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渡辺真仁

J-Physics ニュースレター 7 24 - 25 2019年03月

Charge Transfer Effect under Odd-Parity Crystalline Electric Field: Divergence of Magnetic Toroidal Fluctuation in β-YbAlB4 査読有り

Shinji Watanabe, Kazumasa Miyake

Journal of the Physical Society of Japan 88 ( 3 ) 033701-1 - 033701-5 2019年02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）

A novel property of the quantum critical heavy fermion superconductor β-YbAlB4 is revealed theoretically. By analyzing the crystalline electronic field (CEF) on the basis of the hybridization picture, odd parity CEF is shown to exist because of sevenfold configuration of B atoms around Yb, which breaks the local inversion symmetry. This allows onsite admixture of 4f and 5d wavefunctions with a pure imaginary coefficient, giving rise to the magnetic toroidal (MT) degree of freedom. By constructing a realistic minimal model for β-YbAlB4, we show that onsite 4f-5d Coulomb repulsion drives charge transfer between the 4f and 5d states at Yb, which makes the MT fluctuation as well as the electric dipole fluctuation diverge simultaneously with the critical Yb-valence fluctuation at the quantum critical point of the valence transition.

DOI： 10.7566/JPSJ.88.033701

Kyutacar

Scopus

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Grüneisen Parameter and Thermal Expansion near Magnetic Quantum Critical Points in Itinerant Electron Systems 査読有り

Shinji Watanabe, Kazumasa Miyake

Physical Review B 99 ( 99 ) 035108-1 - 035108-25 2019年01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）

Complete expressions of the thermal-expansion coefficient α and the Grüneisen parameter Γ are derived on the basis of the self-consistent renormalization (SCR) theory. By considering the zero point as well as thermal spin fluctuation under the stationary condition, the specific heat for each class of the magnetic quantum critical point (QCP) specified by the dynamical exponent z=3 [feorromagnetism (FM)] and z=2 [antiferromagnetism (AFM)] and the spatial dimension (d=3 and 2) is shown to be expressed as CV=Ca-Cb, where Ca is dominant at low temperatures, reproducing the past SCR criticality endorsed by the renormalization group theory. Starting from the explicit form of the entropy and using the Maxwell relation, α=αa+αb (with αa and αb being related to Ca and Cb, respectively) is derived, which is proven to be equivalent to α derived from the free energy. The temperature-dependent coefficient found to exist in αb, which is dominant at low temperatures, contributes to the crossover from the quantum-critical regime to the Curie-Weiss regime. For sufficiently low temperatures, the thermal-expansion coefficient at the QCP behaves as α≈αb∼T1/3 (3D FM), T1/2 (3D AFM), -lnT (2D FM), and -ln(-lnT)/ln-TlnT (2D AFM). Based on these correctly calculated CV and α, Grüneisen parameter Γ=Γa+Γb is derived, where Γa and Γb contain αa and αb, respectively. The inverse susceptibility (renormalized by the mode-mode coupling of spin fluctuations) coupled to the volume V in Γb gives rise to the divergence of Γ at the QCP for each class even though the characteristic energy scale of spin fluctuation T0 is finite at the QCP, which gives a finite contribution in Γa=-VT0∂T0∂VT=0. For TT0, the Grüneisen parameter at the QCP behaves as Γ≈Γb∼-T-2/3/lnT (3D FM), T-1/2/(const.-T1/2) (3D AFM), -T-2/3lnT (2D FM), and ln(-lnT)/[TlnTln-TlnT] (2D AFM). General properties of α and Γ including their signs as well as the relation to T0 and the Kondo temperature in temperature-pressure phase diagrams of Ce- and Yb-based heavy electron systems are discussed.

DOI： 10.1103/PhysRevB.99.035108

Scopus

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The Dominant Role of Critical Valence Fluctuations on High Tc Superconductivity in Heavy Fermions 査読有り

Gernot W. Scheerer, Zhi Ren, Shinji Watanabe, Gerard Lapertot, Dai Aoki, Didier Jaccard, Kazumasa Miyake

npj Quantum Materials 3 ( 1 ) 41-1 - 41-6 2018年08月

記述言語：英語掲載種別：研究論文（学術雑誌）

Despite almost 40 years of research, the origin of heavy-fermion superconductivity is still strongly debated. Especially, the pressure-induced enhancement of superconductivity in CeCu2Si2 away from the magnetic breakdown is not sufficiently taken into consideration. As recently reported in CeCu2Si2 and several related compounds, optimal superconductivity occurs at the pressure of a valence crossover, which arises from a virtual critical end point at negative temperature Tcr. In this context, we did a meticulous analysis of a vast set of top-quality high-pressure electrical resistivity data of several Ce-based heavy fermion compounds. The key novelty is the salient correlation between the superconducting transition temperature Tc and the valence instability parameter Tcr, which is in line with theory of enhanced valence fluctuations. Moreover, it is found that, in the pressure region of superconductivity, electrical resistivity is governed by the valence crossover, which most often manifests in scaling behavior. We develop the new idea that the optimum superconducting Tc of a given sample is mainly controlled by the compound’s Tcr and limited by non-magnetic disorder. In this regard, the present study provides compelling evidence for the crucial role of critical valence fluctuations in the formation of Cooper pairs in Ce-based heavy fermion superconductors besides the contribution of spin fluctuations near magnetic quantum critical points, and corroborates a plausible superconducting mechanism in strongly correlated electron systems in general.

DOI： 10.1038/s41535-018-0111-6

Scopus

その他リンク： https://www.nature.com/articles/s41535-018-0111-6#article-info

Anomalous temperature dependence of the relaxation rate measured by muon spin rotation in alpha-YbAl0.986Fe0.014B4 査読有り

Kazumasa Miyake, Shinji Watanabe

Physical Review B 98 ( 7 ) 075125-1 - 075125-6 2018年08月

記述言語：英語掲載種別：研究論文（学術雑誌）

Recently, it was reported by MacLaughlin et al. in Phys. Rev. B 93, 214421 (2016)2469-995010.1103/PhysRevB.93.214421 that α-YbAl0.986Fe0.014B4 exhibits an anomalous temperature dependence in the relaxation rate 1/T1 of μSR, and stressed that such temperature dependence cannot be understood by the scenario based on the quantum critical valence transition (QCVT) while this compound exhibits a series of the non-Fermi-liquid behaviors explained by the theory of the QCVT. In this paper, we point out that the anomalous temperature dependence in 1/T1 can be understood semiquantitatively by assuming that the attraction of a screening cloud of conduction electrons about the μ+ induces a local magnetic moment arising from a 4f hole on the Yb ion, giving rise to the Kondo effect between heavy quasiparticles.

DOI： 10.1103/PhysRevB.98.075125

Scopus

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Effects of Crystalline Electronic Field and Onsite Interorbital Interaction in Yb-based Quasicrystal and Approximant Crystal 招待有り査読有り

S. Watanabe and K. Miyake

J. Phys.: Condens. Matter 30 184001-1 - 184001-11 2018年04月

Yb系準結晶・近似結晶の新しい量子臨界現象招待有り

渡辺真仁

J-Physics ニュースレター 5 10 - 13 2018年03月

Grüneisen Parameter and Thermal Expansion by the Self-Consistent Renormalization Theory of Spin Fluctuations 査読有り

S. Watanabe and K. Miyake

J. Phys. Soc. Jpn. 86 034712-1 - 034712-6 2018年02月

Coincidence of Magnetic and Valence Quantum Critical Points in CeRhIn5 under Pressure 査読有り

Z. Ren, G. W. Scheerer, D. Aoki, K. Miyake, S. Watanabe, and D. Jaccard

Phys. Rev. B 96 184524-1 - 184524-5 2017年11月

New quantum criticality revealed under pressure 招待有り査読有り

Watanabe S., Miyake K.

Japanese Journal of Applied Physics 56 ( 56 ) 05FA01-1 - 05FA01-7 2017年05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）

© 2017 The Japan Society of Applied Physics.Unconventional quantum critical phenomena observed in Yb-based periodic crystals such as YbRh2Si2 and β-YbAlB4 have been one of the central issues in strongly correlated electron systems. The common criticality has been discovered in the quasicrystal Yb15Au51Al34, which surprisingly persists under pressure at least up to P = 1.5 GPa. The T/H scaling where the magnetic susceptibility can be expressed as a single scaling function of the ratio of the temperature T to the magnetic field H has been discovered in the quasicrystal, which is essentially the same as that observed in β-YbAlB4. Recently, the T/H scaling as well as the common criticality has also been observed even in the approximant crystal Yb14Au51Al35 under pressure. The theory of critical Yb-valence fluctuation gives a natural explanation for these striking phenomena in a unified way.

DOI： 10.7567/JJAP.56.05FA01

Scopus

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Ubiquity of Unconventional Phenomena Associated with Critical Valence Fluctuations in Heavy Fermion Metals 招待有り査読有り

K. Miyake, S. Watanabe

Philos. Mag. ( 97 ) 3495 - 3516 2017年04月

Lifetime-Broadening-Suppressed X-ray Absorption Spectrum of beta-YbAlB4 Deduced from Yb 3d→2p Resonant X-ray Emission Spectroscopy 査読有り

Naomi Kawamura, Noriko Kanai, Hisashi Hayashi, Yasuhiro H. Matsuda, Masaichiro Mizumaki, Kentaro Kuga, Satoru Nakatsuji, and Shinji Watanabe

Journal of the Physical Society of Japan 86 ( 86 ) 014711-1 - 014711-7 2016年12月

記述言語：英語掲載種別：研究論文（学術雑誌）

In this work, the Yb 3d → 2p (Yb Lα1,2) resonant X-ray emission spectrum of β-YbAlB4 was acquired using excitation energies around the Yb L3-edge, at 2K. Subsequently, the lifetime-broadening-suppressed (LBS) X-ray absorption structure (XAS) spectrum was obtained using the SIM-RIXS program. This spectrum was found to exhibit clearly resolved pre-edge and shoulder structures. Resonant La1 emission spectra were well reproduced from LBS-XAS profiles over wide ranges of excitation and emission energies. In contrast, noticeable discrepancies appeared between the experimental and simulated Lα2 emission spectra, suggesting an effect resulting from M4M5O1 Coster-Kronig transitions. LBS-XAS, in conjunction with partial fluorescence yield (PFY) XAS and transmission XAS, determined a value for the Yb valence (v) in β-YbAlB4 of 2.76 ± 0.08 at 2K. Despite this relatively large uncertainty in v, each method provided a consistent variation in valence (Sv) as the temperature was raised from 2 to 280 K: 0.060 ± 0.004 (LBS-XAS), 0.061 ± 0.005 (PFY-XAS) and 0.058 ± 0.007 (transmission XAS). The smaller δv associated with LBS-XAS demonstrates the greater precision of this method.

DOI： 10.7566/JPSJ.86.014711

Scopus

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周期系・準周期系重い電子系に普遍的な新しい量子臨界物性の解明

渡辺真仁

J-Physics ニュースレター 2 19 - 19 2016年09月

Origin of quantum criticality in Yb-Al-Au approximant crystal and quasicrystal 査読有り

Watanabe S., Miyake K.

Journal of the Physical Society of Japan 85 ( 6 ) 63703-1 - 63703-5 2016年06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）

©2016 The Physical Society of Japan.To get insight into the mechanism of emergence of unconventional quantum criticality observed in quasicrystal Yb15Al34Au51, the approximant crystal Yb14Al35Au51 is analyzed theoretically. By constructing a minimal model for the approximant crystal, the heavy quasiparticle band is shown to emerge near the Fermi level because of strong correlation of 4f electrons at Yb. We find that charge-transfer mode between 4f electron at Yb on the 3rd shell and 3p electron at Al on the 4th shell in Tsai-type cluster is considerably enhanced with almost flat momentum dependence. The modecoupling theory shows that magnetic as well as valence susceptibility exhibits χ ∼ T-0.5 for zero-field limit and is expressed as a single scaling function of the ratio of temperature to magnetic field T/B over four decades even in the approximant crystal when some condition is satisfied by varying parameters, e.g., by applying pressure. The key origin is clarified to be due to strong locality of the critical Yb-valence fluctuation and small Brillouin zone reflecting the large unit cell, giving rise to the extremely-small characteristic energy scale. This also gives a natural explanation for the quantum criticality in the quasicrystal corresponding to the infinite limit of the unit-cell size.

DOI： 10.7566/JPSJ.85.063703

Scopus

CiNii Article

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f電子系準結晶・近似結晶の量子臨界性

渡辺真仁

日本物理学会講演概要集 ( 一般社団法人日本物理学会 ) 71 ( 1 ) 1753 - 1754 2016年03月

Basic Properties of Conductivity and Normal Hall Effect in the Periodic Anderson Model 査読有り

Shinji Watanabe, Kazumasa Miyake

Journal of the Physical Society of Japan ( 85 ) 044704-1 - 044704-16 2016年03月

Quantum Criticality and Emergence of the T/B Scaling in Strongly Correlated Metals 査読有り

Shinji Watanabe, Kazumasa Miyake

Journal of Magnetism and Magnetic Materials ( 400 ) 13 - 16 2016年02月

Thermal Expansion of a Au-Al-Yb Intermediate Valence Quasicrystal 査読有り

T. Watanukia, S. Kashimoto, T. Ishimasa, A. Machida, S. Yamamoto, Y. Tanaka, M. Mizumaki, N. Kawamura, S. Watanabe

Solid State Communications ( 211 ) 19 - 22 2015年03月

Pressure-Induced Valence Crossover and Novel Metamagnetic Behavior near the Antiferromagnetic Quantum Phase Transition of YbNi3Ga9 査読有り

K. Matsubayashi, T. Hirayama, T. Yamashita, S. Ohara, N. Kawamura, M. Mizumaki, N. Ishimatsu, S. Watanabe, K. Kitagawa, Y. Uwatoko

Physical Review Letters 114 086401-1 - 086401-5 2015年02月

High Pressure Properties for Electrical Resistivity and Ce Valence State of Heavy-Fermion Antiferromagnet Ce2NiGa12 査読有り

N Kawamura, R Sasaki, K Matsubayashi, N Ishimatsu, M Mizumaki, Y Uwatoko, S Ohara and S Watanabe

Journal of Physics: Conference Series 568 042015-1 - 042015-6 2014年12月

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