Hidekazu Shimotani

Tohoku University, Sendai, Kagoshima, Japan

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Publications (93)499.48 Total impact

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    ABSTRACT: As a methodology for controlling the carrier transport of topological insulators (TI's), a flexible tuning in carrier number on the surface states (SS's) of three dimensional TI's by surface modifications using organic molecules is described. The principle of the carrier tuning and its type conversion of TI's presented in this research are based on the charge transfer of holes or electrons at the TI/organic molecule interface. By employing 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4-TCNQ) as an electron acceptor or tetracyanoquinodimethane (TCNQ) as a donor for n- and p- Bi2-xSbxTe3-ySey (BSTS) single crystals, successful carrier conversion from n to p and its reverse mode is demonstrated depending on the electron affinities of the molecules. The present method provides a nondestructive and efficient method for local tuning in carrier density of TI's, and is useful for future applications.
    02/2014;
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    ABSTRACT: Field-effect transistors that employ an electrolyte in place of a gate dielectric layer can accumulate ultrahigh-density carriers not only on a well-defined channel (e.g., a two-dimensional surface) but also on any irregularly shaped channel material. Here, on thin films of 95% pure metallic and semiconducting single-walled carbon nanotubes (SWNTs), the Fermi level is continuously tuned over a very wide range, while their electronic transport and absorption spectra are simultaneously monitored. It is found that the conductivity of not only the semiconducting but also the metallic SWNT thin films steeply changes when the Fermi level reaches the edges of one-dimensional subbands and that the conductivity is almost proportional to the number of subbands crossing the Fermi level, thereby exhibiting a one-dimensional nature of transport even in a tangled network structure and at room temperature.
    Advanced Functional Materials 02/2014; · 9.77 Impact Factor
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    ABSTRACT: Optoelectronic devices based on layered materials like graphene have resulted in significant interest due to their unique properties and potential technological applications. The electric and optoelectronic properties of nano GaTe flakes as layered materials are described in this article. The transistor fabricated from multilayer GaTe shows a p-type action with a hole mobility of about 0.2 cm2V-1s-1. The gate transistor exhibits a high photoresponsivity of 104 A/W, which is greatly better than that of graphene, MoS2 and other layered compounds. Meanwhile, the response speed of 6 ms is also very fast. Both the high photoresponsivity and the fast response time described in the present study strongly suggest that multilayer GaTe is a promising candidate for future optoelectronic and photosensitive device applications.
    ACS Nano 12/2013; · 12.06 Impact Factor
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    ABSTRACT: Since the discovery of room temperature ferromagnetism in (Ti,Co)O2, the mechanism has been under discussion for a decade. Particularly, the central concern has been whether or not the ferromagnetic exchange interaction is mediated by charge carriers like (Ga,Mn)As. Recent two studies on the control of ferromagnetism in anatase (Ti,Co)O2 at room temperature via electric field effect [Y. Yamada et al., Science 332, 1065 (2011)] and chemical doping [Y. Yamada et al., Appl. Phys. Lett. 99, 242502 (2011)] indicate a principal role of electrons in the carrier-mediated exchange interaction. In this article, the authors review fundamental properties of anatase (Ti,Co)O2 and discuss the carrier mediated ferromagnetism.
    08/2013;
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    ABSTRACT: Transition-metal dichalcogenides such as WSe2 and MoS2 have electronic band structures that are ideal for hosting many exotic spin–orbit phenomena. Here we investigate the possibility to generate and modulate a giant Zeeman-type spin polarization in WSe2 under an external electric field. By tuning the perpendicular electric field applied to the WSe2 channel with an electric-double-layer transistor, we observe a systematic crossover from weak localization to weak anti-localization in magnetotransport. Our optical reflection measurements also reveal an electrically tunable exciton splitting. Using first-principles calculations, we propose that these are probably due to the emergence of a merely out-of-plane and momentum-independent spin splitting at and in the vicinity of the vertices of the WSe2 Brillouin zone under electric field. The non-magnetic approach for creating such an intriguing spin splitting keeps the system time-reversally invariant, thereby suggesting a new method for manipulating the spin degrees of freedom of electrons.
    Nature Physics 07/2013; 9(9):563-569. · 19.35 Impact Factor
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    ABSTRACT: We report the synthesis and characterization of two new furan-based biphenyl end-capped oligomers, 2-([1,1′-biphenyl]-4-yl)-5-(5-([1,1′-biphenyl]-4-yl)thiophen-2-yl)furan (BPFT) and 5,5′-di([1,1′-biphenyl]-4-yl)-2,2′-bifuran (BP2F) as candidate semiconductors for organic light-emitting field effect transistors (OLETs). Differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) showed the high thermostability of these furan-based semiconductors. X-Ray crystallography of single crystals grown by physical vapor transfer (PVT) method revealed a complicated herringbone packing of BPFT stacking with unusual flat and bent structures, which is different from that of BP2F and the bithiophene-based analogue 5,5′-di([1,1′-biphenyl]-4-yl)-2,2′-bithiophene (BP2T). BPFT single crystal showed a higher absolute quantum yield (51%) compared to that of BP2F and BP2T. Density Functional Theory (DFT) calculations showed that the different excitation energies between flat and bent structures led to the asymmetric transition dipoles in dark state of BPFT H-aggregates, which explains the highest PLQY of BPFT single crystal. Single crystal FET based on BPFT showed an ambipolar characteristic with high hole and electron mobilities, while single crystal FET based on BP2F exhibited p-type characteristic with a high hole mobility. Light emission was observed from the single-crystal FET based on BPFT.
    J. Mater. Chem. C. 06/2013; 1(26):4163-4170.
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    ABSTRACT: Electrically manipulating electron spins based on Rashba spin-orbit coupling (SOC) is a key pathway for applications of spintronics and spin-based quantum computation. Two-dimensional electron systems (2DES) offer a particularly important SOC platform, where spin polarization can be tuned with an electric field perpendicular to the 2DES. Here, by measuring the tunable circular photogalvanic effect (CPGE), we present a room-temperature electric-field-modulated spin splitting of surface electrons on InN epitaxial thin films which is a good candidate to realize spin injection. The surface band bending and resulting CPGE current are successfully modulated by ionic liquid gating within an electric double-layer transistor configuration. The clear gate voltage dependence of CPGE current indicates that the spin splitting of the surface electron accumulation layer is effectively tuned, providing a way to modulate the injected spin polarization in potential spintronic devices.
    Nano Letters 04/2013; 13(5):2024. · 13.03 Impact Factor
  • 4.84. 04/2013; C 117(16):8072.
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    ABSTRACT: Intercalated compounds of polycyclic aromatic hydrocarbons have been drawing much attention from the view point of new type of organic superconductors. The mechanism of superconductivity in these materials is still unclear, and therefore the true ground states with various carrier concentrations must be understood. The antiferromagnetic ground states were reported particularly on K-doped pentacene, a typical polyacene. In the present study, we focus on the synthesis and the magnetic properties of K-intercalated polyacenes, such as anthracene, tetracene, and pentacene. The improved synthetic method based on the conventional solid state reaction was employed to obtain high quality bulk samples. The X-ray powder diffraction profiles of doped samples showed new stable phases. Interestingly, a pronounced hump at 150 K was observed in the temperature dependence of magnetic susceptibility of K1anthracene. In ESR measurements the linewidth of the signals decreased significantly with a decrease in temperature below 150 K and no Pauli magnetic contribution was detected. These results clearly indicate that charge transfer occurs but the most stable ground state is still insulating via antiferromagnetic interactions. Further discussion will be made among these K-intercalated polyacenes.
    03/2013;
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    ABSTRACT: Zintl phase clathrates, which are featured by the cage framework with guest atoms accommodated inside, are considered as good candidates of thermoelectric materials mainly due to the low thermal conductivity caused by large scattering of the acoustic phonons via the rattling phonons arising from the guest motions [1,2]. The fact has been known so far that, in clathrate Sr8Ga16Ge30 showing off-centered displacement of encapsulated elements, thermal conductivity is suppressed even stronger via the scattering of acoustic phonons by anharmonic rattling phonons. Consequently, further detailed understanding on the anharmonic potentials realized in clathrates is important. In this meeting, we will present our recent studies on low temperature specific heat of type I Ba8Ga16Sn30 and K8Ga8Sn38 in addition to those of Ba8Ga16Ge30 and Sr8Ga16Ge30 reported previously [2]. The discussion will mainly focus on the separation of the apparent linear temperature dependent terms of anharmonic rattling phonons from those of conduction electrons. The electron phonon interaction strength and the tunneling density of anharmonic potentials will be described on a basis of the analyses. [1] J. Tang, et al., Phys. Rev. Lett., 105, 176402 (2010). [2] J.-T. Xu, et al., Phys. Rev. B, 82, 085206 (2010).
    03/2013;
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    ABSTRACT: A topological insulator is a material that behaves as an insulator as a bulk state, while permitting metallicity on its Dirac cone surface state. One of the most serious issues of recent researches in this field, however, has been the fact that the Fermi levels in many TIs actually fall in either the conduction or valence band due to the naturally occurring defects and must be controlled by further doping. We report here that the major electron carriers on the SS of a Bi2-xSbxTe3-ySey(BSTS) single crystal can be converted to the hole carriers via interface control using 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane(F4-TCNQ), with strong electron affinity. The evidence can be elucidated using a detailed three-carrier model. The results apparently demonstrate that the charge transfer at the TI/organic-molecule interface is very efficient in order to control the carrier density of TIs, particularly on the SS. Our present results will be very important for studying the fundamental aspects of TIs as well as their future device applications.
    03/2013;
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    ABSTRACT: Taking advantages of broad tunability of carrier density in electric-double-layer transistors (EDLTs) with ionic-liquid gating, we demonstrate evidence of parallel conduction from both p-type bulk and n-type surface in Mg-doped InN EDLTs by comparing their transport properties, especially Hall effect, with those in non-doped InN. Large anomalous oscillation in Hall coefficients with decreasing gate bias was observed in Mg-doped samples, which can be well simulated by two-carrier model. Our results provide the proof for the p-type bulk conduction in Mg-doped InN by showing its prominent effects on electrical transport.
    Applied Physics Letters 01/2013; 103(25):253508-253508-4. · 3.79 Impact Factor
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    ABSTRACT: We here report the result of in situ magnetization measurements of electrochemical cells at low temperatures. Upon applying voltages between the electrodes of the electrochemical cells, we observed shielding diamagnetic signals from several materials, indicating superconducting transitions. The superconducting states can be induced both electrochemically and electrostatically with appropriate combination of counter electrode materials and electrolytes. The present technique may become a powerful method for searching novel superconductors.
    Journal of Physics Conference Series 12/2012; 400(2):2049-.
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    Applied Physics Letters 09/2012; 101(10). · 3.79 Impact Factor
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    ABSTRACT: In order to clarify its electric double layer (EDL) structure under external voltage, an interface between the ionic liquid N,N-diethyl-N-methyl-N-(2-methoxyethyl)ammonium bis(trifluoromethanesulfonyl)imide and a Au (111) surface was studied by x-ray reflectometry. A clear change in reflectivity was observed as a function of applied voltage. The electron density profile of the ionic liquid around the interface was calculated using liquid structure models. Switching of the external electric field reversed the polarity of the EDL, and the magnitude of the applied voltage controlled the magnitude of the layering structure as well as molecular position fluctuation.
    Applied Physics Letters 08/2012; 101(5). · 3.79 Impact Factor
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    ABSTRACT: Electro-static carrier doping was attempted in a layered transition metal disulphide MoS2 by constructing an electric double-layer transistor with an ionic liquid. With the application of gate voltage VG higher than 3 V, a metallic behavior was observed in the MoS2 channel. We found an onset of electric field-induced superconductivity in the field induced metallic phase. A maximum TC ∼ 9.4 K was observed, which could be higher than those in chemically doped bulk materials.
    Applied Physics Letters 07/2012; 101(4). · 3.79 Impact Factor
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    ABSTRACT: Ambipolar electric double-layer transistors (EDLTs) using organic single crystals and ion-gel electrolytes are successfully created by optimising the fabrication of gel films. The p- and n-type EDLTs enable us to investigate the HOMO-LUMO gap energy of semiconductors, offering a new method with which to measure it.
    Advanced Materials 06/2012; 24(32):4392-7. · 14.83 Impact Factor
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    ABSTRACT: Taking advantage of ultrahigh electric field generated in electric-double-layer transistors (EDLTs), we investigated spin-orbit interaction (SOI) and its modulation in epitaxial trilayer graphene. It was found in magnetotransport that the dephasing length L(φ) and spin relaxation length L(so) of carriers can be effectively modulated with gate bias. As a direct result, SOI-induced weak antilocalization (WAL), together with a crossover from WAL to weak localization (WL), was observed at near-zero magnetic field. Interestingly, among existing localization models, only the Iordanskii-Lyanda-Geller-Pikus theory can successfully reproduce the obtained magnetoconductance well, serving as evidence for gate tuning of the weak but distinct SOI in graphene. Realization of SOI and its large tunability in the trilayer graphene EDLTs provides us with a possibility to electrically manipulate spin precession in graphene systems without ferromagnetics.
    Nano Letters 04/2012; 12(5):2212-6. · 13.03 Impact Factor
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    ABSTRACT: Recent success in isolating single-walled carbon nanotubes (SWNTs) of narrow chirality distribution enabled making pure metallic (m-) and semiconducting (s-) SWNT films. Such films are expected to reflect the nature of individual SWNTs, that is their one dimensional subband structure. Therefore, it is interesting to investigate electronic transport in m- and s-SWNT films by controlling their Fermi level (EF). Chemical doping or FET is unsuitable for the purpose because of the lack of precise and reversible EF controllability, and the narrow controllable EF range, respectively. The problems are solved by our electric double layer transistor technique,^1 where the gate voltage (VG) is applied through an electrolyte. The conductance and optical absorption spectra of the resistance of s- and m-SWNT films were measured at various VG. The conductance of the s-SWNT film showed stepwise change against VG. The absorbance spectra indicate the steps correspond to reaching of the EF to a vHs. Furthermore, even m-SWNT films showed steep increases of conductance, demonstrating that the conductance strongly depend on the subband filling. ^1 H. Shimotani et al., Appl. Phys. Lett. 88, 073104 (2006).
    02/2012;
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    ABSTRACT: Ferromagnetic semiconductor is one of the most attractive materials for semiconductor spintronics because of the controllability of both charge and spin degrees of freedom. Electric field effect of magnetism in the ferromagnetic semiconductors such as (Ga,Mn)As has been demonstrated only at low temperature due to their low Curie temperatures. In this study, we report the electric field manipulation of ferromagnetism in a ferromagnetic semiconductor Ti1-xCoxO2-δ at room temperature [1]. Anatase Ti1-xCoxO2-δ (001) epitaxial film was deposited on TiO2 buffer 5 nm / LaAlO3 (100) substrate in various oxygen pressures in order to vary an electron density by pulsed laser deposition method. An electric double layer transistor was fabricated on a paramagnetic film with an electron density of 1x10^19 cm-3. With increasing gate voltage, the electron density was increased to 7x10^19 cm-3. Ferromagnetic hysteresis loop was observed for VG above 3.0 V in an anomalous Hall resistivity, which is proportional to a magnetization of the film. This result represents that the ferromagnetism was induced at room temperature by an electrostatic charge accumulation, indicating that the ferromagnetism in this compound is mediated by the electron carriers. [4pt] [1] Y. Yamada et al., Science 332, 1065 (2011).
    02/2012;

Publication Stats

391 Citations
499.48 Total Impact Points

Institutions

  • 2004–2013
    • Tohoku University
      • • Department of Physics
      • • Institute for Materials Research
      Sendai, Kagoshima, Japan
  • 1999–2012
    • The University of Tokyo
      • • Center for Quantum-Phase Electronics
      • • Department of Applied Chemistry
      • • Department of Advanced Materials Science
      Tokyo, Tokyo-to, Japan
  • 2011
    • National Institute of Advanced Industrial Science and Technology
      • Nanoelectronics Research Institute
      Tsukuba, Ibaraki, Japan
  • 2001
    • Japan Science and Technology Agency (JST)
      Edo, Tōkyō, Japan