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Last Updated :2024/06/07

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Master

Affiliation (Master)

  • Faculty of Engineering　Applied Physics　Quantum Matter Physics

Affiliation (Master)

  • Faculty of Engineering　Applied Physics　Quantum Matter Physics

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Profile and Settings

Degree

  • PhD（Hokkaido University）

Profile and Settings

  • Name (Japanese)

    Ichimura

  • Name (Kana)

    Koichi

  • Name

    200901020799900776

Achievement

Research Interests

  • superconductivity   spin density wave   charge density wave   organic conductors   scanning tunneling microscopy   low dimensional conductors   tunneling spectroscopy   

Research Areas

  • Natural sciences / Magnetism, superconductivity, and strongly correlated systems

Research Experience

  • 2007/04 - Today Department of Applied Physics, Hokkaido University Associate Professor
  • 1994/05 - 2007/03 Division of Physics, Hokkaido University Research Associate
  • 1992/04 - 1994/03 Division of Physics, Hokkaido University

Education

  •        - 1993  Hokkaido University
  •        - 1993  Hokkaido University  Graduate School, Division of Natural Science
  •        - 1988  Hokkaido University  School of Science
  •        - 1988  Hokkaido University  Faculty of Science

Published Papers

  • Kazuoki Yokoi, Issei Miyazaki, Koichi Ichimura, Satoshi Tanda, Noriaki Matsunaga, Tohru Kurosawa, Migaku Oda
    Solid State Communications 371 115262 - 115262 0038-1098 2023/10
  • Syuma Yasuzuka, Shinya Uji, Shiori Sugiura, Taichi Terashima, Yoshio Nogami, Koichi Ichimura, Satoshi Tanda
    Journal of Superconductivity and Novel Magnetism 33 (4) 953 - 958 1557-1939 2020/04 [Refereed]
     
    Resistivity measurements for a layered transition metal dichalcogenide 2H-NbSe2 were performed to investigate the in-plane anisotropy of the upper critical field (H-c2) for magnetic fields up to 14.5 T at temperatures down to 2.0 K. For fields rotated within the basal plane, the in-plane anisotropy of a characteristic field H* (defined as the field strength at which the resistivity reaches zero) showed twofold symmetry at low temperatures. On the basis of this result, the in-plane anisotropy of H-c2 and the superconducting gap structure in 2H-NbSe2 are discussed.
  • S. Kitou, A. Nakano, S. Kobayashi, K. Sugawara, N. Katayama, N. Maejima, A. Machida, T. Watanuki, K. Ichimura, S. Tanda, T. Nakamura, H. Sawa
    Physical Review B 99 (10) 2469-9950 2019/03/28 [Refereed]
  • Nomura A, Yamaya K, Takayanaci S, Ichimura K, Tanda S
    EPL 124 (6) 0295-5075 2018/12 [Refereed][Not invited]
  • A. Nomura, K. Yamaya, S. Takayanagi, K. Ichimura, T. Matsuura, S. Tanda
    EPL 119 (1) 17005-1 - 17005-7 0295-5075 2017/07 [Refereed][Not invited]
     
    We have synthesized single crystals doped with Cu atoms in TaSe3 which exhibits no Charge Density Wave (CDW) transition, and measured precisely the temperature dependence of the resistance. We discover an anomalous sharp dip in the temperature derivative of the resistance (dR/dT) at about 91 K in Cu-doped TaSe3, which is never observed in pure TaSe3. The dip suggests that there is a phase transition with a relative increase in resistance. In addition, the dip is "gamma"shaped. We reveal that the same "gamma"-shaped dip in dR/dT is commonly observed at the CDW transition temperature in many CDW conductors, which is a universal consequence resulting from the opening and growth of a CDW gap on a Fermi surface. Furthermore, the result of the single-crystal X-ray diffraction (XRD) analysis implies that Cu-doping increases the lattice parameter of the a-axis and c-axis and decreases that of the b-axis, leading to an improvement in the nesting condition. Based on the "gamma"-shaped dip and the result of the single-crystal XRD analysis, we conclude that a CDW emerges by Cu-doping in TaSe3. Copyright (C) EPLA, 2017
  • Takuya Kanno, Takuya Matsumoto, Koichi Ichimura, Toru Matsuura, Satoshi Tanda
    JOURNAL OF LOW TEMPERATURE PHYSICS 183 (1-2) 41 - 49 0022-2291 2016/04 [Refereed][Not invited]
     
    We have observed a metal-insulator transition of a quasi-two-dimensional electronic system in transition metal dichalcogenide 2H-TaSe2 caused by doping iron. The sheet resistance of 2H-FexTaSe2 (0 <= x <= 0.120) single crystals rises about 10(6) times with the increasing of x at the lowest temperature. We investigated the temperature dependence of the resistance and found a metal-insulator transition with a critical sheet resistance 11.7 +/- 5.4 k Omega. The critical exponent of the localization length. is estimated 0.31 +/- 0.18. The values of the critical sheet resistance and. are accordant to those of the chiral unitary class (less than h/1.49e(2) = 17.3 k Omega and 0.35 +/- 0.03, respectively). We suggest that 2H-FexTaSe2 is classified as the chiral unitary class, not as standard unitary class.
  • Yuki Oka, Hiroyoshi Nobukane, Noriaki Matsunaga, Kazushige Nomura, Kazuhiro Katono, Koichi Ichimura, Atsushi Kawamoto
    JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN 84 (6) 0031-9015 2015/06 [Refereed][Not invited]
     
    We performed tunneling spectroscopy (STS) measurement in an organic superconductor, partially deuterated kappa(BEDT-TTF-d[3,3])(2)Cu[N(CN)(2)]Br. We carried out angle-resolved STS on cut lateral surfaces by the focused ion beam (FIB) method as well as as-grown single-crystal surfaces. It was found that the d[3,3]-Br salt is a strong-coupling d-wave superconductor and that the nodal direction is at an angle of pi/4 from the a*-axis, corresponding to the d(x)(-y)(2)(2) symmetry in the bulk superconducting phase. It is understood that the electron correlation in the d[3,3]-Br salt is not strong enough on the basis of the spin fluctuation mechanism. On the other hand, we also observed two types of superconducting gap. This suggests the coexistence of the d(x)(-y)(2)(2) and d(xy) symmetries. This indicates a change in the symmetry from d(x)(-y)(2)(2) in the bulk superconducting phase to d(xy) around the insulating region with increasing electron correlation.
  • K. Katono, T. Taniguchi, K. Ichimura, Y. Kawashima, S. Tanda, K. Yamamoto
    Physical Review B 91 (12) 1098-0121 2015/03/04 [Refereed]
  • T. Kanno, T. Matsumoto, K. Ichimura, T. Matsuura, S. Tanda
    PHYSICA B-CONDENSED MATTER 460 165 - 167 0921-4526 2015/03 [Refereed][Not invited]
     
    We investigated the conduction of electrons in a quasi two-dimensional random magnetic field (RMF) system, namely iron-doped transition metal dichalcogenide 2H-FexTa1-xSe2. We observed a clear metal-insulator transition by increasing the Fe doping rate. The temperature dependence of the resistances is consistent with the unitary class of Anderson localization. However, the observed critical sheet resistance is between 2 and 19 k Omega, which is inconsistent with the unitary class (2h/e(2) = 51.6 k Omega). We propose that chiral symmetry can be present in 2H-FexTa1-xSe2 because the magnetoresistance of the crystals is similar to that of dirty graphites. (C) 2014 Elsevier B.V. All rights reserved.
  • K. Katono, K. Ichimura, T. Taniguchi, Y. Kawashima, K. Yamaya, S. Tanda
    PHYSICA B-CONDENSED MATTER 460 64 - 67 0921-4526 2015/03 [Refereed][Not invited]
     
    We performed scanning tunneling microscopy (STM) at room temperature on a - b surface of alpha-(BETD-TTF)(2)I-3 which undergoes a charge ordering transition at T-CO = 135 K. BEDT-TTF molecular arrangement with a-type was clearly observed. Charge modulation with two fold molecular periodicity was observed along both Columns I and II. The auto correlation of the charge modulation along Column II has extended in the area observed by STM. On the other hand, that along the column I is much weaker than column II although the charge modulation locally develop. The charge modulations along Columns I and II form horizontal stripe structure with some inhomogeneity, whose structure is similar to the charge order below T-CO 135 K. Our result suggests that horizontal charges stripe at room temperature is a precursor of the charge ordered state. (C) 2014 Elsevier B.V. All rights reserved
  • Oka Y, Nobukane H, Matsunaga N, Nomura K, Katono K, Ichimura K, Kawamoto A
    PHYSICA B-CONDENSED MATTER 460 93 - 95 0921-4526 2015/03/01 [Refereed][Not invited]
     
    We performed the STM Spectroscopy (STS) measurement on organic superconductor deuterated κ-(ET-d[3,3])2Cu[N(CN)2]Br to clarify the mechanism of superconductivity. The mechanism of superconductivity in κ-ET2X is predicted as the spin fluctuation mechanism where the gap symmetry changes from dx2-y2 to dxy with increasing the electron correlation. In previous STS measurements, we found that d[2,2]-Br, d[0,0]-Br and Cu(NCS)2 were d-wave superconductors and the symmetry was described by the dx2-y2. The electron correlation of these salts is still weak in the frame of the spin fluctuation model. So, we investigated the d[3,3]-Br which located at stronger electron correlation region than the previously studied salts by the STS. We also investigated the STS on the cut lateral surfaces by the focused ion beam (FIB) method to measure the in-plane anisotropy of superconducting gap in more detail as well as the as-grown surfaces. It was found from the result that d[3,3]-Br was d-wave superconductor and the gap symmetry was dx2-y2 as well as the previously studied ET salts. Therefore, d[3,3]-Br is also still weak in spite of stronger electron correlation than other ET salts. On the other hand, we also observed the two-type superconducting gaps which are the nodal and anti-nodal type gaps at the same angle surface. This suggests the coexistence of dx2-y2 and dxy. It may be the evidence of the change from dx2-y2 to dxy caused by the locally stronger electron correlation around the anti-ferromagnetic insulating region.
  • Yuki Kawashima, Koichi Ichimura, Kazuhiro Katono, Tohru Kurosawa, Migaku Oda, Satoshi Tanda, Yoichi Kamihara, Hideo Hosono
    SOLID STATE COMMUNICATIONS 204 33 - 36 0038-1098 2015/02 [Refereed][Not invited]
     
    We report an electron tunneling study of SmFeAsO1-xFx in the low doping region (x=0, 0.045, 0.046, 0.069) by low temperature UI-IV-STM/STS. Superconducting gaps are observed for each superconducting sample x=0.045 (T-c=12.9 K), x=0.046 (T-c=32.9 K) and x=0.069 (T-c=46.9 K). We obtained corresponding superconducting gap size of Delta(sc) = 9.5 +/- 0.5 meV, 9.75 +/- 0.25 mcV and 11 +/- 1 meV. While T-c increases, Delta(sc) is kept the same. This suggests that the effective attractive interaction is the same and that there is some mechanism that suppresses the superconductivity in the low doping region. On the other hand, similar gap structures were found in a non superconducting sample with x=0 at 7.8 K. The obtained gap size was Delta(N) = 8.5 +/- 1.5 meV, which is almost the same as the superconducting gap in the superconducting samples (x=0.045, 0.046, 0.069). (C) 2014 Published by Elsevier Ltd.
  • Kanno T., Matsumoto T., Ichimura K., Matsuura T., Tanda S.
    Meeting Abstracts of the Physical Society of Japan 一般社団法人日本物理学会 70 1913 - 1913 2189-079X 2015
  • Nakano A., Kitou S., Katayama N., Sawa H., Sagayama H., Kumai R., Matsubayashi K., Okada T., Uwatoko Y., Munakata K., Nakao A., Takagi H., Ichimura K., Tanda S., Sawa H.
    Meeting Abstracts of the Physical Society of Japan 一般社団法人日本物理学会 70 2472 - 2472 2189-079X 2015
  • Yuki Kawashima, Koichi Ichimura, Junya Ishioka, Tohru Kurosawa, Migaku Oda, Kazuhiko Yamaya, Satoshi Tanda
    SOLID STATE COMMUNICATIONS 167 10 - 13 0038-1098 2013/08 [Refereed][Not invited]
     
    The electronic state of Fe1.17Te was investigated with atomic resolution by using STM. We discovered a charge stripe structure at 7.8 K for the first time. The wave vector of the charge stripe structure is along the a-axis or b-axis. Furthermore we found that the charge stripe occurred at the Fe layer. We also found a 9 meV gap structure on the Fermi surface by STS. The gap size is consistent with the mean field approximation with SOW transition temperature of 58 K. (C) 2013 Elsevier Ltd. All rights reserved.
  • J. Ishioka, T. Fujii, K. Katono, K. Ichimura, T. Kurosawa, M. Oda, S. Tanda
    PHYSICAL REVIEW B 86 (24) 1098-0121 2012/12 [Refereed][Not invited]
     
    We are responding to the Comment by J. Wezel on our paper. This study was developed from our previous work [Ishioka et al., Phys. Rev. Lett. 105, 176401 (2010)]. In the PRL paper, H-CDW was defined as a new parameter for expressing CDW chirality for the first time. In his Comment, he claims that H-CDW is ill defined. He also claims that the initial phase phi of the CDW wave function is a more appropriate parameter for expressing chiral CDW, despite our early introduction of phi to explain the experimental data described in the PRL paper. However, we conclude that H-CDW can distinguish the CDW chirality by its sign. Moreover, by considering different H-CDW signs, we had succeeded in demonstrating the difference of the spatial distributions of CDWs as shown in Fig. 4 of the PRB paper [Phys. Rev. B 84, 245125 (2011)]. In our Reply, we discuss the validity of H-CDW. We show that his argument regarding the identification of the CDW with the opposite sign of q is wrong, since the logic is inapplicable to a wave function with a nonzero phi. We also discuss the applicability of H-CDW to two- or three-dimensional CDWs in transition metal dichalcogenides. DOI: 10.1103/PhysRevB.86.247102
  • Satoshi Tsuchiya, Jun-ichi Yamada, Satoshi Tanda, Koichi Ichimura, Taichi Terashima, Nobuyuki Kurita, Kota Kodama, Shinya Uji
    PHYSICAL REVIEW B 85 (22) 220506  1098-0121 2012/06 [Refereed][Not invited]
     
    We report magnetoresistance and magnetic torque measurements for the highly correlated 2D organic superconductor kappa-(BEDT-TTF)(2)Cu(NCS)(2). Under magnetic field parallel to the conducting layers, we obtain convincing evidence of fluctuating superconductivity in a wide temperature and field range, where the magnetic torque shows diamagnetism but the resistance shows no appreciable decrease.
  • K. Katono, K. Ichimura, Y. Kawashima, K. Yamaya, S. Tanda
    PHYSICA B-CONDENSED MATTER 407 (11) 1827 - 1830 0921-4526 2012/06 [Refereed][Not invited]
     
    The density wave phase of alpha-(BEDT-TTF)(2)KHg(SCN)4 was investigated by transport properties and magnetic susceptibility. The density wave transition was observed as a broad increase at T-DW = 9 K by resistance measurement. Temperature dependence of the static magnetic susceptibility chi shows a large Curie tail below 100 K. By subtracting the Curie component, we found that the magnetic susceptibility increases like weak ferromagnetism with decreasing temperature below 7.4 K. The gradual increase of chi below T-DW is not expected in simple CDW or SDW, where the magnetic susceptibility decreases with decreasing temperature due to the reduction of Pauli paramagnetic component. To explain the weak ferromagnetic behavior, we consider the coexistence of CDW and SDW. We propose a model of the mixed density wave, where CDW exists with antiferromagnetically coupled canting spins. (C) 2012 Elsevier B.V. All rights reserved.
  • Y. Kawashima, K. Ichimura, J. Ishioka, T. Kurosawa, M. Oda, K. Yamaya, S. Tanda
    PHYSICA B-CONDENSED MATTER 407 (11) 1796 - 1798 0921-4526 2012/06 [Refereed][Not invited]
     
    We report the STM study on a single-crystalline sample of FeTe at 7.8 K. FeTe is one of the iron-based superconductor. We measured the resistivity and the magnetization of FeTe. FeTe shows SDW transition at 58 K on these measurements. We study the electronic state of FeTe by using STM/STS for observing FeTe from a microscopic viewpoint. We observed the iron layer and the tellurium layer with atomic resolution. Moreover, we discover the charge stripe structure on STM/STS measurement. We find the charge stripe structure is caused by iron atoms from the analysis. The gap structure of 9 meV was observed in tunneling spectra. This gap size is consistent with the SDW gap which is expected from mean field theory with T-N= 58 K. (C) 2012 Elsevier B.V. All rights reserved.
  • Mitsuharu Nagasawa, Tokiko Nagasawa, Koichi Ichimura, Kazushige Nomura
    PHYSICA STATUS SOLIDI C: CURRENT TOPICS IN SOLID STATE PHYSICS, VOL 9, NO 5 9 (5) 1161 - 1163 1862-6351 2012 [Refereed][Not invited]
     
    We measured the electrical conductance along the alpha-direction sigma(a) of (TMTTF)(2)SbF6 under anisotropic external pressure P-Ec which is parallel to the c*-direction. It is expected that P-Ec achieves the uniaxial strain along the c*-direction to the sample. We found that sa is strongly influenced by applying P-Ec and the charge order (CO) transition temperature T-CO decreases with increasing P-Ec. The tendencies of the P-Ec dependence of sigma(a) and TCO is different from that of the anisotropic external pressures which are parallel to the alpha- and the b-directions. We discuss the P-Ec effects on both sigma(a) and the CO transition. (C) 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
  • Yuki Oka, Ryota Abe, Hiroyoshi Nobukane, Noriaki Matsunaga, Kazushige Nomura, Koichi Ichimura, Atsushi Kawamoto
    26TH INTERNATIONAL CONFERENCE ON LOW TEMPERATURE PHYSICS (LT26), PTS 1-5 400 1742-6588 2012 [Refereed][Not invited]
     
    We performed the STM spectroscopy measurement on deuterated kappa-(BEDT-TTF-d[n, n])(2)Cu[N(CN)(2)]Br to clarify the relation between the superconducting state and the strength of the electron correlation in organic superconductors BEDT-TTF family. We report the results for d[2,2]-salt and d[3,3]-salt with the stronger electron correlation than d[0,0]-salt. The superconducting gap observed as the differential conductance varied systematically depending on the direction of the lateral surface. From the analysis of angular dependent gap function, we found that the node direction of d-wave in d[2,2] and d[3,3] were along a* +/- c* same as d[0,0]. These directions were supported by the observation of the ZBCP near the node direction. It suggests that the dimerization, corresponding to the electron correlation, in both salts is still weak, although these salts are situated near a Mott boundary. While, the node direction rotates a little toward the c* axis in d[3,3] with larger dimerization.
  • J. Ishioka, T. Fujii, K. Katono, K. Ichimura, T. Kurosawa, M. Oda, S. Tanda
    PHYSICAL REVIEW B 84 (24) 245125  1098-0121 2011/12 [Refereed][Not invited]
     
    We discovered a chiral behavior of Friedel oscillations (FOs), which have both triangular symmetry and inverse chirality from chiral charge-density waves (CDWs) in 1T-TiSe2 by using scanning tunneling microscopy (STM). Surprisingly, the existence of FOs with the opposite chirality to the underlying CDWs suggests that the excess electron induced by impurity reduces the local helicity of CDWs without CDW breaking. This FOs can be explained with a two-branch model with different chiralities, which permits excitation from one branch to another, that is, Dirac-type dispersion. Moreover, we found two states that correspond to electron addition and electron removal. In terms of the chirality of electrons, these two states are the same. This result provides an evidence for the charge-parity (CP) symmetry conservation in CDW systems.
  • Yuuki Kawashima, Koichi Ichimura, Toru Kurosawa, Migaku Oda, Satoshi Tanda, Hiroki Takahashi, Hironari Okada, Yoichi Kamihara, Hideo Hosono
    PHYSICA C-SUPERCONDUCTIVITY AND ITS APPLICATIONS 470 S315 - S316 0921-4534 2010/12 [Refereed][Not invited]
     
    We report the electron tunneling study on SmFeAsO(1-x)F(x) (x = 0, 0.045, 0.046, 0.069) by using low temperature UHV-STM/STS. The superconducting gap and pseudogap structures are observed on x = 0.045, 0.046, 0.069. We also found similar structures on non-superconducting sample of x = 0. The value of 2 Delta(sc)/kT(c) decreases as T(c) increases. (C) 2010 Elsevier B.V. All rights reserved.
  • J. Ishioka, Y. H. Liu, K. Shimatake, T. Kurosawa, K. Ichimura, Y. Toda, M. Oda, S. Tanda
    PHYSICAL REVIEW LETTERS 105 (17) 176401  0031-9007 2010/10 [Refereed][Not invited]
     
    We discovered the chirality of charge-density waves (CDW) in 1T-TiSe(2) by using STM and time-domain optical polarimetry. We found that the CDW intensity becomes Ia(1):Ia(2):Ia(3) 1:0.7 +/- 0.1:0.5 +/- 0.1, where Ia(i) (i = 1, 2, 3) is the amplitude of the tunneling current contributed by the CDWs. There were two states, in which the three intensity peaks of the CDW decrease clockwise and anticlockwise. The chirality in CDW results in the threefold symmetry breaking. Macroscopically, twofold symmetry was indeed observed in optical measurement. We propose the new generalized CDW chirality H(CDW) equivalent to q(1) . (q(2) x q(3)), where q(i) are the CDW q vectors, which is independent of the symmetry of components. The nonzero H(CDW)-the triple-q vectors do not exist in an identical plane in the reciprocal space-should induce a real-space chirality in CDW system.
  • M. Nagasawa, T. Nagasawa, K. Ichimura, K. Nomura
    PHYSICA B-CONDENSED MATTER 405 (11) S113 - S115 0921-4526 2010/06 [Refereed][Not invited]
     
    We measured the conductivity along the a-direction sigma(a)(T) of the quasi one-dimensional organic conductor (TMTTF)(2)SbF6 under several anisotropic pressures of uniaxial strains which are parallel and perpendicular to the a-direction The uniaxial strains were generated by so-called Frozen oil method It was found that the tendencies of the uniaxial strain dependence of the conductivity and the charge order (CO) transition temperature T-CO are different According to T-CO it decreases strongly with increasing uniaxial strain parallel to the a-direction however It is almost constant for that perpendicular to the a-direction We discuss the effects of compressive uniaxial strains on the conductivity and the CO transition of (TMTTF)(2)SbF6 (C) 2010 Elsevier B V All rights reserved
  • J. Ishioka, Y. H. Liu, K. Shimatake, T. Kurosawa, K. Ichimura, Y. Toda, M. Oda, S. Tanda
    PHYSICA B-CONDENSED MATTER 405 (11) S214 - S216 0921-4526 2010/06 [Refereed][Not invited]
     
    We report the chirality of charge density waves (COW) in 1T-TiSe2 by using scanning tunneling microscopy (STM) measurements We found that the COW intensity becomes Ia(1) Ia(2) Ia(3) = 1 07 +/- 0 1 0 5 +/- 0 1 where Ia(1) (I = 1 2 3) is the amplitude of the tunneling current contributed by the CDWs We found two states in which the three intensity peaks of the CDW decrease clockwise and anticlockwise when we index each nesting vector in order of intensity in the Fourier transformation of the STM images We found that this difference arises from COW stacking along the c-axis at intervals of 2c(0)/3 This chirality is attributed to the helical stacking such as cholesteric liquid crystals (C) 2010 Elsevier B V All rights reserved
  • G. Kumagai, T. Matsuura, K. Ichimura, S. Tanda
    PHYSICA B-CONDENSED MATTER 405 (11) S284 - S287 0921-4526 2010/06 [Refereed][Not invited]
     
    We have measured the magnetic torque of the ring-shaped crystals of TaSe3 by using piezoresistive cantilevers in order to investigate the superconducting topological properties We measured three ring samples and we observed that the magnetic torque of the ring crystals oscillates with increasing the external magnetic field The first peak of Sample A (radius 37 9 mu m) was 1 21 Gauss and that of Sample B (24 5 mu m) was 3 70 Gauss and that of Sample C (15 6 mu m) was 4 73 Gauss respectively We found that the first peak depends on the length of the circumference itself If there are such relation between the peak field and the length of the circumference it is natural that vortices in the ring crystal were placed along the circumference From these results we suggest that vortices exist as cylinder vortices in the rings and this phenomenon gives a new experimental evidence of topological effect in superconductor (C) 2010 Elsevier B V All rights reserved
  • Noriaki Matsunaga, Tomonori Akita, Koichi Ichimura, Kazushige Nomura, Atsushi Kawamoto
    PHYSICA B-CONDENSED MATTER 405 (11) S108 - S110 0921-4526 2010/06 [Refereed][Not invited]
     
    The low-frequency dielectric constant was measured in the pressure induced incommensurate SDW phase of (TMTTF)(2)Br in order to investigate the dynamics of the SDW Under the pressure above 0 5 GPa the huge dielectric constant attributed to the motion of the SDW were observed The temperature dependence of the real part of the dielectric constant epsilon whose amplitudes grow with decreasing frequency shows the peak behavior in the Incommensurate SDW phase In contrast of the case of (TMTSF)(2)PF6 the peak temperature of epsilon ' is constant irrespective of the frequency These peak temperature T-P are scaled as T-P/T-SWD similar to 0 3 irrespective of pressure for the pressure larger than 08 GPa and is associated with the sub-phase transition in the incommensurate SOW proposed by the NMR measurement The dielectric properties are probably connected with the co-existence of the CDW component proposed for high temperature sub-phase (C) 2010 Elsevier B V All rights reserved
  • Genki Kumagai, Toru Matsuura, Koichi Ichimura, Kazuhiko Yamaya, Katsuhiko Inagaki, Satoshi Tanda
    PHYSICAL REVIEW B 81 (18) 184506  1098-0121 2010/05 [Refereed][Not invited]
     
    We measured the magnetic torque of the ring-shaped crystals of TaSe3 by using piezoresistive cantilevers to investigate the superconducting topological properties. We measured three ring samples and we observed that the magnetic torque of the ring crystals oscillates with increasing external magnetic field. The magnetic periods of Sample A (radius: 37.9 mu m), Sample B (24.5 mu m), and Sample C (15.6 mu m) were 2.16 +/- 6.6 x 10(-2) G, 4.69 +/- 1.8 x 10(-1) G, and 5.44 +/- 3.8 x 10(-1) G, respectively. We found that hundreds of vortices collectively and simultaneously penetrated the ring crystal. When such a phenomenon occurs, it is natural that the vortices in the ring crystal would be positioned along the circumference. From these results, we suggest that vortices exist as cylinder vortices in the rings, and this phenomenon provides unambiguous experimental evidence of the topological effect in a superconductor.
  • Hiroyoshi Nobukane, Katsuhiko Inagaki, Satoshi Tsuchiya, Yasuhiro Asano, Koichi Ichimura, Kazuhiko Yamaya, Shigeru Takayanagi, Ikuto Kawasaki, Kenichi Tenya, Hiroshi Amitsuka, Satoshi Tanda
    JAPANESE JOURNAL OF APPLIED PHYSICS 49 (2) 020209.1-020209.3  0021-4922 2010 [Refereed][Not invited]
     
    We report our approach to measuring the transport properties in a single chiral order parameter domain of microscale Sr2RuO4 single crystals. Sr2RuO4 single crystals were grown by the solid-phase reaction. Microscale single crystals were deposited from a dispersed liquid, and selected from the result of the chemical composition and crystallinity of the dispersed crystals. The selected crystals attached to gold electrodes show superconducting properties. We also found the anomalous current-voltage characteristics that show that induced voltage is an even function of bias current. (C) 2010 The Japan Society of Applied Physics
  • H. Nobukane, K. Inagaki, K. Ichimura, K. Yamaya, S. Takayanagi, I. Kawasaki, K. Tenya, H. Amitsuka, K. Konno, Y. Asano, S. Tanda
    Physica B 405 (11) S277 - S278 0921-4526 2010 [Refereed][Not invited]
     
    We have found parity-violating current-voltage (I-V) curves in a single domain of Sr2RuO4 superconductors The Induced voltage V is an even function of bias current I below superconducting transition temperature The anomalous I-V curves show that positive voltage is detected regardless of the current direction The result implies the parity violation of I-V curves (C) 2009 Elsevier B V All rights reserved
  • Mitsuharu Nagasawa, Tokiko Nagasawa, Koichi Ichimura, Kazushige Nomura
    PHYSICA B-CONDENSED MATTER 404 (3-4) 392 - 395 0921-4526 2009/03 [Refereed][Not invited]
     
    We measured the specific heat c(p)(T) of single crystals of quasi-one-dimensional conductors K0.3MoO3 and (TMTTF)(2)AsF6 by differential thermal analysis (DTA) method. In K0.3MoO3, a clear discontinuous change Delta c(p0)similar to 1.2J/mol/K was detected at the charge density wave (CDW) transition temperature T-CDW, = 180 K. On the other hand, in (TMTTF)(2)AsF6, c(p)(7) showed a weak anomaly at the charge order (CO) transition temperature T-CO = 100 K. The entropy lost at around T-CO was Delta S similar to 0.24J/mol/K at most, that is much smaller than Rln 2 similar to 5.8J/mol/K, which is the expected magnitude in the case of order-disorder transition. The CO of (TMTTF)(2)AsF6 Might be related to the displacive type ferroelectric. (C) 2008 Elsevier B.V. All rights reserved.
  • Koichi Ichimura, Shigeru Ikeda, Mikiko Kawai, Kazushige Nomura, Junya Ishioka, Satoshi Tanda
    PHYSICA B-CONDENSED MATTER 404 (3-4) 570 - 572 0921-4526 2009/03 [Refereed][Not invited]
     
    The organic conductor theta-(BEDT-TTF)(2)RbZn(SCN)(4), which exhibits charge ordering below T-CO = 190 K was studied by STM to observe the charge ordering pattern in the real space. The conducting a-c plane was investigated. The donor arrangement in the a-c surface was clearly observed in topographic STM images at room temperature. The STM image shows the rectangular pattern with periods of 1 nm and 0.5 nm along the a- and c-direction, respectively. This means that BEDT-TTF donors located at the center of the rectangle were not observed except a certain scan. The observed STM image is naively expected from the crystal structure of theta-(BEDT-TTF)(2)RbZn(SCN)(4). We found that all donors can be imaged by the scan along the a-axis. The STM image at 170 K shows the nearly two-fold periodicity along the c-direction. It corresponds to the horizontal stripe charge order pattern. (C) 2008 Elsevier B.V. All rights reserved.
  • K. Nomura, R. Muraoka, N. Matsunaga, K. Ichimura, J. Yamada
    PHYSICA B-CONDENSED MATTER 404 (3-4) 562 - 564 0921-4526 2009/03 [Refereed][Not invited]
     
    We have investigated the gap symmetry in the superconducting phase of beta-(BDA-TTP)(2)SbF6 with use of the scanning tunneling microscope (STM). The tunneling spectra obtained on the conducting surface show a clear superconducting gap structure. its functional form is of V-shaped similarly to kappa-(BEDT-TTF)(2)X and suggests the anisotropic superconducting gap with line nodes. For lateral surfaces the shape of tunneling spectra varies from the U-shape with relatively large gap to the V-shape with small gap depending on the tunneling direction alternately twice between directional angle 0 and pi. From the analysis of conductance curve taking the k dependence of the tunneling probability into account, it is found that the gap has maximum near the a* and c* axes and the nodes appear along near a* + c* and the a-c* directions. These indicate that the d(x)(-y)(2)(2) like superconducting pair is formed in this system as the case of kappa-(BEDT-TTF)(2)X. This node direction is consistent with the theoretical prediction based on the spin fluctuation mechanism. However, the zero-bias conductance peak has not been observed yet. (C) 2008 Elsevier B.V. All rights reserved.
  • N. Matsunaga, T. Satoh, K. Ichimura, K. Nomura, A. Kawamoto, T. Sasaki
    Physica B 404 (3-4) 389 - 391 0921-4526 2009/03/01 [Refereed][Not invited]
  • Inagaki K, Tsubota M, Ichimura K, Tanda S, Yamamoto K, Hanasaki N, Nogami Y, Ikeda N, Ito T, Toyokawa H
    PHYSICA B-CONDENSED MATTER 404 (3-4) 396 - 398 0921-4526 2009/03/01 [Refereed][Not invited]
  • Inagaki K, Tsubota M, Ichimura K, Tanda S, Yamamoto K, Hanasaki N, Nogami Y, Ikeda N, Ito T, Toyokawa H
    25TH INTERNATIONAL CONFERENCE ON LOW TEMPERATURE PHYSICS (LT25), PART 4 150 1742-6588 2009 [Refereed][Not invited]
  • H. Nobukane, K. Inagaki, K. Ichimura, S. Takayanagi, K. Yamaya, S. Tanda
    Journal of Physics: Conference Series 129 012922-1 - 012022-4 1742-6588 2009 [Refereed][Not invited]
     
    We observed quantum fluctuations of the superconducting phase in single domain Sr2RuO4 single crystals using a transport measurement. The temperature dependence of the resistivity of submicron Sr 2RuO4 shows that a finite resistivity below the superconducting transition temperature of Tc = 1.69 K. The results may suggest that the finite resistivity is occurred by the flow of vortices due to quantum fluctuations of the superconducting phase in the same way as one-dimensional Josephson junction array systems. © 2008 IOP Publishing Ltd.
  • H. Nobukane, K. Inagaki, K. Ichimura, K. Yamaya, S. Takayanagi, I. Kawasaki, K. Tenya, H. Amitsuka, K. Konno, Y. Asano, S. Tanda
    Solid State Communication Elsevier Ltd. 149 (29-30) 1212 - 1215 0038-1098 2009 [Refereed][Not invited]
     
    We observed an unconventional parity-violating vortex in single domain Sr2RuO4 single crystals using a transport measurement. The current-voltage characteristics of submicron Sr2RuO4 shows that the induced voltage has anomalous components which are even functions of the bias current. The results may suggest that the vortex itself has a helical internal structure characterized by a Hopf invariant (a topological invariant). We also discuss that the hydrodynamics of such a helical vortex causes the parity violation to retain the topological invariant.
  • G. Kumagai, T. Matsuura, K. Ichimura, S. Tanda
    25TH INTERNATIONAL CONFERENCE ON LOW TEMPERATURE PHYSICS (LT25), PART 5 150 1742-6588 2009 [Refereed][Not invited]
     
    Topology has many applications in modern condensed matter physics. We report that superconducting properties are changed by topology. We have measured the magnetic torque of the ring-shaped crystals of TaSe(3) by using piezoresistive cantilevers in order to investigate the superconducting topological properties. We measured two ring samples. The outer radius of sample A was 37.9 mu m and that of sample B was 24.5 mu m. We found that the magnetic torque of the ring crystals changes periodically by increasing of the external magnetic field. The periodicity of sample A was 1.85 Gauss and that of sample B was 4.75 Gauss, respectively. We found that the period is proportional to the circumference of the ring crystal rather than the area enclosed by the outer circumference. In that case, it is natural that vortices in the ring crystal were placed along the circumference. From these results, we suggest that vortices exist as cylinder vortices in the rings at the results of topological effects of superconductive ring-shaped crystals, and this matter give a new experimental evidence of topological effect in superconductor.
  • Koichi Ichimura, Makoto Takami, Kazushige Nomura
    JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN 77 (11) 114707.1-114707.6  0031-9015 2008/11 [Not refereed][Not invited]
     
    Conducting and lateral surfaces of two-dimensional organic superconductor kappa-(BEDT-TTF)(2)Cu[N(CN)(2)]Br were studied by electron tunneling using scanning tunneling microscopy. The V-shape tunneling conductance Curve was observed at the conducting plane in the superconducting phase, attributed to file d-wave pairing symmetry. Tunneling spectra were Successfully obtained at various lateral surfaces of crystals. The tunneling conductance curve varies systematically depending oil tunneling direction. The conductance curve and its directional dependence are explained by the d-wave with nodes along the direction near pi/4 from the k(a)- and k(c)-axes. The pairing symmetry is of the d(x2-y2)- wave. (If we take the same convention for the magnetic Brillouin zone as high-T-c cuprates with the d(x2-y2)-symmetry, the pairing syrnmetry of kappa-(BEDT-TTF)(2)Cu[N(CN)(2)]Br is the d(xy)-symmetry.) This is the same symmetry as that in the superconducting gap of kappa-(BEDT-TTF)(2)Cu(NCS)(2). The zero bias conductance peak (ZBCP) was observed at the tunneling along near pi/4 front the a- and c-axes. This ZBCP result reinforces the conclusion of the d(x2-y2)-wave symmetry.
  • Koichi Ichimura, Junya Ishioka, Toru Kurosawa, Katsuhiko Inagaki, Migaku Oda, Satoshi Tanda, Hiroki Takahashi, Hironari Okada, Yoichi Kamihara, Masahiro Hirano, Hideo Hosono
    JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN 77 151 - 152 0031-9015 2008/11 [Refereed][Not invited]
     
    We report the electron tunneling study on LaFeAsO1-xFx (x=0.07) using low temperature UHV-STM/STS. Tunneling spectra below T-c clearly shows the gap structure. We found three anomalies as kinks in the tunneling differential conductance curve: 1) Superconducting gap structure Delta(SC)=6 meV. 2) Small pseudogap Delta(PGS)=20 meV. 3) Large pseudogap Delta(PGL)=100-150 meV. Corresponding value of 2 Delta(SC)/k(B)T(c)=5.8 suggests strong coupling superconductivity. Tunneling spectra at T=80 K show dip structure. The kink around V=150 mV still exist.
  • Mitsuharu Nagasawa, Tokiko Nagasawa, Koichi Ichimura, Kazushige Nomura
    JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN 77 (10) 105002.1-105002.2  0031-9015 2008/10 [Not refereed][Not invited]
  • Katsuhiko Inagaki, Masakatsu Tsubota, Kazuki Higashiyama, Koichi Ichimura, Satoshi Tanda, Kenichiro Yamamoto, Noriaki Hanasaki, Naoshi Ikeda, Yoshio Nogami, Takayoshi Ito, Hidenori Toyokawa
    JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN 77 (9) 093708.1-093708.4  0031-9015 2008/09 [Not refereed][Not invited]
     
    We report a synchrotron X-ray Study of charge density waves (CDWs) in an o-TaS3 crystal. We found that two independent CDWs coexist ill the temperature range of 130-50K. These waves are incommensurate and commensurate CDWs with longitudinal wave vectors q(c) = 0.252c* and 0.250c*, respectively. The temperature and electric current dependences of the intensity of the two CDW satellites were measured. We found that the commensurate CDW was converted to the incommensurate CDW at 80 K by inducing current flow. Our observation was interpreted in terms of the dynamics of topological defects. We determined the edge dislocation configuration from the electric Current dependence of the intensity of the two CDWs. The result implies Cor the first time that discommensurations are induced ill the commensurate CDW by applying an electric field.
  • Koichi Ichimura, Kazushige Nomura
    JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN 75 (5) 051012.1-051012.9  0031-9015 2006/05 [Not refereed][Not invited]
     
    The superconducting phase of organic superconductors K-(BEDT-TTF)(2)X was studied by the electron tunneling using scanning tunneling microscope (STM). Tunneling both at the conducting plane and lateral surfaces was performed on single crystals of K-(BEDT-TTF)(2)Cu(NCS)(2). Tunneling conductance curve obtained at the conducting plane is explained by the d-wave gap symmetry. The temperature dependence of tunneling spectra is also consistent with the d-wave symmetry. Tunneling spectra at lateral surfaces varies depending on the tunneling direction, suggesting the in-plane anisotropy of the gap. The directional dependence is also consistent with the d-wave symmetry with line nodes pi/4 from the k(b)- and k(c)-axes. It indicates that the superconducting pair wave function has the d(x2-y2)-wave symmetry. (If we take the same conventions for the magnetic Brillouin zone as high-T-c cuprates with the d(x2-y2)-wave symmetry, the gap symmetry of K-(BEDT-TTF)(2)Cu(NCS)(2) is the d(xy)-wave symmetry.) Protonated and partially deuterated K-(BEDT-TTF)(2)Cu[N(CN)(2)]Br were also studied. Tunneling spectra are consistent with the d-wave symmetry in all salts. Gap parameters obtained in each salt were discussed in the view point of the effective correlation. The large gap value suggests the strong coupling superconductivity. The pseudogap structure was found above T-c. The zero bias conductance peak, which gives an evidence for nodes of the superconducting gap, was found.
  • K Ichimura, K Nomura, A Kawamoto
    JAPANESE JOURNAL OF APPLIED PHYSICS PART 1-REGULAR PAPERS BRIEF COMMUNICATIONS & REVIEW PAPERS 45 (3B) 2264 - 2267 0021-4922 2006/03 [Not refereed][Not invited]
     
    The superconducting phase of organic superconductors (BEDT-TTF)(2)X was Studied by electron tunneling using scanning tunneling microscope (STM). Tunneling both at the conducting plane and lateral surfaces was performed on single crystals of (BEDT-TTF)(2)Cu(NCS)(2). The tunneling conductance curve obtained at the conducting plane is explained by d-wave gap symmetry. Tunneling spectra at lateral surfaces vary depending on the tunneling direction, suggesting the in-plane anisotropy of the gap. The directional dependence is also consistent with d-wave symmetry with line nodes pi/4 from the k(b)- and k(c)-axes. This indicates that the superconducting pair wave function has d(x2-y2)-wave symmetry. Protonated and partially deuterated (BEDT-TTF)(2)Cu(CN)(2)]Br samples were also studied. Tunneling spectra are consistent with d-wave symmetry in all salts. Gap parameters are obtained as functions of the effective correlation. The large gap value suggests the strong coupling Superconductivity. The zero-bias conductance peak, which is evidence for nodes of the superconducting gap, was found.
  • Ichimura Koichi, Nomura Kazushige, Kawamoto Atsushi
    LOW TEMPERATURE PHYSICS, PTS A AND B 850 607 - + 0094-243X 2006 [Refereed][Not invited]
     
    The superconducting phase of organic superconductor kappa-(BEDT-TTF)(2)Cu[N(CN)(2)]Br was studied by the electron tunneling using STM. Tunneling spectra were obtained at partially deuterated kappa-(BEDT-TTF-d[3,3])(2)Cu[N(CN)(2)]Br. The tunneling conductance curve exhibits clear superconducting gap structure. Tunneling spectra with linear dependence on the energy are explained by d-wave pairing. We obtained the gap parameter as a function of the effective correlation strength, which can be controlled by partial deuteration. The large gap value suggests strong coupling superconductivity. Moreover, a zero bias conductance peak (ZBCP) was found in d[3,3] salt. d-wave pairing in kappa-(BEDT-TTF)(2)X is strongly supported.
  • M Nagasawa, T Nagasawa, K Ichimura, K Nomura
    JOURNAL DE PHYSIQUE IV 131 107 - 110 1155-4339 2005/12 [Refereed][Not invited]
     
    In the charge ordering (CO) state of the quasi-one dimensional conductors (TMTTF)(2)SbF6, we measured the current-voltage relations by the pulse current method at ambient pressure. From the measurement, we found that the conductivity is constant at low electric field regime and increases with field above a certain field. It is similar to the nonlinear conduction due to the ordinary density wave (DW) sliding. However, the magnitude of the threshold electric field was larger than that in the DW system. We will discuss the origin of the non-linear electric conduction in the CO state of (TMTTF)(2)SbF6 and the apparent similarity with it by the DW sliding.
  • K. Ichimura, S. Higashi, K. Nomura, A. Kawamoto
    Synthetic Metals 153 (1-3) 409 - 412 0379-6779 2005/09/21 [Refereed][Not invited]
     
    We report the electron tunneling spectroscopy on partially deuterated κ-(BEDT-TTF-d[3,3])2Cu[N(CN)2]Br using a low temperature STM. In the superconducting state, tunneling spectra show the energy gap structure clearly the conductance around the zero bias voltage is well reduced. V-shaped tunneling conductance curve inside the gap edge suggests the gap anisotropy. The linear dependence on the energy near zero bias is explained by the d-wave pairing with line nodes. By fitting to the d-wave gap model, we obtain the gap parameter as Δ=1.4-3.3 meV. Correspondingly, 2Δ/kTc is estimated as 2.7-6.4. The value is almost the same as that in κ-(BEDT-TTF)2Cu(NCS)2, d[0,0] and d[2,2] salts. The zero bias anomaly, which is an evidence for nodes of the superconducting gap, was found. The d-wave symmetry is strongly supported. © 2005 Elsevier B.V. All rights reserved.
  • K. Ichimura, K. Tamura, M. Ogata, K. Nomura, T. Toshima, S. Tanda
    Journal De Physique. IV : JP 131 235 - 237 1155-4339 2005 [Refereed][Not invited]
     
    NbSe2 nanotubes were studied by scanning tunneling microscopy (STM). Topographic images of NbScj nanotubes on the substrate of HOPG were taken at room temperature. The length of nanotubes is estimated as 300-2000 nm. Diameter of nanotubes is found to be several nm, which is much smaller than that of multi-walled nanotubes, suggesting single-walled one. The bundle structure, which consists of several ten nanotubes, and Y-junction were found similarly to carbon nanotubes. © EDP Sciences.
  • K Ichimura, K Nomura, H Fukuoka, S Yamanaka
    PHYSICA C-SUPERCONDUCTIVITY AND ITS APPLICATIONS 388 577 - 578 0921-4534 2003/05 [Not refereed][Not invited]
     
    Superconducting phase of silicon clathrate Ba8Si46 was studied by the tunneling spectroscopy using STM. The energy gap structure associated with the superconducting state was observed clearly. The functional form of the tunneling spectra is explained by the model, in which the gap is assumed to be finite and has an anisotropy. According to the model, in which the gap varies depending on the direction in k-space, we obtain gap parameters as Delta(min) = 0.45 meV and Delta(max) = 1.5 meV. (C) 2003 Elsevier Science B.V. All rights reserved.
  • K Ichimura, K Suzuki, K Nomura, A Kawamoto
    SYNTHETIC METALS 137 (1-3) 1229 - 1230 0379-6779 2003/04 [Not refereed][Not invited]
     
    We report the electron tunneling spectroscopy on kappa-(BEDT-TTF)(2)Cu[N(CN)(2)]Br using a low temperature STM. Protonated d[0,0] and partially deuterated d[2,2] salts were studied.. In both salts, the functional shape of the tunneling differential conductance curve in the superconducting state is explained by the d-wave pairing symmetry. We obtain the gap parameter Delta as 2.1-3.9 and 3.0-4.8 meV for d[0,01 and d[2,21 salt, respectively. The value of 2Delta/kT(c) in d[2,2] salt is slightly larger than that in d[0,0] salt. The pseudogap is observed below about 50 K in both salts.
  • K. Ichimura, K. Suzuki, K. Nomura, A. Kawamoto
    Synthetic Metals 133-134 213 - 214 0379-6779 2003/03/13 [Not refereed][Not invited]
     
    We report the electron tunneling spectroscopy on κ-(BEDT-TTF)2Cu[N(CN)2]Br using a low temperature STM. In the superconducting state, tunneling spectra show the energy gap structure clearly. The tunneling differential conductance shows linear dependence on energy inside the gap edge. The conductance curve is well fitted by the d-wave calculation with a little broadening. The d-wave gap parameter is obtained as Δ0 = 2.5-3.5 meV, correspondingly, 2Δ0/kTc = 5.0-7.1. The pairing symmetry is consistent with that in κ-(BEDT-TTF)2Cu(NCS)2 as well as the gap parameter. The broad dip structure around the Fermi level is found in tunneling spectra above Tc. The pseudogap structure is observed below 50 K. © 2002 Elsevier Science B.V. All rights reserved.
  • K Ichimura, M Osawa, K Nomura, H Kataura, Y Maniwa, S Suziki, Y Achiba
    PHYSICA B-CONDENSED MATTER 323 (1-4) 230 - 232 0921-4526 2002/10 [Not refereed][Not invited]
     
    Single-wall carbon nanotubes (SWNTs) prepared on the graphite substrate were investigated by a low-temperature scanning tunneling microscope at 77 K. From the tunneling spectra with high-energy resolution obtained at 77 K, both metallic and semiconducting SWNTs were found in the same bundle. For the metallic SWNT, the conductance curve inside the first peak is finite and quite flat. The width between the first peaks is obtained as 1300 meV. Fine structures due to the anisotropy of equi-energy contours around the K point were observed just outside the first peaks. The observed gap width for the semiconducting SWNT is 600 meV. The conductance curve, which has broad peaks around V = + 80 mV was also observed. (C) 2002 Elsevier Science B.V. All rights reserved.
  • K. Ichimura, M. Kobayashi, K. Nomura, T. Hanajiri, J. Yamada, S. Nakatsuji, H. Anzai
    Synthetic Metals 120 (1-3) 897 - 898 0379-6779 2001/03/15 [Not refereed][Not invited]
     
    The SDW phase of (TMTSF)2PF6 was investigated by the electron tunnelling spectroscopy using a low temperature STM. Below TSDW = 12 K, the tunnelling differential conductance shows the energy gap structure associated with the SDW state. The functional form of the spectrum is explained by the mean field theory for quasi-one dimensional conductors. We obtained the SDW gap ΔSDW = 2.3 meV and the imperfect nesting parameter ε0 = 1.8 meV. The temperature dependence of the zero bias tunnelling conductance below TSDW is also explained by the electronic density of states for the imperfect nesting. The pseudogap structure was observed above TSDW.
  • STM/STS on Carbon Nanotubes at Low Temperature (共著)
    AIP Conference Proceedings 590, 197-200 2001 [Not refereed][Not invited]
  • STM Spectroscopy on kappa-(BEDT-TTF)2Cu(NCS)2 (共著)
    Synthetic Metals 120, 707-708 2001 [Not refereed][Not invited]
  • Tunneling Spectroscopy on the Organic Superconductor kappa-(BEDT-TTF)2Cu(NCS)2 (共著)
    Physical Review B 63, 104518 2001 [Not refereed][Not invited]
  • T. Arai, K. Ichimura, K. Nomura, S. Takasaki, J. Yamada, S. Nakatsuji, H. Anzai
    Solid State Communications 116 (12) 679 - 682 0038-1098 2000/11/24 [Not refereed][Not invited]
     
    Scanning tunneling microscope (STM) spectroscopy on the b-c plane of single crystals of an organic superconductor κ-(BEDT-TTF)2Cu(NCS)2 has been performed in superconducting and normal states with varying temperature. The temperature dependence of the zero-bias conductance dI/dV-̄V = 0 is well explained by the d-wave gap model considering the T2 dependence of the broadening parameter of the one-electron level Γ(T), which is obtained from the fitting of the dI/dV-V curves to the d-wave gap model. The dI/dV-V curve shows a broad dip around the Fermi energy above Tc = 10.4 K. This pseudogap structure remains up to about 45 K. The magnitude of the pseudogap is much larger than that of the superconducting gap Δp to approximately 3 meV.
  • K Ichimura, O Abe, K Nomura, S Takasaki, J Yamada, S Nakatsuji, H Anzai
    SYNTHETIC METALS 103 (1-3) 2097 - 2098 0379-6779 1999/06 [Not refereed][Not invited]
     
    The SDW phase of(TMTSF)(2)PF6 was investigated by the electron tunneling spectroscopy using a low temperature STM. At room temperature and 86 K, the tunneling conductance is almost flat indicating metallic state. Below T-SDW=11.5 K, the spectrum shows the clear gap structure associated with the SDW state. The conductance curve is well explained by the mean field approximation for the quasi-one dimensional conductors. The SDW gap parameter Delta(SDW) and is an element of(0) which characterizes the transfer energy along the b-axis, are estimated as Delta(SDW)=2.5-2.9 meV and is an element of(0)=2.0-2.6 meV, respectively. The SDW-insulator-SDW tunneling spectrum was also observed.
  • K Ichimura, K Nomura, T Nakamura, T Takahashi, B Hilti, JS Zambounis
    SYNTHETIC METALS 103 (1-3) 1873 - 1876 0379-6779 1999/06 [Not refereed][Not invited]
     
    The electron tunneling was performed on the superconducting phase of (MDT-TTF)(2)AuI2 using STM. The tunneling differential conductance at T=1.4 K shows the superconducting energy gap structure clearly. The conductance near zero bias voltage is well reduced while the enhancement at the gap edge is observed. The obtained gap Delta(0)=2 meV (2 Delta(0)/kT(c)=12) is larger than that of the weak coupling limit. The shape of the spectrum, which is different from the BCS density of states, suggests the gap anisotropy. The tunneling spectrum is explained by the d-wave. The conductance well above T-c, is flat. On the other hand, the pseudo gap like structure is observed near T-c.
  • K Ichimura, T Arai, K Nomura, S Takasaki, J Yamada, S Nakatsuji, H Anzai
    SYNTHETIC METALS 103 (1-3) 1812 - 1813 0379-6779 1999/06 [Not refereed][Not invited]
     
    The superconducting phase of kappa-(BEDT-TTF)(2)Cu(NCS)(2) was investigated by the electron tunneling spectroscopy using low temperature STM. The tunneling differential conductance obtained at the b-c surface shows V-shaped gap structure consistent with the d-wave symmetry. The lateral surface of single crystals was also investigated. We found that the tunneling spectrum varies its shape depending on the tip direction. This indicates the gap anisotropy. Taking into account the k-dependence of the tunneling transition probability, the in-plane anisotropy of the conductance is well explained by the d-wave symmetry with line nodes along the direction 45 degrees from the c-axis. The d(x)(2)-(2)(y)-wave pairing is strongly suggested in this material.
  • K Ichimura, T Arai, K Nomura, S Takasaki, J Yamada, S Nakatsuji, H Anzai, T Nakamura, T Takahashi, B Hilti, JS Zambounis
    JOURNAL OF SUPERCONDUCTIVITY 12 (3) 519 - 523 0896-1107 1999/06 [Not refereed][Not invited]
     
    The superconducting phase of organic superconductors kappa-(BEDT-TTF)(2)Cu(NCS)(2) and (MDT-TTF)(2)AuI2 was investigated by the electron tunneling spectroscopy using low temperature STM. The tunneling differential conductance at the lateral surface of kappa-(BEDT-TTF)(2)Cu(NCS)(2) varies its shape depending on the tip direction. The in-plane anisotropy of the conductance is well explained by the d-wave symmetry with line nodes along the direction 45 degrees from the c-axis. For (MDT-TTF)(2)AuI2, the tunneling conductance at T = 1.4 K shows the superconducting energy gap structure clearly. The finite conductance inside the gap edge suggests the gap anisotropy. The tunneling spectrum is explained by the d-wave pairing. The obtained gap Delta(0) = 2 meV (2 Delta(0)/kT(c) = 12) is larger than that of the weak coupling limit. The pseudogap structure is observed near T-c.
  • Koji Suzuki, Koichi Ichimura, Kazushige Nomura, Shunji Takekawa
    Journal of Physics Condensed Matter 11 (15) 3133 - 3139 0953-8984 1999/04/19 [Refereed][Not invited]
     
    We have carried out electron tunnelling spectroscopy on Bi2Sr2CaCu2O8 single crystals with the use of a scanning tunnelling microscope at 4.2 K. A conductance peak structure was observed near zero bias voltage for the tunnelling along the Cu-O bonding direction. A clear superconducting gap was observed for the tunnelling along the Cu-Cu direction. Curves along the Cu-O and Cu-Cu directions are consistent with the dx2-y2-wave pairing symmetry which has line-nodes along the Cu-Cu direction, taking into account the periodic arrangement of surface oxygen atoms. © 1999 IOP Publishing Ltd.
  • K. Suzuki, K. Ichimura, K. Nomura, S. Takekawa
    Physical Review Letters 83 (3) 616 - 619 1079-7114 1999/01/01 [Not refereed][Not invited]
     
    We have carried out electron tunneling spectroscopy on Bi2Sr2CaCu2O8 single crystals with use of STM at 4.2 K. The tunneling differential conductance in the superconducting gap was observed with varying tunneling direction in the a-b plane. Tunneling spectra observed along directions intermediate between Cu-O and Cu-Cu directions are well fitted with the dx2-y2 wave model, taking into account the k dependence of the tunneling transition probability. This result strongly indicates that the superconducting gap has the dx2-y2 wave symmetry with line nodes along the Cu-Cu direction. © 1999 The American Physical Society.
  • K. Ichimura, T. Arai, K. Nomura, S. Takasaki, J. Yamada, S. Nakatsuji, H. Anzai
    Physica C: Superconductivity and its Applications 282-287 (3) 1895 - 1896 0921-4534 1997 [Not refereed][Not invited]
     
    The superconducting phase of κ-(BEDT-TTF)2Cu(NCS)2 was investigated by the electron tunneling spectroscopy using low temperature STM. The tunneling differential conductance obtained at the b-c surface shows V-shaped gap structure consistent with the d-wave symmetry. The lateral surface of single crystals was also investigated. We found that the tunneling spectrum varies its shape depending on the tip direction. This indicates the gap anisotropy. Taking into account the k-dependence of the tunneling transition probability, the inplane anisotropy of the conductance is well explained by the d-wave symmetry with line nodes along the direction φ=45° from the c-axis. The d-wave pairing is strongly suggested in this material.
  • Synthetic Metals 85, 1543-1544 1997 [Not refereed][Not invited]
  • K Ichimura, T Arai, K Nomura, S Takasaki, J Yamada, S Nakatsuji, H Anzai
    SUPERLATTICES AND MICROSTRUCTURES 21 (3) 305 - 313 0749-6036 1997 [Not refereed][Not invited]
     
    Electron tunneling spectroscopy of the organic superconductor kappa-(BEDT-TTF)(2)Cu(NCS)(2) using low temperature scanning tunneling microscope (STM) is reported. The tunneling differential conductance in the superconducting phase was obtained in the b-c plane of a single crystal, by varying the tip position on the sample surface;The differential conductance is reduced near zero bias voltage and enhanced at the gap edge, associated with the superconducting gap structure below T-c similar or equal to 10 K. The gap width differs slightly from sample to sample, while the overall functional shape of the conductance is sample-independent. The tunneling conductance is reduced to almost zero near zero bias voltage, while it is finite inside the gap edge. The curve obtained cannot be fit to the BCS density of states with s-wave pairing symmetry, even if the life-time broadening of one-electron levels is taken into account. Finite conductance inside the gap edge suggests anisotropy of the gap. However, the conductance curve obtained is not explained by a simple d-wave symmetry for Delta(k). The reduced conductance near zero bias voltage suggests a finite gap. An anisotropic model with a finite gap, in which Delta(k) varies depending on the direction in k-space, is examined. The tunneling conductance in the low-energy region is almost fit by the model with Delta(min) = 2 meV and Delta(max) = 6 meV. The finite conductance is explained by introducing a small effect of life time broadening. We conclude that the gap is anisotropic and is finite (at least Delta(min) = 2 meV) on the entire Fermi surface. (C) 1997 Academic Press Limited.
  • K. Nomura, K. Nagao, K. Ichimura, N. Matsunaga
    Synthetic Metals 70 (1-3) 911 - 912 0379-6779 1995/03
  • K. Ichimura, K. Suzuki, K. Nomura, S. Takekawa
    Journal of Physics: Condensed Matter 7 (42) L545 - L551 0953-8984 1995 [Not refereed][Not invited]
     
    We carried out electron tunnelling spectroscopy for single crystals of Bi2Sr2CaCu2O8 in the superconducting phase with use of STM. The tunnelling differential conductance curve with low noise was obtained for the cleaved surface and compared with that for the lateral surface. The two spectra were found to be essentially identical. It is concluded that tunnelling spectra obtained by our STM experiment contain contributions from every wave number and represent the total electronic density of states. From the functional form of the tunnelling conductance, the simple d wave was excluded for the symmetry of the superconducting pair wave function. The conductance curve obtained was well explained by the model, in which the gap is finite on the whole Fermi surface while it is highly anisotropic in k-space. It was suggested that the symmetry of the superconducting pair wave function is of the mixed symmetry of the s and d waves.
  • K ICHIMURA, K NOMURA, S TAKEKAWA
    PHYSICA C 235 1825 - 1826 0921-4534 1994/12 [Not refereed][Not invited]
     
    The electron tunneling study was done on both Bi2Sr2CaCu2O8 and Bi2Sr2CaCu2-xNixO8 with use of STM and temperature dependence of the magnetic field penetration depth was also investigated. The tunneling conductance obtained at the lateral surface of single crystal of Bi2Sr2CaCu2O8 indicates the finite gap on the whole Fermi surface, while the considerable anisotropy is found. The temperature dependence of the penetration depth is of the thermal activation type. These results indicate symmetry of the pair wave function is the predominant s-wave mixed with the d-wave. For Bi2Sr2CaCu2-xNixO8, any clear energy gap structure was not found in the tunneling conductance at the cleaved surface. The penetration depth exhibits the T-2 temperature dependence.
  • K. Nomura, K. Ichimura
    Physica B: Physics of Condensed Matter 194-196 (2) 2189 - 2190 0921-4526 1994/02/02 [Not refereed][Not invited]
     
    The tunneling conductance was measured in Bi2Sr2CaCu8 with use of the Scanning Tunneling Microscope. The electronic density of states in the superconducting phase was correctly obtained at lateral surface of single crystal. The superconducting gap was found to be finite on the whole Fermi surface, while it showed a considerable anisotropy. It was understood that the attractive interaction bringing about the superconductivity has predominant s-wave character mixed with d-wave one. © 1994.
  • Anisotropic Superconducting Gap of Bi2Sr2CaCu2O8 : Tunneling Spectroscopy with STM (共著)
    Journal de Physique IV 3, C2-281-284 1993 [Not refereed][Not invited]
  • Koichi Ichimura, Kazushige Nomura
    Journal of the Physical Society of Japan 62 (10) 3661 - 3679 1347-4073 1993 [Refereed][Not invited]
     
    The electron tunneling study was carried out on Bi2Sr2CaCu208 with use of the scanning tunneling microscope. Both the cleaved and lateral surface of single crystal were investigated. The electronic density of states for the superconducting phase was correctly obtained at the lateral surface. It shows the finite gap on the whole Fermi surface clearly, while the gap has a considerable anisotropy. The temperature dependence of the magnetic field penetration depth was also measured and its thermally activated behavior confirmed the finite gap again. These results indicate the large 5-wave component in the pair wave function and the fair contribution of d-wave as well. The attractive force between electrons is not only due to the on-site interaction, but also the extended one. © 1993, THE PHYSICAL SOCIETY OF JAPAN. All rights reserved.
  • Koichi Ichimura, Kazushige Nomura, Fujio Minami, Shunji Takekawa
    Solid State Communications 82 (3) 171 - 175 0038-1098 1992 [Refereed][Not invited]
     
    Tunneling spectroscopic measurement was carried out in the superconducting phase of Bi2Sr2CaCu2O8 with use of a low temperature scanning tunneling microscope. Both the cleaved Bi-O surface and the lateral surface of a single crystal were investigated. The tunneling conductance shows an explicit superconducting gap structure at both surfaces. For the lateral surface the conductance curve is independent of the tip distance from the sample surface and gives directly the electronic density of states in the superconducting phase. The obtained gap structure is in good agreement with the BCS prediction, although it seems that the gap is slightly anisotropic. The obtained gap-ratio 2Δ/kBTc=6.1 indicates the strong-coupling nature of this superconductor. For the Bi-O surface, on the other hand, the normalized differential conductance increases with increasing the tip distance, especially at the outside of the superconducting gap as reported by Hasegawa et al. Such a dependence on the tip distance is attributed to series barriers of Bi-O, Sr-O layers and vacuum for the tunneling between the Cu-O layer and the tip metal. © 1992.
  • Koichi Ichimura, Kazushige Nomura, Fujio Minami, Shunji Takekawa
    Physica C: Superconductivity and its applications 185-189 (2) 941 - 942 0921-4534 1991/12/01 [Refereed][Not invited]
     
    Tunneling measurement was carried out on single crystals of superconducting oxide Bi2Sr2CaCu2O6-8 with use of the STM. Tunneling conductance varies with the distance between the tip and the sample surface at the outside region of superconducting gap, while the midgap conductance is independent of the tip distance. The superconducting gap parameter was obtained as Δ=26±4 meV, and correspondingly 2Δ/kTc=7±1. Temperature dependence of gap parameter is consistent with the BCS. © 1991.
  • K. Ichimura, K. Nomura, F. Minami, S. Takekawa
    Journal of Physics: Condensed Matter 2 (49) 9961 - 9965 0953-8984 1990 [Refereed][Not invited]
     
    The authors carried out a tunnelling spectroscopy measurement on a single crystal of superconducting oxide Bi2Sr2CaCu 2O8- delta using a low temperature scanning tunnelling microscope. The superconducting gap parameter shows a spatial variation over a relatively small area ( approximately 10 nm). The authors obtained the energy gap parameter Delta (4.2 K)=26 meV, the most observable value, and correspondingly 2 Delta /kBTc=7, larger than the BCS value. The shape of the (dI/dV)-V curve suggests an anisotropy of the energy gap. Thermal smearing of the gap structure is observed below Tc. Temperature dependence of the gap parameter is consistent with BCS. Even above Tc the conductance curve is not flat.
  • K NOMURA, K ICHIMURA
    JOURNAL OF VACUUM SCIENCE & TECHNOLOGY A-VACUUM SURFACES AND FILMS 8 (1) 504 - 507 0734-2101 1990/01 [Refereed][Not invited]
  • K NOMURA, T SHIMIZU, K ICHIMURA, T SAMBONGI, M TOKUMOTO, H ANZAI, N KINOSHITA
    SOLID STATE COMMUNICATIONS 72 (11) 1123 - 1126 0038-1098 1989/12 [Refereed][Not invited]
  • T SAMBONGI, K NOMURA, T SHIMIZU, K ICHIMURA, N KINOSHITA, M TOKUMOTO, H ANZAI
    SOLID STATE COMMUNICATIONS 72 (8) 817 - 819 0038-1098 1989/11 [Refereed][Not invited]
  • K NOMURA, K ICHIMURA
    SOLID STATE COMMUNICATIONS 71 (2) 149 - 152 0038-1098 1989/07 [Refereed][Not invited]
  • STM Observation of CDW Depinning at the Surface of K0.3MoO3 (共著)
    Fizika 21, 95-97 1989 [Not refereed][Not invited]
  • Non-Linear Conductivity in SDW Phase of (TMTSF)2ClO4 (共著)
    Fizika 21, 60-62 1989 [Not refereed][Not invited]

MISC

Presentations

Association Memberships

  • 日本物理学会   The Physical Society of Japan   

Works

  • カーボンナノチューブのSTM/STS
    1999
  • STM/STS on Carbon Nanotubes
    1999
  • Bi系酸化物の超伝導ギャップ
    1990 -1998
  • Superconducting Gap in Bi-Based Oxides
    1990 -1998
  • 有機超伝導体のSTM分光
    1996
  • STM Spectroscopy on Organic Superconductors
    1996

Research Projects

  • Japan Society for the Promotion of Science：Grants-in-Aid for Scientific Research
    Date (from‐to) : 2022/04 -2025/03 
    Author : 市村 晃一, 黒澤 徹
  • Japan Society for the Promotion of Science：Grants-in-Aid for Scientific Research
    Date (from‐to) : 2017/04 -2023/03 
    Author : 市村 晃一, 松浦 徹
     
    前年度に引き続き有機導体beta"-(BEDT-TTF)4[(H3O)Ga(C2O4)3]C6H5NOの単結晶を電解法で作製した。原料試薬の精製過程を増やすことにより試料を良質化を行った。得られた単結晶試料はX線回折により当該化合物であることを同定したうえで、電気抵抗と磁化率の温度依存性を測定し電子物性を評価した。 超伝導相(超伝導転移温度Tc=7.5 K)を重点的に調べるために、主に1.3 Kでの走査トンネル分光(STS)測定を行い、測定データの良質化を図るとともに再現性の確認を目指した。信号検出系を改良しSTS測定時のノイズの低減を図った。これを用い超伝導相である1.3 KにおいてSTS測定を行った。前年度までと同様に、典型的なトンネルスペクトルは絶縁体的な1 eV程度のギャップ構造を示し、超伝導に特徴的な1 meV程度のギャップ構造の観測には至らなかった。ギャップの空間変化の系統性を調べるためにSTSマッピングを行った。これまでよりもトンネルスペクトルのノイズは低減され絶縁体的な1 eV程度のギャップの空間変化が示唆されたが、その周期の定量化までは至らなかった。さらなるデータの蓄積により、超伝導状態での電荷不均化が解明されるものと期待される。 超伝導に電荷秩序と磁気秩序が関連するFeSe0.25Te0.75 (Tc=10.5 K)との比較という観点で議論を進めた。両者に共通することとして、超伝導状態において電荷秩序が形成されていることが見出されたが、超伝導と電荷秩序が協調的な関係なのか何らかの相分離による共存なのかという新たな課題が生じた。これを解明することで強相関電子系の超伝導における電荷ゆらぎの役割が明らかになり新たな超伝導発現機構に言及できるものと期待される。
  • Japan Society for the Promotion of Science：Grants-in-Aid for Scientific Research
    Date (from‐to) : 2012 -2012 
    Author : ICHIMURA Koichi
     
    A scanning tunneling microscope (STM) under high pressure and low temperature was developed. The electronic state of organic conductors, which are materials with the highly correlated electron system, were studied by STM. The role of the long range Coulomb interaction was discussed. STM measurements were performed on three organic conductors with the alpha-type donor arrangement, in which the system tends to undergo the charge ordering state. The charge disproportionation with a stripe structure was found at high temperature in three salts irrespective of its ground states. Moreover, the stripe pattern varied depending on the anisotropy of the electronic band structure. The results suggest that a dimerization has an important role in forming the charge ordering as well as the long range Coulomb interaction. The finding obtained by the present study contributes to elucidate the origin of the charge ordering state, which characterizes highly correlated electron systems.
  • 文部科学省：科学研究費補助金(新学術領域研究(研究領域提案型))
    Date (from‐to) : 2009 -2010 
    Author : 市村 晃一
     
    本研究は、高圧力下で動作する走査トンネル顕微鏡(STM)を新規に開発し、これを用いて分子性導体の基底状態を調べることを目的としている。1.高圧STMのための装置開発ヘリウムガスを用いてボンベ圧(～10MPa)から増圧し100MPaまで加圧可能なヘリウムガス圧発生装置を整備した。これに接続するSTM用圧力セルを新規に製作し室温で100MPaまでの加圧試験を行った。この高圧セル内に、小型のシェアピエゾによる粗動用アクチュエーターおよび探針走査用のチュープスキャナーなどからなるSTMヘッドを組み込み常圧下でのSTMの動作を確認した。2.分子性導体の単結晶試料作成分子性導体α-(BEDT-TTF)_2KHg(SCN)_4の低温での密度波状態に着目し、電解法を用いて、この試料の単結晶を作成した。原料試薬を再結晶精製することにより良質の単結晶が得られた。3.単結晶試料の常圧下での基底状態得られたα-(BEDT-TTF)_2KHg(SCN)_4単結晶試料の常圧下での電子物性を調べるために、電気抵抗率と磁化率の温度依存性を測定した。直流4端子法による電気抵抗率は、低温で緩やかな増大を示し、この試料の密度波転移温度が8Kであることがわかった。SQUID磁束計による磁化率測定では、サンプルホルダーを改良することにより1mg以下の微量試料の磁化率を精密に測定することができた。その結果、従来は高温側...
  • 文部科学省：科学研究費補助金(特定領域研究)
    Date (from‐to) : 2006 -2007 
    Author : 野村 一成, 市村 晃一, 松永 悟明
     
    ドナー分子の中心にTCF骨格を持たないβ-(BDA-TTP)_2SbF6_(T_c=7.5K)の超伝導相においてSTMを用いたトンネル分光測定を行った。この系は近隣め物質の振る舞いからκ-(BEDT-TTF)_2X系と同様に電子相関が強い2次元電子系と考えられるが、β-型の構造を持つためκ-(BEDT-TTF>_2Xのようなモット絶縁体に隣接した超伝導ではないと考えられ、そのルカニズムに興味が持たれる。伝導面でのトンネルスペクトルはT_cよりも十分低温では、ゼロバイアス付近のコンダクタンスがほとんどゼロにまで減少する明確な超伝導ギャップ構造を示す。その関数系はκ-(BEDT-TTF)_2X系と同様にV-型を示しており、ラインノードを持った異方的超伝導であることを示している。一方、側面でのトンネルスペクトルは、面の角度に応じて比較的大きなギャップを持ったU-型から小さなギャップのV-型に角度πの間に2回変化することが明らかになった。トンネル遷移確率の方向依存性を考慮し軸近辺でギャップ最大となり、軸近辺でギャップ最大となり、a^*+c^* a^*-c^*方向近辺でノードを持つことが強く示唆される。上述の結晶系のため、純粋なd(x^2-y^2)波対称性ではな いかもしれないがd(x^2-y^2)波的対称性が実現していると結論される。このノードの方向はフェルミ面の部分ネスティングに...
  • Ministry of Education, Culture, Sports, Science and Technology：Grants-in-Aid for Scientific Research(基盤研究(C))
    Date (from‐to) : 2005 -2006 
    Author : Koichi ICHIMURA, 松永 悟明
     
    Layered compound NbSe_2 nanotubes were studied by low temperature scanning tunneling microscope (STM). NbSe_2 nanotube is an interesting material since it is the first nanotube material, of which bulk phase undergoes the condensed state such as the density wave and superconductivity. NbSe_2 nanotubes were synthesized by the chemical vapor transport method. The nanotube sample for STM measurement was prepared by dropping NbSe_2 nanotubes agitated ultrasonically in 2-propanol on cleaved highly oriented pyrolytic graphite (HOPG).Topographic STM images of NbSe_2 nanotubes on the substrate of HO...
  • 文部科学省：科学研究費補助金(特定領域研究)
    Date (from‐to) : 2004 -2005 
    Author : 野村 一成, 市村 晃一, 松永 悟明
     
    圧力下で誘起される擬一次元有機導体(TMTTF)_2Brのスピン密度波(SDW)相において、圧力を詳細に変化させた非線形電気伝導の測定により、SDWのスライディングのダイナミクスを電子バンドの1次元性との関わりから調べた。いずれの圧力でもSDW転移温度T_以下の温度での電流電圧特性において、電場の増加とともにしきい電場E_Tを伴った鋭い伝導度の増大が観測され、ピン止めをはずしたSDWのスライディングが確認された。静止摩擦に相当するしきい電場E_Tの絶対値は低圧領域において、電子バンドの1次元性が小さい(TMTSF)_2PF_6に比べて相当大きいが、圧力を増加させると急速に減少した。E_Tの温度依存は試料によって多少異なるが、典型的な試料では0.3T_近傍の温度で鋭いピークを示し、低温では急激に減少した。これに対してスライディングの動摩擦によって与えられる余剰伝導度は、0.3T_で緩やかなディップを持ち低温で急激に増大した。一方別の試料では、しきい電場の温度変化に明確なピークは見られなかったが、0.3T_付近から急激に減少する振る舞いは共通に観測された。これに対して、余剰伝導度は試料によらずほぼ共通の振る舞いを示した。これらの結果より、SDWのスライディングに対して、静止摩擦と動摩擦は異なるメカニズムにより与えられていることが分かった。さ...
  • STM/STS on topological crystals
    Date (from‐to) : 2003
  • 文部科学省：科学研究費補助金(奨励研究(A))
    Date (from‐to) : 2000 -2001 
    Author : 市村 晃一
     
    本研究の目的は、有機超伝導体BEDT-TTF塩における走査トンネル顕微鏡(STM)を用いた電子トンネル分光測定により超伝導発現機構に関する知見を得ることである。ドナーの部分重水素置換による電子相関の系統的な変化に対する、超伝導および擬ギャップ状態の変化を明らかにすることを目指し、今年度は水素体(d[0,0])および、これよりも電子相関の強い部分重水素体(d[2,2])の(BEDT-TTF)_2Cu[N(CN)_2]BrにおいてSTM測定を行った。超伝導状態においては、水素体の(BEDT-TTF-d[0,0])_2Cu[N(CN)_2]Br (T_c=11.5K)と部分重水素化した(BEDT-TTF-d[2,2])_2Cu[N(CN)_2]Br (T_c=12.0K)の両者ともに明確な超伝導ギャップが観測された。トンネルコンダクタンスカーブの形状はd-波のギャップ対称性でよく説明された。数多くの測定を重ねデータを収集することにより、それぞれの塩でギャップパラメターΔを決定することができた。得られた値を以下に示す。Δ=2.1-3.9meV 2Δ/kT_c=4.3-7.9 for(BEDT-TTF-d[0,0])_2Cu[N(CN)_2]BrΔ=3.0-4.8meV 2Δ/kT_c=5.8-9.3 for(BEDT-TTF-d[2,2])_2Cu[N(CN)_2]Brこのことから、...
  • 文部科学省：科学研究費補助金(特定領域研究(A))
    Date (from‐to) : 1999 -2000 
    Author : 野村 一成, 松永 悟明, 市村 晃一
     
    カーボンナノチューブのカイラリティーと電子状態の対応を明らかにするため、低温トンネル顕微鏡装置を用いてSTM像観察およびSTS測定を行なった。前年に引き続き低温でのSTM装置の安定動作化を行い、装置の測定ノイズを低減した。また、新たに導入したガス循環精製装置付き真空グローブボックスを用い、酸素および水分のない雰囲気下でナノチューブ試料を扱うことにより、試料表面の清浄化を行なった。金属触媒を用いたアーク放電法により作成された単層カーボンナノチューブ試料のSTM像測定を室温で行い、そのバンドル構造を観測した。また、プロファイルからこれらがおよそ1nmの単層カーボンナノチューブからなることを確認した。STM測定においてバンドルが確認された場所で77KでSTS測定を行い、1.3meV〜1.8meVの有限で平坦な電子状態密度をはさんでVan Hove Singularityの発散的ピークを示す金属ナノチューブの振る舞いを観測した。また、別の場所では0.4meV〜0.6meV程度のギャップを持つ半導体ナノチューブの振る舞いも観測した。これらは、同一のバンドルに属する別のナノチューブで観測されることから、1つのバンドルの中には異なるカイラリティーを持つナノチューブが存在することが明らかになった。さらに、金属ナノチューブのVan Hove Singularityは複数のピークに分裂することも...
  • 文部科学省：科学研究費補助金(奨励研究(A))
    Date (from‐to) : 1997 -1998 
    Author : 市村 晃一
     
    本研究は、走査トンネル顕微鏡(STM)を用いて擬一次元導体のスピン密度波(SDW)および電荷密度波(CDW)相での電子状態を明らかにすることを目的としている。今年度は有機導体(TMTSF)_2PF_6の単結晶作成とSDW相におけるSTM分光測定を重点的に行った。擬一次元有機導体(TMTSF)_2PF_6の単結晶をH型結晶成長セルを用い電解法により作成した。低湿度雰囲気を保つことと、結晶成長の度合いに応じて流す電流量を制御することにより、10×2×0.1mm^3ほどの大きさの良質な単結晶が多数作成された。得られた単結晶試料に対しSTM分光測定を行ったところ、SDW転移温度T_=12K以下ではSDWにともなう明確なギャップ構造が観測された。トンネル微分コンダクタンスカーブの形は擬一次元導体に対する平均場理論から予測される状態密度スペクトルと一致することが分かった。一方、SDW転移温度以上においては、ギャップ構造は完全には消失せずゼロバイアス付近にくぼみを持つことが明らかになった。ゼロバイアスでのコンダクタンスは温度の上昇とともに増加し、40K以上ではコンダクタンスカーブは平坦となりノーマル金属相であることを示す。擬キャップ的振る舞いの原因としては、電子系の低次元性によるゆらぎの効果が考えられるがまだ明らかにはなっていない。電気抵抗、磁化率、比熱、NMR緩和率等の物理量と...
  • 文部科学省：科学研究費補助金(重点領域研究)
    Date (from‐to) : 1996 -1996 
    Author : 市村 晃一, 松永 悟明, 野村 一成
     
    酸化物高温超伝導体Bi_2Sr_2CaCu_2O_8において、低温用走査型トンネル顕微鏡(STM)を用い4.2Kにおいてトンネル分光測定を行った。単結晶側面において測定を行い超伝導ギャップの面内異方性を直接的に調べ、ギャップの対称性を明らかにすることを目的とした。異なる2つの方向からトンネル探針をアプローチし、ノイズの少ないトンネル微分コンダクタンスが得られた。1.[110]方向(Cu-O-Cu結合方向から45°)でのトンネルゼロバイアス付近のコンダクタンスは十分に減少し、明確な超伝導ギャップ構造が観測された。そのコンダクタンスカーブは50nm程度の範囲内では場所によらず同一である。コンダクタンスの関数形は以前の測定とほぼ一致する。ギャップの大きさはΔ_=21meVと求まり、以前の結果(Δ_=25meV)より若干小さいが、これは試料依存性と考えられる。2.[100]方向(Cu-O-Cu結合方向)でのトンネル[110方向でのトンネルとは対照的に、ゼロバイアス付近のコンダクタンスは十分に減少せず、明確なギャップ構造は観測されなかった。方向によりトンネルスペクトルが変化することは、STM測定においてトンネル遷移確率の波数依存性が強いことと、超伝導ギャップが異方的であることを意味する。また、ゼロバイアス付近でコンダクタンスが増加するスペクトルもしばしば観測された。...
  • 文部科学省：科学研究費補助金(奨励研究(A))
    Date (from‐to) : 1996 -1996 
    Author : 市村 晃一
     
    これまで詳細な熱電能の測定が行われていなかった、擬一次元有機導体において熱電能の温度依存性を調べることを目的とした。そして、熱電能という物理量がスピン密度波(SDW)の多相構造を明らかにする新たなプローブとなることを期待した。そこで、低温領域まで測定可能な熱電能測定クライオスタットを製作し、擬一次元有機導体TMTSF塩の熱電能の温度依存性の測定を試みた。1.熱電能測定システムの整備既に熱電能の温度依存性が知られている参照資料と、未知試料とで熱電対を形成し、その両端に適当な温度勾配を与えた際の起電力を計測し未知試料の熱電能を求めた。参照試料として鉛を用いた。クライオスタットは自作した。ガラス製ヘリウムデュワ-内に納められ、室温から4.2K、更に減圧により1.4Kまで連続的に降温可能である。未知試料として銅を用いて動作試験を行ったが、その熱電能の絶対値及び温度依存性は文献値とよい一致を示した。2.TMTSF塩の熱電能測定室温から1.4Kまでの温度領域で(TMTSF)_2ClO_4の熱電能を測定した。しかしながら、ノイズが大きく、再現性のあるデータは現在のところ得られていない。この原因は、参照試料の鉛と試料との接合部(熱電対の片端)にあると思われる。機械的接触により接合させているが、温度を変化させた際試料の熱収縮(膨張)等により接合が不安定になることが分かった。このため、圧電素子を...
  • Ministry of Education, Culture, Sports, Science and Technology：Grants-in-Aid for Scientific Research(一般研究(B), 基盤研究(B))
    Date (from‐to) : 1995 -1996 
    Author : Kazushige NOMURA, 市村 晃一
     
    We have developed the apparatus of the scanning tunneling microscope (STM) for the investigation of the electronic state of the organic superconductor and carried out the tunneling spectroscopy measurement with this apparatus. We have constructed the apparatus for the measurement under the magnitic field by combining the STM unit with the superconducting magnet. We have also performed the tunneling spectroscopic measurement in the superconducting phase of the organic superconductor (BEDT-TTF)_2Cu (NCS)_2For the construction of the STM apparatus, we purchased the STM unit and the superconduc...
  • 文部科学省：科学研究費補助金(奨励研究(A))
    Date (from‐to) : 1995 -1995 
    Author : 市村 晃一
     
    本発明の目的は、低温用走査型トンネル顕微鏡(STM)を用いて擬一次元導体における電荷密度波(CDW)相の電子構造を明らかにすること、及び電荷密度波の並進運動を試料表面において直接観測することである。今年度は、STMを用いた電子トンネル分光測定を重点的に行い電荷密度波相での電子構造の詳細を明らかにすることをめざした。電解法で作成したK_<0.3>MoO_3(ブルーブロンズ)単結晶試料を用い、劈開面においてSTM測定を行った。トンネル電流の検出にロックインアンプに加えその前段に同調型アンプを用いることにより、ノイズは大幅に低減した。室温ではゼロバイアス付近のトンネル微分コンダクタンスは有限であり、金属相であることが示唆される。しかしながら、コンダクタンスカーブは通常の金属のように平坦ではなく、ゼロバイアスにくぼみを持つ構造が見られた。CDW転移温度(180K)以下の77Kでは、コンダクタンスはゼロバイアス付近でほとんどゼロにまで減少した。コンダクタンスカーブの形状はギャップ端のはっきりしないV字型をしており、従来金属の超伝導相で見られるようなBCS状態密度とは大きく異なっている。これは一次元系で揺らぎが大きいことに起因すると考えられる。今後はNbSe_3など他の擬一次元導体においてもSTM分光測定を行い、電子状態密度の形状と一次元性との関連について考察していきたい。また、コンダク...
  • カーボンナノチューブのSTM/STS
  • 低次元導体のSTM分光
  • STM/STS on Carbon Nantubes
  • STM Spectroscopy on Low Dimensional Conductor

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