M. Kawasaki

University of Hamburg, Hamburg, Hamburg, Germany

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Publications (694)2388.69 Total impact

  • Applied Physics Letters 01/2016; 105(16). · 3.52 Impact Factor
  • 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.
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    ABSTRACT: Heterojunctions offer a tremendous opportunity for fundamental as well as applied research, ranging from the unique electronic phases in between oxides to the contact issues in semiconductor devices. Despite their pivotal roles, determining individual building atom of matter in heterojunctions is still challenging, especially for those between highly dissimilar structures, in which breaking of symmetry, chemistry, and bonds may give rise to complex reconstruction and intermixing at the junction. Here, we combine electron microscopy, spectroscopy, and first-principles calculations to determine individual reconstructed atomic columns and their charge states in a complex, multicomponent heterojunction between the delafossite CuScO2 and spinel MgAl2O4. The high resolution enables us to demonstrate that the reconstructed region can accommodate a highly selective intermixing of Cu cations at specific Sc cation sites with half atomic density, forming a complex ordered superstructure. Such ability to resolve reconstructed heterojunctions to the atomic dimensions helps elucidate atomistic mechanisms and discover novel properties with applications in a diverse range of scientific disciplines.
    Nano Letters 10/2014; · 12.94 Impact Factor
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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.
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    ABSTRACT: Strongly correlated electron systems often exhibit very strong interactions between structural and electronic degrees of freedom that lead to complex and interesting phase diagrams. For technological applications of these materials it is important to learn how to drive transitions from one phase to another. A key question here is the ultimate speed of such phase transitions, and to understand how a phase transition evolves in the time domain. Here we apply time-resolved X-ray diffraction to directly measure the changes in long-range order during ultrafast melting of the charge and orbitally ordered phase in a perovskite manganite. We find that although the actual change in crystal symmetry associated with this transition occurs over different timescales characteristic of the many electronic and vibrational coordinates of the system, the dynamics of the phase transformation can be well described using a single time-dependent 'order parameter' that depends exclusively on the electronic excitation.
    Nature Material 08/2014; · 36.43 Impact Factor
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    ABSTRACT: We studied the charge-orbital ordering in the superlattice of charge-ordered insulating Pr0.5Ca0.5MnO3 and ferromagnetic metallic La0.5Sr0.5MnO3 by resonant soft x-ray diffraction (RSXD) and hard x-ray photoemission spectroscopy (HXPES). A temperature-dependent incommensurability is found in the orbital ordering by RSXD. In addition, a large hysteresis is observed that is caused by phase competition between the insulating charge ordered and metallic ferromagnetic states. No magnetic phase transitions are observed in contrast to pure Pr0.5Ca0.5MnO3 thin films, confirming the unique character of the superlattice. Mn HXPES spectra revealed a hysteresis in the metalicity, supporting the picture of phase competition. The deviation from the commensurate orbital order can be directly related to the decrease of ordered-layer thickness that leads to dimensional crossover from three-dimensional to two-dimensional orbital ordering.
    New Journal of Physics 07/2014; 16(7):073044. · 3.67 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; · 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 the surface conduction of a BaTiO3 thin film using electric double layer transistor (EDLT) structure. A transistor operation was observed at 220 K with an on/off ratio exceeding 105, demonstrating that ionic liquid gating is effective to induce carriers at the surface of ferroelectric materials. Temperature dependence of channel resistance exhibited a metallic behavior down to 150 K. EDLT structure is also fabricated using a commercial BaTiO3 bulk single crystal for comparison, which shows abrupt resistance increase across the orthorhombic to rhombohedral transition temperature at 183 K. This result indicates that the epitaxial strain is effective to maintain low resistance in this material with keeping the single domain structure.
    Applied Physics Letters 06/2014; 104(22):222101-222101-4. · 3.52 Impact Factor
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    ABSTRACT: The low-energy electron dynamics of the quasi-two-dimensional electron-hole multiorbital system Sr2RuO4 is investigated by time-domain terahertz spectroscopy for a thin film sample. The direct observation of the real and imaginary part of the optical conductivity below 8 meV clearly demonstrates a narrowing of the coherent quasiparticle response with decreasing temperature. The optical conductivity spectra, dc conductivity, and Hall coefficient can be quantitatively well described by the multiband Drude-Lorentz model, from which band dependent scattering rates can be extracted. The resulting modest band dependent scattering rates, which are lower than 2 meV at 4 K, support a rather isotropic relaxation regime.
    03/2014; 89(16).
  • Journal of the Physical Society of Japan 03/2014; 83(3):032001. · 1.48 Impact Factor
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    ABSTRACT: We report the study of magnetic and orbital order in epitaxial thin films grown on –. Resonant soft x-ray scattering revealed significant modifications of the magnetic order in the film as compared to the bulk. Namely (i) a different magnetic ordering wave vector, (ii) different spin directions and (iii) an additional magnetic reordering transition. We demonstrate that an analysis of the resonant scattering which is based solely on local symmetries and which does not involve a modeling of energy-dependent lineshapes allows to extract this detailed microscopic information. This approach significantly simplifies the analysis and interpretation of resonant scattering data.
    New Journal of Physics 03/2014; 16(3):033006. · 3.67 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 dynamics of photogenerated electrons and holes in undoped anatase TiO2 were studied by femtosecond absorption spectroscopy from the visible to mid-infrared region (0.1–2.0 eV). The transient absorption spectra exhibited clear metallic responses, which were well reproduced by a simple Drude model. No mid-gap absorptions originating from photocarrier localization were observed. The reduced optical mass of the photocarriers obtained from the Drude-model analysis is comparable to theoretically expected one. These results demonstrate that both photogenerated holes and electrons act as mobile carriers in anatase TiO2. We also discuss scattering and recombination dynamics of photogenerated electrons and holes on the basis of the time dependence of absorption changes.
    Journal of Applied Physics 02/2014; 115(5):053514-053514-8. · 2.19 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
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    ABSTRACT: We have investigated the charge transport and low-energy dynamics originating from the antiferromagnetic (AF) domain wall (DW) in pyrochlore-type Nd2Ir2O7 with the all-in all-out type AF ground state by means of the magneto-resistance measurement and the terahertz time-domain spectroscopy. The resistivity is much lower in the multi-magnetic-domain state than in the nearly single-magnetic-domain one. Furthermore, the terahertz optical conductivity spectrum unravels a Drude-like response only in the multidomain state; the Drude-like absorption is strongly suppressed with increasing field and irreversibly disappears above 5 T. These observations suggest that the AF DW is highly conductive with a minimal damping constant of 2 meV, contrary to the fully-gapped (45 meV) AF insulating state of bulk. We discuss the nature of metallic states at AF DWs and their possible relevance to the gapless edge state inherent to the Weyl semimetal.
    01/2014; 89(7).
  • 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

Publication Stats

16k Citations
2,388.69 Total Impact Points


  • 2014
    • University of Hamburg
      • Institute of Applied Physics
      Hamburg, Hamburg, Germany
    • University of Waterloo
      • Department of Physics and Astronomy
      Waterloo, Ontario, Canada
  • 2013–2014
    • Wako University
      Edo, Tōkyō, Japan
  • 2004–2014
    • RIKEN
      • Strong Correlation Interface Research Group
      Вако, Saitama, Japan
    • Beijing Jiaotong University
      Peping, Beijing, China
  • 1987–2014
    • The University of Tokyo
      • • Department of Advanced Materials Science
      • • Department of Applied Physics
      • • Center for Quantum-Phase Electronics
      • • Department of Physics
      • • Department of Applied Chemistry
      Edo, Tōkyō, Japan
  • 2001–2013
    • National Institute of Advanced Industrial Science and Technology
      • Nanoelectronics Research Institute
      Tsukuba, Ibaraki, Japan
  • 2012
    • Nankai University
      • Department of Physics
      T’ien-ching-shih, Tianjin Shi, China
    • Paul Scherrer Institut
      Aargau, Switzerland
  • 2000–2012
    • Tohoku University
      • Institute for Materials Research
    • Tata Institute of Fundamental Research
      • Department of Astronomy and Astrophysics
      Mumbai, Mahārāshtra, India
  • 2011
    • University of Southampton
      Southampton, England, United Kingdom
  • 2010
    • Stanford University
      • Department of Physics
      Stanford, CA, United States
  • 2008
    • Hankuk University of Foreign Studies
      Sŏul, Seoul, South Korea
  • 2000–2008
    • National Institute for Materials Science
      • Advanced Materials Laboratory
      Tsukuba, Ibaraki, Japan
  • 2004–2007
    • Japan Science and Technology Agency (JST)
      Edo, Tōkyō, Japan
  • 2006
    • University of Warsaw
      Warszawa, Masovian Voivodeship, Poland
  • 1989–2003
    • Tokyo Institute of Technology
      • • Materials and Structures Laboratory
      • • Department of Innovative and Engineered Materials
      Tokyo, Tokyo-to, Japan
  • 2002
    • Waseda University
      Edo, Tōkyō, Japan
  • 1998
    • JEOL
      Edo, Tōkyō, Japan
  • 1996
    • The Hong Kong University of Science and Technology
      • Department of Physics
      Kowloon, Hong Kong