Hidekazu Shimotani

Tohoku University, Miyagi, Japan

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Publications (98)553 Total impact

  • [Show abstract] [Hide abstract]
    ABSTRACT: High-quality polycrystalline polyacenes (PLAs) of anthracene (AN), tetracene (TN), and pentacene (PN) doped with potassium in the 1:1 and 1:2 stoichiometries are prepared and their physical properties are systematically studied by magnetic measurements. The magnetic susceptibilities as a function of temperature for Kx(PLAs) are interpreted in the framework of a band-filling model. Clear evidence for the occurrence of charge transfer from K to the organic semiconductors is provided. K1(AN) prepared by a solid-state diffusion process exhibits a pronounced magnetic hump at 140 K, which is associated with antiferromagnetic interactions ascribed to the on-site Coulomb repulsion in a Mott insulating state, whereas both K1(TN) and K1(PN) show a conventional Curie–Weiss behavior at low temperatures. We successfully demonstrate the tuning of the electronic states of PLA-type semiconductors in bulk. Comments are given about the possibility of metallic states in potassium-doped PLAs, which have recently been reported for other polycyclic aromatic hydrocarbons.
    Berichte der deutschen chemischen Gesellschaft 08/2014; 2014(24). DOI:10.1002/ejic.201402381
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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 06/2014; 24(22). DOI:10.1002/adfm.201303566 · 10.44 Impact Factor
  • Physical Review B 06/2014; 89(21). DOI:10.1103/PhysRevB.89.214301 · 3.66 Impact Factor
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    ABSTRACT: High quality bulk samples of anthracene (AN) doped with potassium (K) in 1 : 1 and 2 : 1 stoichiometries were successfully prepared by a method involving a room temperature solid-state mechanical diffusion process prior to intercalation reactions during heat treatment, and their physical properties were studied using both magnetic and optical measurements. The transfer of almost one electron from K to AN in K1(AN) was confirmed by SQUID and ESR measurements. A pronounced magnetic hump centered at 150 K associated with antiferromagnetic interactions was observed, which can most likely be interpreted in terms of on-site Coulomb repulsions of the Mott insulating states. Optical spectra of K1(AN) clearly showed the insulating states, as well as the electron occupation of the LUMO-derived band of AN. Our results demonstrated tuning of the ground state of a typical bulk hydrocarbon by alkali metal intercalation.
    Dalton Transactions 05/2014; 43(26):10040. DOI:10.1039/c4dt00071d · 4.10 Impact Factor
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    ABSTRACT: Electric field tuning of superconductivity has been a long-standing issue in solid state physics since the invention of the field-effect transistor (FET) in 1960. Owing to limited available carrier density in conventional FET devices, electric-field-induced superconductivity was believed to be possible in principle but impossible in practice. However, in the past several years, this limitation has been overcome by the introduction of an electrochemical concept, and electric-field-induced superconductivity has been realized. In the electric double layer (EDL) formed at the electrochemical interfaces, an extremely high electric field is generated and hence high-density charge carriers sufficient to induce superconductivity exist and are collectively used as a charge accumulation device known as an EDL capacitor. Field-induced superconductivity has been used to establish the relationship between T-c and carrier density and can now be used to search for new superconductors. Here, we review electric-field-induced superconductivity using an FET device, with a particular focus on the latest advances in EDL transistors.
    Journal of the Physical Society of Japan 03/2014; 83(3):032001. DOI:10.7566/JPSJ.83.032001 · 1.48 Impact Factor
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    ABSTRACT: As a methodology for controlling the carrier transport of topological insulators (TIs), a flexible tuning in carrier number on the surface states (SSs) of three-dimensional TIs by surface modifications using organic molecules is described. The principle of the carrier tuning and its type conversion of TIs presented in this research are based on the charge transfer of holes or electrons at the TI–organic molecule interface. When 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4-TCNQ) as an electron acceptor or tetracyanoquinodimethane (TCNQ) as a donor is employed for n- and p-Bi2–xSbxTe3–ySey (BSTS) single crystals, successful carrier conversion from n- to p-type 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 TIs and is useful for future applications.
    The Journal of Physical Chemistry C 02/2014; 118(7):3533–3538. DOI:10.1021/jp409715s · 4.84 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; 8(1). DOI:10.1021/nn4054039 · 12.03 Impact Factor
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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 12/2013; 103(25):253508-253508-4. DOI:10.1063/1.4852175 · 3.52 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.
    ChemInform 08/2013; 45(7). DOI:10.1142/S2010324712300058
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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. DOI:10.1038/nphys2691 · 20.60 Impact Factor
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    ABSTRACT: Resistivity, Hall resistivity, thermopower, thermal conductivity, and magnetization are reported for polycrystalline Ba8Ni5Ge41. Ba8Ni5Ge41 is diamagnetic with susceptibility χ dia = (−2.4 to −2.82) × 10−7 emu/g. Semiconductor-like behavior was observed for the resistivity. The thermopower shows positive values for a wide temperature range. The Hall resistivity indicates the dominance of electrons, suggesting the existence of multiband conductance. At room temperature, the thermal conductivity is 1.78(5) W/K m. The highest ZT of Ba8Ni5Ge41 is 0.0016 at about 278 K.
    Journal of Electronic Materials 07/2013; 42(7). DOI:10.1007/s11664-013-2509-z · 1.68 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.
    06/2013; 1(26):4163-4170. DOI:10.1039/C3TC30220B
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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. DOI:10.1021/nl400153p · 12.94 Impact Factor
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    ABSTRACT: We theoretically analyze the optoelectronic properties of single crystals of 2,5-bis(4-biphenylyl) bithiophene (BP2T) and 2-(4-biphenyl)-5-[5-(4-biphenyl)-2-thienyl] furan (BPFT) molecules, aiming to provide a guiding principle for the material design of organic light-emitting transistors. The X-ray structure analysis and the density functional theory (DFT) calculations indicate that half of the BPFT molecules bend the π-conjugation plane in the crystal. The Marcus theory parametrized by the DFT calculations indicates anisotropic charge mobilities. The emission spectra of the BP2T and BPFT crystals are analyzed by the time-dependent DFT calculations in conjunction with the Frenkel exciton model and the vibronic coupling analysis. We revealed that the high photoluminescence efficiency of the BPFT crystal originates from the symmetry breaking of the H-aggregate, where the transition dipole of the dark state does not cancel out.
    The Journal of Physical Chemistry C 04/2013; C 117(16):8072. DOI:10.1021/jp400646n · 4.84 Impact Factor
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    ECS Transactions 03/2013; 50(4):53-57. DOI:10.1149/05004.0053ecst
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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.
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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).
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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.
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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-. DOI:10.1088/1742-6596/400/2/022049
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    Applied Physics Letters 09/2012; 101(10). DOI:10.1063/1.4750245 · 3.52 Impact Factor

Publication Stats

2k Citations
553.00 Total Impact Points

Institutions

  • 2004–2014
    • Tohoku University
      • • Department of Physics
      • • Institute for Materials Research
      Miyagi, Japan
    • Japan Advanced Institute of Science and Technology
      KMQ, Ishikawa, Japan
  • 1999–2013
    • The University of Tokyo
      • • Department of Applied Physics
      • • Center for Quantum-Phase Electronics
      • • Department of Advanced Materials Science
      • • Department of Applied Chemistry
      Tōkyō, Japan
  • 2007
    • High Energy Accelerator Research Organization
      • Institute of Materials Structure Science
      Tsukuba, Ibaraki, Japan
  • 2001
    • Japan Science and Technology Agency (JST)
      Edo, Tōkyō, Japan
  • 2000
    • University of Bucharest
      • Department of Physical Chemistry
      Bucureşti, Bucureşti, Romania