Kenjiro Miyano

The University of Tokyo, Tokyo, Tokyo-to, Japan

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Publications (112)353.97 Total impact

  • Naoki Ogawa, Yasushi Ogimoto, Kenjiro Miyano
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    ABSTRACT: Ultrafast photoinduced modulation is demonstrated in the orbital-order-induced polarization in manganite thin films. The evolution of the polarization modulation and the lattice temperature rise after photoexcitation are tracked separately by monitoring the relevant nonlinear optical susceptibility tensor elements. Substantial portion of the polarization disappears within the excitation pulse width (∼120 fs), indicating the purely electronic origin triggered by an interband excitation. We propose oxide heterostructure devices incorporating polar gating layers capable of ultrafast optical control of electronic phases.
    Applied Physics Letters 06/2013; 102(25). · 3.52 Impact Factor
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    ABSTRACT: Electron correlation often produces a variety of electrically insulating states caused by self-organization of electrons, which are particularly stable at commensurate fillings. Although collapsing such ordered states by minute external stimuli has been a key strategy toward device applications, it is difficult to access their true electronic phase boundaries due to the necessity of fine-tuning of material parameters. Here, we demonstrate the ambipolar resistance switching in Pr1-xSrxMnO3 thin films (x = 0.5; an effectively 1/4-filled state) by quasi-continuous control of the doping level x and band-width W using gate-voltage and magnetic field, enabled by the extreme electric-field formed at the nanoscale interface generated in an electrolyte-gated transistor. An electroresistance peak with unprecedented steepness emerges on approaching a critical point in the x-W phase diagram. The technique opens a new route to Mott-insulator based transistors and to discovering singularities hitherto unnoticed in conventional bulk studies of strongly correlated electron systems.
    Scientific Reports 01/2013; 3:2904. · 5.08 Impact Factor
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    ABSTRACT: Polar states are realized in pseudocubic manganite films fabricated on high-index substrates, in which a Jahn-Teller (JT) distortion remains an active variable. Several types of orbital orders (OOs) were found to develop large optical second harmonics, signaling broken-inversion symmetry distinct from their bulk forms and films on (100) substrates. The observed symmetry lifting and first-principles calculation both indicate that the modified JT q2 mode drives Mn-site off centering, which can be controlled by a magnetic-field-induced phase transition via a coupling of OO and spin orders.
    Physical Review Letters 04/2012; 108(15):157603. · 7.73 Impact Factor
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    ABSTRACT: We found static polarization in perovskite manganite films when they are under shear stress. The phenomenon is omnipresent in films deformed in (at least) monoclinic fashion due to the substrate-imposed strain, whereas it is absent in bulk crystals even though they are distorted in a similar manner in thermal equilibrium. The substrate stress of low symmetry is clearly the driving force for the appearance of the polarization. Optical second-harmonic generation (SHG) confirms the loss of inversion symmetry in strained films and pyroelectricity was detected in insulating films confirming the presence of the static polarization. DFT calculations show that the stable atomic positions in the experimentally observed structure is polar with the shift of the center of gravity of anions relative to that of cations as much as 10-2 å. The calculated polar structure is consistent with the symmetry obtained from the SHG polarimetry.
    02/2012;
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    ABSTRACT: The competition between the substrate-induced strain and the electrostatic polarization in improper ferroelectric films (hexagonal YMnO3) has been studied using optical second-harmonic generation techniques. Deposited on a (111) surface of yttria-stabilized zirconia substrate, the films exhibit (1) strong strain with negligible polarization, (2) relaxation accompanied by the nonlinear buildup of the polarization, and (3) bulklike polar behavior, in the order of increasing film thickness. The role of the substrate is distinctively different from that in proper ferroelectrics.
    Physical review. B, Condensed matter 10/2011; 84(14). · 3.66 Impact Factor
  • Kenjiro Miyano, Naoki Ogawa, Yasushi Ogimoto
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    ABSTRACT: Perovskite manganites develop various ordering patterns of charge, orbital, and spin, whose geometrical correlation brings out a new order, e.g., electronic polarization. As an example in pseudo-cubic systems, the CE-type charge/orbital order (CO/OO) was predicted to be multiferroic. With the use of nonlinear optics, we show that Pr0.5Sr0.5MnO3 thin films exhibit symmetry breaking in the OO phase without CO. Ultrathin films prepared on LSAT(110) substrates reproduce bulk-like properties, showing successive phase transitions from paramagnetic metal, to ferromagnetic metal, and finally to A-type antiferromagnetic (AF) insulator upon cooling. Below TOO=TN, we detect SHG from these films and visualize the formation of AF domains with the size of several mum, which is much larger than that of phase-separated manganites. With careful examination of the magnetic point group, we can ascribe the broken symmetry to the AF spin order under the monoclinic lattice distortion concomitant with the OO, which also manifests the direction of the AF vector.
    03/2011;
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    ABSTRACT: Photoinduced phase transitions are of special interest in condensed matter physics because they can be used to change complex macroscopic material properties on the ultrafast timescale. Cooperative interactions between microscopic degrees of freedom greatly enhance the number and nature of accessible states, making it possible to switch electronic, magnetic or structural properties in new ways. Photons with high energies, of the order of electron volts, in particular are able to access electronic states that may differ greatly from states produced with stimuli close to equilibrium. In this study we report the photoinduced change in the lattice structure of a charge and orbitally ordered Nd(0.5)Sr(0.5)MnO(3) thin film using picosecond time-resolved X-ray diffraction. The photoinduced state is structurally ordered, homogeneous, metastable and has crystallographic parameters different from any thermodynamically accessible state. A femtosecond time-resolved spectroscopic study shows the formation of an electronic gap in this state. In addition, the threshold-like behaviour and high efficiency in photo-generation yield of this gapped state highlight the important role of cooperative interactions in the formation process. These combined observations point towards a 'hidden insulating phase' distinct from that found in the hitherto known phase diagram.
    Nature Material 02/2011; 10(2):101-5. · 35.75 Impact Factor
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    ABSTRACT: We characterized the optical absorption and solar cell characteristics of high-density InAs self-assembled quantum dots (QDs) grown on GaAs (001) substrates by atomic hydrogen-assisted molecular beam epitaxy. The GaNAs material can be used as strain-compensating layer (SCL) thereby minimizing the net strain, and thus is advantageous for multi-stacking of InAs QDs structures. As a result, dislocations and coalesced islands were not observed in 100 layer-stacked QDs. For QD solar cell characterization, the short-circuit current density of QDSC increases with increasing number of QD stacks, and reaches as high as 26.4 mA/cm2 for 50 layer-stacked sample under air-mass 1.5 condition. However, the light absorption by QD superlattice as determined by optical absorption measurements at is limited to ∼ 10% even for 100 layer-stacked samples. (© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
    physica status solidi (c) 01/2011;
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    ABSTRACT: Gigantic responses arising from the melting of charge-orbital ordering (COO) in perovskite manganites have been attractive both in material science and technologies. We review the essentials to realize such phenomena in a thin film form aiming at a possible application to correlated electron devices, namely, the role of epitaxial strain on (1 1 0) substrates. The crucial issues in designing the correlated electron interfaces of (0 0 1), (1 1 0), and (2 1 0) are also discussed. In addition, the epitaxial growth of high-Tco materials such as (Bi, Sr)MnO3 and A-site ordered SmBaMn2O6 thin films is described, which is indispensable towards room temperature operation of correlated electron devices.
    ChemInform 11/2010;
  • Advanced Materials 11/2010; 22(48):5507-11. · 14.83 Impact Factor
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    ABSTRACT: The critical thickness has been a fundamental issue in thin films of ferroelectric materials--the absence of which was recently predicted theoretically for improper ferroelectric materials of hexagonal YMnO3 (YMO). [1] Here we report second harmonic generation (SHG) studies on YMO ultrathin films, revealing experimentally this prediction. The YMO ultrathin films were deposited on YSZ (111) substrates by pulsed laser ablation. The samples were single crystalline having atomically flat surfaces and epitaxial relation ([10-10]YMO //[-110]YSZ), as characterized by in-situ RHEED, AFM, XRD, and HR-TEM. It is found that even a film with 1.5-unit-cell thickness shows substantial SHG intensity, indicating the existence of spontaneous polarization. In addition to the polarization along the c-axis, we found in-plane components which are not observed in single crystals. The details of the crystallographic symmetry and possible origins of the in-plane components will be discussed. [4pt] [1] N. Sai, et al., Phys. Rev. Lett. 102, 107601 (2009).
    03/2010;
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    ABSTRACT: We have investigated the structural feature of orbital-ordering manganite thin films fabricated on the perovskite (011) substrates by means of synchrotron x-ray diffraction technique. Like bulk manganites, these films show large jumps in lattice parameters at the orbital ordering transition temperature, which clearly show the transitions are first order. However, the magnitude of the volume change at the transition temperature for the films is much smaller than that for the orbital ordering transition in bulk manganite. This difference is understood by taking into account the stress from the substrate.
    Journal of Physics Conference Series 02/2010; 211(1):012004.
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    ABSTRACT: Electronic and magnetic dynamics at a single interface of antiferromagnetic manganites, LaMnO3/SrMnO3, were studied with time-resolved second-harmonic generation and time-resolved nonlinear magneto-optical Kerr effect. The temporal evolution of the respective signals shows clear differences; an optical pump makes an instantaneous response of the electrons and, in contrast, induces slow demagnetization, which points to a possibility of two-dimensional half-metallic ferromagnetism for this heterointerface. Detailed analysis with various pump and probe polarizations reveals possible pathways of the optical charge-transfer excitation between Mn d orbitals, leading to a subsequent perturbation to the interface spin system.
    Physical review. B, Condensed matter 12/2009; 80(24). · 3.66 Impact Factor
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    ABSTRACT: Lattice polarization at the SrTiO3/LaAlO3 interface was investigated by optical second harmonic generation. Superlattices with varying periodicity were employed to study the evolution of interface polarization, while separating substrate contributions. We observed large perpendicular optical nonlinearity, which abruptly increases when the sublattice thickness goes above 3 unit cells. The polarization is primarily in SrTiO3 and develops up to 8 unit cells from the interface.
    Physical review. B, Condensed matter 07/2009; 80(8):81106. · 3.66 Impact Factor
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    ABSTRACT: An electronic effect on a macroscopic domain structure is found in a strongly correlated half-doped manganite film Nd$_{0.5}$Sr$_{0.5}$MnO3 grown on a (011) surface of SrTiO3. The sample has a high-temperature (HT) phase free from distortion above 180K and two low-temperature (LT) phases with a large shear-mode strain and a concomitant twin structure. One LT phase has a large itinerancy (A-type), and the other has a small itinerancy (CE-type), while the lattice distortions they cause are almost equal. Our x ray diffraction measurement shows that the domain size of the LT phase made by the HT-CE transition is much smaller than that by the HT-A transition, indicating that the difference in domain size is caused by the electronic states of the LT phases.
    Physical review. B, Condensed matter 03/2009; 79(22). · 3.66 Impact Factor
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    ABSTRACT: Transition-metal compounds with spin, charge, and orbital degrees of freedom tend to have frustrated electronic states coupled with local lattice distortions and to show drastic response to external stimuli such as photo-excitation. We have studied the charge-orbital states in perovskite-type Pr0.55(Ca1-ySry)0.45MnO3 thin films (PCSMO) and spinel-type CuIr2S4 using photoemission spectroscopy combined with additional laser illumination. PCSMO and CuIr2S4 are clear-cut examples of transition-metal compounds showing photo-induced metallic conductivities but the charge-orbital states in the two systems show contrasting responses to the photo-excitation. The charge-orbital states in PCSMO are stabilized by Jahn-Teller or Breathing-type lattice distortions and can be destroyed by photo-excitation. On the other hand, the charge-orbital states in CuIr2S4 are stabilized by dimer formation and tend to be robust against photo-excitation.
    Journal of Physics Conference Series 03/2009; 148(1):012047.
  • Kenjiro Miyano
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    ABSTRACT: Photoinduced phase change in manganite thin films are described, which are bidirectional between metallic and insulating states and persistent within the laboratory time scale. The phenomena are due to the strong cross-coupling between electronic, magnetic, and elastic interactions. Among them, the elastic part is particularly important to make the process slow and the effect persistent or at least metastable. Observations supporting this view are presented.
    Journal of Physics Conference Series 03/2009; 148(1):012014.
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    ABSTRACT: The electronic symmetry of the SrTiO3/LaAlO3 interface was investigated by optical second harmonic generation, using superlattices with varying periodicity to study the evolution of the electronic reconstruction while avoiding substrate contributions. The superlattices show large perpendicular optical nonlinearity, which abruptly increases when the sublattice thickness goes above 3 unit cells, revealing substantial effects of the polar-nonpolar interface. The nonlinear 'active' area is primarily in SrTiO3, develops with increasing thickness, and extends up to 8 unit cells from the interface.
    03/2009;
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    ABSTRACT: Thin films of strongly correlated materials are studied intensively because of their potential of device application. Those materials in bulk form show various fascinating properties such as metal-insulator transition. However, clear phase transitions are often suppressed under the strain from the substrates. We have studied Nd0.5Sr0.5MnO3 thin film on SrTiO3 (011) substrate, which is a unique film that has clear orbital-ordering (OO) transition, by x-ray scattering under magnetic field up to 8T. As reported earlier [1], this system show three phase transitions, paramagnetic (PM), ferromagnetic (F), A-type OO (A) to CE-type OO (CE) with cooling in zero field, and at F-A transition temperature (170K), the symmetry lowers and twin occurs. The phase sequence was changed to PM, F to CE above 4T, and above this field, considerable amount of the FM phase remains down to 10K. This field induced phase separation is attributable to the martensitic accommodation strain at the domain boundary. [1]Y.W. et al., Phys. Rev. Lett. 96 017202 (2006), J.Phys.Soc.Jpn. 77, 014712 (2008).
    03/2009;
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    ABSTRACT: We report the lattice dynamics in the photoinduced phase of Nd0.5Sr0.5MnO3 thin film by means of time-resolved x-ray diffraction measurements at the NW14A beamline in the Photon Factory Advanced Ring. We observed the decrease of intensity after the photoirradiation for a superlattice reflection which is associated with Jahn-Teller distortion.
    Journal of Physics Conference Series 02/2009; 148(1).

Publication Stats

2k Citations
353.97 Total Impact Points

Institutions

  • 1992–2012
    • The University of Tokyo
      • • Research Center for Advanced Science and Technology
      • • Department of Applied Physics
      Tokyo, Tokyo-to, Japan
  • 2006
    • High Energy Accelerator Research Organization
      • Institute of Materials Structure Science
      Tsukuba, Ibaraki-ken, Japan
  • 2003
    • National Institute for Materials Science
      Tsukuba, Ibaraki, Japan
  • 2000
    • Japan Science and Technology Agency (JST)
      Edo, Tōkyō, Japan