M. Lang

Goethe-Universität Frankfurt am Main, Frankfurt, Hesse, Germany

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Publications (210)517.69 Total impact

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    ABSTRACT: We present temperature-dependent in-plane resistivity measurements on FeSe single crystals under He-gas pressure up to 800 MPa and magnetic fields $B \leq$ 10 T. A sharp phase transition anomaly is revealed at the tetragonal-to-orthorhombic transition at $T_s$ slightly below 90 K. $T_s$ becomes reduced with increasing pressure in a linear fashion at a rate d$T_{s}$/d$P$ $\simeq$ -31 K/GPa. This is accompanied by a $P$-linear increase of the superconducting transition temperature at $T_c \sim$ 8.6 K with d$T_{c}$/d$P$ $\simeq$ +5.8 K/GPa. Pressure studies of the normal-state resistivity highlight two distinctly different regimes: for $T > T_s$, i.e., in the tetragonal phase, the in-plane resistivity changes strongly with pressure. This contrasts with the state deep in the orthorhombic phase at $T \ll T_s$, preceding the superconducting transition. Here a $T$-linear resistivity is observed the slope of which does not change with pressure. Resistivity studies in varying magnetic fields both at ambient and finite pressure reveal clear changes of the magnetoresistance, $\Delta \rho \propto B^{2}$, upon cooling through $T_s$. Our data are consistent with a reconstruction of the Fermi surface accompanying the structural transition.
    Physical Review B 03/2015; 91(17). DOI:10.1103/PhysRevB.91.174510 · 3.66 Impact Factor
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    ABSTRACT: We investigate the structural, electronic and magnetic properties of the newly synthesized mineral barlowite Cu4(OH)6FBr which contains Cu2+ ions in a perfect kagome arrangement. In contrast to the spin-liquid candidate herbertsmithite ZnCu3(OH)6Cl2, kagome layers in barlowite are perfectly aligned due to the different bonding environments adopted by F- and Br- compared to Cl-. We perform density functional theory calculations to obtain the Heisenberg Hamiltonian parameters of Cu4(OH)6FBr which has a Cu2+ site coupling the kagome layers. The 3D network of exchange couplings together with a substantial Dzyaloshinskii-Moriya coupling lead to canted antiferromagnetic ordering of this compound at TN=15 K as observed by magnetic susceptibility measurements on single crystals.
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    ABSTRACT: Low temperature scanning tunneling spectroscopy reveals the local density of states of the organic superconductor $\kappa$-(BEDT-TTF)$_2$Cu[N(CN)$_2$]Br, that was cut in-situ in ultra-high vacuum perpendicular to the superconducting BEDT-TTF layers. The spectra confirm that superconductivity is confined to the conducting BEDT-TTF layers, while the Cu[N(CN)$_2$]Br anion layers are insulating. The density of states comprises a twofold superconducting gap, which is attributed to the two separated bands crossing the Fermi surface.
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    ABSTRACT: Two-dimensional (2D) systems with continuous symmetry lack conventional long-range order because of thermal fluctuations. Instead, as pointed out by Berezinskii, Kosterlitz and Thouless (BKT), 2D systems may exhibit so-called topological order driven by the binding of vortex-antivortex pairs. Signatures of the BKT mechanism have been observed in thin films, specially designed heterostructures, layered magnets and trapped atomic gases. Here we report on an alternative approach for studying BKT physics by using a chemically constructed multilayer magnet. The novelty of this approach is to use molecular-based pairs of spin S=½ ions, which, by the application of a magnetic field, provide a gas of magnetic excitations. On the basis of measurements of the magnetic susceptibility and specific heat on a so-designed material, combined with density functional theory and quantum Monte Carlo calculations, we conclude that these excitations have a distinct 2D character, consistent with a BKT scenario, implying the emergence of vortices and antivortices.
    Nature Communications 10/2014; 5:5169. DOI:10.1038/ncomms6169 · 10.74 Impact Factor
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    ABSTRACT: Powder X-ray diffraction (PXRD) and single-crystal neutron scattering were used to study in detail the structural properties of the Cs2CuCl(4-x)Br(x) series, good realizations of layered triangular antiferromagnets. Detailed temperature-dependent PXRD reveal a pronounced anisotropy of the thermal expansion for the three different crystal directions of the orthorhombic structure without any structural phase transition down to 20 K. Remarkably, the anisotropy of the thermal expansion varies for different $x$, leading to distinct changes of the geometry of the local Cu environment as a function of temperature and composition. The refinement of the atomic positions confirms that for x=1 and 2, the Br atoms occupy distinct halogen sites in the [CuX4]-tetrahedra (X = Cl, Br). The precise structure data are used to calculate the magnetic exchange couplings using density functional methods for x=0. We observe a pronounced temperature dependence of the calculated magnetic exchange couplings, reflected in the strong sensitivity of the magnetic exchange couplings on structural details. These calculations are in good agreement with the experimentally established values for Cs2CuCl4 if one takes the low-temperature structure data as a starting point.
    Physical Review B 10/2014; 91(3). DOI:10.1103/PhysRevB.91.035124 · 3.66 Impact Factor
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    ABSTRACT: The density of states of the organic superconductor $\kappa$-(BEDT-TTF)$_2$Cu[N(CN)$_2$]Br, measured by scanning tunneling spectroscopy on \textit{in-situ} cleaved surfaces, reveals a logarithmic suppression near the Fermi edge persisting above the critical temperature $T_\mathrm{c}$. A soft Hubbard gap as predicted by the Anderson-Hubbard model for systems with disorder exactly describes the experimentally observed suppression. The electronic disorder also explains the diminished coherence peaks of the quasiparticle density of states below $T_\mathrm{c}$.
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    ABSTRACT: This article reviews some recent developments for new cooling technologies in the fields of condensed matter physics and cold gases, both from an experimental and theoretical point of view. The main idea is to make use of distinct many-body interactions of the system to be cooled which can be some cooling stage or the material of interest itself, as is the case in ultracold gases. For condensed matter systems, we discuss magnetic cooling schemes based on a large magnetocaloric effect as a result of a nearby quantum phase transition and consider effects of geometrical frustration. For ultracold gases, we review many-body cooling techniques, such as spin-gradient and Pomeranchuk cooling, which can be applied in the presence of an optical lattice. We compare the cooling performance of these new techniques with that of conventional approaches and discuss state-of-the-art applications.
    International Journal of Modern Physics B 10/2014; 28(26):1430017. DOI:10.1142/S0217979214300175 · 0.46 Impact Factor
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    ABSTRACT: The spin excitations in the spin-liquid phase of the anisotropic triangular lattice quantum antiferromagnet Cs$_2$CuCl$_4$ have been shown to propagate dominantly along the crystallographic $b$-axis [L. Balents, Nature (London) 464, 199 (2010)]. To test this dimensional reduction scenario, we have performed ultrasound experiments in the spin-liquid phase of Cs$_2$CuCl$_4$ probing the elastic constant $c_{22}$ and the sound attenuation along the $b$-axis as a function of an external magnetic field along the $a$-axis. We show that our data can be quantitatively explained within the framework of a nearest neighbor spin-1/2 Heisenberg chain, where fermions are introduced via the Jordan-Wigner transformation and the spin-phonon interaction arises from the usual exchange-striction mechanism.
    Physical Review B 09/2014; 91(4). DOI:10.1103/PhysRevB.91.041108 · 3.66 Impact Factor
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    ABSTRACT: The effect of hydrostatic pressure and partial Na substitution on the normal-state properties and the superconducting transition temperature ($T_c$) of K$_{1-x}$Na$_x$Fe$_2$As$_2$ single crystals were investigated. It was found that a partial Na substitution leads to a deviation from the standard $T^2$ Fermi-liquid behavior in the temperature dependence of the normal-state resistivity. It was demonstrated that non-Fermi liquid like behavior of the resistivity for K$_{1-x}$Na$_{x}$Fe$_2$As$_2$ and some KFe$_2$As$_2$ samples can be explained by disorder effect in the multiband system with rather different quasiparticle effective masses. Concerning the superconducting state our data support the presence of a shallow minimum around 2 GPa in the pressure dependence of $T_c$ for stoichiometric KFe$_2$As$_2$. The analysis of $T_c$ in the K$_{1-x}$Na$_{x}$Fe$_2$As$_2$ at pressures below 1.5 GPa showed, that the reduction of $T_c$ with Na substitution follows the Abrikosov-Gor'kov law with the critical temperature $T_{c0}$ of the clean system (without pair-breaking) which linearly depends on the pressure. Our observations, also, suggest that $T_c$ of K$_{1-x}$Na$_x$Fe$_2$As$_2$ is nearly independent of the lattice compression produced by the Na substitution. Further, we theoretically analyzed the behavior of the band structure under pressure within the generalized gradient approximation (GGA). A qualitative agreement between the calculated and the recently in de Haas-van Alphen experiments [T. Terashima et al., Phys.Rev.B89, 134520(2014)] measured pressure dependencies of the Fermi-surface cross-sections has been found. These calculations, also, indicate that the observed minimum around 2~GPa in the pressure dependence of $T_c$ may occur without a change of the pairing symmetry.
    Physical Review B 09/2014; 90(9). DOI:10.1103/PhysRevB.90.094511 · 3.66 Impact Factor
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    ABSTRACT: In the present study we report the synthesis of some novel nitronyl nitroxide biradical systems 1–4c with various π-bridges between the radical centres. UV-Vis, IR, EPR and X-ray diffraction studies, along with MS and NMR data where appropriate, are described. Magnetic measurements revealed that the biradicals 1c, 3c and 4c exhibit a moderately strong antiferromagnetic intra-molecular exchange, whereas nitroxide 2c shows a significantly higher exchange coupling, which can only be explained by the presence of strong inter-molecular interactions. From DFT calculations performed on the basis of the X-ray crystal structure of compound 4c, a theoretical value of the intra-dimer coupling constant Jintra = −8.6 K is obtained. Direct proof also for inter-molecular arrangement with Jinter −2 K was provided by the low temperature AC studies of biradical 4c. According to the magnetic characterization, the nitronyl biradical 4c is a promising candidate for a purely organic-based low-dimensional quantum magnet.
    07/2014; 2(32). DOI:10.1039/C4TC00399C
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    ABSTRACT: A detailed investigation of the out-of-plane electrical properties of charge-ordered alpha-(BEDT-TTF)2I3 reveals clear indications for relaxor ferroelectricity. Similar to multiferroic kappa-(BEDT-TTF)2Cu[N(CN)2]Cl, the polar order in this material is ascribed to the occurrence of bond- and site-centered charge order. Cluster-like polar order is observed deep in the charge-ordered state, which is explained by the existence of polar and nonpolar stacks of the organic molecules in this material, preventing long-range ferroelectricity. The results are discussed in relation to the formation or absence of electronic polar order in related charge-transfer salts.
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    ABSTRACT: The coupling of magnetic and electronic degrees of freedom to the crystal lattice in the ferromagnetic semimetal EuB$_6$, which exhibits a complex ferromagnetic order and a colossal magnetoresistance (CMR) effect, %, very likely involving magnetic polarons, is studied by high-resolution thermal expansion and magnetostriction experiments. EuB$_6$ may be viewed as a model system, where pure magnetism-tuned transport and the response of the crystal lattice can be studied in a comparatively simple environment,i.e., not influenced by strong crystal-electric field effects and Jahn-Teller distortions. We find a very large lattice response, quantified by (i) the magnetic Gr\"uneisen parameter, (ii) the spontaneous strain when entering the ferromagnetic region and (iii) the magnetostriction in the paramagnetic temperature regime. Our analysis reveals that a significant part of the lattice effects originates in the magnetically-driven delocalization of charge carriers, consistent with the scenario of percolating magnetic polarons. A strong effect of the formation and dynamics of local magnetic clusters on the lattice parameters is suggested to be a general feature of CMR materials.
    Physical Review Letters 04/2014; 113(6). DOI:10.1103/PhysRevLett.113.067202 · 7.73 Impact Factor
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    ABSTRACT: We present results of ultrasonic measurements on a single crystal of the distorted diamond-chain compound azurite Cu$_3$(CO$_3$)$_2$(OH)$_2$. Pronounced elastic anomalies are observed in the temperature dependence of the longitudinal elastic mode $c_{22}$ which can be assigned to the relevant magnetic interactions in the system and their couplings to the lattice degrees of freedom. From a quantitative analysis of the magnetic contribution to $c_{22}$ the magneto-elastic coupling $G$ = $\partial J_2$/$\partial \epsilon_b$ can be determined, where $J_2$ is the intra-dimer coupling constant and $\epsilon_b$ the strain along the intra-chain $b$ axis. We find an exceptionally large coupling constant of $|G| \sim ($3650 $\pm$ 150) K highlighting an extraordinarily strong sensitivity of $J_2$ against changes of the $b$-axis lattice parameter. These results are complemented by measurements of the hydrostatic pressure dependence of $J_2$ by means of thermal expansion and magnetic susceptibility measurements performed both at ambient and finite hydrostatic pressure. We propose that a structural peculiarity of this compound, in which Cu$_2$O$_6$ dimer units are incorporated in an unusually stretched manner, is responsible for the anomalously large magneto-elastic coupling.
    Physical Review B 02/2014; 89(17). DOI:10.1103/PhysRevB.89.174427 · 3.66 Impact Factor
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    ABSTRACT: Core-level photoemission spectra of the Fabre salts with X = SbF6 and PF6 were taken using hard X-rays from PETRA III, Hamburg. In these salts TMTTF layers show a significant stack dimerization with a charge transfer of 1e per dimer to the anion SbF6 or PF6. At room temperature and slightly below the core-level spectra exhibit single lines, characteristic for a well-screened metallic state. At reduced temperatures progressive charge localization sets in, followed by a 2nd order phase transition into a charge-ordered ground state. In both salts groups of new core-level signals occur, shifted towards lower kinetic energies. This is indicative of a reduced transverse-conductivity across the anion layers, visible as layer-dependent charge depletion for both samples. The surface potential was traced via shifts of core-level signals of an adsorbate. A well-defined potential could be established by a conducting cap layer of 5 nm aluminum which appears " transparent " due to the large probing depth of HAXPES (8–10 nm). At the transition into the charge-ordered phase the fluorine 1s line of (TMTTF)2SbF6 shifts by 2.8 eV to higher binding energy. This is a spectroscopic fingerprint of the loss of inversion symmetry accompanied by a cooperative shift of the SbF6 anions towards the more positively charged TMTTF donors. This shift does not occur for the X = PF6 compound, most likely due to smaller charge disproportion or due to the presence of charge disorder.
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    ABSTRACT: Depending on the crystal growth conditions, an orthorhombic (O-type) or a tetragonal (T-type) structure can be found in the solid solution Cs2CuCl4-xBrx (0 < x < 4). Here we present measurements of the temperature-dependent magnetic susceptibility and isothermal magnetization on the T-type compounds x = 1.6 and 1.8 and compare these results with the magnetic properties recently derived for the O-type variant by Cong et al., Phys. Rev. B 83, 064425 (2011). The systems were found to exhibit quite dissimilar magnetic properties which can be assigned to differences in the Cu coordination in these two structural variants. Whereas the tetragonal compounds can be classified as quasi-2D ferromagnets, characterized by ferromagnetic layers with a weak antiferromagnetic inter-layer coupling, the orthorhombic materials, notably the border compounds x = 0 and 4, are model systems for frustrated 2D Heisenberg antiferromagnets
    IEEE Transactions on Magnetics 11/2013; DOI:10.1109/TMAG.2014.2298496 · 1.21 Impact Factor
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    ABSTRACT: The recently proposed multiferroic state of the charge-transfer salt {\kappa}-(BEDT-TTF)$_2$Cu[N(CN)$_2$]Cl [P. Lunkenheimer et al., Nature Mater., vol. 11, pp. 755-758, Sept. 2012] has been studied by dc-conductivity, magnetic susceptibility and measurements of the dielectric constant on various, differently prepared single crystals. In the majority of crystals we confirm the existence of an order-disorder-type ferroelectric state which coincides with antiferromagnetic order. This phenomenology rules out scenarios which consider an inhomogeneous, short-range-ordered ferroelectric state. Measurements of the dielectric constant and the magnetic susceptibility on the same crystals reveal that both transitions lie very close to each other or even collapse, indicating that both types of order are intimately coupled to each other. We address issues of the frequency dependence of the dielectric constant {\epsilon}' and the dielectric loss {\epsilon}'' and discuss sample-to-sample variations.
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    ABSTRACT: The organic charge-transfer salt EtMe$_3$P[Pd(dmit)$_2$]$_2$ is a quasi-two-dimensional Mott insulator with localized spins $S$ = 1/2 residing on a distorted triangular lattice. Here we report measurements of the uniaxial thermal expansion coefficients $\alpha_i$ along the in-plane i = $a$- and c-axis as well as along the out-of-plane b-axis for temperatures 1.4\,K $\leq T \leq$ 200\,K. Particular attention is paid to the lattice effects around the phase transition at $T_{VBS}$ = 25\,K into a low-temperature valence-bond-solid phase and the paramagnetic regime above where effects of short-range antiferromagnetic correlations can be expected. The salient results of our study include (i) the observation of strongly anisotropic lattice distortions accompanying the formation of the valence-bond-solid, and (ii) a distinct maximum in the thermal expansion coefficients in the paramagnetic regime around 40\,K. Our results demonstrate that upon cooling through $T_{VBS}$ the in-plane c-axis, along which the valence bonds form, contracts while the second in-plane a-axis elongates by the same relative amount. Surprisingly, the dominant effect is observed for the out-of-plane b-axis which shrinks significantly upon cooling through $T_{VBS}$. The pronounced anomaly in $\alpha_i$ around 40\,K is attributed to short-range magnetic correlations. It is argued that the position of this maximum, relative to that in the magnetic susceptibility around 70\,K, speaks in favor of a more anisotropic triangular-lattice scenario for this compound than previously thought.
    Physical Review B 09/2013; 89(4). DOI:10.1103/PhysRevB.89.045113 · 3.66 Impact Factor
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    ABSTRACT: We present a detailed low-temperature investigation of the statics and dynamics of the anions and methyl groups in the organic conductors (TMTSF)2PF6 and (TMTSF)2AsF6 (TMTSF: tetramethyl-tetraselenafulvalene). The 4 K neutron-scattering structure refinement of the fully deuterated (TMTSF)2PF6-D12 salt allows locating precisely the methyl groups at 4 K. This structure is compared to the one of the fully hydrogenated (TMTSF)2PF6-H12 salt previously determined at the same temperature. Surprisingly, it is found that deuteration corresponds to the application of a negative pressure of 5×102 MPa to the H12 salt. Accurate measurements of the Bragg intensity show anomalous thermal variations at low temperature both in the deuterated PF6 and AsF6 salts. Two different thermal behaviors have been distinguished. Small Bragg-angle measurements reflect the presence of low-frequency modes at characteristic energies θE = 8.3 K and θE = 6.7 K for the PF6-D12 and AsF6-D12 salts, respectively. These modes correspond to the low-temperature methyl group motion. Large Bragg-angle measurements evidence an unexpected structural change around 55 K, which probably corresponds to the linkage of the anions to the methyl groups via the formation of F…D-CD2 bonds observed in the 4 K structural refinement. Finally we show that the thermal expansion coefficient of (TMTSF)2PF6 is dominated by the librational motion of the PF6 units. We quantitatively analyze the low-temperature variation of the lattice expansion via the contribution of Einstein oscillators, which allows us to determine for the first time the characteristic frequency of the PF6 librations: θE ≈ 50 K and θE = 76 K for the PF6-D12 and PF6-H12 salts, respectively.
    Physical review. B, Condensed matter 07/2013; 88(2). DOI:10.1103/PhysRevB.88.024105 · 3.66 Impact Factor
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    ABSTRACT: Materials close to a quantum-critical point – a zero-temperature phase transition – exhibit anomalous thermodynamic properties also at finite temperatures. Close to a magnetic field-induced quantum-critical point, for example, the finite-temperature entropy S_T shows strong variations upon varying the magnetic field. Here we discuss the possibility to use this accumulation of entropy around a field-induced quantum-critical point for realizing an efficient magnetic cooling. Our proof-of-principle demonstration is based on measurements and theoretical calculations of the magnetocaloric properties of low-dimensional spin-1/2 antiferromagnets close to their field-induced quantum-critical points. We present results of the magnetocaloric effect Γ_B = T−1(∂T/∂B)_S ≈ const as a function of both field and temperature in the vicinity of the quantum-critical point and discuss various performance characteristics, such as range of operation, efficiency and hold time. These figures are compared with those of a state-of-the-art paramagnetic coolant.
    physica status solidi (b) 03/2013; 250(3):457-463. DOI:10.1002/pssb.201200794 · 1.61 Impact Factor
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    ABSTRACT: We report on measurements of the magnetic susceptibility and electrical resistance under He-gas pressure on single crystals of Ca(Fe1−xCox)2As2. We find that for properly heat-treated crystals with modest Co concentration, x = 0.028, the salient ground states associated with iron-arsenide superconductors, i.e., orthorhombic/antiferromagnetic (o/afm), superconducting, and nonmagnetic collapsed-tetragonal (cT) states can be accessed all in one sample with reasonably small and truly hydrostatic pressure. This is possible owing to the extreme sensitivity of the o/afm (for T≤ Ts,N) and superconducting (T≤Tc) states against variation of pressure, disclosing pressure coefficients of dTs,N/dP=−(1100±50) K/GPa and dTc/dP=−(60±3) K/GPa, respectively. Systematic investigations of the various phase transitions and ground states via pressure tuning revealed no coexistence of bulk superconductivity (sc) with the o/afm state which we link to the strongly first-order character of the corresponding structural/magnetic transition in this compound. Our results, together with literature results, indicate that preserving fluctuations associated with the o/afm transition to low enough temperatures is vital for sc to form.
    Physical review. B, Condensed matter 12/2012; 86(22). DOI:10.1103/PhysRevB.86.220511 · 3.66 Impact Factor

Publication Stats

3k Citations
517.69 Total Impact Points

Institutions

  • 2002–2014
    • Goethe-Universität Frankfurt am Main
      • • Institute of Biophysics
      • • Institut für Theoretische Physik (ITP)
      • • Institut für Anorganische und Analytische Chemie
      Frankfurt, Hesse, Germany
  • 1997–2012
    • University Hospital Frankfurt
      Frankfurt, Hesse, Germany
  • 1987–2007
    • Darmstadt University of Applied Sciences
      Darmstadt, Hesse, Germany
  • 2006
    • Università degli Studi di Siena
      Siena, Tuscany, Italy
  • 1998–2003
    • Max Planck Institute for Chemical Physics of Solids
      Dresden, Saxony, Germany
  • 2001
    • Universität Stuttgart
      Stuttgart, Baden-Württemberg, Germany
  • 1998–2000
    • Niigata University
      • Department of Material Science and Technology
      Niahi-niigata, Niigata, Japan
  • 1999
    • Japan Advanced Institute of Science and Technology
      KMQ, Ishikawa, Japan
  • 1992–1994
    • Tohoku University
      • Institute for Materials Research
      Miyagi, Japan
  • 1991
    • Nippon Telegraph and Telephone
      Edo, Tōkyō, Japan
  • 1988–1990
    • Technical University Darmstadt
      • Institute of Solid States Physics
      Darmstadt, Hesse, Germany