A. Tsukazaki

Japan Science and Technology Agency (JST), Edo, Tōkyō, Japan

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Publications (115)579.46 Total impact

  • [Show abstract] [Hide abstract]
    ABSTRACT: We report magnetotransport measurements on a high-mobility two-dimensional electron system at the nonmagnetic MgZnO/ZnO heterointerface showing distinct behavior for electrons with spin-up and spin-down orientations. The low-field Shubnikov-de Haas oscillations manifest alternating resistance peak heights which can be attributed to distinct scattering rates for different spin orientations. The tilt-field measurements at a half-integer filling factor reveal that the majority spins show usual diffusive behavior, i.e., peaks with the magnitude proportional to the index of the Landau level at the Fermi energy. By contrast, the minority spins develop "plateaus" with the magnitude of dissipative resistivity that is fairly independent of the Landau level index and is of the order of the zero-field resistivity.
    Physical Review Letters 11/2015; 115(19). DOI:10.1103/PhysRevLett.115.197601 · 7.51 Impact Factor
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    ABSTRACT: Quantum anomalous Hall effect (QAHE), which generates dissipation-less edge current without external magnetic field, is observed in magnetic-ion doped topological insulators (TIs), such as Cr- and V-doped (Bi,Sb)2Te3. The QAHE emerges when the Fermi level is inside the magnetically induced gap around the original Dirac point of the TI surface state. Although the size of gap is reported to be about 50 meV, the observable temperature of QAHE has been limited below 300 mK. We attempt magnetic-Cr modulation doping into topological insulator (Bi,Sb)2Te3 films to increase the observable temperature of QAHE. By introducing the rich-Cr-doped thin (1 nm) layers at the vicinity of the both surfaces based on non-Cr-doped (Bi,Sb)2Te3 films, we have succeeded in observing the QAHE up to 2 K. The improvement in the observable temperature achieved by this modulation-doping appears to be originating from the suppression of the disorder in the surface state interacting with the rich magnetic moments. Such a superlattice designing of the stabilized QAHE may pave a way to dissipation-less electronics based on the highertemperature and zero magnetic-field quantum conduction.
    Applied Physics Letters 11/2015; 107(18):182401. DOI:10.1063/1.4935075 · 3.30 Impact Factor
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    ABSTRACT: The spin-momentum locking at the Dirac surface state of a topological insulator (TI) offers a distinct possibility of a highly efficient charge-to-spin current (C-S) conversion compared with spin Hall effects in conventional paramagnetic metals. For the development of TI-based spin current devices, it is essential to evaluate its conversion efficiency quantitatively as a function of the Fermi level EF position. Here we exemplify a coefficient of qICS to characterize the interface C-S conversion effect by using spin torque ferromagnetic resonance (ST-FMR) for (Bi1-xSbx)2Te3 thin films whose EF is tuned across the band gap. In bulk insulating conditions, interface C-S conversion effect via Dirac surface state is evaluated as nearly constant large values of qICS, reflecting that the qICS is inversely proportional to the Fermi velocity vF that is almost constant. However, when EF traverses through the Dirac point, the qICS is remarkably suppressed possibly due to the degeneracy of surface spins or instability of helical spin structure. These results demonstrate that the fine tuning of the EF in TI based heterostructures is critical to maximizing the efficiency using the spin-momentum locking mechanism.
  • J. Shiogai · Y. Ito · T. Mitsuhashi · T. Nojima · A. Tsukazaki ·
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    ABSTRACT: Among the recently discovered iron-based superconductors, ultrathin films of FeSe grown on SrTiO3 substrates have uniquely evolved into a high superconducting-transition-temperature (TC) material. The mechanisms for the high-TC superconductivity are ongoing debate mainly with the superconducting gap characterized with in-situ analysis for FeSe films grown by bottom-up molecular-beam epitaxy. Here, we demonstrate the alternative access to investigate the high-TC superconductivity in ultrathin FeSe with top-down electrochemical etching technique in three-terminal transistor configuration. In addition to the high-TC FeSe on SrTiO3, the electrochemically etched ultrathin FeSe transistor on MgO also exhibits superconductivity around 40 K, implying that the application of electric-field effectively contributes to the high-TC superconductivity in ultrathin FeSe regardless of substrate material. Moreover, the observable critical thickness for the high-TC superconductivity is expanded up to 10-unit-cells under applying electric-field and the insulator-superconductor transition is electrostatically controlled. The present demonstration implies that the electric-field effect on both conduction and valence bands plays a crucial role for inducing high-TC superconductivity in FeSe.
    Nature Physics 10/2015; DOI:10.1038/nphys3530 · 20.15 Impact Factor
  • J. Falson · Y. Kozuka · J.H. Smet · T. Arima · A. Tsukazaki · M. Kawasaki ·
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    ABSTRACT: The remarkable historic advances experienced in condensed matter physics have been enabled through the continued exploration and proliferation of increasingly richer and cleaner material systems. In this work, we report on the scattering times of charge carriers confined in state-of-the-art MgZnO/ZnO heterostructures displaying electron mobilities in excess of 106 cm2/V s. Through an examination of low field quantum oscillations, we obtain the effective mass of charge carriers, along with the transport and quantum scattering times. These times compare favorably with high mobility AlGaAs/GaAs heterostructures, suggesting the quality of MgZnO/ZnO heterostructures now rivals that of traditional semiconductors.
    Applied Physics Letters 08/2015; 107(8). DOI:10.1063/1.4929381 · 3.30 Impact Factor
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    ABSTRACT: By breaking the time-reversal-symmetry in three-dimensional topological insulators with introduction of spontaneous magnetization or application of magnetic field, the surface states become gapped, leading to quantum anomalous Hall effect or quantum Hall effect, when the chemical potential locates inside the gap. Further breaking of inversion symmetry is possible by employing magnetic topological insulator heterostructures that host nondegenerate top and bottom surface states. Here, we demonstrate the tailored-material approach for the realization of robust quantum Hall states in the bilayer system, in which the cooperative or cancelling combination of the anomalous and ordinary Hall responses from the respective magnetic and non-magnetic layers is exemplified. The appearance of quantum Hall states at filling factor 0 and +1 can be understood by the relationship of energy band diagrams for the two independent surface states. The designable heterostructures of magnetic topological insulator may explore a new arena for intriguing topological transport and functionality.
    Nature Communications 07/2015; 6. DOI:10.1038/ncomms9530 · 11.47 Impact Factor
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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
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    ABSTRACT: We investigate skyrmion formation in both a single crystalline bulk and epitaxial thin films of MnSi by measurements of planar Hall effect. A prominent stepwise field profile of planar Hall effect is observed in the well-established skyrmion phase region in the bulk sample, which is assigned to anisotropic magnetoresistance effect with respect to the magnetic modulation direction. We also detect the characteristic planar Hall anomalies in the thin films under the in-plane magnetic field at low temperatures, which indicates the formation of skyrmion strings lying in the film plane. Uniaxial magnetic anisotropy plays an important role in stabilizing the in-plane skyrmions in the MnSi thin film.
    Journal of the Physical Society of Japan 06/2015; 84(10). DOI:10.7566/JPSJ.84.104708 · 1.59 Impact Factor
  • Y. Ohuchi · Y. Kozuka · M. Uchida · K. Ueno · A. Tsukazaki · M. Kawasaki ·
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    ABSTRACT: We report on the topological Hall effect (THE) in centrosymmetric EuO thin films. This THE signal persists down to the lowest temperature in the metallic region below 50 K for the films thinner than 200 nm. The signal rapidly disappears by tilting the applied magnetic field from surface normal, suggestive of noncoplanar spin configuration such as two-dimensional skyrmions. This observation possibly substantiates the theoretical proposal of magnetic skyrmions in 2D Heisenberg ferromagnets in marked contrast to better established B20-type chiral helimagnets.
    Physical Review B 06/2015; 91(24). DOI:10.1103/PhysRevB.91.245115 · 3.74 Impact Factor
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    T C Fujita · Y Kozuka · M Uchida · A Tsukazaki · T Arima · M Kawasaki ·
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    ABSTRACT: A new class of materials termed topological insulators have been intensively investigated due to their unique Dirac surface state carrying dissipationless edge spin currents. Recently, it has been theoretically proposed that the three dimensional analogue of this type of band structure, the Weyl Semimetal phase, is materialized in pyrochlore oxides with strong spin-orbit coupling, accompanied by all-in-all-out spin ordering. Here, we report on the fabrication and magnetotransport of Eu2Ir2O7 single crystalline thin films. We reveal that one of the two degenerate all-in-all-out domain structures, which are connected by time-reversal operation, can be selectively formed by the polarity of the cooling magnetic field. Once formed, the domain is robust against an oppositely polarised magnetic field, as evidenced by an unusual odd field dependent term in the magnetoresistance and an anomalous term in the Hall resistance. Our findings pave the way for exploring the predicted novel quantum transport phenomenon at the surfaces/interfaces or magnetic domain walls of pyrochlore iridates.
    Scientific Reports 05/2015; 5:9711. DOI:10.1038/srep09711 · 5.58 Impact Factor
  • J. Falson · D. Maryenko · B. Friess · D. Zhang · Y. Kozuka · A. Tsukazaki · J. H. Smet · M. Kawasaki ·
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    ABSTRACT: The fractional quantum Hall (FQH) effect emerges in high-quality two-dimensional electron systems exposed to a magnetic field when the Landau-level filling factor, νe, takes on a rational value. Although the overwhelming majority of FQH states have odd-denominator fillings, the physical properties of the rare and fragile even-denominator states are most tantalizing in view of their potential relevance for topological quantum computation. For decades, GaAs has been the preferred host for studying these even-denominator states, where they occur at νe = 5/2 and 7/2. Here we report an anomalous series of quantized even-denominator FQH states outside the realm of III–V semiconductors in the MgZnO/ZnO 2DES electron at νe = 3/2 and 7/2, with precursor features at 9/2; all while the 5/2 state is absent. The effect in this material occurs concomitantly with tunability of the orbital character of electrons at the chemical potential, thereby realizing a new experimental means for investigating these exotic ground states.
    Nature Physics 03/2015; 11(4). DOI:10.1038/nphys3259 · 20.15 Impact Factor
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    ABSTRACT: Low-temperature photoluminescence and reflectance measurements were employed to study the optical transitions present in two-dimensional electron systems confined at Mg x Zn 1– x O/ZnO heterojunctions. Transitions involving A- and B-holes and electrons from the two lowest subbands formed within the confinement potential are detected. In the studied density range of 2.0–6.5 × 1011 cm−2, the inter-subband splitting is measured and the first excited electron subband is shown to be empty of electrons.
    Applied Physics Letters 02/2015; 106(8):082102. DOI:10.1063/1.4913313 · 3.30 Impact Factor
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    ABSTRACT: The in-plane Mg doping distribution in molecular beam epitaxy grown MgxZn1−xO/ZnO heterostructures is mapped by low-temperature photoluminescence measurements in an effort to evaluate and control the resultant inhomogeneity formed during the growth process. In an unrotated sample, the independent configuration effects of the O3 and Mg source cells are clearly demonstrated in a composition spread due to flux gradients, while this inhomogeneity is suppressed by sample rotation during the growth. The present mapping results provide an important means for investigating improved doping regimes with the aim of enhancing the quality of quantum transport observable at the MgxZn1−xO/ZnO heterointerface.
    Japanese Journal of Applied Physics 02/2015; 54(2):028004. DOI:10.7567/JJAP.54.028004 · 1.13 Impact Factor
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    ABSTRACT: The plasma, magnetoplasma, and edge magnetoplasma excitations were investigated in two-dimensional electron systems hosted at the heterointerface of MgZnO/ZnO structures using optical detection of resonant microwave absorption. The magnetodispersion of the plasma excitations allowed the extraction of the electron effective mass. It was found to exhibit a surprisingly large dependence on the electron density, which is difficult to account for just from nonparabolicity effects.
    Physical Review B 02/2015; 91(8). DOI:10.1103/PhysRevB.91.085304 · 3.74 Impact Factor
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    ABSTRACT: We investigate the skyrmion formation process in nano-structured FeGe Hall-bar devices by measurements of topological Hall effect, which extracts the winding number of a spin texture as an emergent magnetic field. Step-wise profiles of topological Hall resistivity are observed in the course of varying the applied magnetic field, which arise from instantaneous changes in the magnetic nano-structure such as creation, annihilation, and jittering motion of skyrmions. The discrete changes in topological Hall resistivity demonstrate the quantized nature of emergent magnetic flux inherent in each skyrmion, which had been indistinguishable in many-skyrmion systems on a macroscopic scale.
    Physical Review B 01/2015; 91(4). DOI:10.1103/PhysRevB.91.041122 · 3.74 Impact Factor
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    D. Maryenko · J. Falson · Y. Kozuka · A. Tsukazaki · M. Kawasaki ·
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    ABSTRACT: We report electrical transport measurements in tilted magnetic field on a high-mobility two- dimensional electron system (2DES) confined at the MgZnO/ZnO heterointerface. The observation of multiple crossing events of spin-resolved electron Landau levels enables the mapping of the sequence of electronic states in the magnetic field and the detailed study of level crossing of each Landau level. It shows the absence of the first coincidence event even when the magnetic field has only a perpendicular component to the 2DES plane. This is consistent with the enhanced spin-susceptibility from our previous reports. We further observe a non-linear dependence of paramagnetic spin-susceptibility on total magnetic field and estimate spin-susceptibility at zero field.
    Physical Review B 09/2014; 90(24). DOI:10.1103/PhysRevB.90.245303 · 3.74 Impact Factor
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    ABSTRACT: The three-dimensional (3D) topological insulator (TI) is a novel state of matter as characterized by two-dimensional (2D) metallic Dirac states on its surface. Bi-based chalcogenides such as Bi2Se3, Bi2Te3, Sb2Te3 and their combined/mixed compounds like Bi2Se2Te and (Bi1-xSbx)2Te3 are typical members of 3D-TIs which have been intensively studied in forms of bulk single crystals and thin films to verify the topological nature of the surface states. Here, we report the realization of the Quantum Hall effect (QHE) on the surface Dirac states in (Bi1-xSbx)2Te3 films (x = 0.84 and 0.88). With electrostatic gate-tuning of the Fermi level in the bulk band gap under magnetic fields, the quantum Hall states with filling factor \nu = \pm 1 are resolved with quantized Hall resistance of Ryx = h/e2 and zero longitudinal resistance, owing to chiral edge modes at top/bottom surface Dirac states. Furthermore, the appearance of a \nu = 0 state (\sigma xy = 0) reflects a pseudo-spin Hall insulator state when the Fermi level is tuned in between the energy levels of the non-degenerate top and bottom surface Dirac points. The observation of the QHE in 3D TI films may pave a way toward TI-based electronics.
    Nature Communications 09/2014; 6. DOI:10.1038/ncomms7627 · 11.47 Impact Factor
  • T. Tambo · J. Falson · D. Maryenko · Y. Kozuka · A. Tsukazaki · M. Kawasaki ·
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    ABSTRACT: The adaptation of "air-gap" dielectric based field-effect transistor technology to controlling the MgZnO/ZnO heterointerface confined two-dimensional electron system (2DES) is reported. We find it possible to tune the charge density of the 2DES via a gate electrode spatially separated from the heterostructure surface by a distance of 5 mu m. Under static gating, the observation of the quantum Hall effect suggests that the charge carrier density remains homogeneous, with the 2DES in the 3 mm square sample the sole conductor. The availability of this technology enables the exploration of the charge carrier density degree of freedom in the pristine sample limit. (c) 2014 AIP Publishing LLC.
    Journal of Applied Physics 08/2014; 116(8):084310. DOI:10.1063/1.4894155 · 2.18 Impact Factor
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    ABSTRACT: Topological insulators are bulk electronic insulators which possess symmetry protected gapless modes on their surfaces. Breaking the symmetries that underlie the gapless nature of the surface modes is predicted to give rise to exotic new states of matter. In particular, it has recently been predicted and shown that breaking of time reversal symmetry in the form of ferromagnetism can give rise to a gapped state characterized by a zero magnetic field quantized Hall response and dissipationless longitudinal transport known as the Quantum Anomalous Hall (QAH) state. A key question that has thus far remained experimentally unexplored is the relationship of this new type of quantum Hall state with the previously known orbitally driven quantum Hall states. Here, we show experimentally that a ferromagnetic topological insulator exhibiting the QAH state is well described by the global phase diagram of the quantum Hall effect. By mapping the behavior of the conductivity tensor in the parameter space of temperature, magnetic field, and chemical potential in the vicinity of the QAH phase, we find evidence for quantum criticality and delocalization behavior that can quantitatively be described by the renormalization group properties of the quantum Hall ground state. This result demonstrates that the QAH state observed in ferromagnetic topological insulators can be understood within the context of the law of corresponding states which governs the quantum Hall state. This suggests a roadmap for studying the QAH effect including transitions to possible adjacent topologically non-trivial states and a possible universality class for the QAH transition.
    Nature Physics 06/2014; 10(10). DOI:10.1038/nphys3053 · 20.15 Impact Factor
  • K. Imasaka · J. Falson · Y. Kozuka · A. Tsukazaki · M. Kawasaki ·
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    ABSTRACT: Atomic reconstruction at the interface of MgZnO and ZnO in molecular beam epitaxy grown heterostructures is investigated. Using secondary ion mass spectroscopy, we experimentally find that Mg atomic reconstruction depends on the polarity of the interface; it is not observed in n-type interfaces (MgZnO on Zn-polar ZnO) owing to electron accumulation, while in p-type interfaces (ZnO on Zn-polar MgZnO), Mg drastically redistributes into the ZnO layer. Combined with self-consistent calculation of band profiles and carrier distributions, we reveal that the observed Mg reconstruction is not due to thermal diffusion but consequences in order to avoid hole accumulation. This tendency implies inherent significant asymmetry of energy scales of atomic and electronic reconstructions between n-type and p-type interfaces.
    Applied Physics Letters 06/2014; 104(24):242112-242112-5. DOI:10.1063/1.4884383 · 3.30 Impact Factor

Publication Stats

4k Citations
579.46 Total Impact Points


  • 2010-2015
    • Japan Science and Technology Agency (JST)
      • Exploratory Research for Advanced Technology (ERATO)
      Edo, Tōkyō, Japan
  • 2002-2015
    • Tohoku University
      • • Institute for Materials Research
      • • Graduate School of Engineering
      Miyagi, Japan
  • 2014
    • University of Hamburg
      • Institute of Applied Physics
      Hamburg, Hamburg, Germany
  • 2010-2013
    • The University of Tokyo
      • • Department of Advanced Materials Science
      • • Department of Applied Physics
      Edo, Tōkyō, Japan
  • 2000-2002
    • Tokyo Institute of Technology
      • • Department of Innovative and Engineered Materials
      • • Materials and Structures Laboratory
      Tokyo, Tokyo-to, Japan