. The relationship with experimental results in the Pr skutterudites, especially in PrOs4Sb12 and PrFe4As12, is discussed in view of the nature of quadrupolar and magnetic interactions. It is shown that the complex non-collinear magnetic structure of Fe ions observed in the latter material is reproduced well by the effective d-f interaction.

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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）

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）

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）

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）

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）

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）

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）

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）

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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