Y. Ihara

Kyoto University, Kyoto, Kyoto-fu, Japan

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Publications (43)79.23 Total impact

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    ABSTRACT: The discovery in 1991 of high temperature superconductivity (SC) in A3C60 compounds, where A is an alkali ion, has been initially ascribed to a BCS mechanism, with a weak incidence of electron correlations. However various experimental evidences taken for compounds with distinct alkali content established the interplay of strong correlations and Jahn Teller distortions of the C60 ball. The importance of electronic correlations even in A3C60 has been highlighted by the recent discovery of two expanded fulleride Cs3C60 isomeric phases that are Mott insulators at ambient pressure. Both phases undergo a pressure induced first order Mott transition to SC with a (p, T) phase diagram displaying a dome shaped SC, a common situation encountered nowadays in correlated electron systems. NMR experiments allowed us to establish that the bipartite A15 phase displays N\'eel order at 47K, while magnetic freezing only occurs at lower temperature in the fcc phase. NMR data do permit us to conclude that well above the critical pressure, the singlet superconductivity found for light alkalis is recovered. However deviations from BCS expectations linked with electronic correlations are found near the Mott transition. So, although SC involves an electron-phonon mechanism, correlations have a significant incidence on the electronic properties, as had been anticipated from DMFT calculations.
    Journal of Physics Conference Series 10/2013; 449(1).
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    ABSTRACT: In order to determine the superconducting paring state in the ferromagnetic superconductor UCoGe, ^{59}Co NMR Knight shift, which is directly related to the microscopic spin susceptibility, was measured in the superconducting state under magnetic fields perpendicular to spontaneous magnetization axis: ^{59}K^{a, b}. ^{59}K^{a, b} shows to be constant, but does not decrease below a superconducting transition. These behaviors as well as the invariance of the internal field at the Co site in the superconducting state exclude the spin-singlet pairing, and can be interpreted with the equal-spin pairing state with a large exchange field along the c axis, which was studied by Mineev [Phys. Rev. B 81, 180504 (2010)].
    Physical Review B 08/2013; 88(8). · 3.77 Impact Factor
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    ABSTRACT: Identification of pairing mechanisms leading to the unconventional superconductivity realized in copper-oxide, heavy-fermions, and organic compounds is one of the most challenging issues in condensed-matter physics. Clear evidence for an electron-phonon mechanism in conventional superconductors is seen by the isotope effect on the superconducting transition temperatures $T_{\rm SC}$, since isotopic substitution varies the phonon frequency without affecting the electronic states. In unconventional superconductors, magnetic fluctuations have been proposed to mediate superconductivity, and considerable efforts have been made to unravel relationships between normal-state magnetic fluctuations and superconductivity. Here, we show that characteristic experimental results on the ferromagnetic (FM) superconductor UCoGe ($T_{\rm Curie} \sim 2.5 $ K and $T_{\rm SC} \sim 0.6$ K) can be understood consistently within a scenario of the spin-triplet superconductivity induced by FM spin fluctuations. Temperature and angle dependencies of the upper critical magnetic field of the superconductivity ($H_{c2}$) are calculated on the basis of the above scenario by solving the Eliashberg equation. Calculated $H_{c2}$ well agrees with the characteristic experimental results observed in UCoGe. This is a first example that FM fluctuations are shown to be a pairing glue of superconductivity.
    Journal of Physics Conference Series 02/2013;
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    ABSTRACT: We have performed 59Co NQR / NMR measurements on the single-crystalline UCoGe, in order to investigate the relationship between ferromagnetism and superconductivity. The measurements of Knight-shift and nuclear spin-lattice relaxation rate provide clear evidence that both static and dynamic susceptibilities are ferromagnetic with Ising anisotropy. In addition, Hc2 also shows extremely large anisotropy which can not be explained by the GL model with anisotropy of effective mass. These data suggest intimate relationship between Ising magnetization and anisotropic superconductivity in UCoGe.
    Journal of Physics Conference Series 12/2012; 391(1):2017-.
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    ABSTRACT: From detailed angle-resolved NMR and Meissner measurements on a ferromagnetic (FM) superconductor UCoGe (T(Curie)∼2.5  K and T(SC)∼0.6  K), we show that superconductivity in UCoGe is tightly coupled with longitudinal FM spin fluctuations along the c axis. We found that magnetic fields along the c axis (H∥c) strongly suppress the FM fluctuations and that the superconductivity is observed in the limited magnetic-field region where the longitudinal FM spin fluctuations are active. These results, combined with model calculations, strongly suggest that the longitudinal FM spin fluctuations tuned by H∥c induce the unique spin-triplet superconductivity in UCoGe. This is the first clear example that FM fluctuations are intimately related with superconductivity.
    Physical Review Letters 02/2012; 108(6):066403. · 7.94 Impact Factor
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    ABSTRACT: Recent transport measurements [Churchill et al. Nature Phys. 5, 321 (2009)] found a surprisingly large, 2-3 orders of magnitude larger than usual (13)C hyperfine coupling (HFC) in (13)C enriched single-wall carbon nanotubes. We formulate the theory of the nuclear relaxation time in the framework of the Tomonaga-Luttinger liquid theory to enable the determination of the HFC from recent data by Ihara et al. [Europhys. Lett. 90, 17,004 (2010)]. Though we find that 1/T(1) is orders of magnitude enhanced with respect to a Fermi-liquid behavior, the HFC has its usual, small value. Then, we reexamine the theoretical description used to extract the HFC from transport experiments and show that similar features could be obtained with HFC-independent system parameters.
    Physical Review Letters 10/2011; 107(18):187204. · 7.94 Impact Factor
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    Yoshihiko Ihara, Kenji Ishida
    07/2011; , ISBN: 978-953-307-151-0
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    ABSTRACT: We have performed ^{59}Co-nuclear quadrupole resonance (NQR) and nuclear magnetic resonance (NMR) studies on YCoGe, which is a reference compound of ferromagnetic superconductor UCoGe, in order to investigate the magnetic properties at the Co site. Magnetic and superconducting transitions were not observed down to 0.3 K, but a conventional metallic behavior was found in YCoGe, although its crystal structure is similar to that of UCoGe. From the comparison between experimental results of two compounds, the ferromagnetism and superconductivity observed in UCoGe originate from the U-5f electrons.
    Journal of the Physical Society of Japan 04/2011; 80(6). · 2.09 Impact Factor
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    ABSTRACT: We present here ^{13}C and ^{133}Cs NMR spin lattice relaxation T_{1} data in the A15 and fcc-Cs_{3}C_{60} phases for increasing hydrostatic pressure through the transition at p_{c} from a Mott insulator to a superconductor. We evidence that for p>> p_{c} the (T_{1}T)^{-1} data above T_{c} display metallic like Korringa constant values which match quantitatively previous data taken on other A_{3}C_{60} compounds. However below the pressure for which T_{c} goes through a maximum, (T_{1}T)^{-1} is markedly increased with respect to the Korringa values expected in a simple BCS scenario. This points out the importance of electronic correlations near the Mott transition. For p > p_{c} singular T dependences of (T_{1}T)^{-1} are detected for T >> T_{c}. It will be shown that they can be ascribed to a large variation with temperature of the Mott transition pressure p_{c} towards a liquid-gas like critical point, as found at high T for usual Mott transitions.
    02/2011;
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    ABSTRACT: We present here 13C and 133Cs NMR spin lattice relaxation T1 data in the A15 and fcc-Cs3C60 phases for increasing hydrostatic pressure through the transition at pc from a Mott insulator to a superconductor. We evidence that for pGtpc the (T1T)- 1 data above Tc display metallic-like Korringa constant values which match quantitatively previous data taken on other A3C60 compounds. However below the pressure for which Tc goes through a maximum, (T1T)- 1 is markedly increased with respect to the Korringa values expected in a simple BCS scenario. This points out the importance of electronic correlations near the Mott transition. For pgsimpc singular T-dependences of (T1T)- 1 are detected for TGtTc. It will be shown that they can be ascribed to a large variation with temperature of the Mott transition pressure pc towards a liquid-gas-like critical point, as found at high T for usual Mott transitions.
    EPL (Europhysics Letters) 01/2011; 94(3). · 2.26 Impact Factor
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    ABSTRACT: We have carried out direction-dependent 59Co NMR experiments on a single crystal sample of the ferromagnetic superconductor UCoGe in order to study the magnetic properties in the normal state. The Knight-shift and nuclear spin-lattice relaxation rate measurements provide microscopic evidence that both static and dynamic susceptibilities are ferromagnetic with strong Ising anisotropy. We discuss that superconductivity induced by these magnetic fluctuations prefers spin-triplet pairing state.
    Physical Review Letters 11/2010; 105(20):206403. · 7.94 Impact Factor
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    ABSTRACT: Magnetic measurements on optimally doped single crystals of BaFe2(As1−xPx)2 (x≈0.35) with magnetic fields applied along different crystallographic axes were performed under pressure, enabling the pressure evolution of coherence lengths and the anisotropy factor to be followed. Despite a decrease in the superconducting critical temperature, our studies reveal that the superconducting properties become more anisotropic under pressure. With appropriate scaling, we directly compare these properties with the values obtained for BaFe2(As1−xPx)2 as a function of phosphorus content.
    Physical review. B, Condensed matter 06/2010; 82(9). · 3.77 Impact Factor
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    ABSTRACT: We report a NMR and magnetometry study on the expanded intercalated fulleride Cs3C60 in both its A15 and face centered cubic structures. NMR allowed us to evidence that both exhibit a first-order Mott transition to a superconducting state, occurring at distinct critical pressures p{c} and temperatures T{c}. Though the ground state magnetism of the Mott phases differs, their high T paramagnetic and superconducting properties are found similar, and the phase diagrams versus unit volume per C60 are superimposed. Thus, as expected for a strongly correlated system, the interball distance is the relevant parameter driving the electronic behavior and quantum transitions of these systems.
    Physical Review Letters 06/2010; 104(25):256402. · 7.94 Impact Factor
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    ABSTRACT: We report 13C nuclear magnetic resonance measurements on single wall carbon nanotube (SWCNT) bundles. The temperature dependence of the nuclear spin-lattice relaxation rate, 1/T1, exhibits a power-law variation, as expected for a Tomonage-Luttinger liquid (TLL). The observed exponent is smaller than that expected for the two band TLL model. A departure from the power law is observed only at low T, where thermal and electronic Zeeman energy merge. Extrapolation to zero magnetic field indicates gapless spin excitations. The wide T range on which power-law behavior is observed suggests that SWCNT is so far the best realization of a one-dimensional quantum metal. Comment: 5 pages, 4 figures
    EPL (Europhysics Letters) 10/2009; · 2.26 Impact Factor
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    ABSTRACT: In order to investigate the relationship between superconductivity and magnetism in bilayer-hydrate cobaltate Nax(H3O)zCoO2syH2O Co nuclear magnetic resonance (NMR) and nuclear quadrupole resonance (NQR) measurements were performed on three different samples, which demonstrate various ground states at low temperatures. The appearance of small internal fields is observed in the NQR spectra below approximately 6 K on one of the samples that possesses the largest c -axis length and the highest NQR frequency. The other two samples exhibit superconducting transition in zero magnetic field, while these two samples show different ground states in the magnetic fields greater than 5 T. The comparison of the NMR spectra of these two samples obtained in high magnetic fields reveals the appearance of static internal magnetic fields at the Co site below 4 K in the sample that possesses the intermediate c -axis length and the NQR frequency.
    Physical review. B, Condensed matter 01/2009; 79(2). · 3.77 Impact Factor
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    ABSTRACT: We have performed 69,71Ga nuclear magnetic resonance (NMR), nuclear quadrupole resonance (NQR), and muon spin rotation and resonance on the quasi-two-dimensional antiferromagnet NiGa2S4, in order to investigate its spin dynamics and magnetic state at low temperatures. Although there exists only one crystallographic site for Ga in NiGa2S4, we found two distinct Ga signals by NMR and NQR. The origin of the two Ga signals is not fully understood, but possibly due to stacking faults along the c axis which induce additional broad Ga NMR and NQR signals with different local symmetries. We found the spin freezing occurring at Tf, at which the specific heat shows a maximum, from a clear divergent behavior of the nuclear spin-lattice relaxation rate 1∕T1 and nuclear spin-spin relaxation rate 1∕T2 measured by Ga-NQR as well as the muon spin relaxation rate λ. The main sharp NQR peaks exhibit a stronger tendency of divergence, compared with the weak broader spectral peaks, indicating that the spin freezing is intrinsic in NiGa2S4. The behavior of these relaxation rates strongly suggests that the Ni spin fluctuations slow down towards Tf, and the temperature range of the divergence is anomalously wider than that in a conventional magnetic ordering. A broad structureless spectrum and multicomponent T1 were observed below 2 K, indicating that a static magnetic state with incommensurate magnetic correlations or inhomogeneously distributed moments is realized at low temperatures. However, the wide temperature region between 2 K and Tf, where the NQR signal was not observed, suggests that the Ni spins do not freeze immediately below Tf, but keep fluctuating down to 2 K with the MHz frequency range. Below 0.5 K, all components of 1∕T1 follow a T3 behavior. We also found that 1∕T1 and 1∕T2 show the same temperature dependence above Tf but different temperature dependence below 0.8 K. These results suggest that the spin dynamics is isotropic above Tf, which is characteristic of the Heisenberg spin system, and becomes anisotropic below 0.8 K.
    Physical Review B. 02/2008; 77(5).
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    ABSTRACT: Co nuclear magnetic resonance (NMR) and nuclear quadrupole resonance (NQR) studies were performed in the recently discovered UCoGe, in which the ferromagnetic and superconducting (SC) transitions were reported to occur at $T_{\rm Curie} \sim 3$ K and $T_S \sim 0.8$ K (N. T. Huy {\it et al.}, Phys. Rev. Lett. {\bf 99} (2007) 067006), in order to investigate the coexistence of ferromagnetism and superconductivity as well as the normal-state and SC properties from a microscopic point of view. From the nuclear spin-lattice relaxation rate $1/T_1$ and Knight-shift measurements, we confirmed that ferromagnetic fluctuations which possess a quantum critical character are present above $T_{\rm Curie}$ and the occurrence of ferromagnetic transition at 2.5 K in our polycrystalline sample. The magnetic fluctuations in the normal state show that UCoGe is an itinerant ferromagnet similar to ZrZn$_2$ and YCo$_2$. The onset SC transition was identified at $T_S \sim 0.7$ K, below which $1/T_1$ of 30 % of the volume fraction starts to decrease due to the opening of the SC gap. This component of $1/T_1$, which follows a $T^3$ dependence in the temperature range of $0.3 - 0.1$ K, coexists with the magnetic components of $1/T_1$ showing a $\sqrt{T}$ dependence below $T_S$. From the NQR measurements in the SC state, we suggest that the self-induced vortex state is realized in UCoGe.
    Journal of the Physical Society of Japan 01/2008; · 2.09 Impact Factor
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    ABSTRACT: Co nuclear quadrupole resonance (NQR) measurements were performed on the bilayer hydrate cobaltate compound Nax(H3O)zCoO2·yH2O under hydrostatic pressures to study the pressure effect on the superconducting (SC) and magnetic properties. We found that the hydrostatic pressure increases the NQR frequency νQ and suppresses the SC transition temperature Tc. The relationship between νQ and Tc observed in the present pressure-dependent measurements is different from that observed in sample-dependent NQR measurements. Nuclear spin–lattice relaxation rate 1/T1 was also measured under several pressures. The results indicate that the magnetic fluctuations are suppressed with applying pressures. The pressure effect on Tc can be understood as the suppression of magnetic fluctuations, which act as the glue to form the cooper pairs. From the pressure effect on asymmetric parameter η of the electric-field gradient, we suggest a possible explanation for the difference between sample dependence and pressure dependence.
    Journal of Physics and Chemistry of Solids 01/2008; 69(12):3132-3135. · 1.53 Impact Factor
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    ABSTRACT: We performed Co nuclear quadrupole resonance (NQR) measurements on bilayered hydrate cobaltate Nax(H3O)CoO2·yH2O. The sample we used demonstrates a magnetic transition at TM˜6K, which is observed from susceptibility, nuclear spin-lattice relaxation rate, and NQR spectrum measurements. We observed inhomogeneous broadening of the NQR spectrum below TM. In order to interpret the internal field distribution, we numerically solved a nuclear-spin Hamiltonian including the nuclear quadrupole interaction and the Zeeman interaction, and estimated the internal fields from the observed Co-NQR spectrum. It is considered that the internal fields possess weak modulation. The obtained internal-field distribution suggests the occurrence of the spin-density-wave ordering.
    Physica B Condensed Matter 01/2008; 403:1089-1091. · 1.33 Impact Factor
  • Physica B Condensed Matter 01/2008; 403:1089-1091. · 1.33 Impact Factor

Publication Stats

265 Citations
79.23 Total Impact Points

Institutions

  • 2005–2012
    • Kyoto University
      • Department of Physics II
      Kyoto, Kyoto-fu, Japan
  • 2009–2011
    • Université Paris-Sud 11
      • Laboratoire de Physique des Solides
      Orsay, Île-de-France, France
  • 2006–2007
    • National Institute for Materials Science
      • Advanced Materials Laboratory
      Tsukuba, Ibaraki-ken, Japan