A. Tsukada

Stanford University, Palo Alto, CA, United States

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Publications (48)68.36 Total impact

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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: High quality potassium tungsten oxide (K0.33 WOy) films were synthesized by pulsed laser deposition followed by annealing in vacuum. Oxygen concentration modulated anomalous correlation of metal-insulator and superconductivity transitions were studied; a similar scenario was recently suggested in the literature [1] for polycrystalline rubidium tungsten oxide samples. Detailed studies of the transport properties below and above the superconducting transition temperature exhibit a diversity of unexpected behavior. Some of our results can be interpreted as a signature of reduced dimensionality in the ab-plane in oxygen-rich insulating samples, implying a formation of spatially inhomogeneous electronic structure. We compare such phenomenology to the behavior of other materials with strong electron-phonon interactions, and discuss its implication for the possible high temperature superconducting anomaly in sodium tungsten oxides reported in the literature [2]. [1] D. C. Ling et. al., J. Phys. Conf. Ser. 150, 052141 (2009). [2] S. Reich, and Y. Tsabba, Eur. Phys. J. B 9, 1 (1999).
    02/2012;
  • 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: We report the effects of growth conditions on the superconducting properties of FeSe films epitaxially grown on LaAlO3 substrates by pulsed laser deposition (PLD). Customary materials characterization techniques [X-ray diffraction (XRD), in-situ X-ray photoelectron spectroscopy (XPS), in-situ ultra-violet photoelectron spectroscopy (UPS), and scanning electron microscopy (SEM)] revealed the films had a c-axis oriented tetragonal structure with lattice constants dependent on the growth temperature (varied from 100 to 600°C). The standard four-point probe method was used to measure the resistivity and superconducting transitions. Films grown at 400–550°C showed a clear superconducting onset but no zero resistance down to 2 K. The highest superconducting onset temperature (TconsetT_{\mathrm{c}}^{\mathrm{onset}}) of 8 K was observed in films grown at 500°C and the onset temperature was clearly correlated to the ratio of the lattice constants (c/a). As the thickness of the FeSe films increased from 27 nm to 480 nm, TconsetT_{\mathrm{c}}^{\mathrm{onset}} also increased as the strain in the system was relaxed.
    Applied Physics A 01/2011; 104(1):311-318. · 1.69 Impact Factor
  • Physical review. B, Condensed matter 03/2010; 81(9). · 3.77 Impact Factor
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    ABSTRACT: Hall effect in tungsten-oxide thin-films is investigated. Tungsten oxides (MxWO3-y) have long been studied for their interesting structural, electronic and electro-chromic properties. A pure tungsten trioxide (WO3) is an insulator and doping of M (M = alkali or alkali-earth element) or oxygen deficiency makes the system conducting and sometimes superconducting. Indeed, even some very high Tc anomalies have been reported. We have succeeded in growth of superconducting KxWO3-y thin films and noticed that oxygen composition strongly affect the superconducting properties. Few reports for oxygen non-stoichiometry effects on the superconductivity in tungsten oxides are available, and the relationship between these normal state electronic properties and the superconducting properties of these materials has not yet been fully clarified. In this presentation, we will discuss relationship between superconductivity, oxygen non-stoichiometry and Hall coefficient.
    03/2010;
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    ABSTRACT: We present a detailed temperature-dependent Raman light scattering study of optical phonons in molecular-beam-epitaxy-grown films of the electron-doped superconductor La2 -x Ce x CuO4 close to optimal doping (x ~ 0.08, T c = 29 K and x ~ 0.1, T c = 27 K). The main focus of this work is a detailed characterization and microstructural analysis of the films. Based on micro-Raman spectroscopy in combination with X-ray diffraction, energy-dispersive X-ray analysis, and scanning electron microscopy, some of the observed phonon modes can be attributed to micron-sized inclusions of Cu2O. In the slightly underdoped film (x ~ 0.08), both the Cu2O modes and others that can be assigned to the La2 -x Ce x CuO4 matrix show pronounced softening and narrowing upon cooling below T ~ T c . Based on control measurements on commercial Cu2O powders and on a comparison to prior Raman scattering studies of other high-temperature superconductors, we speculate that proximity effects at La2 -x Ce x CuO4/Cu2O interfaces may be responsible for these anomalies. Experiments on the slightly overdoped La2 -x Ce x CuO4 film (x ~ 0.1) did not reveal comparable phonon anomalies.
    Physics of Condensed Matter 01/2010; 75(4):461-467. · 1.28 Impact Factor
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    ABSTRACT: Recently, in contrast to the general consensus, there has been accumulating evidence indicating a series of parent compounds T′-RE2CuO4 [RE=Pr, Nd, Sm, Eu, and Gd] prepared by metal–organic decomposition show superconductivity with Tc even exceeding 30K [O. Matsumoto, A. Utsuki, A. Tsukada, H. Yamamoto, T. Manabe, M. Naito, Phys. Rev. B 79 (2009) 100508(R) [1]]. Here, we report the preparation of superconducting films of T′-RE2CuO4 by MBE with a post-reduction process. Using the MBE-grown films, we also performed photoemission experiments, the results of which suggest the superconducting carriers in the parent compounds arise not from accidental doping but from their intrinsically metallic nature.
    Physica C Superconductivity 01/2010; 470. · 0.72 Impact Factor
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    ABSTRACT: We investigated the generic phase diagram of the electron doped superconductor, Nd2−xCexCuO4, using films prepared by metal organic decomposition. After careful oxygen reduction treatment to remove interstitial Oap atoms, we found that the Tc increases monotonically from 24 K to 29 K with decreasing x from 0.15 to 0.00, demonstrating a quite different phase diagram from the previous bulk one. Based on the discussion on the tremendous influence of Oap “impurities” on superconductivity in T′ cuprates, we conclude that our result represents the generic phase diagram for oxygen-stoichiometric Nd2−xCexCuO4.
    Physica C Superconductivity 01/2010; 470. · 0.72 Impact Factor
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    ABSTRACT: We report the synthesis of infinite-layer LaNiO2 thin films by metal organic decomposition. Our work is aimed to synthesize perovskite-like oxides with 3d9 electronic configuration, which high-Tc copper oxides commonly take. The 3d9 configuration is very rare in oxides other than cuprates. Ni1+ oxides, even though Ni1+ is an unusual oxidation state, may be one of very few candidates. One example is infinite-layer LaNiO2. The bulk synthesis of LaNiO2 is difficult, but we demonstrate in this article that the thin-film synthesis of LaNiO2 by metal organic decomposition is rather easy. This is due to the advantage of thin films with a large-surface-to-volume ratio, which makes oxygen diffusion prompt. Although superconductivity has not been observed yet, resistivity measurements indicate that LaNiO2 is conductive.
    Physica C Superconductivity 10/2009; · 0.72 Impact Factor
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    ABSTRACT: Dynamic fluctuation effects of electron-doped La2-xCexCuO4 superconducting thin films for a wide range of the Ce concentration (x=0.075 to 0.15) are studied through the microwave spectroscopic measurements in zero magnetic field. Dynamic scaling analysis of the fluctuation-induced microwave conductivity shows that the three-dimensional (3D) XY critical behavior is observed in a wide range of electron dopings, in contrast to our previous results on the hole-doped La2-xSrxCuO4 thin films where three kinds of critical charge dynamics and two kinds of dimensional crossovers were observed by hole doping. Our results clearly indicate that the critical behaviors between hole-doped and electron-doped regions in the phase diagram of high-Tc cuprates are not symmetric intrinsically. Thus, correct theories describing high-temperature superconductivity in cuprates should meet this criterion.
    Journal of Physics Conference Series 03/2009; 150(5):052111.
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    ABSTRACT: We report the search for superconductivity in surface-doped WO3 films. Possible evidence for high temperature superconductivity in this system has been reported in the literature. In our work, WO3 films were grown by MBE and characterized by in-situ XPS, UPS, and ex-situ XRD and resistivity measurements. For some films, Na was deposited on the surface at room temperature, and the resultant Na+WO3-x films annealed in vacuum (˜ 10-8 Torr) for 1h at various temperatures (300 -- 800C). With increasing thickness of Na, the intensity of the Na1s peak in XPS spectra and area of the W5d state in UPS spectra increased, suggesting some charge transfer to the WO3 film. After annealing below 500C, XPS and UPS spectra did not change, while after annealing above 500C, the area of the W5d state drastically increased. The same behavior was observed for pure WO3-x films, however, suggesting that oxygen vacancies are created during the annealing. The films started to decompose above 700C. The resistivity of our films decreased with increasing annealing temperature. However, so far we have not observed any sign of superconductivity down to 2 K in any of our films. This work is supported by Air Force Office of Scientific Research.
    03/2009;
  • H. Oyanagi, C. Zhang, A. Tsukada, M. Naito
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    ABSTRACT: Lattice instability in the planar Cu–O bond in high-temperature superconducting cuprates is probed by the Cu K-edge extended X-ray absorption fine structure (EXAFS). Refined temperature-dependent polarized EXAFS data for high-quality La1.85Sr0.15CuO4 (LSCO) single crystals grown by MBE and TSFZ methods are analyzed and compared with the transport properties. Temperature-dependent oxygen displacement shows a signature of bond splitting at T * and a sharp drop due to superconductivity coherence at T c. Detailed analysis of the anomalous oxygen displacement demonstrates the Cu–O bond splitting, i.e., long and short bonds (ΔR≈0.12 Å), which is in good agreement with the recent cluster calculation. The results indicate that the origin of pseudogap is related to the formation of charged dynamical lattice distortions (polarons) that are coexistent with metallic (superconducting) domains.
    Journal of Superconductivity and Novel Magnetism 01/2009; 22(2):165-168. · 0.70 Impact Factor
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    ABSTRACT: Phase stability diagram and boundary of a- and c-axis orientation of SmBa2Cu3Oy films grown by pulsed laser deposition were studied. In the study, anomalously long-c tetragonal SmBa2Cu3Oy were discovered in films grown in low oxygen pressure. The long-c phase has 1% longer c-axis lattice constant compared to normal-c orthorhombic SmBa2Cu3Oy and has lower superconducting transition temperature ( ~ 70 K). The long-c phase is able to coexist in the normal-c matrix over a range of oxygen pressure and growth temperature.
    IEEE Transactions on Applied Superconductivity 01/2009; 19(3):3383-3386. · 1.20 Impact Factor
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    ABSTRACT: The parent compounds of high-Tc cuprates have long been considered to be antiferromagnetic insulators. In this Rapid Communication, however, we report that superconductivity is achieved in T′-R2CuO4 (R=Pr, Nd, Sm, Eu, Gd). Superconducting T′-R2CuO4 films were synthesized by using metal-organic decomposition. The highest Tc of T′-R2CuO4 is over 30 K, substantially higher than “electron-doped” analogs. Remarkably, Gd2CuO4, even the derivatives of which have not shown superconductivity so far, gets superconducting with Tconset as high as ∼20 K. The implication of our results is briefly discussed.
    Physical review. B, Condensed matter 01/2009; 79(10). · 3.77 Impact Factor
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    ABSTRACT: We report the synthesis of infinite-layer LaNiO2 thin films by metal organic decomposition. Our work is aimed to synthesize perovskite-like oxides with 3d9 electronic configuration, which is common to high-Tc copper oxides. The 3d9 configuration is very rare in oxides other than cuprates. Ni1+ oxides, even though Ni1+ is an unusual oxidation state, may be one of very few candidates. One example of the Ni1+ phases is infinite-layer LaNiO2. The bulk synthesis of LaNiO2 is difficult, but we demonstrate in this article that the thin-film synthesis of LaNiO2 by metal organic decomposition is rather easy. This is due to the advantage of thin films with a large-surface-to-volume ratio, which makes oxygen diffusion prompt. Resistivity measurements indicate that LaNiO2 is essentially a metal but unfortunately with no trace of superconductivity yet.
    12/2008;
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    ABSTRACT: We investigated the generic phase diagram of the electron doped superconductor, Nd2−xCexCuO4, using films prepared by metal organic decomposition. After careful oxygen reduction treatment to remove interstitial Oap atoms, we found that the Tc increases monotonically from 24 K to 29 K with decreasing x from 0.15 to 0.00, demonstrating a quite different phase diagram from the previous bulk one. The implication of our results is discussed on the basis of tremendous influence of Oap “impurities” on superconductivity and also magnetism in T′ cuprates. Then we conclude that our result represents the generic phase diagram for oxygen-stoichiometric Nd2−xCexCuO4.
    Physica C Superconductivity 12/2008; 469:924-927. · 0.72 Impact Factor
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    ABSTRACT: We have recently achieved superconductivity in undoped T'-RE2CuO4 (RE: Pr, Nd, Sm, Eu, and Gd), using epitaxial thin films by metal organic decomposition. The key recipes to achieve superconductivity are low-PO2 firing and subsequent vacuum reduction to minimize the amount of impurity oxygen atoms, which are very harmful to high-Tc superconductivity. In this article, we report our investigation on the reduction dependence of superconductivity of T'-RE2CuO4. For thin films, the amount of remnant Oap atoms is difficult to evaluate but we propose that one good measure for this may be the c-axis lattice constant, which tells us whether the reduction is insufficient or excessive.
    12/2008;
  • H. Oyanagi, A. Tsukada, M. Naito
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    ABSTRACT: Undoped T′-type cuprates are believed to be charge transfer (Mott–Hubbard) insulators. Recent observation of undoped superconducting T′-type cuprates suggests half-filling weakly correlated metallic electron states (Tsukada et al.), which is against the fundamental picture. Here we report the local structures of undoped T′-(La3+,Y3+)2CuO4 and doped T′-(La3+,Ce4+)2CuO4 thin-film single crystals by polarized extended X-ray absorption fine structure (EXAS). The in-plane (E//ab) polarized Cu K-EXAFS data shows a remarkable Cu–O bond distance displacement. The broadened Cu–O distribution suggests a large local lattice distortion in T′-type structures, which might influence the electronic structure.
    Journal of Physics and Chemistry of Solids 09/2008; · 1.53 Impact Factor
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    ABSTRACT: We use quasiparticle tunneling across La2-xCexCuO4 grain boundary junctions to probe the superconducting state and its disappearance with increasing temperature and magnetic field. A zero bias conductance peak due to zero energy surface Andreev bound states is a clear signature of the phase coherence of the superconducting state. Hence, such a peak must disappear at or below the upper critical field Bc2(T). For La2-xCexCuO4 this approach sets a lower bound for Bc2(0) approximately 25 T which is substantially higher than reported previously. The method of probing the superconducting state via Andreev bound states should also be applicable to other cuprate superconductors.
    Physical Review Letters 06/2008; 100(22):227001. · 7.73 Impact Factor

Publication Stats

201 Citations
68.36 Total Impact Points

Institutions

  • 2008–2011
    • Stanford University
      • Geballe Laboratory for Advanced Materials
      Palo Alto, CA, United States
  • 2006–2009
    • Tokyo University of Agriculture and Technology
      • Department of Applied Physics
      Tokyo, Tokyo-to, Japan
  • 2007
    • Tokyo University of Agriculture
      Edo, Tōkyō, Japan
  • 2003
    • University of Tuebingen
      • Institute of Physical and Theoretical Chemistry
      Tübingen, Baden-Wuerttemberg, Germany
    • The Ohio State University
      • Department of Physics
      Columbus, OH, United States
  • 2001–2003
    • Tokyo University of Science
      Edo, Tōkyō, Japan