Pohl, T.*; Sun, Y. L.*; Obertelli, A.*; Lee, J.*; Gmez-Ramos, M.*; 緒方 一介*; 吉田 数貴; Cai, B. S.*; Yuan, C. X.*; Brown, B. A.*; et al.
Physical Review Letters, 130(17), p.172501_1 - 172501_8, 2023/04
Sheng, Q.*; 金子 竜也*; Yamakawa, Kohtaro*; Guguchia, Z.*; Gong, Z.*; Zhao, G.*; Dai, G.*; Jin, C.*; Guo, S.*; Fu, L.*; et al.
Physical Review Research (Internet), 4(3), p.033172_1 - 033172_14, 2022/09
A pyrite system NiSSe exhibits a bandwidth controlled Mott transition via (S,Se) substitutions in a two-step process: the antiferromagnetic insulator (AFI) to antiferromagnetic metal (AFM) transition at 0.45 followed by the AFM to paramagnetic metal (PMM) transition at 1.0. Among a few other Mott systems which exhibit similar two-step transitions, Ni(S,Se) is of particular interest because a large intermediate AFM region in the phase diagram would provide unique opportunities to study the interplay between the spin and charge order. By comparing and combining our muon spin relaxation studies and previous neutron scattering studies, here we propose a picture where the spin order is maintained by the percolation of "nonmetallic" localized and dangling Ni moments surrounded by S, while the charge transition from AFI to AFM is caused by the percolation of the conducting paths generated by the Ni-Se-Ni bonds.
Bao, S.*; Wang, W.*; Shangguan, Y.*; Cai, Z.*; Dong, Z.-Y.*; Huang, Z.*; Si, W.*; Ma, Z.*; 梶本 亮一; 池内 和彦*; et al.
Physical Review X, 12(1), p.011022_1 - 011022_15, 2022/02
In the local or itinerant extreme, magnetic excitations can be described by the Heisenberg model which treats electron spins as localized moments, or by the itinerant-electron model where the exchange interaction between electrons leads to unequal numbers of electrons with up and down spins. However, the nature of the magnetic excitations has been elusive when both local moments and itinerant electrons are present in the intermediate range. Using inelastic neutron scattering, we provide direct spectroscopic evidence on the coexistence of and interplay between local moments and itinerant electrons in a van der Waals metallic ferromagnet FeGeTe, which can sustain tunable room-temperature ferromagnetism down to the monolayer limit. We find that there exist ferromagnetic spin-wave excitations dispersing from the zone center at low energies resulting from local moments and a columnlike broad continuum at the zone boundary at high energies up to over 100 meV resulting from itinerant electrons. Unlike the two-dimensional crystal structure, the low-energy mode exhibits a three-dimensional nature, and the high-energy mode also has an out-of-plane dependence. Both modes persist well above the Curie temperature of 160 K. Our neutron spectroscopic data reveal that the low-energy spin waves at 100 K are more coherent than those at 4 K, which is evidence of the weakening of the Kondo screening at high temperatures. These results unambiguously demonstrate the coexistence of local moments and itinerant electrons and the Kondo effect between these two components in FeGeTe. Such behaviors are generally expected in heavy-fermion systems with heavy electrons but are rarely clearly observed in materials with light electrons. These findings shed light on the understanding of magnetism in transition-metal compounds.
Shangguan, Y.*; Bao, S.*; Dong, Z.-Y.*; Cai, Z.*; Wang, W.*; Huang, Z.*; Ma, Z.*; Liao, J.*; Zhao, X.*; 梶本 亮一; et al.
Physical Review B, 104(22), p.224430_1 - 224430_8, 2021/12
Dimerized magnets forming alternating Heisenberg chains exhibit quantum coherence and entanglement and thus can find potential applications in quantum information and computation. However, magnetic systems typically undergo thermal decoherence at finite temperatures. Here, we show inelastic neutron scattering results on an alternating antiferromagnetic-ferromagnetic chain compound NaCuTeO that the excited quasiparticles can counter thermal decoherence and maintain strong correlations at elevated temperatures. At low temperatures, we observe clear dispersive singlet-triplet excitations arising from the dimers formed along the crystalline -axis. The excitation gap is of 18 meV and the bandwidth is about half of the gap. The band top energy has a weak modulation along the  direction, indicative of a small interchain coupling. The gap increases while the bandwidth decreases with increasing temperature, leading to a strong reduction in the available phase space for the triplons. As a result, the Lorentzian-type energy broadening becomes highly asymmetric as the temperature is raised. These results are associated with a strongly correlated state resulting from hard-core constraint and quasiparticle interactions. We consider these results to be not only evidence for strong correlations at finite temperatures in NaCuTeO, but also for the universality of the strongly correlated state in a broad range of quantum magnetic systems.
Yao, Y.*; Cai, R.*; Yang, S.-H.*; Xing, W.*; Ma, Y.*; 森 道康; Ji, Y.*; 前川 禎通; Xie, X.-C.*; Han, W.*
Physical Review B, 104(10), p.104414_1 - 104414_6, 2021/09
We report the experimental observation of half-integer Shapiro steps in the strong ferromagnetic Josephson junction (Nb-NiFe-Nb) by investigating the current-phase relation under radio-frequency microwave excitation. The half-integer Shapiro steps are robust in a wide temperature range from T = 4 to 7 K. The half-integer Shapiro steps could be attributed to co-existence of 0- and -states in the strong ferromagnetic NiFe Josephson junctions with the spatial variation of the NiFe thickness. This scenario is also supported by the high-resolution transmission electron microscopy characterization of the Nb/NiFe/Nb junction.
Lam, T.-N.*; Lee, S. Y.*; Tsou, N.-T.*; Chou, H.-S.*; Lai, B.-H.*; Chang, Y.-J.*; Feng, R.*; 川崎 卓郎; Harjo, S.; Liaw, P. K.*; et al.
Acta Materialia, 201, p.412 - 424, 2020/12
We examined fatigue-crack-growth behaviors of CoCrFeMnNi high-entropy alloys (HEAs) under as-fatigued and tensile-overloaded conditions using neutron-diffraction measurements coupled with diffraction peak-profile analyses. We applied both high-resolution transmission electron microscopy (HRTEM) and neutron-diffraction strain mapping for the complementary microstructure examinations. Immediately after a single tensile overload, the crack-growth-retardation period was obtained by enhancing the fatigue resistance, as compared to the as-fatigued condition. The combined mechanisms of the overload-induced larger plastic deformation, the enlarged compressive residual stresses and plastic-zone size, the crack-tip blunting ahead of the crack tip, and deformation twinning governed the pronounced macroscopic crack-growth-retardation behavior following the tensile overload.
Bao, S.*; Cai, Z.*; Si, W.*; Wang, W.*; Wang, X.*; Shangguan, Y.*; Ma, Z.*; Dong, Z.-Y.*; 梶本 亮一; 池内 和彦*; et al.
Physical Review B, 101(21), p.214419_1 - 214419_8, 2020/06
We perform thermodynamic and inelastic neutron scattering (INS) measurements to study the lattice dynamics (phonons) of a cubic collinear antiferromagnet CuTeO which hosts topological spin excitations (magnons). While the specific heat and thermal conductivity results show that the thermal transport is dominated by phonons, the deviation of the thermal conductivity from a pure phononic model indicates that there is a strong coupling between magnons and phonons. In the INS measurements, we find a mode in the excitation spectra at 4.5 K, which exhibits a slight downward dispersion around the Brillouin zone center. This mode disappears above the Nel temperature and thus cannot be a phonon. Furthermore, the dispersion is distinct from that of a magnon. Instead, it can be explained by the magnon-polaron mode, collective excitations resulting from the hybridization between magnons and phonons. We consider the suppression of the thermal conductivity and emergence of the magnon-polaron mode to be evidence for magnon-phonon coupling in CuTeO.
Cai, Z.*; Bao, S.*; Wang, W.*; Ma, Z.*; Dong, Z.-Y.*; Shangguan, Y.*; Wang, J.*; Ran, K.*; Li, S.*; 蒲沢 和也*; et al.
Physical Review B, 101(13), p.134408_1 - 134408_10, 2020/04
Wang, J.*; Ran, K.*; Li, S.*; Ma, Z.*; Bao, S.*; Cai, Z.*; Zhang, Y.*; 中島 健次; 河村 聖子; ermk, P.*; et al.
Nature Communications (Internet), 10, p.2802_1 - 2802_6, 2019/06
Superconductivity mediated by phonons is typically conventional, exhibiting a momentum-independent -wave pairing function, due to the isotropic interactions between electrons and phonons along different crystalline directions. Here, by performing inelastic neutron scattering measurements on a superconducting single crystal of SrBiSe, a prime candidate for realizing topological superconductivity by doping the topo-logical insulator BiSe, we found that there exist singular phonons, with the linewidths of the acoustic phonons increasing substantially at long wavelengths, but only for those along the  direction. This observation indicates a large and singular electron-phonon coupling at small momenta, which we propose to give rise to the exotic -wave nematic superconducting pairing in the MxBiSe (M = Cu, Sr, Nb) superconductor family. Therefore, we show that these superconductors may be the first examples where electron-phonon interaction can induce more exotic superconducting pairing than the -wave.
Tam, D. M.*; Song, Y.*; Man, H.*; Cheung, S. C.*; Yin, Z.*; Lu, X.*; Wang, W.*; Frandsen, B. A.*; Liu, L.*; Gong, Z.*; et al.
Physical Review B, 95(6), p.060505_1 - 060505_6, 2017/02
We use neutron diffraction and muon spin relaxation to study the effect of in-plane uniaxial pressure on the antiferromagnetic (AF) orthorhombic phase in BaFeTAs and its Co- and Ni-substituted members near optimal superconductivity. In the low-temperature AF ordered state, uniaxial pressure necessary to detwin the orthorhombic crystals also increases the magnetic ordered moment, reaching an 11% increase under 40 MPa for BaFeCoAs, and a 15% increase for BaFeNiAs. We also observe an increase of the AF ordering temperature () of about 0.25 K/MPa in all compounds, consistent with density functional theory calculations that reveal better Fermi surface nesting for itinerant electrons under uniaxial pressure. The doping dependence of the magnetic ordered moment is captured by combining dynamical mean field theory with density functional theory, suggesting that the pressure-induced moment increase near optimal superconductivity is closely related to quantum fluctuations and the nearby electronic nematic phase.
Frandsen, B. A.*; Liu, L.*; Cheung, S. C.*; Guguchia, Z.*; Khasanov, R.*; Morenzoni, E.*; Munsie, T. J. S.*; Hallas, A. M.*; Wilson, M. N.*; Cai, Y.*; et al.
Nature Communications (Internet), 7, p.12519_1 - 12519_8, 2016/08
RENiO (RE = rare-earth element) and VO are archetypal Mott insulator systems. When tuned by chemical substitution (RENiO) or pressure (VO), they exhibit a quantum phase transition (QPT) between an antiferromagnetic Mott insulating state and a paramagnetic metallic state. Because novel physics often appears near a Mott QPT, the details of this transition, such as whether it is first or second order, are important. Here, we demonstrate through muon spin relaxation/rotation experiments that the QPT in RENiO and VO is first order: the magnetically ordered volume fraction decreases to zero at the QPT, resulting in a broad region of intrinsic phase separation, while the ordered magnetic moment retains its full value until it is suddenly destroyed at the QPT. These findings bring to light a surprising universality of the pressure-driven Mott transition, revealing the importance of phase separation and calling for further investigation into the nature of quantum fluctuations underlying the transition.
石井 賢司; 筒井 健二; 池内 和彦*; Jarrige, I.; 水木 純一郎; 平賀 晴弘*; 山田 和芳*; 遠山 貴己*; 前川 禎通; 遠藤 康夫*; et al.
Physical Review B, 85(10), p.104509_1 - 104509_5, 2012/03
We perform a resonant inelastic X-ray scattering (RIXS) study of Ni-substituted LaCuO and Cu-substituted LaNiO.Resonantly enhanced charge-transfer excitations are successfully observed by tuning the incident photon energy to the -edge of the substituted element. These excitations display a very weak momentum dependence, evidencing their local character. We find a shift in the energy of the onset edge of these features compared with the non-substituted compounds LaCuO and LaNiO. This shift is quantitatively reproduced by exact diagonalization calculations of the RIXS spectra. RIXS is shown to be a powerful tool to probe the electronic states of substituted elements and to understand their interaction with surrounding atoms.
石井 賢司; 池内 和彦*; Jarrige, I.; 水木 純一郎; 平賀 晴弘*; 山田 和芳*; 筒井 健二; 遠山 貴己*; 前川 禎通*; 遠藤 康夫*; et al.
Physica C, 470(Suppl.1), p.S155 - S157, 2010/12
Substitution of a specific element for a component is often used to elucidate its role in the superconductors. For a complete understanding of the substitution effect not only on the superconductivity but also on the electronic states, element-selective experiments are required. In this respect, resonant inelastic X-ray scattering (RIXS) has proven to be a powerful experimental technique enabling element-selective diagnosis of charge excitations. By tuning the incident photon energy to a given absorption edge of the element, the excitations involving the element are resonantly enhanced. In addition, RIXS has the advantage that momentum dependence can be measured. Using RIXS, we have measured the electronic excitations at the substituting sites in LaCuNiO and LaNiCuO. Near the absorption edge of the substituting element, namely the Ni -edge for LaCuNiO and the Cu -edge for LaNiCuO, charge transfer excitations from the 2 level of neighboring oxygen to the upper Hubbard band of the substituted Ni/Cu are observed. Experimental spectra are qualitatively reproduced by a theoretical calculation based on a - model.
Jarrige, I.; Cai, Y. Q.*; 石井 啓文*; 平岡 望*; Bleuzen, A.*
Applied Physics Letters, 93(5), p.054101_1 - 054101_3, 2008/08
Charge-transfer excitation is at the source of the photoinduced magnetism observed in several Prussian blue molecule-based magnets. Using resonant inelastic X-ray scattering and X-ray absorption spectroscopy, we probe directly the thermally-activated charge transfer in a photomagnetic Fe-Co cyanide, CsCo[Fe(CN)][V]. 16HO, where [V] represents [Fe(CN)] vacancies. The temperature dependence of both Co and Fe valence ratios is estimated for the first time in one cooling run, thus yielding a more complete picture of the temperature-induced cooperative electronic modifications. This novel approach, benefiting from relatively short acquisition times, opens the possibility for realtime characterization of the photoinduced magnetism in molecule-based magnets.
Jarrige, I.; Rueff, J.-P.*; Shieh, S.*; 田口 宗孝*; 大石 泰生*; 松村 隆*; 石井 啓文*; 平岡 望*; Cai, Y.*
no journal, ,
The pressure-induced mixed-valent state in TmTe is investigated up to 10.6 GPa using resonant inelastic X-ray scattering (RIXS) and the Anderson impurity model (AIM). The volume collapse transition (VCT) which takes place over the 2-4.3 GPa range is accompanied by a strong increase in the 4-5 hybridization. The valence in the collapsed state is found to be 2.5, showing that the VCT is inconsistent with the commonly accepted picture of Tm to Tm conversion. At 6 GPa, a transition in both magnetic and electronic ground states occurs at constant valence and hybridization. This could be indicative of a non-trivial instability of the 4 ground state such as a non Fermi liquid behavior.
Jarrige, I.; Rueff, J.-P.*; Shieh, S.*; Wang, J.*; 松村 隆*; 石井 啓文*; 平岡 望*; Cai, Y.*
no journal, ,
The pressure-induced mixed-valent state in TmTe is investigated up to 10.6 GPa using resonant inelastic X-ray scattering (RIXS) and the Anderson impurity model (AIM). The volume collapse transition (VCT) which takes place over the 24.3 GPa range is accompanied by a strong increase in the 4f-5d hybridization. The valence in the collapsed state is found to be 2.5, showing that the VCT is inconsistent with the commonly accepted picture of Tm to Tm conversion. At 6 GPa, a transition in both magnetic and electronic ground states occurs at constant valence and hybridization. This could be indicative of a non-trivial instability of the 4f ground state such as a non Fermi liquid behavior.
Jarrige, I.; Shieh, S.*; 石井 啓文*; 平岡 望*; Cai, Y.*
no journal, ,
Here we report on a spectroscopic investigation of the intricate interplays between electronic, magnetic and structural properties of LaMnO across its pressure-induced insulator to metal transition (IMT) at 32 GPa. We measure the Mn K-beta fluorescence line by X-ray emission spectroscopy (XES) and the Mn K-egde by X-ray absorption spectroscopy in the partial fluorescence yield mode (PFY-XAS). The XES spectra disclose a high-spin to low-spin transition accompanying the IMT that points to strongly interwoven charge and spin degrees of freedom, which is reminiscent of the Mott perspective. On the other hand, close insight is gained on the structure from the Mn K pre-edge region, where evidence is found for a complete suppression of the Jahn-Teller distortions at the IMT. This is congruent with the scenario that the gap closure results from the progressive pressure-induced degeneracy of the majority-spin eg band, thus suggesting that the IMT in LaMnO is not a plain Mott transition.
石井 賢司; 池内 和彦; Jarrige, I.; 水木 純一郎; 平賀 晴弘*; 山田 和芳*; 筒井 健二; 遠山 貴己*; 前川 禎通*; 石井 啓文*; et al.
no journal, ,
これまでに高温超伝導体のCuをNiに置換した系は非常に数多くの研究がなされてきた。その多くはCu()をNi()に置換したと考え、Niを磁性不純物として着目したものであったが、Ni置換は電子状態にも何らかの影響があると予想される。今回、共鳴非弾性X線散乱(RIXS)の元素選択性を利用することによりNi周りの電子状態を抽出することに成功したのでその結果について報告する。本研究で用いた吸収端のRIXSでは、CuまたはNiの-遷移に対応するエネルギーのX線を利用することで、電荷ギャップを越える励起が観測できる。例えば、Niの吸収端でLaCuNiOの測定を行うと、中間状態で生成されたNiの正孔を遮蔽するために周りの酸素軌道からNiの軌道へ電子が遷移する。酸素軌道にできたホールはCuとZhang-Rice siglet(ZRS)を形成するので、ZRSとNi()が存在するような励起状態を観測することになる。同様に、LaCuOをCuの吸収端で測定すると、ZRSとCu()が生じる励起状態が観測される。今回、LaCuNiOのの測定を行い、既に報告されているのスペクトルと比較することで、Ni, Cuそれぞれに特徴的な電子状態の議論を行う。
Jarrige, I.; Rueff, J.-P.*; Shieh, S.*; 田口 宗孝*; 石井 啓文*; Cai, Y. Q.*
no journal, ,
Identifying the mechanism of Kondo phenomena is arguably one of the most exciting and widely studied issues in condensed matter physics. Intermediate-valent Tm compounds stand out as potential candidates for multichannel Kondo (ncK) effect, since both integer valences Tm and Tm are magnetic. Using resonant inelastic X-ray scattering and the Anderson impurity model, we probed the Tm valence and 4f-5d hybridization in the high-pressure intermediate-valent TmTe compound. Our results reveal that the f delocalization is discontinuous, characterized by a previously unobserved plateau-like behavior over the 4.36.5 GPa range. This is at odds with the usual, continuous Kondo-like f delocalization, and is interpreted as an experimental observation of the effect of culminating ncK scattering of d electrons from local moments on the electronic structure.
Jarrige, I.; Rueff, J.-P.*; Cai, Y. Q.*; Shieh, S.*; 石井 啓文*; 田口 宗孝*
no journal, ,
Among numerous f electron systems which present Kondo properties, intermediate-valent Tm compounds stand out as potential candidates for multichannel Kondo (ncK) effect, since both integer valences Tm2+ and Tm3+ are magnetic. Using resonant inelastic X-ray scattering and the Anderson impurity model, we probed the Tm valence and 4f-5d hybridization in the high-pressure intermediate-valent TmTe compound. Our results reveal that the f delocalization is discontinuous, characterized by a previously unobserved plateau-like behavior between the volume collapse transition and the structural transition (4.36.5 GPa), with an associated valence (2.55) at odds with the near-trivalency previously reported. This is at odds with the usual, continuous Kondo-like f delocalization, and is interpreted as an experimental observation of the effect of culminating ncK scattering of d electrons from local moments on the electronic structure.