A. Tsukazaki

University of Hamburg, Hamburg, Hamburg, Germany

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Publications (95)453.8 Total impact

  • N. Kanazawa, M. Kubota, A. Tsukazaki, Y. Kozuka, K. S. Takahashi, M. Kawasaki, M. Ichikawa, F. Kagawa, Y. Tokura
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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.
    01/2015;
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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). · 3.66 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.
    09/2014;
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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; · 20.60 Impact Factor
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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. · 3.52 Impact Factor
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    ABSTRACT: We report on the sign inversion of the anomalous Hall effect (AHE) in EuO thin films along with photoirradiation as well as a temperature scan across ̃25 K that is well below the Curie temperature (TC̃80 K). The former gives an enhancement of the mobile electron density (n) by more than 30%, but the latter gives a negligible modification of n of only 3% with a significant enhancement in mobility. It is found, in addition to the universal scaling law between longitudinal conductivity (σxx) and anomalous Hall conductivity (σAH) as |σAH|∝σxx1.6, that there is a critical value of about 102 S cm-1 in σxx that gives a boundary in the sign inversion of σAH. If n solely governs the sign of σAH, the phenomenon could be explained by a Fermi level shift across the singularity in the band structure. However, our band calculation shows that, within any realistic adjustment of band parameters, the sign inversion of AHE never occurs. Thus, we conclude that other mechanisms of AHE are necessary to account for the AHE of EuO.
    Physical Review B 02/2014; 89(12). · 3.66 Impact Factor
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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. · 36.43 Impact Factor
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    ABSTRACT: The magnetic skyrmion, i.e., the nanometric swirling spin vortexlike object with the topolgical charge, is broadly observed in chiral-lattice cubic magnets, typically MnSi; where the cylindrical-shape skyrmions form the two-dimensional hexagonal crystal, in a very narrow temperature-magentic field window for bulk crystals but in a much wider one for tens of nanometer thin films under the perpendicular magnetic field applied. We have investigated the stability of two-dimensional skyrmion states emerging in epitaxial thin films of Mn1-xFexSi with various thicknesses t and iron contents x (t =10, 15, and 20 nm; x =0, 0.02, and 0.04) by changing the magnetic-field direction. Topological Hall effect arising from the skyrmions is critically suppressed in the course of tilting the applied magnetic field from the normal vector, indicating the collapse of the skyrmion state. Utilizing this observation, the stable region of the skyrmions, which depends on the film thickness relative to the helimagnetic period, can be mapped out in the temperature-magnetic field plane.
    Physical Review B 01/2014; 89(6). · 3.66 Impact Factor
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    ABSTRACT: Torque magnetometry at low temperature and in high magnetic fields B is performed on MgZnO/ZnO heterostructures incorporating high-mobility two-dimensional electron systems. We find a sawtoothlike quantum oscillatory magnetization M (B), i.e., the de Haas-van Alphen (dHvA) effect. At the same time, nonequilibrium currents and unexpected spikelike overshoots in M are observed which allow us to identify the microscopic nature and density of the residual disorder. The acceptorlike 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 heterostructures.
    Physical Review B 01/2014; 89(7). · 3.66 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.
    Journal of Physics D Applied Physics 01/2014; 47(7). · 2.52 Impact Factor
  • A Tsukazaki, A Ohtomo, M Kawasaki
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    ABSTRACT: ZnO and related alloys are an important class of materials to realize transparent electronics because of their characteristic wide band-gap and high mobility, and also because of their practical advantages, such as: available n-type materials and bulk single crystals, low-cost production, and absence of toxicity. Our studies have been conducted for more than a decade and they have enabled surface and interface engineering on an atomic scale, presenting a promising technology for developing electrical devices of various kinds. The quality of the epitaxial films was improved drastically when grown on high-temperature annealed buffer layers prepared on lattice-matched ScAlMgO4 substrates using pulsed-laser deposition. We carefully investigated the growth temperature dependence of surface morphology and electrical properties. Electron mobility was recorded as 440 cm2 V−1 s−1 at room temperature and 5500 cm2 V−1 s−1 at 1 K, leading to observation of the integer quantum Hall-effect (QHE) in abrupt ZnO/Mgx Zn1−x O interfaces. Two-dimensional electron gas (2DEG) was formed spontaneously in the interface because of the polarization mismatch between the layers. The observation of QHE enables us to access the direct determination of the interfacial electronic structure. In addition, the field-effect control of 2DEG has been demonstrated using lattice-matched interfaces as high-mobility channels.
    Journal of Physics D Applied Physics 01/2014; 47(3). · 2.52 Impact Factor
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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. · 1.36 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. · 2.00 Impact Factor
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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). · 3.66 Impact Factor
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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). · 3.66 Impact Factor
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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. · 7.73 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). · 7.73 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). · 2.19 Impact Factor

Publication Stats

3k Citations
453.80 Total Impact Points

Institutions

  • 2014
    • University of Hamburg
      • Institute of Applied Physics
      Hamburg, Hamburg, Germany
  • 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
    • Nankai University
      • Department of Physics
      T’ien-ching-shih, Tianjin Shi, China
  • 2000–2002
    • Tokyo Institute of Technology
      • • Department of Innovative and Engineered Materials
      • • Materials and Structures Laboratory
      Tokyo, Tokyo-to, Japan