S. Nakamura

Tohoku University, Sendai, Kagoshima, Japan

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Publications (63)194.98 Total impact

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    ABSTRACT: We have investigated magneto-transport properties in a single crystal of pyrochore-type Nd2Ir2O7. The metallic conduction is observed on the antiferromagnetic domain walls of the all-in all-out type Ir-5d moment ordered insulating bulk state, that can be finely controlled by external magnetic field along [111]. On the other hand, an applied field along [001] induces the bulk phase transition from insulator to semimetal as a consequence of the ?eld-induced modification of Nd-4f and Ir-5d moment con?gurations. A theoretical calculation consistently describing the experimentally observed features suggests a variety of exotic topological states as functions of electron correlation and Ir-5d moment orders which can be finely tuned by choice of rare-earth ion and by magnetic field, respectively.
    Physical Review Letters 06/2015; 115(5). DOI:10.1103/PhysRevLett.115.056402 · 7.51 Impact Factor
  • S. Nakamura · S. Toyoshima · N. Kabeya · K. Katoh · T. Nojima · A. Ochiai ·
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    ABSTRACT: We have synthesized single crystals of ternary intermetallic Yb3Ru4Al12 with a distorted kagome lattice structure, and investigated the low-temperature resistivity, specific heat, magnetization, and magnetic phase transitions. Yb3Ru4Al12 is the first 4f system that has a Gd3Ru4Al12-type crystal structure where antiferromagnetic interaction acts on the spin. The crystal electric field (CEF) ground state of this compound is determined as a well isolated twofold degenerate state that is subjected to a strong easy-plane-type magnetic anisotropy. In the present study, the spin system of Yb3Ru4Al12 is regarded as an AFM XY model of S=1/2. This compound undergoes successive magnetic phase transitions at 1.5 and 1.6 K, and the resistivity exhibits T2 behavior below 1 K. The ratio of the coefficient of the T2 term in the resistivity A, and that of the electronic specific heat coefficient γ0, deviates from the Kadowaki-Woods (KW) law. The successive phase transitions and low-temperature properties of Yb3Ru4Al12 where geometrical frustration and heavy fermion behavior occur are discussed.
    Physical Review B 06/2015; 91(21). DOI:10.1103/PhysRevB.91.214426 · 3.74 Impact Factor
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    ABSTRACT: To investigate the role of multipoles in the phase transitions of NdPd3S4, measurements of specific heat, magnetization, and electrical resistivity were performed for single-crystalline samples. We also carried out powder neutron diffraction experiments to identify a magnetically ordered structure. At zero magnetic field, an antiferromagnetic (AFM) transition occurs at TN = 1.8 K. When a magnetic field is applied, the AFM ordered phase, characterized by a propagation vector k = (1, 0, 0), switches into field-induced phases. We propose that the field-induced phases are associated with quadrupolar ordering.
    Journal of Physics Conference Series 12/2012; 391(1):2076-. DOI:10.1088/1742-6596/391/1/012076
  • A. Ochiai · S. Nakamura · T. Nojima · K. Katoh ·
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    ABSTRACT: Magnetic properties of YbAl3C3 with the hexagonal ScAl3C3-type structure have been investigated by the magnetization (M) and specific-heat (C) measurements under magnetic fields (H). YbAl3C3 is reported to show a spin-gap state, which is considered to be ascribed to a magnetic dimer formed in the orthorhombic phase below the structural transition temperature (Ts= 77 K). Present study has revealed history-dependent magnetic properties below Ts and field-induced anomalous magnetic states at low temperatures. The former is considered to be attributed to the cross correlation between the structural deformation and the magnetic field similar to those observed in multiferroic materials, although YbAl3C3 below Ts is not in the ferromagnetic state but certainly in the dimer state. The latter is partially similar to that observed in the field-induced ordered phase (FIOP) of d-electron dimer systems. The M versus H curve at low temperatures exhibits a kink at a certain magnetic field and then it increases in proportion to the field like that of FIOP. However, neither kink nor peak suggesting the emergence of FIOP is observed in the temperature dependence of both M(T) and C(T). Instead, C/T shows an anomalous increase proportional to −lnT with decreasing temperature in a finite field range, suggesting an anomalous disordered state such as a non-Fermi-liquid state in a strongly correlated f-electron system. These anomalous magnetic states may be partially relevant to characteristics of f-electron dimer system.
    Physical review. B, Condensed matter 04/2012; 85(14). DOI:10.1103/PhysRevB.85.144416 · 3.66 Impact Factor
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    ABSTRACT: We report magnetotransport properties of a two-dimensional electron gas confined at MgZnO/ZnO heterointerface in a high magnetic field up to 26 T. High electron mobility and low charge carrier density enabled the observation of the fractional quantum Hall state ν = 1/3. For an even lower charge carrier density, we observe a transition from quantum Hall liquid to an insulator below the filling factor 1/3. Because of the large electron effective mass in ZnO, we suggest the MgZnO/ZnO heterostructures to be a prototype system for highly correlated quantum Hall physics.
    Physical review. B, Condensed matter 07/2011; 84(3). DOI:10.1103/PhysRevB.84.033304 · 3.66 Impact Factor
  • T. Nojima · H. Tada · S. Nakamura · N. Kobayashi · H. Shimotani · Y. Iwasa ·
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    ABSTRACT: We demonstrated extreme hole reduction and electron accumulation in YBa2Cu3Oy films by an electrochemical technique. Following the transition from superconductor to insulator, we succeeded in doping electrons as evidenced by the decrease in resistivity and sign change of the Hall coefficient. Further doping resulted in metallic n-type YBa2Cu3Oy with a carrier density of ˜2.5 × 1020 cm-3, but without any sign of superconductivity. The oxygen vacancies electrochemically induced in the CuO2 planes played a major role in the p-n transition.
    Physical review. B, Condensed matter 07/2011; 84(2). DOI:10.1103/PhysRevB.84.020502 · 3.66 Impact Factor

  • K Ueno · S Nakamura · H Shimotani · H T Yuan · N Kimura · T Nojima · H Aoki · Y Iwasa · M Kawasaki ·
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    ABSTRACT: Superconductivity at interfaces has been investigated since the first demonstration of electric-field-tunable superconductivity in ultrathin films in 1960(1). So far, research on interface superconductivity has focused on materials that are known to be superconductors in bulk. Here, we show that electrostatic carrier doping can induce superconductivity in KTaO(3), a material in which superconductivity has not been observed before. Taking advantage of the large capacitance of the self-organized electric double layer that forms at the interface between an ionic liquid and KTaO(3) (ref. 12), we achieve a charge carrier density that is an order of magnitude larger than the density that can be achieved with conventional chemical doping. Superconductivity emerges in KTaO(3) at 50 mK for two-dimensional carrier densities in the range 2.3 × 10(14) to 3.7 × 10(14) cm(-2). The present result clearly shows that electrostatic carrier doping can lead to new states of matter at nanoscale interfaces.
    Nature Nanotechnology 05/2011; 6(7):408-12. DOI:10.1038/nnano.2011.78 · 34.05 Impact Factor
  • K. Ueno · S. Nakamura · H. Shimotani · T. Nojima · Y. Iwasa · M. Kawasaki ·
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    ABSTRACT: We briefly review principles and main features of an electric double layer transistor (EDLT) as well as electric field induced superconductivity in SrTiO3. EDLT is a field-effect transistor that employs an electrolyte as a gate dielectric. An electric double layer between a semiconductor and the electrolyte attains much higher breakdown field than the maximum of a solid gate dielectric, resulting in high density charge accumulation up to 1014cm-2. That density is sufficient for inducing new physical phases, such as superconductivity and ferromagnetism, on various oxide systems. We employed a surface of a SrTiO3 single crystal as a semiconductor channel. We have demonstrated insulator-tosuperconductor transition by electric field-effect without chemical doping. Charge carrier density was linearly increased from zero to 1014 cm-2 with increasing gate bias to 3.5 V. Superconducting critical parameters, such as critical temperature Tc, critical magnetic field Hc, and critical current density Jc were examined as a function of carrier density by varying gate bias. Tc was almost constant as a function of the carrier density, contrasting to bell-shaped dependence of Hc and Jc. Temperature dependence of I-V curve shows the BKT-type transition, which indicates two-dimensional superconductivity in the electric field induced superconductivity.
    Proceedings of SPIE - The International Society for Optical Engineering 02/2011; 7940. DOI:10.1117/12.879922 · 0.20 Impact Factor
  • S Nakamura · T Iwashita · T Nojima · A Yoshihara · S Ohnuma · H Fujimori ·
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    ABSTRACT: We investigate the transport and magnetic properties of ferromagnetic Fe–Al–O and Co–Al–O granular films. A negative linear magnetoresistivity is found at least up to 27 T for Co–Al–O films. The temperature dependence of the magnetization of Fe–Al–O films can be explained in terms of the T1.5-law for three dimensional spin waves theory, but that of Co–Al–O films does not obey the simple power law expected from this theory. The coercive field of a Co–Al–O film is strongly dependent on temperature.
    Journal of Physics Conference Series 01/2011; 266(1):012019. DOI:10.1088/1742-6596/266/1/012019
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    ABSTRACT: A quantum spin system is seldom observed in f-electron compounds, probably because their magnetic ground states are represented by the total angular momentum J which is usually much larger than 1/2. One of exceptional cases is Yb4As3, which behaves as a one-dimensional S=1/2 Heisenberg antiferromagnet. Recently, a new candidate of the quantum spin system YbAl3C3 with the ScAl3C3 type crystal structure has been found, where a magnetic dimer ground state is realized at low temperatures. To investigate the mechanism of the magnetic dimer formation in YbAl3C3, we have prepared several rare earth compounds RAl3C3with the ScAl3C3 type crystal structure and compared their physical properties with those of YbAl3C3.
    Journal of Physics Conference Series 01/2010; 200(2). DOI:10.1088/1742-6596/200/2/022040
  • S. Nakamura · T. Iwashita · T. Nojima · A. Yoshihara · S. Ohnuma · H. Fujimori ·
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    ABSTRACT: We investigate the transport properties of ferromagnetic granular Co-Al-O films. The electrical resistivity of these films has a T2 dependence at low temperatures, which can be explained in terms of scattering of electrons by spin waves. The resistivity of these films was found to have a logarithmic temperature dependence. The negative magnetoresistivity has long tails in high field ranges.
    Journal of Physics Conference Series 01/2010; 200(1):2141-. DOI:10.1088/1742-6596/200/1/012141
  • E. Matsuoka · D. Usui · S. Nakamura · T. Nojima · S. Takagi · H. Onodera ·
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    ABSTRACT: The magnetic properties of single-crystalline RPd3S4 (R=Ho, Er) have been examined by the measurements of the specific heat C(T, B), magnetization M(T, B) and magnetic susceptibility M/B(T). The previous study using polycrystalline samples has reported that HoPd3S4 and ErPd3S4 show successive phase transitions above 0.5K. However, the present measurements using the single-crystalline samples have revealed that HoPd3S4 and ErPd3S4 compounds do not show any phase transitions down to 0.5K. The anomalies appeared in C(T, B) of both compounds could be understood as a Schottky specific heat of R nuclei and 4f electrons.
    Physica B Condensed Matter 10/2009; 404(19):3297-3299. DOI:10.1016/j.physb.2009.07.095 · 1.32 Impact Factor
  • H Tada · T Nojima · S Nakamura · H Shimotani · Y Iwasa · N Kobayashi ·
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    ABSTRACT: We report on the continuous carrier control from holes to the electrons in YBa2Cu3Oy (YBCO) films by an electrochemical reaction method. In this method, the reduction in the hole concentration or the electron doping occurs at room temperature simply by applying a voltage of ~1 V between the YBCO film and the Pt electrode in the electrolyte solution of AC1O4 /polyethylene oxide, where A = K or Cs. When the initial sample is slightly overdoped, a dome-like increase and decrease of Tc and an increase of resistance R and Hall coefficient RH, which correspond to a reduction in the hole concentration to ~1021 cm-3, are observed with increasing voltage or application time. These results suggest that this electrochemical method is a straightforward and effective method for electron doping. Starting with a heavily underdoped sample, which is not superconductive, we succeeded in changing the sign of RH from positive to negative and in rapidly reducing R.
    Journal of Physics Conference Series 03/2009; 150(5). DOI:10.1088/1742-6596/150/5/052255
  • H. Tada · T. Nojima · S. Nakamura · H. Shimotani · Y. Iwasa · N. Kobayashi ·
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    ABSTRACT: Art. No.: 052255 , Cited By (since 1996): 3 , Export Date: 4 August 2012 , Source: Scopus
    Journal of Physics Conference Series 01/2009; 150(5).
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    K Ueno · S Nakamura · H Shimotani · A Ohtomo · N Kimura · T Nojima · H Aoki · Y Iwasa · M Kawasaki ·
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    ABSTRACT: Electric field control of charge carrier density has long been a key technology to tune the physical properties of condensed matter, exploring the modern semiconductor industry. One of the big challenges is to increase the maximum attainable carrier density so that we can induce superconductivity in field-effect-transistor geometry. However, such experiments have so far been limited to modulation of the critical temperature in originally conducting samples because of dielectric breakdown. Here we report electric-field-induced superconductivity in an insulator by using an electric-double-layer gating in an organic electrolyte. Sheet carrier density was enhanced from zero to 10(14) cm(-2) by applying a gate voltage of up to 3.5 V to a pristine SrTiO(3) single-crystal channel. A two-dimensional superconducting state emerged below a critical temperature of 0.4 K, comparable to the maximum value for chemically doped bulk crystals, indicating this method as promising for searching for unprecedented superconducting states.
    Nature Material 11/2008; 7(11):855-8. DOI:10.1038/nmat2298 · 36.50 Impact Factor
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    ABSTRACT: We have measured the electrical resistivity of Pr(Os1-xRux)4Sb12(Os1-xRux)4Sb12, mixed crystal of the unconventional superconductor PrOs4Sb12PrOs4Sb12 and BCS-type superconductor PrRu4Sb12PrRu4Sb12, and determined the coefficients of the T2T2-term (A-value) in the normal states. The A-value obtained decreases with increasing x in the range of 0<x<0.60<x<0.6 and becomes constant when x>0.6x>0.6. This suggests that the mass enhancement takes place in the vicinity of x=0.6x=0.6 as well as the unconventional superconductivity.
    Journal of Magnetism and Magnetic Materials 03/2007; 310(2):258-259. DOI:10.1016/j.jmmm.2006.10.036 · 1.97 Impact Factor
  • S Nakamura · M Endo · H Yamamoto · T Isshiki · N Kimura · H Aoki · T Nojima · S Otani · S Kunii ·
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    ABSTRACT: We report unusual evolution of the conduction-electron state in the localized f electron system CexLa1-xB6 from normal electron state to heavy Fermi liquid (FL) state through local FL and non-FL states with increasing Ce concentration and/or with increasing magnetic field. The effective mass of quasiparticle or the coefficient A of T2 term of resistivity is found to increase divergently near the boundary between FL state and non-FL state. The features of the non-FL state are also different from those of the typical non-FL systems previously observed or theoretically predicted.
    Physical Review Letters 01/2007; 97(23):237204. DOI:10.1103/PhysRevLett.97.237204 · 7.51 Impact Factor
  • A. Ochiai · H. Akita · G. Yoshino · S. Nakamura · T. Nojima ·
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    ABSTRACT: We have measured the electrical resistivity of Pr(Os1-xRu)Sb12, mixed crystal of the unconventional superconductor PrOs4Sb12 and BCS-type superconductor PrRu4Sb12, and determined the coefficients of the T-term (A-value) in the normal states. The A-value obtained decreases with increasing x in the range of 0
    Journal of Magnetism and Magnetic Materials 01/2007; 310:258-259. · 1.97 Impact Factor
  • T. Nojima · T. Hyodo · S. Nakamura · N. Kobayashi ·
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    ABSTRACT: We have studied the voltage-current characteristics in La0.7Ca0.3MnO3/YBa2Cu3Oy/Au, ferromagnet/superconductor/nonmagnetic metal, double tunnel junctions. Near Tc, the tunnel conductance G(V) reveals a step-like decrease, which can be ascribed to the phenomenon of the phase transition from superconducting state to normal one caused by the spin accumulation effect. With decreasing temperature T, the step in G(V) changes to the sharp peak. Our results indicate that the spin-injection-induced phase transition has a nature of a first order transition at low T.
    09/2006; DOI:10.1063/1.2354987

Publication Stats

862 Citations
194.98 Total Impact Points


  • 1992-2012
    • Tohoku University
      • • Institute for Materials Research
      • • Department of Physics
      Sendai, Kagoshima, Japan
    • Okayama University
      • Medical School
      Okayama, Okayama, Japan
  • 1997-2000
    • Niigata University
      • Department of Physics
      Niahi-niigata, Niigata, Japan