M. Naito

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

Are you M. Naito?

Claim your profile

Publications (176)313.67 Total impact

  • [Show abstract] [Hide abstract]
    ABSTRACT: Iron nitride films were grown by atomic-nitrogen-assisted molecular beam epitaxy. ZnS-type FeN (γ″-FeN) was synthesized with low Fe evaporation rates (≤ 0.2 Å/s) at low growth temperatures below 210 °C whereas γ′-Fe4N was synthesized with high Fe evaporation rates (≥1.0 Å/s) at 130–415 °C. Our results indicate that the stabilization of ZnS-type FeN requires not only the fulfillment of thermodynamic constraints but also the control of a delicate balance of kinetically driven competition. The use of lattice-matched GaN(0001) substrates enables the growth of epitaxial films of ZnS-type FeN.
    No preview · Article · Apr 2015 · Journal of Crystal Growth
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Superconducting Pr$_2$CuO$_x$, $x\simeq 4$ films with $T^\prime$ structure and a $T_c$ of 27 K have been investigated by millimeter-wave transmission and broadband (infrared-to-ultraviolet) reflectivity measurements in the normal and superconducting state. The results obtained by both experimental methods show a consistent picture of the superconducting condensate formation below $T_c$. An Eliashberg analysis of the data proves $d$-wave superconductivity and unitary-limit impurity scattering of the charge carriers below $T_{c}$. The derived electron-exchange boson interaction spectral function $I^2\chi(\omega)$ shows only marginal changes at the superconducting transition with the mass enhancement factor $\lambda$, the first inverse moment of $I^2\chi(\omega)$, being equal to 4.16 at 30 K and to 4.25 at 4 K.
    Full-text · Article · Jan 2015 · Journal of Physics Condensed Matter
  • [Show abstract] [Hide abstract]
    ABSTRACT: We report on one-step growth of superconducting SmFeAs(O,F) films without post-growth fluorine diffusion. One-step growth is a prerequisite for multilayer deposition, which is required for fabricating sandwich-type Josephson junctions; however, it has not been achieved because of a lack of a suitable fluorine source. We found that FeF2, a ‘soft’ fluoride, decomposes at relatively low temperatures (∼650 °C) and supplies fluorine as well as Fe to films. The use of FeF2 has enabled us to perform reproducible growth of SmFeAs(O,F) films with a systematic change of the film’s properties by varying growth parameters. Now, single-phase single-crystalline SmFeAs(O,F) films with &${{T}_{{\rm c}}}^{{\rm end}}\;\gt \;50\;{\rm K}$; are routinely prepared. Furthermore, the c-axis lattice parameter of the films are varied in a systematic manner with the FeF2 deposition rate, indicating that the F doping level seems to be controllable. The comparison of one-step growth and our previous two-step growth demonstrates that a more homogeneous F concentration is achieved by one-step growth, whereas higher F concentration is achieved by two-step growth.
    No preview · Article · Jan 2015 · Superconductor Science and Technology
  • Ai Ikeda · Takaaki Manabe · Michio Naito
    [Show abstract] [Hide abstract]
    ABSTRACT: Reduction agents, such as an activated carbon, TiH2, and CaH2 powders, were compared from a viewpoint of the facility for the topotactic reduction of LaNiO3 to LaNiO2 films. The activated carbon did not yield infinite-layer LaNiO2 whereas both of TiH2 and CaH2 yielded infinite-layer LaNiO2 with low resistivity (∼ 1 mΩcm at 300 K) as well as metallic behavior down to 70 K. Thermal desorption spectroscopy indicated that H2 released from metal hydrides plays a dominant role in the topotactic reduction.
    No preview · Article · Nov 2014 · Physica C Superconductivity
  • Ai Ikeda · Takaaki Manabe · Michio Naito
    [Show abstract] [Hide abstract]
    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.
    No preview · Article · Oct 2014 · Physica C Superconductivity
  • Source
    [Show abstract] [Hide abstract]
    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.
    Full-text · Article · Jun 2014 · Physical review. B, Condensed matter
  • Source
    [Show abstract] [Hide abstract]
    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.
    Full-text · Article · May 2014 · Physical Review B
  • [Show abstract] [Hide abstract]
    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.
    No preview · Article · Feb 2014 · Proceedings of SPIE - The International Society for Optical Engineering
  • [Show abstract] [Hide abstract]
    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.
    No preview · Article · Feb 2014 · Applied Physics Letters
  • Ai Ikeda · Takaaki Manabe · Michio Naito
    [Show abstract] [Hide abstract]
    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.
    No preview · Article · Dec 2013 · Physica C Superconductivity
  • Keisuke Yamazaki · Michio Naito
    [Show abstract] [Hide abstract]
    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.
    No preview · Article · Dec 2013 · Physica C Superconductivity
  • [Show abstract] [Hide abstract]
    ABSTRACT: Molecular beam epitaxy (MBE) has been an excellent tool to design artificial heterostructures in the research field of semiconductors by means of an alternate stacking of existing lattices. This article describes further capabilities of MBE as a synthesis tool sui generis, especially for multi-cation oxides, due to low-temperature reaction and the pseudomorphic stabilization effect by epitaxy. Single-crystalline Sr0.9La0.1CuO2 films exhibiting metallic conduction and superconductivity are successfully prepared by MBE. A new phase, T⁎-La2CuO4, is also stabilized on a DyScO3 substrate. Methods of high-precision rate control of each constituent element, which is prerequisite for a reproducible growth of the multi-cation oxide films, are also discussed.
    No preview · Article · Sep 2013 · Journal of Crystal Growth
  • Source
    [Show abstract] [Hide abstract]
    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.
    Preview · Article · Jul 2013 · Scientific Reports
  • Source
    [Show abstract] [Hide abstract]
    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.
    Full-text · Article · Jul 2013 · Scientific Reports
  • [Show abstract] [Hide abstract]
    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.
    No preview · Article · May 2012 · Physical review. B, Condensed matter
  • Shinya Ueda · Soichiro Takeda · Shiro Takano · Michio Naito
    [Show abstract] [Hide abstract]
    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).
    No preview · Article · May 2012 · Applied Physics Express
  • S. Takano · S. Ueda · S. Takeda · H. Sugawara · M. Naito
    [Show abstract] [Hide abstract]
    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.
    No preview · Article · May 2012 · Physica C Superconductivity
  • Source
    S Takeda · S Ueda · S Takano · A Yamamoto · M Naito
    [Show abstract] [Hide abstract]
    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.
    Preview · Article · Mar 2012 · Superconductor Science and Technology
  • [Show abstract] [Hide abstract]
    ABSTRACT: Although high-Tc superconductivity has been reported for several different material families, as exemplified by MgB2 and iron-based compounds, the record in Tc is held by cuprate superconductors. The electronic phase diagram of the cuprates, however, remains enigmatic and is still a key ingredient to understand the mechanism of high-Tc superconductivity. In this review, we present our combined efforts to synthesize MBEand MOD- 214-type cuprates obtained within more than 20 years. Intended to promote the comprehension of high-Tc superconductivity, our experimental efforts are summarized with respect to materials-research-related aspects as well as to the physical properties. A revised view of the electronic phase diagram, derived from our experimental observations on T and T' cuprates, is presented as the summary.
    No preview · Article · Mar 2012
  • H. Oyanagi · A. Tsukada · M. Naito
    [Show abstract] [Hide abstract]
    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.
    No preview · Article · Feb 2012 · Proceedings of SPIE - The International Society for Optical Engineering

Publication Stats

2k Citations
313.67 Total Impact Points

Institutions

  • 2005-2015
    • Tokyo University of Agriculture and Technology
      • Department of Applied Physics
      Edo, Tokyo, Japan
  • 2013-2014
    • NTT DATA Corporation
      Edo, Tōkyō, Japan
  • 2010-2012
    • Japan Science and Technology Agency (JST)
      Edo, Tōkyō, Japan
  • 2011
    • The University of Tokyo
      白山, Tōkyō, Japan
  • 2008
    • National Institute of Advanced Industrial Science and Technology
      Tsukuba, Ibaraki, Japan
  • 1998-2008
    • NTT DOCOMO
      Edo, Tōkyō, Japan
  • 2007
    • Tokyo University of Agriculture
      Edo, Tōkyō, Japan
  • 2002-2004
    • Tokyo University of Science
      • Department of Applied Physics
      Edo, Tōkyō, Japan
    • The Ohio State University
      Columbus, Ohio, United States
  • 2001
    • Tsinghua University
      • Department of Physics
      Peping, Beijing, China
    • University of Cologne
      • II. Institute of Physics
      Köln, North Rhine-Westphalia, Germany
  • 1997
    • Nippon Telegraph and Telephone
      Edo, Tōkyō, Japan