A. Tsukazaki

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

Are you A. Tsukazaki?

Claim your profile

Publications (123)582.76 Total impact

  • Source

    Preview · Article · Jan 2016 · Science Advances
  • [Show abstract] [Hide abstract]
    ABSTRACT: We achieve the enhancement of circular photogalvanic effect arising from the photo-injection of spins in topological insulator thin films by tuning the Fermi level ($E_{\rm F}$). A series of (Bi$_{1-x}$Sb$_x$)$_2$Te$_3$ thin films were tailored so that the Fermi energy ranges above 0.34 eV to below 0.29 eV of the Dirac point, i.e., from the bulk conduction band bottom to the valence band top through the bulk in-gap surface-Dirac cone. The circular photogalvanic current, indicating a flow of spin-polarized surface-Dirac electrons, shows a pronounced peak when the $E_{\rm F}$ is set near the Dirac point and is also correlated with the carrier mobility. Our observation reveals that there are substantial scatterings between the surface-Dirac and bulkstate electrons in the generation process of spin-polarized photocurrent, which can be avoided by designing the electronic structure in topological insulators.
    No preview · Article · Jan 2016
  • [Show abstract] [Hide abstract]
    ABSTRACT: Microwave induced resistance and photovoltage oscillations were investigated in MgxZn1−xO/ZnO heterostructures. The physics of these oscillations is controlled significantly by scattering mechanisms, and therefore these experiments were motivated by the recently achieved high quality levels in this material and the apparent dominance of large angle, short-range scattering, which is distinct from the prevailing small angle scattering in state-of-the-art GaAs structures. Within the studied frequency range of 35–120 GHz, up to four oscillations were resolved at 1.4 K temperature, but only in high density samples. This allowed us to extract the value of the effective electron mass m*=(0.35±0.01)m0, which is enhanced over the bare band mass, and estimate a local quantum scattering time of about 5 ps.
    No preview · Article · Jan 2016
  • T. C. Fujita · M. Uchida · Y. Kozuka · S. Ogawa · A. Tsukazaki · T. Arima · M. Kawasaki
    [Show abstract] [Hide abstract]
    ABSTRACT: Pyrochlore iridates have attracted growing attention because of a theoretical prediction of a possible topological semimetal phase originating from all-in-all-out spin ordering. Related to the topological band structure, recent findings of the magnetic domain wall conduction have stimulated investigations of magnetic domain distribution in this system. Here, we investigate the size of magnetic domains in Eu$_2$Ir$_2$O$_7$ single crystalline thin films by magnetoresistance (MR) using microscale Hall bars. Two distinct magnetic domains of the all-in-all-out spin structure are known to exhibit linear MR but with opposite signs, which enables us to estimate the ratio of the two domains in the patterned channel. The linear MR for 80 ${\times}$ 60 ${\mu}$m$^2$ channel is nearly zero after zero-field cooling, suggesting random distribution of domains smaller than the channel size. In contrast, the wide distribution of the value of the linear MR is detected in 2 ${\times}$ 2 ${\mu}$m$^2$ channel, reflecting the detectable domain size depending on each cooling-cycle. Compared to simulation results, we estimate the average size of a single all-in-all-out magnetic domain as 1-2 ${\mu}$m.
    No preview · Article · Jan 2016 · Applied Physics Letters
  • [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.
    No preview · Article · Nov 2015 · Physical Review Letters
  • Source
    [Show abstract] [Hide abstract]
    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 both the 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 higher-temperature and zero magnetic-field quantum conduction.
    Preview · Article · Nov 2015 · Applied Physics Letters
  • [Show abstract] [Hide abstract]
    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.
    No preview · Article · Oct 2015
  • Source
    J. Shiogai · Y. Ito · T. Mitsuhashi · T. Nojima · A. Tsukazaki
    [Show abstract] [Hide abstract]
    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.
    Preview · Article · Oct 2015 · Nature Physics
  • J. Falson · Y. Kozuka · J.H. Smet · T. Arima · A. Tsukazaki · M. Kawasaki
    [Show abstract] [Hide abstract]
    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.
    No preview · Article · Aug 2015 · Applied Physics Letters
  • Source
    [Show abstract] [Hide abstract]
    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.
    Preview · Article · Jul 2015 · Nature Communications
  • Source
    [Show abstract] [Hide abstract]
    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.
    Preview · Article · Jun 2015 · Physical Review Letters
  • Source
    [Show abstract] [Hide abstract]
    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.
    Preview · Article · Jun 2015 · Journal of the Physical Society of Japan
  • Y. Ohuchi · Y. Kozuka · M. Uchida · K. Ueno · A. Tsukazaki · M. Kawasaki
    [Show abstract] [Hide abstract]
    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.
    No preview · Article · Jun 2015 · Physical Review B
  • Source
    T C Fujita · Y Kozuka · M Uchida · A Tsukazaki · T Arima · M Kawasaki
    [Show abstract] [Hide abstract]
    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.
    Preview · Article · May 2015 · Scientific Reports
  • [Show abstract] [Hide abstract]
    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.
    No preview · Article · Mar 2015 · Nature Physics
  • [Show abstract] [Hide abstract]
    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.
    No preview · Article · Feb 2015 · Applied Physics Letters
  • [Show abstract] [Hide abstract]
    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.
    No preview · Article · Feb 2015 · Physical Review B
  • [Show abstract] [Hide abstract]
    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.
    No preview · Article · Feb 2015 · Japanese Journal of Applied Physics
  • Source
    [Show abstract] [Hide abstract]
    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.
    Preview · Article · Jan 2015 · Physical Review B
  • Source
    D. Maryenko · J. Falson · Y. Kozuka · A. Tsukazaki · M. Kawasaki
    [Show abstract] [Hide abstract]
    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.
    Preview · Article · Sep 2014 · Physical Review B

Publication Stats

5k Citations
582.76 Total Impact Points

Institutions

  • 2010-2015
    • Japan Science and Technology Agency (JST)
      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