Youichi Murakami

High Energy Accelerator Research Organization

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Youichi Murakami has a total of 1000 co-authors. Here are the top co-authors by number of shared publications:

Hironori Nakao
Photon Factory, Institute of Materials Structure Science, High Energy Accelerator Research Organization, Tsukuba, Ibaraki 305-0801, Japan
138 shared publications
Reiji Kumai
Institute of Materials Structure Science, High Energy Accelerator Research Organization (KEK) 1-1 Oho
54 shared publications
Y. Tokura
Department of Applied Physics and Quantum Phase Electronics Center, University of Tokyo, Tokyo 113-8656, Japan
33 shared publications
Kenji Ishii
National Institutes for Quantum Science and Technology
28 shared publications
Yuuichi Yamasaki
National Institute for Materials Science
27 shared publications
Yusuke Wakabayashi
Tohoku University
26 shared publications

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Cited by 10
- 4 December 2020
- Published by American Physical Society (APS) in Physical Review B
Magnetism induced by interlayer electrons in the quasi-two-dimensional electride $Y_{2} C$ : Inelastic neutron scattering study
by Hiromu Tamatsukuri, Youichi Murakami, Yoshio Kuramoto, Hajime Sagayama, Masato Matsuura, Yukinobu Kawakita, Satoru Matsuishi, Yasuhito Washio, Takeshi Inoshita, Noriaki Hamada, et al.
Magnetic excitations in layered electride $Y_{2} C$ have been found by inelastic neutron scattering. We have observed weak but clear magnetic scattering around the wave number $Q = 0$ , but no magnetic order down to the lowest temperature measured (7 K). The imaginary part of the dynamical susceptibility deduced is well described by the Lorentz function of energy $E$ for each momentum $Q$ . The width $Γ (Q)$ of the Lorentzian is proportional to $Q (Q^{2} + κ^{2})$ with $κ^{- 1} \sim 4 Å$ at $T = 7$ K. We have also found that with increasing $Q$ the magnetic form factor decays faster than that of a $4 d$ electron in a single Y atom, which indicates a more extended magnetic moment in $Y_{2} C$ . These results provide experimental evidence that the itinerant magnetism in $Y_{2} C$ originates from the anionic electrons that reside in the interlayers. The Curie-Weiss-like behavior of the magnetic susceptibility reported in $Y_{2} C$ is ascribed to the mode coupling effects of spin fluctuations.
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- 4 December 2020
- Published by American Physical Society (APS) in Physical Review B
Magnetism induced by interlayer electrons in the quasi-two-dimensional electride $Y_{2} C$ : Inelastic neutron scattering study
by Hiromu Tamatsukuri, Youichi Murakami, Yoshio Kuramoto, Hajime Sagayama, Masato Matsuura, Yukinobu Kawakita, Satoru Matsuishi, Yasuhito Washio, Takeshi Inoshita, Noriaki Hamada, et al.
Magnetic excitations in layered electride $Y_{2} C$ have been found by inelastic neutron scattering. We have observed weak but clear magnetic scattering around the wave number $Q = 0$ , but no magnetic order down to the lowest temperature measured (7 K). The imaginary part of the dynamical susceptibility deduced is well described by the Lorentz function of energy $E$ for each momentum $Q$ . The width $Γ (Q)$ of the Lorentzian is proportional to $Q (Q^{2} + κ^{2})$ with $κ^{- 1} \sim 4 Å$ at $T = 7$ K. We have also found that with increasing $Q$ the magnetic form factor decays faster than that of a $4 d$ electron in a single Y atom, which indicates a more extended magnetic moment in $Y_{2} C$ . These results provide experimental evidence that the itinerant magnetism in $Y_{2} C$ originates from the anionic electrons that reside in the interlayers. The Curie-Weiss-like behavior of the magnetic susceptibility reported in $Y_{2} C$ is ascribed to the mode coupling effects of spin fluctuations.
Cited by 10
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Cited by 20
- 5 August 2020
- Published by American Physical Society (APS) in Physical Review B
Metamagnetic transitions and magnetoelectric responses in the chiral polar helimagnet ${Ni}_{2} InSb O_{6}$
by Y. Araki, T. Sato, Y. Fujima, N. Abe, M. Tokunaga, S. Kimura, D. Morikawa, V. Ukleev, Y. Yamasaki, C. Tabata, et al.
In this study, the magnetic field effect was investigated on the magnetic and electric properties of a chiral-polar-ordered corundum, ${Ni}_{2} InSb O_{6}$ . Single-crystal soft x-ray and neutron diffraction measurements were used to verify the magnetic modulations with long wavelengths. The modulation direction tends to align along the magnetic field when it is applied perpendicular to the polar axis, suggesting that the nearly proper-screw-type helicoid should be formed below 77 K owing to the dominant activation of chirality-induced uniform Dzyaloshinskii-Moriya interactions despite the existence of both chirality and polarity. When a high magnetic field is applied perpendicular to the polar axis, a helix-to-canted antiferromagnetic transition is observed through the intermediate soliton lattice type state. In contrast, a magnetic field applied along the polar axis induces a first-order metamagnetic transition. These metamagnetic transitions accompany a change in the electric polarization along the polar axis.
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- 5 August 2020
- Published by American Physical Society (APS) in Physical Review B
Metamagnetic transitions and magnetoelectric responses in the chiral polar helimagnet ${Ni}_{2} InSb O_{6}$
by Y. Araki, T. Sato, Y. Fujima, N. Abe, M. Tokunaga, S. Kimura, D. Morikawa, V. Ukleev, Y. Yamasaki, C. Tabata, et al.
In this study, the magnetic field effect was investigated on the magnetic and electric properties of a chiral-polar-ordered corundum, ${Ni}_{2} InSb O_{6}$ . Single-crystal soft x-ray and neutron diffraction measurements were used to verify the magnetic modulations with long wavelengths. The modulation direction tends to align along the magnetic field when it is applied perpendicular to the polar axis, suggesting that the nearly proper-screw-type helicoid should be formed below 77 K owing to the dominant activation of chirality-induced uniform Dzyaloshinskii-Moriya interactions despite the existence of both chirality and polarity. When a high magnetic field is applied perpendicular to the polar axis, a helix-to-canted antiferromagnetic transition is observed through the intermediate soliton lattice type state. In contrast, a magnetic field applied along the polar axis induces a first-order metamagnetic transition. These metamagnetic transitions accompany a change in the electric polarization along the polar axis.
Cited by 20
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Cited by 6
- 29 June 2020
- Published by American Physical Society (APS) in Physical Review B
Electronic charge transfer driven by spin cycloidal structure
by Y. Ishii, S. Horio, Y. Noda, M. Hiraishi, H. Okabe, M. Miyazaki, S. Takeshita, A. Koda, K. M. Kojima, R. Kadono, et al.
Muon spin rotation and resonant soft x-ray scattering experiments on prototype multiferroics $R {Mn}_{2} O_{5}$ ( $R$ = Y, Sm) are used to demonstrate that the local electric displacements are driven by the spin-current (SC) mechanism. Small local electric displacements were evaluated by observing spin polarization at ligand O ions, for which implanted muons served as an extremely sensitive probe. Our results for ${YMn}_{2} O_{5}$ provide evidence that the spin polarization of O ions forming a spin cycloid chain with Mn spins increases in proportion to the vector spin chirality ( $S_{i} \times S_{j}$ ) of the Mn ions. This relationship strongly indicates that the charge transfer between O and Mn ions is driven by the SC mechanism, which leads to the ferroelectricity accompanying O spin polarization.
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- 29 June 2020
- Published by American Physical Society (APS) in Physical Review B
Electronic charge transfer driven by spin cycloidal structure
by Y. Ishii, S. Horio, Y. Noda, M. Hiraishi, H. Okabe, M. Miyazaki, S. Takeshita, A. Koda, K. M. Kojima, R. Kadono, et al.
Muon spin rotation and resonant soft x-ray scattering experiments on prototype multiferroics $R {Mn}_{2} O_{5}$ ( $R$ = Y, Sm) are used to demonstrate that the local electric displacements are driven by the spin-current (SC) mechanism. Small local electric displacements were evaluated by observing spin polarization at ligand O ions, for which implanted muons served as an extremely sensitive probe. Our results for ${YMn}_{2} O_{5}$ provide evidence that the spin polarization of O ions forming a spin cycloid chain with Mn spins increases in proportion to the vector spin chirality ( $S_{i} \times S_{j}$ ) of the Mn ions. This relationship strongly indicates that the charge transfer between O and Mn ions is driven by the SC mechanism, which leads to the ferroelectricity accompanying O spin polarization.
Cited by 6
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Cited by 16
- 6 May 2020
- Published by American Physical Society (APS) in Physical Review B
Field-induced spin reorientation in the antiferromagnetic Dirac material ${EuMnBi}_{2}$ revealed by neutron and resonant x-ray diffraction
by H. Masuda, H. Sakai, H. Takahashi, Y. Yamasaki, A. Nakao, T. Moyoshi, H. Nakao, Y. Murakami, T. Arima, S. Ishiwata
Field-dependent magnetic structures of a layered Dirac material ${EuMnBi}_{2}$ were investigated in detail by the single crystal neutron diffraction and the resonant x-ray magnetic diffraction techniques. On the basis of the reflection conditions in the antiferromagnetic phase at zero field, the Eu moments were found to be ordered ferromagnetically within the $a b$ plane and stacked antiferromagnetically along the $c$ axis in the sequence of up-up-down-down. Upon the spin-flop transition under the magnetic field parallel to the $c$ axis, the Eu moments are reoriented from the $c$ to the $a$ or $b$ directions forming two kinds of spin-flop domains, whereas the antiferromagnetic structure of the Mn sublattice remains intact as revealed by the quantitative analysis of the change in the neutron diffraction intensities. The present study provides a concrete basis to discuss the dominant role of the Eu sublattice on the enhanced two-dimensionality of the Dirac fermion transport in ${EuMnBi}_{2}$ .
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- 6 May 2020
- Published by American Physical Society (APS) in Physical Review B
Field-induced spin reorientation in the antiferromagnetic Dirac material ${EuMnBi}_{2}$ revealed by neutron and resonant x-ray diffraction
by H. Masuda, H. Sakai, H. Takahashi, Y. Yamasaki, A. Nakao, T. Moyoshi, H. Nakao, Y. Murakami, T. Arima, S. Ishiwata
Field-dependent magnetic structures of a layered Dirac material ${EuMnBi}_{2}$ were investigated in detail by the single crystal neutron diffraction and the resonant x-ray magnetic diffraction techniques. On the basis of the reflection conditions in the antiferromagnetic phase at zero field, the Eu moments were found to be ordered ferromagnetically within the $a b$ plane and stacked antiferromagnetically along the $c$ axis in the sequence of up-up-down-down. Upon the spin-flop transition under the magnetic field parallel to the $c$ axis, the Eu moments are reoriented from the $c$ to the $a$ or $b$ directions forming two kinds of spin-flop domains, whereas the antiferromagnetic structure of the Mn sublattice remains intact as revealed by the quantitative analysis of the change in the neutron diffraction intensities. The present study provides a concrete basis to discuss the dominant role of the Eu sublattice on the enhanced two-dimensionality of the Dirac fermion transport in ${EuMnBi}_{2}$ .
Cited by 16
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Cited by 24
- 15 April 2020
- Published by Physical Society of Japan in Journal of the Physical Society of Japan
Current-induced Giant Lattice Deformation in the Mott Insulator Ca₂RuO₄
by Ryuji Okazaki, Kensuke Kobayashi, Reiji Kumai, Hironori Nakao, Youichi Murakami, Fumihiko Nakamura, Hiroki Taniguchi, Ichiro Terasaki
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- 15 April 2020
- Published by Physical Society of Japan in Journal of the Physical Society of Japan
Current-induced Giant Lattice Deformation in the Mott Insulator Ca₂RuO₄
by Ryuji Okazaki, Kensuke Kobayashi, Reiji Kumai, Hironori Nakao, Youichi Murakami, Fumihiko Nakamura, Hiroki Taniguchi, Ichiro Terasaki
Cited by 24
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Cited by 11
- 8 January 2020
- Published by Royal Society of Chemistry (RSC) in Chemical Communications
Polar nano-region structure in the oxynitride perovskite LaTiO₂N
by Jun-Ichi Yamaura, Sachiko Maki, Takashi Honda, Yoshio Matsui, Alfian Noviyanto, Toshiya Otomo, Hitoshi Abe, Youichi Murakami, Naoki Ohashi
Polar nano-region formation in the oxynitride perovskite LaTiO₂N is demonstrated from PDF analysis.
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- 8 January 2020
- Published by Royal Society of Chemistry (RSC) in Chemical Communications
Open Access
Polar nano-region structure in the oxynitride perovskite LaTiO₂N
by Jun-Ichi Yamaura, Sachiko Maki, Takashi Honda, Yoshio Matsui, Alfian Noviyanto, Toshiya Otomo, Hitoshi Abe, Youichi Murakami, Naoki Ohashi
Polar nano-region formation in the oxynitride perovskite LaTiO₂N is demonstrated from PDF analysis.
Cited by 11
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Cited by 54
- 24 October 2019
- Published by American Chemical Society (ACS) in The Journal of Physical Chemistry Letters
Structural and Thermal Properties in Formamidinium and Cs-Mixed Lead Halides
by Shiro Kawachi, Mika Atsumi, Noriko Saito, Naoki Ohashi, Youichi Murakami, Jun-Ichi Yamaura
Formamidinium [FA, HC(NH₂)₂⁺] lead iodide and its cation mixture have attracted interests as potential applications for efficient solar cell superior to well-known methy-lammonium lead iodide. We investigated the crystal structure and thermodynamic properties of high-quality single crystals of FA_1−xCs_xPbI₃ for x = 0 and 0.1 through X-ray diffraction and heat capacity measurements. Both α–FA_0.9Cs_0.1PbI₃ as well as α–FAPbI₃ crystallize in cubic Pm−3m structure with orientationally disordered FA molecules confined in the non-distorted Pb−I framework. In FAPbI₃, we observed a second-order transition at 280 K and two first-order transitions at 141.2 and 130.2 K in between β and γ-phases instead of previously known single β−γ transition. After doping with 10% Cs, the multiple first-order transitions disappear, leading to phase transitions emerging at 300 and 149 K with second-order character. We moreover observed low-energy localized modes for both compounds, which is presumably tied to anomalous thermal motion, rattling, of the FA molecule.
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- 24 October 2019
- Published by American Chemical Society (ACS) in The Journal of Physical Chemistry Letters
Open Access
Structural and Thermal Properties in Formamidinium and Cs-Mixed Lead Halides
by Shiro Kawachi, Mika Atsumi, Noriko Saito, Naoki Ohashi, Youichi Murakami, Jun-Ichi Yamaura
Formamidinium [FA, HC(NH₂)₂⁺] lead iodide and its cation mixture have attracted interests as potential applications for efficient solar cell superior to well-known methy-lammonium lead iodide. We investigated the crystal structure and thermodynamic properties of high-quality single crystals of FA_1−xCs_xPbI₃ for x = 0 and 0.1 through X-ray diffraction and heat capacity measurements. Both α–FA_0.9Cs_0.1PbI₃ as well as α–FAPbI₃ crystallize in cubic Pm−3m structure with orientationally disordered FA molecules confined in the non-distorted Pb−I framework. In FAPbI₃, we observed a second-order transition at 280 K and two first-order transitions at 141.2 and 130.2 K in between β and γ-phases instead of previously known single β−γ transition. After doping with 10% Cs, the multiple first-order transitions disappear, leading to phase transitions emerging at 300 and 149 K with second-order character. We moreover observed low-energy localized modes for both compounds, which is presumably tied to anomalous thermal motion, rattling, of the FA molecule.
Cited by 54
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Cited by 3
- 12 September 2019
- Published by American Physical Society (APS) in Physical Review B
Isotropic magnetoelectric effect in ${Tb}_{1 - x} {Gd}_{x} {Mn}_{2} O_{5}$ studied by resonant x-ray scattering
by Y. Ishii, Y. Murakoshi, N. Sato, Y. Noda, T. Honda, H. Nakao, Y. Murakami, H. Kimura
A new type of magnetoelectric (ME) effect was discovered in ${Tb}_{1 - x} {Gd}_{x} {Mn}_{2} O_{5}$ in which Gd ions are substituted into ${TbMn}_{2} O_{5}$ . The substitution of Gd ions increased to $x = 0.5$ was found to completely suppress increases in electric polarization below $T \sim 14 K$ , which is clearly observed in ${TbMn}_{2} O_{5}$ . The electric polarization of ${Tb}_{0.5} {Gd}_{0.5} {Mn}_{2} O_{5}$ reappears below $T \sim 14 K$ in response to an external magnetic field applied along each crystallographic axis. This isotropic ME effect is in distinction from those exhibited by the $R {Mn}_{2} O_{5}$ family and by other multiferroic compounds. Resonant x-ray scattering was used to investigate the mechanism of the ME effect, and the data clearly demonstrate that this phenomenon can be attributed to competition between two different magnetic orderings. Furthermore, the magnetic-field dependence of the resonant intensity around the O $K$ edge implies that charge transfer between O and Mn ions microscopically contributes to the ME effects. These findings indicate potential new applications for multiferroic materials.
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- 12 September 2019
- Published by American Physical Society (APS) in Physical Review B
Isotropic magnetoelectric effect in ${Tb}_{1 - x} {Gd}_{x} {Mn}_{2} O_{5}$ studied by resonant x-ray scattering
by Y. Ishii, Y. Murakoshi, N. Sato, Y. Noda, T. Honda, H. Nakao, Y. Murakami, H. Kimura
A new type of magnetoelectric (ME) effect was discovered in ${Tb}_{1 - x} {Gd}_{x} {Mn}_{2} O_{5}$ in which Gd ions are substituted into ${TbMn}_{2} O_{5}$ . The substitution of Gd ions increased to $x = 0.5$ was found to completely suppress increases in electric polarization below $T \sim 14 K$ , which is clearly observed in ${TbMn}_{2} O_{5}$ . The electric polarization of ${Tb}_{0.5} {Gd}_{0.5} {Mn}_{2} O_{5}$ reappears below $T \sim 14 K$ in response to an external magnetic field applied along each crystallographic axis. This isotropic ME effect is in distinction from those exhibited by the $R {Mn}_{2} O_{5}$ family and by other multiferroic compounds. Resonant x-ray scattering was used to investigate the mechanism of the ME effect, and the data clearly demonstrate that this phenomenon can be attributed to competition between two different magnetic orderings. Furthermore, the magnetic-field dependence of the resonant intensity around the O $K$ edge implies that charge transfer between O and Mn ions microscopically contributes to the ME effects. These findings indicate potential new applications for multiferroic materials.
Cited by 3
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Cited by 4
- 13 June 2019
- Published by American Physical Society (APS) in Physical Review B
Quantum dynamics of hydrogen in the iron-based superconductor ${LaFeAsO}_{0.9} D_{0.1}$ measured with inelastic neutron spectroscopy
by Jun-Ichi Yamaura, Haruhiro Hiraka, Soshi Iimura, Yoshinori Muraba, Joonho Bang, Kazuhiko Ikeuchi, Mitsutaka Nakamura, Yasuhiro Inamura, Takashi Honda, Masatoshi Hiraishi, et al.
Inelastic neutron scattering was performed for an iron-based superconductor ${LaFeAsO}_{0.9} D_{0.1}$ , where most of D (deuterium) replaces oxygen, while a tiny amount goes into the interstitial sites. By first-principles calculation, we characterize the interstitial sites for D (and for H slightly mixed) with four equivalent potential minima. Below the superconducting transition temperature $T_{c} = 26 K$ , excitations emerge in the range 5–15 meV, while they are absent in the reference system ${LaFeAsO}_{0.9} F_{0.1}$ . The strong excitations at 14.5 and 11.1 meV broaden rapidly around 15 and 20 K, respectively, where each energy becomes comparable to twice the superconducting gap. The strong excitations are ascribed to a quantum rattling, or a band motion of hydrogen, which arises only if the number of potential minima is larger than two.
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- 13 June 2019
- Published by American Physical Society (APS) in Physical Review B
Quantum dynamics of hydrogen in the iron-based superconductor ${LaFeAsO}_{0.9} D_{0.1}$ measured with inelastic neutron spectroscopy
by Jun-Ichi Yamaura, Haruhiro Hiraka, Soshi Iimura, Yoshinori Muraba, Joonho Bang, Kazuhiko Ikeuchi, Mitsutaka Nakamura, Yasuhiro Inamura, Takashi Honda, Masatoshi Hiraishi, et al.
Inelastic neutron scattering was performed for an iron-based superconductor ${LaFeAsO}_{0.9} D_{0.1}$ , where most of D (deuterium) replaces oxygen, while a tiny amount goes into the interstitial sites. By first-principles calculation, we characterize the interstitial sites for D (and for H slightly mixed) with four equivalent potential minima. Below the superconducting transition temperature $T_{c} = 26 K$ , excitations emerge in the range 5–15 meV, while they are absent in the reference system ${LaFeAsO}_{0.9} F_{0.1}$ . The strong excitations at 14.5 and 11.1 meV broaden rapidly around 15 and 20 K, respectively, where each energy becomes comparable to twice the superconducting gap. The strong excitations are ascribed to a quantum rattling, or a band motion of hydrogen, which arises only if the number of potential minima is larger than two.
Cited by 4
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Cited by 9
- 11 June 2019
- Published by Royal Society of Chemistry (RSC) in RSC Advances
Pressure-induced hydrogen localization coupled to a semiconductor–insulator transition in a hydrogen-bonded molecular conductor
by Akira Ueda, Kouki Kishimoto, Takayuki Isono, Shota Yamada, Hiromichi Kamo, Kensuke Kobayashi, Reiji Kumai, Youichi Murakami, Jun Gouchi, Yoshiya Uwatoko, et al.
Purely organic crystals, κ-X₃(Cat-EDT-TTF)₂ [X = H or D, Cat-EDT-TTF = catechol-fused tetrathiafulvalene], are a new type of molecular conductor with hydrogen dynamics. In this work, hydrostatic pressure effects on these materials were investigated in terms of the electrical resistivity and crystal structure. The results indicate that the pressure induces and promotes hydrogen (deuterium) localization in the hydrogen bond, in contrast to the case of the conventional hydrogen-bonded materials (where pressure prevents hydrogen localization), and consequently leads to a significant change in the electrical conducting properties (i.e., the occurrence of a semiconductor–insulator transition). Therefore, we have successfully found a new type of pressure-induced phase transition where the cooperation of the hydrogen dynamics and π-electron interactions plays a crucial role.
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- 11 June 2019
- Published by Royal Society of Chemistry (RSC) in RSC Advances
Open Access
Pressure-induced hydrogen localization coupled to a semiconductor–insulator transition in a hydrogen-bonded molecular conductor
by Akira Ueda, Kouki Kishimoto, Takayuki Isono, Shota Yamada, Hiromichi Kamo, Kensuke Kobayashi, Reiji Kumai, Youichi Murakami, Jun Gouchi, Yoshiya Uwatoko, et al.
Purely organic crystals, κ-X₃(Cat-EDT-TTF)₂ [X = H or D, Cat-EDT-TTF = catechol-fused tetrathiafulvalene], are a new type of molecular conductor with hydrogen dynamics. In this work, hydrostatic pressure effects on these materials were investigated in terms of the electrical resistivity and crystal structure. The results indicate that the pressure induces and promotes hydrogen (deuterium) localization in the hydrogen bond, in contrast to the case of the conventional hydrogen-bonded materials (where pressure prevents hydrogen localization), and consequently leads to a significant change in the electrical conducting properties (i.e., the occurrence of a semiconductor–insulator transition). Therefore, we have successfully found a new type of pressure-induced phase transition where the cooperation of the hydrogen dynamics and π-electron interactions plays a crucial role.
Cited by 9

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