研究者詳細 - 渡辺　真仁

論文 - 渡辺　真仁

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Dynamical and static structure factors in hedgehog-antihedgehog order in an icosahedral 1/1 approximant crystal 査読有り国際誌

Shinji Watanabe

Physical Review B 109 184404-1 - 184404-7 2024年05月

Magnetic Dynamics and Nonreciprocal Excitation in Uniform Hedgehog Order in Icosahedral 1/1 Approximant Crystal 査読有り国際誌

Shinji Watanabe

Scientific Reports 13 14438-1 - 14438-11 2023年09月

Crystalline electric field and magnetic anisotropy in Dy-based icosahedral quasicrystal and approximant 査読有り国際誌

Shinji Watanabe, Tatsuya Iwasaki

Physical Review B 108 045110-1 - 045110-6 2023年07月

Magnetism and topological property in icosahedral quasicrystal 査読有り国際誌

Shinji Watanabe

Journal of Physics: Conference Series 2461 012011-1 - 012011-8 2023年03月

Temperature Dependence of Yb Valence in an Au–Al–Yb Intermediate-Valence Quasicrystal Investigated by Yb L3-Edge X-ray Absorption Near-Edge Structure Spectroscopy 査読有り国際誌

Tetsu Watanuki, Masaichiro Mizumaki, Shinji Watanabe, Naomi Kawamura, Kiyofumi Nitta, Akihiko Machida, Tsutomu Ishimasa

MATERIALS TRANSACTIONS ( 公益社団法人日本金属学会 ) 63 ( 10 ) 1380 - 1383 2022年09月

記述言語：英語掲載種別：研究論文（学術雑誌）

In this paper, we investigate the temperature dependence of Yb valence in an Au-Al-Yb intermediate-valence quasicrystal by Yb L3-edge synchrotron X-ray absorption near-edge structure spectroscopy. The Yb valence of 2.61+ at room temperature decreases monotonically as the temperature decreases, indicating that the Yb ionic radius increases as the temperature decreases and is consistent with the anomalous suppression of the thermal shrinkage of this quasicrystal by cooling, as previously reported. The Yb valence changes by -0.053 on cooling from 298 to 2 K, and its change is more gradual on the high-temperature side than on the low-temperature side with the boundary around 200 K. Additionally, the effect of the changes in the Yb valence on the volume is quantitatively examined. The ratio of the volume changes due to the change in Yb valence per Yb atom, ΔVYb, to the amount of change in Yb valence, Δν, which is expressed as ΔVYb/Δν, was evaluated. The value of ΔVYb/Δν was estimated to be -2.8 Å3/unit Yb valence/Yb atom, whose value was close to that evaluated for YbInCu4

DOI： 10.2320/matertrans.MT-M2022064

Scopus

CiNii Research

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Magnetic dynamics of hedgehog in icosahedral quasicrystal 査読有り国際誌

Shinji Watanabe

Scientific Reports 12 ( 1 ) 15514-1 - 15514-8 2022年09月

担当区分：筆頭著者,　責任著者記述言語：英語掲載種別：研究論文（学術雑誌）

Quasicrystals (QCs) possess a unique lattice structure without translational invariance, which is characterized by the rotational symmetry forbidden in periodic crystals such as the 5-fold rotation. Recent discovery of the ferromagnetic (FM) long-range order in the terbium-based QC has brought about breakthrough but the magnetic structure and dynamics remain unresolved. Here, we reveal the dynamical as well as static structure of the FM hedgehog state in the icosahedral QC. The FM hedgehog is shown to be characterized by the triple-Q state in the reciprocal-lattice q space. Dynamical structure factor is shown to exhibit highly structured q and energy dependences. We find a unique magnetic excitation mode along the 5-fold direction exhibiting the streak fine structure in the q-energy plane, which is characteristic of the hedgehog in the icosahedral QC. Non-reciprocal magnetic excitations are shown to arise from the FM hedgehog order, which emerge in the vast extent of the q-energy plane.

DOI： 10.1038/s41598-022-19870-6

Kyutacar

Scopus

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Magnetic dynamics of ferromagnetic long range order in icosahedral quasicrystal 査読有り国際誌

Shinji Watanabe

Scientific Reports 12 ( 1 ) 10792-1 - 10792-9 2022年06月

担当区分：筆頭著者,　責任著者記述言語：英語掲載種別：研究論文（学術雑誌）

Quasicrystals lack translational symmetry and have unique lattice structures with rotational symmetry forbidden in periodic crystals. The electric state and physical property are far from complete understanding, which are the frontiers of modern matter physics. Recent discovery of the ferromagnetic long-range order in the rare-earth based icosahedral quasicrystal has made the breakthrough. Here, we first reveal the dynamical as well as static magnetic structure in the ferromagnetic long-range order in the terbium-based quasicrystal. The dynamical structure factor exhibits highly structured energy and wavenumber dependences beyond the crystalline-electric-field excitation. We find the presence of the magnetic excitation mode analog to magnon with unique hierarchical structure as well as the localized magnetic excitation with high degeneracy in the quasicrystal. Non-collinear and non-coplanar magnetic structure on the icosahedron is discovered to give rise to non-reciprocal magnetic excitation in the quasicrystal as well as non-reciprocal magnon in the periodic cubic 1/1 approximant. These findings afford illuminating insight into the magnetic dynamics in the broad range of the rare-earth-based quasicrystals and approximants.

DOI： 10.1038/s41598-022-14796-5

Kyutacar

Scopus

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Pressure dependence of the magnetic ground state in CePtSi2 査読有り国際誌

S. E. Dissanayake, F. Ye, W. Tian, M. Matsuda, H. Muto, S. Suzuki, T. Nakano, S. Watanabe, J. Gouchi, and Y. Uwatoko

Physical Review B 105 ( 24 ) 245111-1 - 245111-6 2022年06月

記述言語：英語掲載種別：研究論文（学術雑誌）

CePtSi2 was reported to exhibit an antiferromagnetic order below T∗=1.8 K at ambient pressure, a valence state change at ∼1.2 GPa, and superconductivity in the range between 1.4 and 2.1 GPa with a maximum transition temperature of 0.14 K [T. Nakano, Phys. Rev. B 79, 172507 (2009)1098-012110.1103/PhysRevB.79.172507]. We have performed polycrystalline and single-crystal neutron diffraction experiments to determine the magnetic structure under ambient and high pressures. We found incommensurate magnetic peaks with a magnetic propagation vector of (0.32,0,0.11) at ambient pressure below TSDW∼1.25 K. Those magnetic peaks originate from a spin-density-wave order with the easy axis along the c axis and an averaged ordered moment of 0.45(5)μB, suggesting that there may be an intermediate phase between T∗ and TSDW. Applying pressures, the magnetic propagation vector shows no change and the magnetic order disappears around 1.0 GPa, which is much lower than the critical pressure for the superconducting phase. The results suggest that other than magnetic fluctuations may play a primary role in the superconducting pairing mechanism.

DOI： 10.1103/PhysRevB.105.245111

Scopus

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Topological magnetic textures and long-range orders in terbium-based quasicrystal and approximant 査読有り国際誌

Shinji Watanabe

Proceedings of the National Academy of Sciences of the United States of America 118(43) ( 43 ) e2112202118-1 - e2112202118-5 2021年10月

担当区分：筆頭著者記述言語：英語掲載種別：研究論文（学術雑誌）

The quasicrystal (QC) possesses a unique lattice structure with rotational symmetry forbidden in periodic crystals. The electric property is far from complete understanding. It has been a longstanding issue whether magnetic long-range order is realized in the QC. Here, we report our theoretical discovery of the ferromagnetic long-range order in the Tb-based QC. The difficulty in past theoretical studies on the QC was lack of the microscopic theory of the crystalline electric field (CEF), which is crucially important in the rare earth systems. By analyzing the CEF in the Tb-based QC, we clarify that magnetic anisotropy plays a key role in realizing unique magnetic textures in the Tb-based QC and approximant crystal (AC). By constructing the minimal model, we show that various magnetic textures on the icosahedron, at whose vertices Tb atoms are located, are realized. We find that the hedgehog state is characterized by the topological charge of one and the whirling-moment state is characterized by an unusually large topological charge of three. The hedgehog and whirling-moment states are shown to be realized as antiferromagnetic orders transcribed as the emergent monopole and antimonopole in the 1/1 AC. We find that these states exhibit the topological Hall effect under applied magnetic field accompanied by the topological as well as metamagnetic transition. Our model and the determined phase diagram are expected to be relevant to the broad range of the rare earth–based QCs and ACs with strong magnetic anisotropy, which are useful not only to understand magnetism but also, to explore topological properties.

DOI： 10.1073/pnas.2112202118

Scopus

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Magnetism and topology in Tb-based icosahedral quasicrystal 査読有り国際誌

Shinji Watanabe

Scientific Reports 11 ( 11 ) 17679-1 - 17679-8 2021年09月

担当区分：筆頭著者記述言語：英語掲載種別：研究論文（学術雑誌）

Quasicrystal (QC) possesses a unique lattice structure with rotational symmetry forbidden in conventional crystals. The electric property is far from complete understanding and it has been a long-standing issue whether the magnetic long-range order is realized in the QC. The main difficulty was lack of microscopic theory to analyze the effect of the crystalline electric field (CEF) at the rare-earth atom in QCs. Here we show the full microscopic analysis of the CEF in Tb-based QCs. We find that magnetic anisotropy arising from the CEF plays a key role in realizing unique magnetic textures on the icosahedron whose vertices Tb atoms are located at. Our analysis of the minimal model based on the magnetic anisotropy suggests that the long-range order of the hedgehog characterized by the topological charge of one is stabilized in the Tb-based QC. We also find that the whirling-moment state is characterized by unusually large topological charge of three. The magnetic textures as well as the topological states are shown to be switched by controlling compositions of the non-rare-earth elements in the ternary compounds. Our model is useful to understand the magnetism as well as the topological property in the rare-earth-based QCs and approximant crystals.

DOI： 10.1038/s41598-021-97024-w

Kyutacar

Scopus

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Elastic Softening in Quantum Critical Yb-Al-Au Approximant Crystal and Quasicrystal 査読有り国際誌

Shinji Watanabe

Solid State Communications 336 ( 336 ) 114425-1 - 114425-6 2021年06月

担当区分：筆頭著者記述言語：英語掲載種別：研究論文（学術雑誌）

The elastic property of quantum critical quasicrystal (QC) Yb15Al34Au51 is analyzed theoretically on the basis of the approximant crystal (AC) Yb14Al35Au51. By constructing the realistic effective model in the AC, we evaluate the 4f-5d Coulomb repulsion at Yb as Ufd≈1.46eV realizing the quantum critical point (QCP) of the Yb-valence transition. The RPA analysis of the QCP shows that softening in elastic constants occurs remarkably for bulk modulus and longitudinal mode at low temperatures. Possible relevance of these results to the QC as well as the pressure-tuned AC is discussed.

DOI： 10.1016/j.ssc.2021.114425

Scopus

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Crystalline Electronic Field in Rare-Earth Based Quasicrystal and Approximant: Analysis of Quantum Critical Au-Al-Yb Quasicrystal and Approximant 査読有り

Shinji Watanabe, Mina Kawamoto

J. Phys. Soc. Jpn. 90 ( 6 ) 063701-1 - 063701-5 2021年04月

担当区分：筆頭著者記述言語：英語掲載種別：研究論文（学術雑誌）

On the basis of the point charge model, we formulate the crystalline electronic field (CEF) Hamiltonian HCEF in rareearth based quasicrystal (QC) and approximant crystal (AC) with ligand ions located at pseudo 5-fold configurations by using the operator equivalent method. By setting the total angular momentum J = 7=2, the CEF in the quantum critical QC Au51Al34Yb15 and the 1=1 AC Au51Al35Yb14 is analyzed with consideration for the effect of Al=Au mixed sites. We find that the ratio of the valences of ligand ions x = ZAl=ZAu plays an important role in characterizing the CEF ground state. As x decreases from x = 3, the 4f wave function of the CEF ground state with the flat shape lying in the mirror plane is deformed around x ≈ 0.8 to the flat shape perpendicular to the pseudo 5-fold axis at x = 0. The formulated HCEF by J is generally applicable to rare-earth-based QCs and ACs, which is useful to analyze the CEF.

DOI： 10.7566/JPSJ.90.063701

Kyutacar

Scopus

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Interplay of Valence and Semimetal-to-Insulator Transitions in SmS 査読有り

Shinji Watanabe

J. Phys. Soc. Jpn. 90 ( 2 ) 023706-1 - 023706-5 2021年01月

担当区分：筆頭著者記述言語：英語掲載種別：研究論文（学術雑誌）

Recent discoveries of a new type of quantum criticality arising from Yb-valence fluctuations in Yb-based metal in periodic crystal and quasicrystal have opened a new class of quantum critical phenomena in correlated electron systems. To clarify whether this new concept can be generalized to other rare-earth-based semimetal and insulator, we study SmS which exhibits golden-black phase transition under pressure. By constructing the model for SmS, we show that Coulomb repulsion between 4 f and 5d orbitals at Sm drives first-order valence transition (FOVT) and semimetal-to-insulator transition (MIT) simultaneously, which explains the golden-black phase transition. We clarify the ground-state phase diagram for the FOVT and MIT by identifying the quantum critical point of the FOVT. We find that exciton condensates in both semimetal and insulator phases. Our result explains measured peak anomalies in the specific heat and compressibility in pressurized golden SmS and provides a cue to clarify recently-observed anomalies in black SmS.

DOI： 10.7566/JPSJ.90.023706

Kyutacar

Scopus

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Ultrasound Response in Quantum Critical β-YbAlB4 and α-YbAl0.986Fe0.014B4 査読有り

Shinji Watanabe

J. Phys. Soc. Jpn. 89 ( 7 ) 073702-1 - 073702-5 2020年06月

担当区分：筆頭著者記述言語：英語掲載種別：研究論文（学術雑誌）

We analyze the key origin of quantum valence criticality in the heavy electron metal β-YbAlB4 evidenced in the sister compound α-YbAl0.986Fe0.014B4. By constructing a realistic canonical model for β-YbAlB4, we evaluate Coulomb repulsion between the 4f and 5d electrons at Yb Ufd≈6.2 eV realizing the quantum critical point (QCP) of the Ybvalence transition. To reveal the Yb 5d contribution to the quantum critical state, we propose ultrasound measurement. We find that softening of elastic constants of not only the bulk modulus but also the shear moduli is caused by electric quadrupole fluctuations enhanced by critical 4f and 5d charge fluctuations for low temperatures at the valence QCP. Possible relevance of these results to β-YbAlB4 and also α-YbAl1-xFexB4 is discussed.

DOI： 10.7566/JPSJ.89.073702

Kyutacar

Scopus

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Theory for Non-Fermi Liquid Temperature Dependence in Resistivity of CexLa1-xCu5.62Au0.38 (x=0.02-0.10) on the Local Quantum Valence Criticality of Ce Impurities 査読有り

K. Miyake, S. Watanabe

JPS Conf. Proc. 30 011027-1 - 011027-7 2020年03月

Quantum Valence Criticality in Heavy Fermions on Periodic and Aperiodic Crystals 査読有り

S. Watanabe, K. Miyake

JPS Conf. Proc. 30 011023-1 - 011023-12 2020年03月

価数転移に由来する新しい量子現象査読有り

渡辺真仁

J-Physics ニュースレター 9 2020年03月

磁気量子臨界点近傍の比熱、熱膨張係数、グリュナイゼンパラメーター招待有り査読有り

渡辺真仁

J-Physics ニュースレター 9 2020年03月

Charge Transfer Effect under Odd-Parity Crystalline Electric Field 査読有り

S. Watanabe, K. Miyake

JPS Conf. Proc. 29 014001-1 - 014001-10 2020年02月

Non-Divergent Grüneisen Parameter in Quantum Critical Quasicrystal Yb15Al34Au51: Reflection of Robustness of Quantum Criticality under Pressure 査読有り国際誌

Shinji Watanabe, Kazumasa Miyake

Solid State Communications ( ELSEVIER ) 306 113774-1 - 113774-6 2019年11月

担当区分：責任著者記述言語：英語掲載種別：研究論文（学術雑誌）

The mechanism of not diverging Grüneisen parameter in the quantum critical heavy-fermion quasicrystal (QC) Yb15Al34Au51 is analyzed. We construct the formalism for calculating the specific heat CV(T), the thermal-expansion coefficient α(T), and the Grüneisen parameter Γ(T) near the quantum critical point of the Yb valence transition. By applying the framework to the QC, we calculate CV(T), α(T), and Γ(T), which explains the measurements. Not diverging Γ(T) is attributed to the robustness of the quantum criticality in the QC under pressure. The difference in Γ(T) at the lowest temperature between the QC and approximant crystal is shown to reflect the difference in the volume derivative of characteristic energy scales of the critical Yb-valence fluctuation and the Kondo temperature. Possible implications of our theory to future experiments are also discussed.

DOI： 10.1016/j.ssc.2019.113774

Kyutacar

Scopus

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