M. Naito

Tokyo University of Agriculture and Technology, Edo, Tōkyō, Japan

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Publications (193)311.92 Total impact

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    ABSTRACT: Superconducting Pr$_2$CuO$_x$, $x\simeq 4$ (PCO) films with $T^\prime$ structure and a $T_c$ of 27 K have been investigated by various optical methods in a wide frequency (7 - 55000 cm$^{-1}$) and temperature (2 to 300 K) range. The optical spectra do not reveal any indication of a normal-state gap formation. A Drude-like peak centered at zero frequency dominates the optical conductivity below 150 K. At higher temperatures, it shifts to finite frequencies. The detailed analysis of the low-frequency conductivity reveals that the Drude peak and a far-infrared (FIR) peak centered at about 300 cm$^{-1}$ persist at all temperatures. The FIR-peak spectral weight is found to grow at the expense of the Drude spectral weight with increasing temperature. The temperature dependence of the penetration depth follows a behavior typical for $d$-wave superconductors. The absolute value of the penetration depth for zero temperature is 1.6 $\mu$m, indicating a rather low density of the superconducting condensate.
    Physical review. B, Condensed matter 06/2014; 90:024503. · 3.77 Impact Factor
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    ABSTRACT: The Meissner effect has been directly demonstrated by depth-resolved muon spin rotation measurements in high-quality thin films of the T'-structured cuprate, T'-La$_{1.9}$Y$_{0.1}$CuO$_4$, to confirm bulk superconductivity ($T_c\simeq21$ K) in its {\sl undoped} state. The gradual expelling of an external magnetic field is observed over a depth range of $\sim$100 nm in films with a thickness of 275(15) nm, from which the penetration depth is deduced to be 466(22) nm. Based on this result, we argue that the true ground state of the "parent" compound of the $n$-type cuprates is not a Mott insulator but a strongly correlated metal with colossal sensitivity to apical oxygen impurities.
    05/2014;
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    ABSTRACT: Multi-cation oxides with crystalline perfection known from single crystals of Si or Ge are a challenge not only for basic research but also towards a revolution of oxide electronic materials. Here, we present our approach for the synthesis of high-quality thin films of multi-cation oxides. We show that our synthesis method, using state-of the-art molecular beam epitaxy (MBE), facilitates for the design of new materials. We geared our MBE system with a precise rate control system of each constituent cation flux as well as activated oxygen (O*) and ozone (O3). The resulting performances of our MBE setup are unmatched with respect to high-quality film growth as well as multi-cation flexibility by demonstrating growth of various cuprate-, scandate-, argentate-, titanate-, and ruthenate thin films. Such augmented methods are key to novel materials and go well beyond the artificial stacking of known materials and lattices.
    02/2014;
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    ABSTRACT: The microstructures of SmFeAs(O,F), Ba(Fe,Co)2As2, and FeTe0.5Se0.5 prepared on CaF2 substrates were investigated using transmission electron microscopy. The SmFeAs(O,F)/CaF2 interface is steep, without a disordered layer. By contrast, a chemical reaction occurs at the interface in the cases of Ba(Fe,Co)2As2 and FeTe0.5Se0.5. The reaction layers are located on opposite sides of the interface for Ba(Fe,Co)2As2 and FeTe0.5Se0.5. We found that the lattice distortion of the three superconducting films on the CaF2 substrates enhances the TC values compared with films prepared on oxide substrates. The origin of this lattice deformation varies depending on the superconducting material.
    Applied Physics Letters 02/2014; 104(12). · 3.79 Impact Factor
  • Ai Ikeda, Takaaki Manabe, Michio Naito
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    ABSTRACT: We report the synthesis of perovskite RENiO3 films (RE = La, Pr, Nd, Sm, and Eu) by metal organic decomposition (MOD). The RENiO3 family is an ideal system for studying the metal–insulator transition due to the simplicity of the materials. One of the drawbacks is that the bulk synthesis of the RENiO3 requires processing at high oxygen pressures to stabilize Ni3+. Fundamentally, MOD is similar to solid-state reaction, but it turned out that the MOD synthesis tends to stabilize RENiO3 without the need for high oxygen pressure. The films prepared by MOD show high crystallinity and low resistivity. Furthermore, we have investigated the epitaxial strain effect and observed a dramatic effect in PrNiO3 and NdNiO3 films on LaAlO3 substrates. The metal–insulator transition in the PrNiO3 films on LaAlO3 is fully suppressed, whereas the metal–insulator transition temperature is considerably lowered in the NdNiO3 films on LaAlO3.
    Physica C Superconductivity 01/2014; 505:24–31. · 0.72 Impact Factor
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    ABSTRACT: In many of today's most interesting materials, strong interactions prevail upon the magnetic moments, the electrons, and the crystal lattice, forming strong links between these different aspects of the system. Particularly, in two-dimensional cuprates, where copper is either five- or six-fold coordinated, superconductivity is commonly induced by chemical doping which is deemed to be mandatory by destruction of long-range antiferromagnetic order of 3d(9) Cu(2+) moments. Here we show that superconductivity can be induced in Pr2CuO4, where copper is four-fold coordinated. We induced this novel quantum state of Pr2CuO4 by realizing pristine square-planar coordinated copper in the copper-oxygen planes, thus, resulting in critical superconducting temperatures even higher than by chemical doping. Our results demonstrate new degrees of freedom, i.e., coordination of copper, for the manipulation of magnetic and superconducting order parameters in quantum materials.
    Scientific Reports 07/2013; 3:2235. · 5.08 Impact Factor
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    ABSTRACT: The recently discovered oxypnictide superconductor SmFeAs(O,F) is the most attractive material among the Fe-based superconductors due to its highest transition temperature of 56 K and potential for high-field performance. In order to exploit this new material for superconducting applications, the knowledge and understanding of its electro-magnetic properties are needed. Recent success in fabricating epitaxial SmFeAs(O,F) thin films opens a great opportunity to explore their transport properties. Here we report on a high critical current density of over 10(5) A/cm(2) at 45 T and 4.2 K for both main field orientations, feature favourable for high-field magnet applications. Additionally, by investigating the pinning properties, we observed a dimensional crossover between the superconducting coherence length and the FeAs interlayer distance at 30-40 K, indicative of a possible intrinsic Josephson junction in SmFeAs(O,F) at low temperatures that can be employed in electronics applications such as a terahertz radiation source and a superconducting Qubit.
    Scientific Reports 07/2013; 3:2139. · 5.08 Impact Factor
  • Ai Ikeda, Takaaki Manabe, Michio Naito
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    ABSTRACT: Infinite-layer LaNiO2 thin films were synthesized by metal organic decomposition and subsequent topotactic reduction in hydrogen, and their transport properties were investigated. LaNiO2 is isostructural to SrCuO2, the parent compound of high-Tc Sr0.9La0.1CuO2 with Tc = 44 K, and has 3d9 configuration, which is very rare in oxides but common to high-Tc copper oxides. The bulk synthesis of LaNiO2 is not easy, but we demonstrate in this article that the thin-film synthesis of LaNiO2 is rather easy, thanks to a large-surface-to-volume ratio, which makes oxygen diffusion prompt. Our refined synthesis conditions produced highly conducting films of LaNiO2. The resistivity of the best film is as low as 640 μΩ cm at 295 K and decreases with temperature down to 230 K but it shows a gradual upturn at lower temperatures.
    Physica C Superconductivity 01/2013; 495:134–140. · 0.72 Impact Factor
  • Keisuke Yamazaki, Michio Naito
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    ABSTRACT: Recently we reported the growth of MgB2 films by pyrolysis of decaborane (B10H14) in Mg vapor. In this article, we discuss the intrinsic problem at the MgB2/Al2O3 interface, which was encountered during our optimization of the growth temperature. Mg is a stronger reductant than Al, and takes out oxygen from Al2O3. Increasing the growth temperature, this reaction becomes noticeable and leads to the formation of MgO and Mg1−xAlxB2, which deteriorates the properties of resultant MgB2 films. Our observations present a general issue in the choice of substrate and barrier materials in thin film growth and/or junction fabrication of compounds containing reactive cations such as Mg.
    Physica C Superconductivity 01/2013; 495:84–87. · 0.72 Impact Factor
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    ABSTRACT: We report a study of transport and magnetic properties of high-quality c axis-oriented superconducting T′-RE2−xCexCuO4 (RE = La, Pr, Nd) thin films grown by molecular beam epitaxy at optimal as well as x = 0.00 doping levels. While the superconducting coherence length ξGL decreases from RE = Nd to La, it remains nearly constant as a function of doping. For optimally doped T′-cuprates, the electronic conductivity is higher than the dopant-free sample, while Tc is higher for the dopant-free samples for RE = Nd and Pr. We estimated the superconducting penetration depth λab by using a relationship obtained from optical measurements and compared it to the diamagnetic response, which corresponds to the superfluid density ns. Our data suggest that electron doping increases the superfluid density ns significantly, but the pairing interaction is not affected. In the normal state, the temperature dependence of the resistivity was used to deduce the Fermi temperature TF, from which we estimated the charge-carrier concentration n. A systematic relationship between n, ns, and the superconducting transition temperature Tc was not observed. We finally conclude that the superconducting ground state is merely influenced by Ce doping with respect to Tc and ξGL but solely by the RE element.
    Physical review. B, Condensed matter 05/2012; 85(18). · 3.77 Impact Factor
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    ABSTRACT: As-grown superconducting thin films of SmFeAs(O,F) with Tc higher than 50 K were successfully grown by molecular beam epitaxy. In the growth of the films, we codeposited Sm, SmF3, Fe, and As in oxygen atmosphere. The most crucial factor for obtaining superconducting films has turned out to be the substrate choice. We have found that CaF2-buffered LaAlO3 is much more suitable for the growth of SmFeAs(O,F) films than bare LaAlO3 or CaF2 and yielded nearly single-phased SmFeAs(O,F) films with Tcon (Tcend) ˜ 52 K (50 K).
    Applied Physics Express 05/2012; 5(5):3101-. · 2.73 Impact Factor
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    ABSTRACT: Superconducting SmFeAs(O,F) films were prepared on various substrates by molecular beam epitaxy and subsequent fluorine diffusion from an overlayer of SmF3. We have performed a detailed comparison of films on different substrates in the structure and superconductivity. In general, fluoride substrates are more suitable than oxide substrates in obtaining better superconductivity of SmFeAs(O,F) films. The best substrate so far is CaF2, which yielded record high Tc, Tcon(Tcend) = 57.8 K (56.4 K). There is a clear correlation between Tc and epitaxial strain in SmFeAs(O,F) films on CaF2: “the less epitaxial strain, the higher Tc”. The implication of this correlation is briefly discussed.
    Physica C Superconductivity 05/2012; 475:10–13. · 0.72 Impact Factor
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    ABSTRACT: We have investigated the in-plane penetration depth, λ(T), of nominally undoped superconducting Pr2CuO4 films (Tc = 27.5 K) by measuring the phase-sensitive millimeter-wave transmission at temperatures between 2 and 30 K. The overall temperature dependence of the superfluid density, ns = λ2(0 K)/λ2(T), resembles the behavior typical for the cuprate superconductors [ns(T) ∝ 1 − (T/Tc)2]. However, a closer look on the penetration depth at low temperatures reveals flattening of its temperature dependence. We find λ(T) ∝ Tn with n = 2.8 ± 0.2.
    Physica C Superconductivity 02/2012; 473:11–13. · 0.72 Impact Factor
  • H. Oyanagi, A. Tsukada, M. Naito
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    ABSTRACT: The electronic state of T'-type cuprates which exhibit superconductivity without doping was investigated by polarized x-ray absorption spectroscopy (XAS) for T'-La2CuO4 and T'-(La,Y)2CuO4 thin film single-crystals. The effect of oxygenation and deoxygenation on the near-edge structures evidences the two processes create and remove apical oxygen defects that strongly suppress superconductivity. The near-edge spectra further indicate that the deoxygenation, well known as a common prerequisite for superconductivity, also creates in-plane oxygen defects, whose contribution to the n-type conduction and superconductivity without doping is not ruled out. The observed local lattice distortion consistent with the neutron scattering experiment may influence a long-range magnetic order favoring a metallic state without doping.
    Proc SPIE 02/2012;
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    ABSTRACT: Superconducting Nd1.85Ce0.15CuO4 (Tczero = 24 K) and Nd2CuO4 (Tczero = 25 K) thin films have been grown by molecular beam epitaxy and their magneto-transport and structural properties have been investigated. The as-grown films are insulators irrespective of the substitution level, and superconductivity is induced after the samples are treated by an annealing process under reducing atmospheres. Though the metallic conductivity is higher in the Ce4+ substituted sample, the superconducting properties are quite similar between Ce4+ substituted and substitution-free samples. A similar upper critical magnetic field as well as a similar superconducting transition temperature of Nd1.85Ce0.15CuO4 and Nd2CuO4 shows that the addition of electrons merely influences the superconducting state. Consequently, the appearance of an antiferromagnetic Mott insulating state solely depends on the annealing process, not on the electron doping or cerium substitution level.
    Japanese Journal of Applied Physics 01/2012; 51(1):0106-. · 1.07 Impact Factor
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    ABSTRACT: We report on our growth of superconducting SmFeAs(O, F) films by F diffusion. In our process, F-free SmFeAsO films were grown by molecular beam epitaxy (MBE) first, and subsequently F was introduced to the films via F diffusion from an overlayer of SmF3. We compared the growth conditions and also the properties of resultant films for CaF2 and LaAlO3 substrates. The best films on CaF2 exhibited a high transition temperature, K (56.4 K) at the highest, which may exceed the highest Tc ever reported for bulk samples. Furthermore, the films on CaF2 also showed high critical current density over 1 MA cm−2 in self-field at 5 K. On the other hand, the of the film on LaAlO3 was 50.3 K (49.3 K). The deteriorated superconducting properties on LaAlO3 appear to be due to oxygen diffusion from LaAlO3 to films.
    Superconductor Science and Technology 01/2012; 25(3). · 2.76 Impact Factor
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    ABSTRACT: We report the molecular beam epitaxy (MBE) growth of the iron-based superconductors, Ba1-xKxFe2As2 and SmFeAs(O,F). In the growth of Ba1-xKxFe2As2 films, the key to incorporating volatile K in films is low-temperature (≤350 °C) growth in reduced As flux. The highest Tc thus far obtained is Tc{}on (Tc{}end) = 38.0 K (35.8 K). In the growth of superconducting SmFeAs(O,F), we adopted two methods. In the first method, we first grew pristine SmFeAsO films, and subsequently introduced F into the films by diffusion from an overlayer of SmF3. In the second method, we grew as-grown superconducting SmFeAs(O,F) films by coevaporating Sm, SmF3, Fe, and As. Thus far, better results have been obtained by the first F diffusion method. The films prepared by F diffusion showed Tc{}on (Tc{}end) = 56.5 K (55.3 K), whereas the as-grown films showed Tc{}on (Tc{}end) = 51.5 K (48.0 K).
    Japanese Journal of Applied Physics 01/2012; 51(1):0103-. · 1.07 Impact Factor
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    ABSTRACT: Superconducting thin films of SmFeAs(O,F) were prepared by molecular beam epitaxy on fluoride substrates. In our process, F-free SmFeAsO films were grown first, and F was subsequently introduced to the films by diffusion from an overlayer of SmF3. By this simple process, record high Tc, namely, Tcon (Tcend) = 57.8 K (56.4 K) was obtained in a film on CaF2. Furthermore, the films on CaF2 showed high critical current density over 1 MA/cm2 in the self-field at 5 K. The correlation between superconductivity and epitaxial strain in SmFeAs(O,F) films is discussed.
    Applied Physics Letters 12/2011; 99(23). · 3.79 Impact Factor
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    ABSTRACT: Single-crystalline films of superconducting Ba1-xKxFe2As2 and Sr1-xKxFe2As2 were grown at temperatures below 350 °C under reduced As flux by molecular beam epitaxy. We performed a systematic study of the doping dependence of Tc in Ba1-xKxFe2As2 for x = 0.0 to 1.0 and Sr1-xKxFe2As2 for x = 0.0 to 0.5. The highest Tcon (Tcend) so far attained is 38.3 K (35.5 K) at x ˜ 0.3 for Ba1-xKxFe2As2, and 33.4 K (31.0 K) at x ˜ 0.4 for Sr1-xKxFe2As2. The optimum K-doping level inducing the highest Tc is different in the two systems. The implication of the results is briefly discussed.
    Physica C Superconductivity 11/2011; 471(21):1177-1180. · 0.72 Impact Factor
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    ABSTRACT: We report the growth of MgB2 films by thermally decomposing decaborane (B10H14) in Mg vapor. B10H14, solid at ambient temperature and stable in air, is much more convenient to use as a boron source compared to highly explosive diborane (B2H6) which is commonly used in the hybrid physical--chemical vapor deposition of MgB2 films. MgB2 films were grown on c-cut sapphire substrates held at 400--450 °C by supplying vaporized B10H14 into a pocket heater with high Mg vapor pressure. The resultant films were phase-pure, c-axis oriented, and in-plane aligned, and had self-field critical current density of ˜3× 107 A/cm2 at 20 K.
    Applied Physics Express - APPL PHYS EXPRESS. 01/2011;

Publication Stats

1k Citations
311.92 Total Impact Points

Institutions

  • 2005–2014
    • Tokyo University of Agriculture and Technology
      • Department of Applied Physics
      Edo, Tōkyō, Japan
  • 2010
    • Kitasato University
      Edo, Tōkyō, Japan
  • 2009
    • Stanford University
      • Geballe Laboratory for Advanced Materials
      Palo Alto, CA, United States
  • 2007
    • Tokyo University of Agriculture
      Edo, Tōkyō, Japan
  • 1990–2005
    • Nippon Telegraph and Telephone
      • NTT Basic Research Laboratories
      Edo, Tōkyō, Japan
  • 2004
    • Tokyo University of Science
      Edo, Tōkyō, Japan
  • 2003
    • Bavarian Academy of Sciences and Humanities
      Arching, Bavaria, Germany
  • 2002–2003
    • The Ohio State University
      • Department of Physics
      Columbus, OH, United States
  • 1998–2001
    • University of Cologne
      • II. Institute of Physics
      Köln, North Rhine-Westphalia, Germany
  • 1983–2000
    • The University of Tokyo
      • • Department of Applied Physics
      • • Department of Chemistry
      Tokyo, Tokyo-to, Japan
  • 1996
    • University of Tuebingen
      • Institute of Physical and Theoretical Chemistry
      Tübingen, Baden-Wuerttemberg, Germany
  • 1989
    • NTT DOCOMO
      Edo, Tōkyō, Japan