A. Tsukazaki

The University of Tokyo, Edo, Tōkyō, Japan

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Publications (88)375.04 Total impact

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    ABSTRACT: Topological insulators are a class of semiconductor exhibiting charge-gapped insulating behaviour in the bulk, but hosting a spin-polarized massless Dirac electron state at the surface. The presence of a topologically protected helical edge channel has been verified for the vacuum-facing surface of several topological insulators by means of angle-resolved photoemission spectroscopy and scanning tunnelling microscopy. By performing tunnelling spectroscopy on heterojunction devices composed of p-type topological insulator (Bi1-xSbx)2Te3 and n-type conventional semiconductor InP, we report the observation of such states at the solid-state interface. Under an applied magnetic field, we observe a resonance in the tunnelling conductance through the heterojunction due to the formation of Landau levels of two-dimensional Dirac electrons at the interface. Moreover, resonant tunnelling spectroscopy reveals a systematic dependence of the Fermi velocity and Dirac point energy on the composition x. The successful formation of robust non-trivial edge channels at a solid-state interface is an essential step towards functional junctions based on topological insulators.
    Nature Material 02/2014; 13(3):253-257. · 35.75 Impact Factor
  • A. Tsukazaki, A. Ohtomo, M. Kawasaki
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    ABSTRACT: The full text of this article is available in the PDF provided.
    01/2014; 47(7).
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    Y. Kozuka, A. Tsukazaki, M. Kawasaki
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    ABSTRACT: Recent technological advancement in ZnO heterostructures has expanded the possibility of device functionalities to various kinds of applications. In order to extract novel device functionalities in the heterostructures, one needs to fabricate high quality films and interfaces with minimal impurities, defects, and disorder. With employing molecular-beam epitaxy (MBE) and single crystal ZnO substrates, the density of residual impurities and defects can be drastically reduced and the optical and electrical properties have been dramatically improved for the ZnO films and heterostructures with MgxZn1-xO. Here, we overview such recent technological advancement from various aspects of application. Towards optoelectronic devices such as a light emitter and a photodetector in an ultraviolet region, the development of p-type ZnO and the fabrication of excellent Schottky contact, respectively, have been subjected to intensive studies for years. For the former, the fine tuning of the growth conditions to make MgxZn1-xO as intrinsic as possible has opened the possibilities of making p-type MgxZn1-xO through NH3 doping method. For the latter, conducting and transparent polymer films spin-coated on MgxZn1-xO was shown to give almost ideal Schottky junctions. The wavelength-selective detection can be realized with varying the Mg content. From the viewpoint of electronic devices, two-dimensional electrons confined at the MgxZn1-xO/ZnO interfaces are promising candidate for quantum devices because of high electron mobility and strong electron-electron correlation effect. These wonderful features and tremendous opportunities in ZnO-based heterostructures make this system unique and promising in oxide electronics and will lead to new quantum functionalities in optoelectronic devices and electronic applications with lower energy consumption and high performance.
    11/2013; 1(1).
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    ABSTRACT: The magnetoluminescence of the two-dimensional electron system in a single Mg x Zn1 - x O/ZnO hetero-junction ( x = 0.02) at a temperature of 0.3 K in magnetic fields up to 14 T has been studied. The concentration of two-dimensional electrons in the structure under study has been determined from the oscillations of the luminescence intensity as a function of the magnetic field. The value thus obtained is close to the one derived from transport measurements. The resonance corresponding to plasma excitations of two-dimensional electrons has been observed using optical detection of microwave absorption. In a magnetic field, the lower branch of magnetoplasma excitations, which corresponds to the propagation of edge magnetoplasmons in a structure with nearly square geometry, has been observed and investigated.
    JETP Letters 10/2013; 98(4):223-226. · 1.52 Impact Factor
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    ABSTRACT: Iron garnets are one of the most well-studied magnetic materials that enabled magnetic bubble memories and magneto-optical devices employing films with a perpendicular easy axis. However, most studies have been conducted on rather thick films (>1 μm), and it has not been elucidated whether it is possible to align the magnetic easy axis perpendicular to the film plane for much thinner (<100 nm) films by overcoming shape anisotropy. We studied the effects of epitaxial strain and film composition on the magnetic properties of 50-nm-thick garnet thin films grown by pulsed-laser deposition. Y3Fe5O12 was selected as the most prototypical garnet and Sm3−xTmxFe5O12 (x=1, 2, 3) was selected in view of its negatively large magnetostriction constants. We employed (111) planes of single crystalline Gd3Ga5O12 and (CaGd)3(MgGaZr)5O12 substrates to tune the epitaxial strain. Thin films with a pseudomorphic structure were fabricated with the in-plane strain (ε//) ranging from −1.5% to +0.5%, corresponding to the stress-induced anisotropy field (HA) ranging from −40 kOe to +25 kOe, respectively. The magnetization ratio of the out-of-plane to in-plane component (M⊥/M//) systematically varied in accord with HA, yielding M⊥/M// >1 for thin films with HA values larger than 20 kOe. Among the films grown, Tm3Fe5O12 on Gd3Ga5O12 showed the largest ε// and HA values of +0.5% and +25 kOe, respectively, to realize an apparently perpendicular easy axis, confirmed by a large M⊥/M// value of 7.8. Further, magnetic force microscope images showed a maze pattern typical of a perpendicularly magnetized film. These results reveal a method for tailoring the magnetic anisotropy of garnet ultrathin films by utilizing epitaxial strain. These thin films may be utilized to obtain nanoscale magnetic bubbles for use in novel devices.
    Journal of Magnetism and Magnetic Materials 08/2013; 339:63–70. · 1.83 Impact Factor
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    ABSTRACT: Torque magnetometry at low temperature and in high magnetic fields B is performed on a MgZnO/ZnO heterostructure incorporating a high-mobility two-dimensional electron system. We find a sawtooth-like quantum oscillatory magnetization M(B), i.e., the de Haas-van Alphen (dHvA) effect. At the same time, unexpected spike-like overshoots in M and non-equilibrium currents are observed which allow us to identify the microscopic nature and density of the residual disorder. The acceptor-like scatterers give rise to a magnetic thaw down effect which enhances the dHvA amplitude beyond the electron-electron interaction effects being present in the MgZnO/ZnO heterostructure
    07/2013;
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    ABSTRACT: We report the experimental determination of Rashba spin-orbit interaction (SOI) strength in two-dimensional electrons in a MgZnO/ZnO heterostructure using electrically detected electron spin resonance. The Rashba parameter is determined to be 7.0×10−14 eV m, which is the second smallest value among prevalent semiconductor heterostructures, following Si/SiGe. A long transverse spin relaxation time was derived to show a maximum value of 27 ns, owing to weak SOI. Our study demonstrates that the ZnO heterostructure is a promising candidate for spintronic devices utilizing long spin coherence.
    Physical review. B, Condensed matter 05/2013; 87(20).
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    ABSTRACT: We report on the photoluminescence (PL) properties of MgZnO/ZnO heterojunctions grown by plasma-assisted molecular-beam epitaxy. Influence of the applied magnetic field (B) on the radiative recombination of the two-dimensional electron gas (2DEG) is investigated up to 54 T. An increase in magnetic field in the range of B <= 20 T results in a redshift in the PL. Abrupt lineshape changes in the PL spectra are observed at higher magnetic fields, in correlation with the integer quantum Hall states. We attempt to interpret these features using the conventional model for the 2DEG-related PL based on the transition between the 2DEG and a hole as well as a model taking a bound state effect into account, i.e., a charged exciton. The comparison about the adequateness of these models was made, being in favor of the charged exciton model.
    Physical review. B, Condensed matter 02/2013; 87(8).
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    ABSTRACT: We performed combined magnetotransport and cyclotron resonance experiments on two-dimensional electron systems confined in the Mg_{x}Zn_{1-x}O/ZnO heterostructures over a wide range of carrier densities, from 1.9 to 12×10^{11} cm^{-2} (3.5≲r_{s}≲10, where r_{s} is the Wigner-Seitz radius). As the carrier density was reduced, the transport mass m_{tr}^{*} was strongly enhanced. In marked contrast, the effective masses determined from the cyclotron resonance m_{CR}^{*} were found to be independent of the carrier density and as large as the bulk effective mass. The large enhancement of m_{tr}^{*}, which exceeds m_{CR}^{*} by ∼60%, at the lowest carrier density with r_{s}∼10 is purely attributed to the strong electron correlation.
    Physical Review Letters 12/2012; 109(24):246401. · 7.94 Impact Factor
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    ABSTRACT: We have grown pseudomorphic Tm3Fe5O12 films (46--350 nm in thickness) with perpendicular magnetization on (111) Gd3Ga5O12 substrates. Among various garnets, Tm3Fe5O12 is selected because of a negatively large magnetostriction constant to overcome strong shape anisotropy in very thin films. A stress-induced anisotropy field as large as +25 kOe is estimated by calculation under a moderate in-plane tensile strain of +0.49%. The magnetization hysteresis loop and magnetic domain structure indicate the perpendicular easy axis. The domain size (W) in its maze pattern varies from 500 to 960 nm with increasing thickness (t) and agrees well with a scaling law of W\propto\sqrt{t}.
    Applied Physics Express 10/2012; 5(10):3002-. · 2.73 Impact Factor
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    ABSTRACT: Magneto-transport properties have been investigated for epitaxial thin films of B20-type MnSi grown on Si(111) substrates. Both Lorentz transmission electron microscopy (TEM) images and topological Hall effect (THE) clearly point to the robust formation of skyrmions over a wide temperature-magnetic field region. New features distinct from those of bulk MnSi are observed for epitaxial MnSi films: a shorter (nearly half) period of the spin helix and skyrmions, and an opposite sign of THE. These observations suggest versatile features of skyrmion-induced THE beyond the current understanding.
    Physical Review Letters 09/2012; 110(11). · 7.94 Impact Factor
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    ABSTRACT: The growth techniques for MgxZn1-xO thin films have advanced at a rapid pace in recent years, enabling the application of this material to a wide range of optical and electrical applications. In designing structures and optimizing device performances, it is crucial that the Mg content of the alloy be controllable and precisely determined. In this study, we have established laboratory-based methods to determine the Mg content of MgxZn1-xO thin films grown on ZnO substrates, ranging from the solubility limit of x ~ 0.4 to the dilute limit of x < 0.01. For the absolute determination of Mg content, Rutherford backscattering spectroscopy is used for the high Mg region above x = 0.14, while secondary ion mass spectroscopy is employed to quantify low Mg content. As a lab-based method to determine the Mg content, c-axis length is measured by X-ray diffraction and is well associated with Mg content. The interpolation enables the determination of Mg content to x = 0.023, where the peak from the ZnO substrate overlaps the MgxZn1-xO peak in standard laboratory equipment, and thus quantitative determination. At dilute Mg contents below x = 0.023, the localized exciton peak energy of the MgxZn1-xO films as measured by photoluminescence is found to show a linear Mg content dependence, which is well resolved from the free exciton peak of ZnO substrate down to x = 0.0043. Our results demonstrate that X-ray diffraction and photoluminescence in combination are appropriate methods to determine Mg content in a wide Mg range from x = 0.004 to 0.40 in a laboratory environment.
    Journal of Applied Physics 08/2012; 112(4). · 2.21 Impact Factor
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    ABSTRACT: All-optical pump-probe detection of magnetization precession has been performed for ferromagnetic EuO thin films at 10 K. We demonstrate that the circularly-polarized light can be used to control the magnetization precession on an ultrafast time scale. This takes place within the 100 fs duration of a single laser pulse, through combined contribution from two nonthermal photomagnetic effects, i.e., enhancement of the magnetization and an inverse Faraday effect. From the magnetic field dependences of the frequency and the Gilbert damping parameter, the intrinsic Gilbert damping coefficient is evaluated to be {\alpha} \approx 3\times10^-3.
    Physical review. B, Condensed matter 07/2012; 86(6).
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    ABSTRACT: Topological insulators are quantum materials comprised of an insulating bulk gap and topologically protected metallic surfaces with a Dirac-like band dispersion. To access the Dirac point by transport measurements is a very challenging issue faced in current investigations of these materials. Here we report the electronic state modulation in topological insulator (Bi1−xSbx)2Te3 thin films by means of an ionic-liquid gating. The films with 20 nm thickness were grown on lattice-matched semi-insulating InP substrates by molecular beam epitaxy; the temperature dependencies of resistance of these films clearly indicate their insulating bulk and metallic surface characteristics. The surface state carriers were systematically controlled by using electric-double-layer transistor (EDLT) configurations with ionic-liquid dielectrics. It was demonstrated that the surface state in the (Bi1−xSbx)2Te3-based EDLTs is tuned across the Dirac point, showing ambipolar transport in a topological transport regime.
    Physical review. B, Condensed matter 07/2012; 86(4).
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    ABSTRACT: We have investigated the ultrafast spin dynamics in EuO thin films by time-resolved Faraday rotation spectroscopy. The photoinduced magnetization is found to be increased in a transient manner, accompanied with subsequent demagnetization. The dynamical magnetization enhancement showed a maximum slightly below the Curie temperature with prolonged tails toward both lower and higher temperatures and dominates the demagnetization counterpart at 55 K. The magnetization enhancement component decays in ~1 ns. The realization of the transient collective ordering is attributable to the enhancement of the f-d exchange interaction.
    Physical Review Letters 05/2012; 108(25). · 7.94 Impact Factor
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    ABSTRACT: The sequence of prominent fractional quantum Hall states up to ν=5/11 around ν=1/2 in a high-mobility two-dimensional electron system confined at oxide heterointerface (ZnO) is analyzed in terms of the composite fermion model. The temperature dependence of R(xx) oscillations around ν=1/2 yields an estimation of the composite fermion effective mass, which increases linearly with the magnetic field. This mass is of similar value to an enhanced electron effective mass, which in itself arises from strong electron interaction. The energy gaps of fractional states and the temperature dependence of R(xx) at ν=1/2 point to large residual interactions between composite fermions.
    Physical Review Letters 05/2012; 108(18):186803. · 7.94 Impact Factor
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    ABSTRACT: We investigate the spin susceptibility (g*m*) of dilute two-dimensional (2D) electrons confined at the MgxZn1−xO/ZnO heterointerface. Magnetotransport measurements show a four-fold enhancement of g*m*, dominated by the increase in the Landé g-factor. The g-factor enhancement leads to a ferromagnetic instability of the electron gas as evidenced by sharp resistance spikes. At high magnetic field, the large g*m* leads to full spin polarization, where we found sudden increase in resistance around the filling factors of half-integer, accompanied by complete disappearance of fractional quantum Hall (QH) states. Along with its large effective mass and the high electron mobility, our result indicates that the ZnO 2D system is ideal for investigating the effect of electron correlations in the QH regime.
    Physical review. B, Condensed matter 02/2012; 85(7).
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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).
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    ABSTRACT: We have found that there is a narrow but distinct window in oxygen pressure for growing phase-pure epitaxial EuO films by a pulsed laser deposition. With finely decreasing the oxygen pressure, the electrical property is varied from insulating to metallic with an enhancement in Curie temperature from 70 to 120 K. The anomalous Hall contribution was clearly observed in Hall resistance at 5 K in the highest electron density sample. The saturated anomalous Hall conductivity (0.2 S/cm) is rather high in comparison with those of the other ferromagnetic oxides, probably due to strong spin-orbit coupling in EuO.
    Applied Physics Letters 03/2011; · 3.79 Impact Factor
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    ABSTRACT: We present a fabrication procedure for ZnO channel waveguides intended for nonlinear optical applications. Ar ion milling was used to etch the single crystal thin film samples, and the effects of bias power, chamber pressure and Ar flow rate were investigated, finding optimal parameters for waveguide fabrication. The effect of sidewall roughness was estimated by comparing the results of cut-back measurements and an analytical model. We show an easy and effective method for the fabrication of ZnO channel waveguides.
    Proc SPIE 02/2011;

Publication Stats

536 Citations
207 Downloads
375.04 Total Impact Points

Institutions

  • 2010–2014
    • The University of Tokyo
      • • Department of Advanced Materials Science
      • • Center for Quantum-Phase Electronics
      Edo, Tōkyō, Japan
  • 2002–2014
    • Tohoku University
      • Institute for Materials Research
      Japan
  • 2012
    • RIKEN
      • Strong Correlation Interface Research Group
      Wako, Saitama-ken, Japan
  • 2000–2002
    • Tokyo Institute of Technology
      • • Department of Innovative and Engineered Materials
      • • Materials and Structures Laboratory
      Tokyo, Tokyo-to, Japan