Y. Kitaoka
Osaka University
33 shared publications
M. Yashima
Graduate School of Engineering Science, Osaka University, Osaka 560-8531, Japan
28 shared publications
Akira Iyo
National Institute of Advanced Industrial Science and Technology (AIST)
12 shared publications
S. Tajima
Graduate School of Science, Osaka University, Osaka 560-0043, Japan
12 shared publications
S. Miyasaka
Graduate School of Science, Osaka University, Osaka 560-0043, Japan
11 shared publications
H. Eisaki
National Institute of Advanced Industrial Science and Technology (AIST) 305-8568 Tsukuba Ibaraki
10 shared publications

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- 30 December 2025
- Published by National Academy of Sciences in Proceedings of the National Academy of Sciences
Frustrated random-singlet state with ice-type structural fluctuation in spinel titanates
by Noriaki Hanasaki, Takayuki Hattori, Takumi Komoda, Kouei Minamoto, Shuhei Torigoe, Satoshi Yamashita, Yasuhiro Nakazawa, Takehito Nakano, Kouhei Yoshimi, Mitsuharu Yashima, et al.
In ice, it is well known that the orientation of H ₂ O molecules is disordered by geometrical frustration. Ice-analogous materials having a pyrochlore lattice display interesting phenomena such as the spin-ice state and the magnetic monopole. In the spinel titanate MgTi ₂ O ₄ , the Ti ions have a quantum spin in the pyrochlore lattice. The Ti ions are displaced, accompanied by the spin-singlet formation. Since this displacement pattern follows the ice rule, the title compound is a material analogous to ice. When a small quantity of Ti ions are replaced with Mg ions, the ice-type structural fluctuation exists. In this structural ice-type state, the spins are also fluctuating at a nanosecond scale down to 0.3 K. We ascribed this phenomenon to the gapless frustrated random-singlet state, in which the spin-singlet pairs are resonating, and the orphan spins are hopping.
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- 30 December 2025
- Published by National Academy of Sciences in Proceedings of the National Academy of Sciences
Open Access
Frustrated random-singlet state with ice-type structural fluctuation in spinel titanates
by Noriaki Hanasaki, Takayuki Hattori, Takumi Komoda, Kouei Minamoto, Shuhei Torigoe, Satoshi Yamashita, Yasuhiro Nakazawa, Takehito Nakano, Kouhei Yoshimi, Mitsuharu Yashima, et al.
In ice, it is well known that the orientation of H ₂ O molecules is disordered by geometrical frustration. Ice-analogous materials having a pyrochlore lattice display interesting phenomena such as the spin-ice state and the magnetic monopole. In the spinel titanate MgTi ₂ O ₄ , the Ti ions have a quantum spin in the pyrochlore lattice. The Ti ions are displaced, accompanied by the spin-singlet formation. Since this displacement pattern follows the ice rule, the title compound is a material analogous to ice. When a small quantity of Ti ions are replaced with Mg ions, the ice-type structural fluctuation exists. In this structural ice-type state, the spins are also fluctuating at a nanosecond scale down to 0.3 K. We ascribed this phenomenon to the gapless frustrated random-singlet state, in which the spin-singlet pairs are resonating, and the orphan spins are hopping.
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- 15 May 2024
- Published by Physical Society of Japan in Journal of the Physical Society of Japan
Role of Vanadium-Oxide Layer in Electronic State of Sr₂VFeAsO₃₋_δ with Oxygen Deficiency
by Masamichi Nakajima, Hiroaki Yokota, Taihei Wakimura, Tetsuya Takeuchi, Koya Nakamura, Mitsuharu Yashima, Hidekazu Mukuda, Shigeki Miyasaka, Setsuko Tajima
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- 15 May 2024
- Published by Physical Society of Japan in Journal of the Physical Society of Japan
Role of Vanadium-Oxide Layer in Electronic State of Sr₂VFeAsO₃₋_δ with Oxygen Deficiency
by Masamichi Nakajima, Hiroaki Yokota, Taihei Wakimura, Tetsuya Takeuchi, Koya Nakamura, Mitsuharu Yashima, Hidekazu Mukuda, Shigeki Miyasaka, Setsuko Tajima
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Cited by 143
- 15 May 2024
- Published by Physical Society of Japan in Journal of the Physical Society of Japan
Multiband Metallic Ground State in Multilayered Nickelates La₃Ni₂O₇ and La₄Ni₃O₁₀ Probed by ¹³⁹La-NMR at Ambient Pressure
by Masataka Kakoi, Takashi Oi, Yujiro Ohshita, Mitsuharu Yashima, Kazuhiko Kuroki, Takeru Kato, Hidefumi Takahashi, Shintaro Ishiwata, Yoshinobu Adachi, Naoyuki Hatada, et al.
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- 15 May 2024
- Published by Physical Society of Japan in Journal of the Physical Society of Japan
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Multiband Metallic Ground State in Multilayered Nickelates La₃Ni₂O₇ and La₄Ni₃O₁₀ Probed by ¹³⁹La-NMR at Ambient Pressure
by Masataka Kakoi, Takashi Oi, Yujiro Ohshita, Mitsuharu Yashima, Kazuhiko Kuroki, Takeru Kato, Hidefumi Takahashi, Shintaro Ishiwata, Yoshinobu Adachi, Naoyuki Hatada, et al.
Cited by 143
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- 1 August 2022
- Published by IOP Publishing in Journal of Physics: Conference Series
⁷⁵As NMR studies on iron-based La1111 superconductors in lightly and heavily electron doped states
by T Kouchi, K Yoshinaga, S Nishioka, T Shiota, M Yashima, H Mukuda, K Kuroki, S Miyasaka, S Tajima
We report ⁷⁵As NMR experiments in heavily electron-doped LaFePnO_0.75H_0.25 (Pn=As_1−xSb_x and As_1−x′P_x′) compounds with the maximum T c max ( ∼ 33.1 K ) , and compare with the previous results in lightly electron-doped LaFePn(O,F) compounds. The T_c of this series can be sensitively controlled by the pnictogen height (h_Pn ) through the substitution at Pn site, and the electron doping level through the substitution at the O site with H or F. In heavily electron-doped LaFePnO(La1111) compounds, we found that spin fluctuations at low-energies were moderately suppressed upon cooling in the Sb-substituted high T_c compounds with high h_Pn (x ≥ 0), although they are completely suppressed in P-substituted non-superconducting compounds (x′ ≥ 0.2) with lower h_Pn . This feature is largely different from that in the lightly doped La1111 compounds with the well-nested Fermi surfaces, where the spin fluctuations are critically enhanced upon cooling. Here, we present the characteristics of spin fluctuations over wide doping region of La1111-based compounds, and discuss the relationship with the superconductivity.
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- 1 August 2022
- Published by IOP Publishing in Journal of Physics: Conference Series
Open Access
⁷⁵As NMR studies on iron-based La1111 superconductors in lightly and heavily electron doped states
by T Kouchi, K Yoshinaga, S Nishioka, T Shiota, M Yashima, H Mukuda, K Kuroki, S Miyasaka, S Tajima
We report ⁷⁵As NMR experiments in heavily electron-doped LaFePnO_0.75H_0.25 (Pn=As_1−xSb_x and As_1−x′P_x′) compounds with the maximum T c max ( ∼ 33.1 K ) , and compare with the previous results in lightly electron-doped LaFePn(O,F) compounds. The T_c of this series can be sensitively controlled by the pnictogen height (h_Pn ) through the substitution at Pn site, and the electron doping level through the substitution at the O site with H or F. In heavily electron-doped LaFePnO(La1111) compounds, we found that spin fluctuations at low-energies were moderately suppressed upon cooling in the Sb-substituted high T_c compounds with high h_Pn (x ≥ 0), although they are completely suppressed in P-substituted non-superconducting compounds (x′ ≥ 0.2) with lower h_Pn . This feature is largely different from that in the lightly doped La1111 compounds with the well-nested Fermi surfaces, where the spin fluctuations are critically enhanced upon cooling. Here, we present the characteristics of spin fluctuations over wide doping region of La1111-based compounds, and discuss the relationship with the superconductivity.
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Cited by 5
- 28 April 2022
- Published by American Physical Society (APS) in Physical Review B
Enhanced superconductivity and moderate spin fluctuations suppressed at low energies in heavily electron-doped La1111-based superconductor
by T. Kouchi, S. Nishioka, K. Suzuki, M. Yashima, H. Mukuda, T. Kawashima, H. Tsuji, K. Kuroki, S. Miyasaka, S. Tajima
To elucidate the origin of the reenhanced high- $T_{c}$ phase in the heavily electron-doped Fe pnictides, systematic ${}^{75}{As}$ NMR studies are performed on heavily electron-doped $LaFe P n O_{0.75} H_{0.25}$ by controlling the pnictogen height $(h_{P n})$ from the Fe plane through the substitution at $P n (= As)$ site with Sb or P. The measurements of nuclear spin relaxation rate $(1 / T_{1})$ and Knight shift $(K)$ reveal that the moderate spin fluctuations at high temperatures are suppressed toward low temperatures. Such characteristic spin fluctuations with gaplike feature at low energies are more enlarged in higher $T_{c}$ compounds with higher $h_{P n}$ , while those are totally suppressed in nonsuperconducting compounds with lower $h_{P n}$ . This implies that the contribution of the finite-energy part in the spin-fluctuation spectrum is crucial for enhancing $T_{c}$ in the heavily electron-doped regime. This is in contrast to many cases of typical Fe-based compounds with hole and electron Fermi surfaces of similar sizes, where the spin fluctuations at low energies develop significantly at low temperatures. The features in the heavily electron-doped states are discussed in relation with the characteristics of the faint hole Fermi surface derived from the $d_{x y}$ orbital that rises when $h_{P n}$ is high, together with the enhanced electron correlation effects.
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- 28 April 2022
- Published by American Physical Society (APS) in Physical Review B
Enhanced superconductivity and moderate spin fluctuations suppressed at low energies in heavily electron-doped La1111-based superconductor
by T. Kouchi, S. Nishioka, K. Suzuki, M. Yashima, H. Mukuda, T. Kawashima, H. Tsuji, K. Kuroki, S. Miyasaka, S. Tajima
To elucidate the origin of the reenhanced high- $T_{c}$ phase in the heavily electron-doped Fe pnictides, systematic ${}^{75}{As}$ NMR studies are performed on heavily electron-doped $LaFe P n O_{0.75} H_{0.25}$ by controlling the pnictogen height $(h_{P n})$ from the Fe plane through the substitution at $P n (= As)$ site with Sb or P. The measurements of nuclear spin relaxation rate $(1 / T_{1})$ and Knight shift $(K)$ reveal that the moderate spin fluctuations at high temperatures are suppressed toward low temperatures. Such characteristic spin fluctuations with gaplike feature at low energies are more enlarged in higher $T_{c}$ compounds with higher $h_{P n}$ , while those are totally suppressed in nonsuperconducting compounds with lower $h_{P n}$ . This implies that the contribution of the finite-energy part in the spin-fluctuation spectrum is crucial for enhancing $T_{c}$ in the heavily electron-doped regime. This is in contrast to many cases of typical Fe-based compounds with hole and electron Fermi surfaces of similar sizes, where the spin fluctuations at low energies develop significantly at low temperatures. The features in the heavily electron-doped states are discussed in relation with the characteristics of the faint hole Fermi surface derived from the $d_{x y}$ orbital that rises when $h_{P n}$ is high, together with the enhanced electron correlation effects.
Cited by 5
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- 25 April 2022
- Published by arXiv in arXiv
Enhanced superconductivity and moderate spin fluctuations suppressed at low energies in heavily electron-doped La1111-based superconductor
by T. Kouchi, S. Nishioka, K. Suzuki, M. Yashima, H. Mukuda, T. Kawashima, H. Tsuji, K. Kuroki, S. Miyasaka, S. Tajima
To elucidate the origin of re-enhanced high-$T_c$ phase in the heavily electron-doped Fe-pnictides, systematic $^{75}$As NMR studies are performed on heavily electron-doped LaFe$Pn$O$_{0.75}$H$_{0.25}$ by controlling the pnictogen height ($h_{Pn}$) from the Fe plane through the substitution at $Pn$(=As) site with Sb or P. The measurements of nuclear spin relaxation rate (1/$T_1$) and Knight shift ($K$) reveal that the moderate spin fluctuations at high temperatures are suppressed toward low temperatures. Such characteristic spin fluctuations with gap like feature at low energies are more enlarged in higher $T_c$ compounds with higher $h_{Pn}$, while those are totally suppressed in non-superconducting compounds with lower $h_{Pn}$. This implies that the contribution of the finite energy part in the spin fluctuation spectrum is crucial for enhancing $T_c$ in the heavily electron-doped regime. This is in contrast to many cases of typical Fe-based compounds with hole and electron Fermi surfaces of similar sizes, where the spin fluctuations at low energies develop significantly at low temperatures. The features in the heavily electron-doped states are discussed in relation with the characteristics of the faint hole Fermi surface derived from $d_{xy}$ orbital that rises when $h_{Pn}$ is high, together with the enhanced electron correlation effects.
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- 25 April 2022
- Published by arXiv in arXiv
Open Access
Enhanced superconductivity and moderate spin fluctuations suppressed at low energies in heavily electron-doped La1111-based superconductor
by T. Kouchi, S. Nishioka, K. Suzuki, M. Yashima, H. Mukuda, T. Kawashima, H. Tsuji, K. Kuroki, S. Miyasaka, S. Tajima
To elucidate the origin of re-enhanced high-$T_c$ phase in the heavily electron-doped Fe-pnictides, systematic $^{75}$As NMR studies are performed on heavily electron-doped LaFe$Pn$O$_{0.75}$H$_{0.25}$ by controlling the pnictogen height ($h_{Pn}$) from the Fe plane through the substitution at $Pn$(=As) site with Sb or P. The measurements of nuclear spin relaxation rate (1/$T_1$) and Knight shift ($K$) reveal that the moderate spin fluctuations at high temperatures are suppressed toward low temperatures. Such characteristic spin fluctuations with gap like feature at low energies are more enlarged in higher $T_c$ compounds with higher $h_{Pn}$, while those are totally suppressed in non-superconducting compounds with lower $h_{Pn}$. This implies that the contribution of the finite energy part in the spin fluctuation spectrum is crucial for enhancing $T_c$ in the heavily electron-doped regime. This is in contrast to many cases of typical Fe-based compounds with hole and electron Fermi surfaces of similar sizes, where the spin fluctuations at low energies develop significantly at low temperatures. The features in the heavily electron-doped states are discussed in relation with the characteristics of the faint hole Fermi surface derived from $d_{xy}$ orbital that rises when $h_{Pn}$ is high, together with the enhanced electron correlation effects.
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- 26 March 2022
- Published by Springer Nature in Journal of Low Temperature Physics
Correction to: First Mobility Measurement of Ions Trapped Below the Normal and Superfluid 3He Surface
by T. Shiino, H. Mukuda, K. Kono, W. F. Vinen
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- 26 March 2022
- Published by Springer Nature in Journal of Low Temperature Physics
Open Access
Correction to: First Mobility Measurement of Ions Trapped Below the Normal and Superfluid 3He Surface
by T. Shiino, H. Mukuda, K. Kono, W. F. Vinen
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Cited by 5
- 10 December 2020
- Published by American Physical Society (APS) in Physical Review B
${}^{181}{Ta}$ nuclear quadrupole resonance study of the noncentrosymmetric superconductor ${PbTaSe}_{2}$
by K. Yokoi, M. Yashima, H. Murakawa, H. Mukuda, K. Yamauchi, T. Oguchi, H. Sakai, N. Hanasaki
We report on a pure ${}^{181}{Ta}$ nuclear quadrupole resonance (NQR) measurement of ${PbTaSe}_{2}$ at zero magnetic field, which has the advantage of directly probing the intrinsic superconducting phase and electronic states of the ${TaSe}_{2}$ layer. We observed the ${}^{181}{Ta}$ NQR spectrum of the intrinsic structure with space group $P \bar{6} m 2$ , which agrees well with density functional theory calculations. The nuclear spin relaxation rate $1 / T_{1}$ shows an exponential decrease well below $T_{_{c}}$ , indicating that the superconducting state is fully gapped in the framework of BCS theory. The gap size obtained by ${}^{181}{Ta}$ NQR was smaller than the value in previous reports, which may imply that the Fermi surfaces composed of $Ta 5 d$ orbitals, where the average pairing interactions are expected to be weaker than in the BCS model, are primarily probed. The temperature dependence of $1 / T_{1}$ below $T_{_{c}}$ can be reproduced well by the superposition of quadrupole and magnetic relaxation mechanisms, together with the distribution of the superconducting gap size inherent in multiple Fermi surfaces theoretically proposed in ${PbTaSe}_{2}$ .
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- 10 December 2020
- Published by American Physical Society (APS) in Physical Review B
${}^{181}{Ta}$ nuclear quadrupole resonance study of the noncentrosymmetric superconductor ${PbTaSe}_{2}$
by K. Yokoi, M. Yashima, H. Murakawa, H. Mukuda, K. Yamauchi, T. Oguchi, H. Sakai, N. Hanasaki
We report on a pure ${}^{181}{Ta}$ nuclear quadrupole resonance (NQR) measurement of ${PbTaSe}_{2}$ at zero magnetic field, which has the advantage of directly probing the intrinsic superconducting phase and electronic states of the ${TaSe}_{2}$ layer. We observed the ${}^{181}{Ta}$ NQR spectrum of the intrinsic structure with space group $P \bar{6} m 2$ , which agrees well with density functional theory calculations. The nuclear spin relaxation rate $1 / T_{1}$ shows an exponential decrease well below $T_{_{c}}$ , indicating that the superconducting state is fully gapped in the framework of BCS theory. The gap size obtained by ${}^{181}{Ta}$ NQR was smaller than the value in previous reports, which may imply that the Fermi surfaces composed of $Ta 5 d$ orbitals, where the average pairing interactions are expected to be weaker than in the BCS model, are primarily probed. The temperature dependence of $1 / T_{1}$ below $T_{_{c}}$ can be reproduced well by the superposition of quadrupole and magnetic relaxation mechanisms, together with the distribution of the superconducting gap size inherent in multiple Fermi surfaces theoretically proposed in ${PbTaSe}_{2}$ .
Cited by 5
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Cited by 7
- 15 November 2019
- Published by Physical Society of Japan in Journal of the Physical Society of Japan
Unconventional Multi-gap Superconductivity and Antiferromagnetic Spin Fluctuations in New Iron-arsenide LaFe₂As₂ in Heavily Electron-doped Regime
by Takayoshi Kouchi, Mitsuharu Yashima, Hidekazu Mukuda, Shigeyuki Ishida, Hiroshi Eisaki, Yoshiyuki Yoshida, Kenji Kawashima, Akira Iyo
We report 75As-NMR/NQR results on new iron-arsenide compounds (La0.5−xNa0.5+x)Fe2As2. The parent compound x = 0 exhibits a stripe-type antiferromagnetic (AFM) order below TN = 130 K. The measuremen...
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- 15 November 2019
- Published by Physical Society of Japan in Journal of the Physical Society of Japan
Unconventional Multi-gap Superconductivity and Antiferromagnetic Spin Fluctuations in New Iron-arsenide LaFe₂As₂ in Heavily Electron-doped Regime
by Takayoshi Kouchi, Mitsuharu Yashima, Hidekazu Mukuda, Shigeyuki Ishida, Hiroshi Eisaki, Yoshiyuki Yoshida, Kenji Kawashima, Akira Iyo
We report 75As-NMR/NQR results on new iron-arsenide compounds (La0.5−xNa0.5+x)Fe2As2. The parent compound x = 0 exhibits a stripe-type antiferromagnetic (AFM) order below TN = 130 K. The measuremen...
Cited by 7
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- 3 October 2019
- Published by arXiv in arXiv
Unconventional Multi-gap Superconductivity and Antiferromagnetic Spin Fluctuations in New Iron-arsenide LaFe2As2 in Heavily Electron-doped Regime
by T. Kouchi, M. Yashima, H. Mukuda, S. Ishida, H. Eisaki, Y. Yoshida, K. Kawashima, A. Iyo
We report 75As-NMR/NQR results on new iron-arsenide compounds (La0.5-xNa0.5+x)Fe2As2. The parent compound x=0 exhibits a stripe-type antiferromagnetic (AFM) order below T_N=130 K. The measurement of nuclear spin relaxation rate at hole-doped x=+0.3 and heavily electron-doped x=-0.5 revealed that the normal-state properties are dominated by AFM spin fluctuations (AFMSFs), which are more significant at x=+0.3 than at x=-0.5. Their superconducting (SC) phases are characterized by unconventional multi-gap SC state, where the smaller SC gaps are particularly weaken in common. The experimental results indicate the close relationship between the AFMSFs and the SC from the hole-doped state to heavily electron-doped state, which shed light on a unique SC phase emerged in the heavily electron-doped regime being formally equivalent to non-SC compound Ba(Fe0.5Co0.5)Fe2As2.
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- 3 October 2019
- Published by arXiv in arXiv
Open Access
Unconventional Multi-gap Superconductivity and Antiferromagnetic Spin Fluctuations in New Iron-arsenide LaFe2As2 in Heavily Electron-doped Regime
by T. Kouchi, M. Yashima, H. Mukuda, S. Ishida, H. Eisaki, Y. Yoshida, K. Kawashima, A. Iyo
We report 75As-NMR/NQR results on new iron-arsenide compounds (La0.5-xNa0.5+x)Fe2As2. The parent compound x=0 exhibits a stripe-type antiferromagnetic (AFM) order below T_N=130 K. The measurement of nuclear spin relaxation rate at hole-doped x=+0.3 and heavily electron-doped x=-0.5 revealed that the normal-state properties are dominated by AFM spin fluctuations (AFMSFs), which are more significant at x=+0.3 than at x=-0.5. Their superconducting (SC) phases are characterized by unconventional multi-gap SC state, where the smaller SC gaps are particularly weaken in common. The experimental results indicate the close relationship between the AFMSFs and the SC from the hole-doped state to heavily electron-doped state, which shed light on a unique SC phase emerged in the heavily electron-doped regime being formally equivalent to non-SC compound Ba(Fe0.5Co0.5)Fe2As2.

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