Selected Publications since 2003
2011 2010 2009 2008 2007 2006 2005 2004 2003
2011
 | Evidence for a New Two-Dimensional C 4 H-Type Polymer Based on Hydrogenated Graphene
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 | Direct imaging of Joule heating dynamics and temperature profiling inside a carbon nanotube interconnect
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 | Graphene: Piecing it TogetherGraphene has a multitude of striking properties that make it an exceedingly attractive material for various applications, many of which will emerge over the next decade. However, one of the most promising applications lie in exploiting its peculiar electronic properties which are governed by its electrons obeying a linear dispersion relation. This leads to the observation of half integer quantum hall effect and the absence of localization. The latter is attractive for graphenebased fi eld effect transistors. However, if graphene is to be the material for future electronics, then signifi cant hurdles need to be surmounted, namely, it needs to be mass produced in an economically viable manner and be of high crystalline quality with no or virtually no defects or grains boundaries. Moreover, it will need to be processable with atomic precision. Hence, the future of graphene as a material for electronic based devices will depend heavily on our ability to piece graphene together as a single crystal and defi ne its edges with atomic precision. In this progress report, the properties of graphene that make it so attractive as a material for electronics is introduced to the reader. The focus then centers on current synthesis strategies for graphene and their weaknesses in terms of electronics applications are highlighted. |
 | CCVD Synthesis of Carbon-Encapsulated Cobalt Nanoparticles for Biomedical ApplicationsCarbon-encapsulated ferromagnetic Cobalt nanoparticles (Co@C) have been synthesized by catalytic chemical vapour deposition (CCVD). The nanoparticles, mainly ranging between 10 and 15 nm, are tightly encapsulated by 2–3 concentric graphitic carbon shells and protected from oxidation. Because of their magnetic properties (saturation magnetization of 106 emu/g and a coercivity HC of 250 Oe), Co@C nanoparticles have been investigated for hyperthermia application. Although the observed values of the specifi c absorption rate (28.7 W/gCo@C at 30 kA/m and 215.4 W/gCo@C at 70 kA/m) are slightly lower than required in actual hyperthermia therapies, the observed strong heating effect provides a very promising starting point for future clinical application. It is also demonstrated that these nanoparticles can at the same time be used for magnetic resonance imaging (MRI) with an effi ciency comparable to commercially available T2 contrast agents. |
 | Saturation Field of Frustrated Chain Cuprates: Broad Regions of Predominant Interchain CouplingA thermodynamic method to extract the interchain coupling (IC) of spatially anisotropic 2D or 3D spin-1/2 systems from their empirical saturation field Hs (T=0) is proposed. Using modern theoretical methods we study how Hs is affected by an antiferromagnetic (AFM) IC between frustrated chains described in the J1-J2-spin model with ferromagnetic 1st and AFM 2nd neighbor in-chain exchange. A complex 3D-phase diagram has been found. For Li2CuO2 and Ca2Y2Cu5O10, we show that Hs is solely determined by the IC and predict Hs≈61 T for the latter. With Hs≈55 T from magnetization data one reads out a weak IC for Li2CuO2 close to that obtained from inelastic neutron scattering. |
 | Evidence for a New Two-Dimensional C4H-Type Polymer Based on Hydrogenated GrapheneA new polymer with C4H stoichiometry based on graphene is synthesized in situ using template-induced polymerization of self-organizing hydrogen adsorbates on graphene. The polymerization is observed “live” on the surface of graphene by photoemission spectroscopy. Photoemission spectroscopy allows for an accurate determination of the carbon/ hydrogen stoichiometry, an aspect that is extremely important for understanding functionalized graphene. |
 | Observation of Electronic Ferroelectric Polarization in Multiferroic YMn2O5We report the observation of a magnetic polarization of the O 2p states in YMn2O5 through the use of soft x-ray resonant scattering at the oxygen K edge. Remarkably, we find that the temperature dependence of the integrated intensity of this signal closely follows the macroscopic electric polarization, and hence is proportional to the ferroelectric order parameter. This is in contrast with the temperature dependence observed at the Mn L3 edge, which reflects the Mn magnetic order parameter. First-principles calculations provide a microscopic understanding of these results and show that a spin-dependent hybridization of O 2p and Mn 3d states results in a purely electronic contribution to the ferroelectric polarization, which can exist in the absence of lattice distortions. |
 | Spin Gap in the Zigzag Spin-1/2 Chain Cuprate Sr0.9Ca0.1CuO2We report a comparative study of 63Cu nuclear magnetic resonance spin lattice relaxation rates T1-1 on undoped SrCuO2 and Ca-doped Sr0.9Ca0.1CuO2 spin chain compounds. A temperature independent T1-1 is observed for SrCuO2 as expected for an S=1/2 Heisenberg chain. Surprisingly, we observe an exponential decrease of T1-1 for T<90 K in the Ca-doped sample evidencing the opening of a spin gap. The data analysis within the J1-J2 Heisenberg model employing density-matrix renormalization group calculations suggests an impurity driven small alternation of the J2-exchange coupling as a possible cause of the spin gap. |
 | A New Family of 1D Exchange Biased Heterometal Single Molecule Magnets: Observation of Pronounced Quantum Tunneling Steps in the Hysteresis Loops of Quasi-Linear {Mn2Ni3} ClustersFirst members of a new family of heterometallic Mn/Ni complexes [Mn2Ni3X2L4(LH)2(H2O)2] (X = Cl: 1; X = Br: 2) have been synthesized and characterized. The molecular structures feature a quasi-linear MnIII-NiII-NiII-NiII-MnIII core with six-coordinate metal ions, where elongated axes of all the distorted octahedral coordination polyhedra are aligned parallel and are fixed with respect to each other by intramolecular hydrogen bonds. 1 and 2 exhibit quite strong ferromagnetic exchange interactions throughout (JMn-Ni ≈ 44 K (1) or 42 K (2); JNi-Ni ≈ 19 K (1) or 18 K (2)) that lead to an Stot = 7 ground state. Frequency-tunable high-field ESR, measured on an oriented powder sample of 1 revealed a substantial magnetic anisotropy gap with a negative axial anisotropy D = – 0.55 K which gives evidence for a bistable (easy axis) magnetic ground state of the molecule. The analysis of the ESR data by means of a minimal effective spin Hamiltonian enables an accurate description of the energy level scheme and the spin states of the complex. The observed high spin ground state and the negative axial anisotropy ensure necessary prerequisites for the realization of a single molecular magnet. Indeed, slow relaxation of the magnetization at low temperatures for both 1 and 2 is evident from frequency-dependent peaks in the out-of-phase AC susceptibility and hysteresis loops in magnetization versus DC field measurements. Moreover, pronounced quantum tunnelling steps are observed in the hysteresis loops which are also very characteristic for single molecular magnets. |
 | Manifestation of New Interference Effects in a Superconductor-Ferromagnet Spin ValveSuperconductor/ferromagnet (S/F) spin valve effect theories based on the S/F proximity phenomenon assume that the superconducting transition temperature Tc of F1/F2/S or F1/S/F2 trilayers for parallel magnetizations of the F1- and F2-layers Tc{P} are smaller than for the antiparallel orientations Tc{AP}. We report for CoOx/Fe1/Cu/Fe2/In multilayeres with varying Fe2-layer thickness the sign-changing oscillating behavior of the spin valve effect DeltaTc=Tc{AP}-Tc{P}. We observe the full direct effect with Tc{AP}>Tc{P} for Fe2-layer thickness d{Fe2}<1 nm and the full inverse (Tc{AP}= 1 nm. Interference of Cooper pair wave functions reflected from both surfaces of the Fe2-layer appear as the most probable reason for the observed behavior of DeltaTc. |
2010
 | Sexithiophene encapsulated in single walled carbon nanotube: An in situ Raman spectroelectrochemical study of a peapodThe interaction of singlewalled carbon nanotubes (SWCNTs) and a-sexithiophene (6T) was studied by Raman spectroscopy and by in situ Raman spectroelectrochemistry. The encapsulation of 6T in SWCNT and its interaction causes a bleaching of its photoluminescence, and also small shifts of its Raman bands. The Raman features of the SWCNT with embedded 6T (6T-peapods) change in both intensity and frequency compared to those of pristine SWCNT, which is a consequence of a change of the resonant condition. Electrochemical doping demonstrated that the electrode potential applied to the SWCNT wall causes changes in the embedded 6T. The effects of electrochemical charging on the Raman features of pristine SWCNT and 6T@SWCNT were compared. It is shown that the interaction of SWCNT with 6T also changes the electronic structure of SWCNT in its charged state. This change of electronic structure is demonstrated both for semiACHTUNGTRENUNGconducting and metallic tubes. M. Kalbáč, L. Kavan, S. Gorantla, T. Gemming, L. Dunsch, Chem. Eur. J. 16:38 (2010) 11753–11759 URL |
 | Latent Porosity in Potassium Dodecafluoro-closo-dodecaborate(2−). Structures and Rapid Room Temperature Interconversions of Crystalline K2B12F12, K2(H2O)2B12F12, and K2(H2O)4B12F12 in the Presence of Water VaporStructures of K(2)(H(2)O)(2)B(12)F(12) and K(2)(H(2)O)(4)B(12)F(12) were determined by X-ray diffraction. They contain [K(μ-H(2)O)(2)K](2+) and [(H(2)O)K(μ-H(2)O)(2)K(H(2)O)](2+) dimers, respectively, which interact with superweak B(12)F(12)(2-) anions via multiple K···F(B) interactions and (O)H···F(B) hydrogen bonds (the dimers in K(2)(H(2)O)(4)B(12)F(12) are also linked by (O)H···O hydrogen bonds). DFT calculations show that both dimers are thermodynamically stabilized by the lattice of anions: the predicted ΔE values for the gas-phase dimerization of two K(H(2)O)(+) or K(H(2)O)(2)(+) cations into [K(μ-H(2)O)(2)K](2+) or [(H(2)O)K(μ-H(2)O)(2)K(H(2)O)](2+) are +232 and +205 kJ mol(-1), respectively. The calculations also predict that ΔE for the gas-phase reaction 2 K(+) + 2 H(2)O → [K(μ-H(2)O)(2)K](2+) is +81.0 kJ mol, whereas ΔH for the reversible reaction K(2)B(12)F(12 (s)) + 2 H(2)O((g)) → K(2)(H(2)O)(2)B(12)F(12 (s)) was found to be -111 kJ mol(-1) by differential scanning calorimetry. The K(2)(H(2)O)(0,2,4)B(12)F(12) system is unusual in how rapidly the three crystalline phases (the K(2)B(12)F(12) structure was reported recently) are interconverted, two of them reversibly. Isothermal gravimetric and DSC measurements showed that the reaction K(2)B(12)F(12 (s)) + 2 H(2)O((g)) → K(2)(H(2)O)(2)B(12)F(12 (s)) was complete in as little as 4 min at 25 °C when the sample was exposed to a stream of He or N(2) containing 21 Torr H(2)O((g)). The endothermic reverse reaction required as little as 18 min when K(2)(H(2)O)(2)B(12)F(12) at 25 °C was exposed to a stream of dry He. The products of hydration and dehydration were shown to be crystalline K(2)(H(2)O)(2)B(12)F(12) and K(2)B(12)F(12), respectively, by PXRD, and therefore these reactions are reconstructive solid-state reactions (there is also evidence that they may be single-crystal-to-single-crystal transformations when carried out very slowly). The hydration and dehydration reaction times were both particle-size dependent and carrier-gas flow rate dependent and continued to decrease up to the maximum carrier-gas flow rate of the TGA instrument that was used, demonstrating that the hydration and dehydration reactions were limited by the rate at which H(2)O((g)) was delivered to or swept away from the microcrystal surfaces. Therefore, the rates of absorption and desorption of H(2)O from unit cells at the surface of the microcrystals, and the rate of diffusion of H(2)O across the moving K(2)(H(2)O)(2)B(12)F(12 (s))/K(2)B(12)F(12 (s)) phase boundary, are even faster than the fastest rates of change in sample mass due to hydration and dehydration that were measured. The exchange of 21 Torr H(2)O((g)) with either D(2)O or H(2)(18)O in microcrystalline K(2)(D(2)O)(2)B(12)F(12) or K(2)(H(2)(18)O)(2)B(12)F(12) at 25 °C was also facile and required as little as 45 min to go to completion (H(2)O((g)) replaced both types of isotopically labeled water at the same rate for a given starting sample of K(2)B(12)F(12), demonstrating that water molecules were exchanging, not protons. Significant portions of mass (m) vs time (t) plots for the (1,2)H(2)O((g))/K(2)((2,1)H(2)O)(2)B(12)F(12 (s)) exchange reactions fit the equation m ∝ e(-kt), with 10(3)k = 1.9 s(-1) for one particle size distribution and 10(3)k = 0.50 s(-1) for another. Finally, K(2)(H(2)O)(2)B(12)F(12) was not transformed into K(2)(H(2)O)(4)B(12)F(12) after prolonged exposure to 21 Torr H(2)O((g)) at 25 °C, 37 Torr H(2)O((g)) at 35 °C, or 55 Torr H(2)O((g)) at 45 °C. D. Peryshkov, A. A. Popov, S. H. Strauss, Water Vapor, J. Am .Chem. Soc. 2010, 132 (39), 13902-13913. DOI: 10.1021/ja105522d URL
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 | A Facile Route to Metal Nitride Clusterfullerenes Using Guanidinium Salts: A Selected Organic Solid as the Nitrogen SourceUsing guanidinium salts 1 and 2 as the new nitrogen sources, metal nitride clusterfullerenes (NCFs) based on a variety of metals (Dy, Sc, Y, Gd, Lu, and mixed metals Sc/Dy, Sc/Gd, Sc/Lu, and Lu/Ce) have been synthesized based on a new “selective organic solid” (SOS) route. The synthesis of Dy-NCFs by using Dy/1 was studied in detail, and the optimum molar ratio of 1/Dy/C has been determined to be 2.5:1:10. For several representative metals such as Sc, Y, Gd, Dy, and Sc/Dy, we quantitatively compared the yield of M3N@C80 synthesized by the SOS route with the reported “reactive gas atmosphere” route, thereby indicating that the yield of M3N@C80 by using 1 could be comparable to that obtained by the reactive gas atmosphere route. Three other nitrogen sources (3–5) were also studied for comparison, which were mixed with Dy metal but did not result in the formation of Dy-NCF. A possible reaction scheme for the solid-state reaction of 1, metal, and graphite is proposed. The SOS route appears to be a general route for the synthesis of NCFs that promises both high selectivity of NCFs and high reproducibility of the fullerene yield. Another advantages of the SOS route compared to the reported “trimetallic nitride template” (TNT) process and the reactive gas atmosphere route is that no additional heating pretreatment is needed, thus simplifying the procedure and being much more facile. Shangfeng Yang, Lin Zhang, Wenfeng Zhang, Lothar Dunsch, Chem. Eur. J. 16 (2010) 16:41 (2010) 12398-12405 URL |
 | High-Performance Field Effect Transistors from Solution Processed Carbon NanotubesNanoelectronic field effect transistors (FETs) are produced using solution processed individual carbon nanotubes (CNTs), synthesized by both arc discharge and laser ablation methods. We show that the performance of solution processed FETs approaches that of CVD-grown FETs if the nanotubes have minimal lattice defects and are free from surface contamination. This is achieved by treating the nanotubes to a high-temperature vacuum annealing process and using 1,2-dichloroethane for dispersion. We present CNT FETs with mobilities of up to 3546 cm2/(V s), transconductance of 4.22 µS, on-state conductance of 9.35 µS and on/off ratios as high as 106. High-resolution transmission electron microscopy is used to examine the presence of catalyst particles and amorphous carbon on the surface and Raman spectroscopy is used to examine the lattice defects, both of which lead to reduced device performance. H. Wang, J. Luo, A. Robertson, Y. Ito, W. Yan, V. Lang, M. Zaka, F. Schaeffel, M. H. Ruemmeli, G. A. D. Briggs, J. H. Warner, ACS nano vol. 4, no. 11, 6659–6664 (2010) URL |
 | Full spin switch effect for the superconducting current in a superconductor/ferromagnet thin film heterostructureUsing the spin switch design F1/F2/S theoretically proposed by Oh et al., (Appl. Phys. Lett. 71, 2376 (1997)), that comprises a ferromagnetic bilayer as a ferromagnetic component, and an ordinary superconductor as the second interface component, we have realized a full spin switch effect for the superconducting current. An experimental realization of this spin switch construction was achieved for the CoOx /Fe1/Cu/Fe2/ In multilayer. P. V. Leksin, N. N. Garif’yanov, I. A. Garifullin, J. Schumann, H. Vinzelberg, V. Kataev, R. Klingeler, O. G. Schmidt, B. Büchner, Appl. Phys. Lett. 97, 102505 (2010); doi:10.1063/1.3486687 URL |
 | Tunable Band Gap in Hydrogenated Quasi-Free-Standing GrapheneWe show by angle-resolved photoemission spectroscopy that a tunable gap in quasi-free-standing monolayer graphene on Au can be induced by hydrogenation. The size of the gap can be controlled via hydrogen loading and reaches 1.0 eV for a hydrogen coverage of 8%. The local rehybridization from sp2 to sp3 in the chemical bonding is observed by X-ray photoelectron spectroscopy and X-ray absorption and allows for a determination of the amount of chemisorbed hydrogen. The hydrogen induced gap formation is completely reversible by annealing without damaging the graphene. Calculations of the hydrogen loading dependent core level binding energies and the spectral function of graphene are in excellent agreement with photoemission experiments. Hydrogenation of graphene gives access to tunable electronic and optical properties and thereby provides a model system to study hydrogen storage in carbon materials. D. Haberer, D. V. Vyalikh, S. Taioli, B. Dora, M. Farjam, J. Fink, D. Marchenko, T. Pichler, K. Ziegler, S. Simonucci, M. S. Dresselhaus, M. Knupfer, B. Büchner and A. Grüneis, Nano Lett., DOI: 10.1021/nl101066m URL |
 | Unraveling the electron spin resonance pattern of nonsymmetric radicals with 30 fluorine atoms: electron spin resonance and vis-near-infrared spectroelectrochemistry of the anion radicals and dianions of C60(CF3)(2n) (2n = 2-10) derivatives and density functional theory-assisted assignment.The charged states of C(60)(CF(3))(2n) (2n = 2-10) derivatives have been studied by electron spin resonance (ESR) and vis-near-infrared (NIR) spectroelectrochemistry. The anion radicals and diamagnetic dianions were furthermore described by theoretical calculations. The ESR spectra of anion radicals exhibit complex patterns due to multiple CF(3) groups. Their interpretation is accomplished by DFT calculations with B3LYP functional. It is shown that calculations provide reliable results when the extended aug-cc-pCVTZ basis set is used for fluorine atoms; however, specially tailored basis sets such as EPR-III also give very similar results with only a fraction of the computational cost. Absorption spectra of the anions exhibit NIR absorption bands, whose assignment is provided by time-dependent DFT calculations. A.A. Popov, I.E. Kareev, N.B. Shustova, S.H. Strauss, O.V. Boltalina, L. Dunsch, Journal of the American Chemical Society (2010) Volume: 132 Issue: 33 Pages: 11709-11721 URL |
 | Al3Li4(BH4)(13): A Complex Double-Cation Borohydride with a New StructureThe new double-cation Al-Li-borohydride is an attractive candidate material for hydrogen storage due to a very low hydrogen desorption temperature (approximately 70 degrees C) combined with a high hydrogen density (17.2 wt%). It was synthesised by high-energy ball milling of AlCl(3) and LiBH(4). The structure of the compound was determined from image-plate synchrotron powder diffraction supported by DFT calculations. The material shows a unique 3D framework structure within the borohydrides (space group=P-43n, a=11.3640(3) A). The unexpected composition Al(3)Li(4)(BH(4))(13) can be rationalized on the basis of a complex cation [(BH(4))Li(4)](3+) and a complex anion [Al(BH(4))(4)](-). The refinements from synchrotron powder diffraction of different samples revealed the presence of limited amounts of chloride ions replacing the borohydride on one site. In situ Raman spectroscopy, differential scanning calorimetry (DSC), thermogravimetry (TG) and thermal desorption measurements were used to study the decomposition pathway of the compound. Al-Li-borohydride decomposes at approximately 70 degrees C, forming LiBH(4). The high mass loss of about 20 % during the decomposition indicates the release of not only hydrogen but also diborane. I. Lindemann, R. Domènech Ferrer, L. Dunsch, Y. Filinchuk, R. Cerný, H. Hagemann, V. D'Anna, L.M. Lawson;Daku, L. Schultz, O. Gutfleisch, Chemistry-A European Journal (2010) Volume: 16 Issue: 29 Pages: 8707-8712 URL |
 | Spin-Flow Vibrational Spectroscopy of Molecules with Flexible Spin Density: Electrochemistry, ESR, Cluster and Spin Dynamics, and Bonding in TiSc2N@C80The recently isolated TiSc2N@C80 was used to study the spin state of a Ti3+ ion in a mixed metal nitride cluster in a fullerene cage. The electronic state of the new clusterfullerene is characterized starting with the redox behavior of this structure. It differs markedly from that of homometallic nitride clusterfullerenes in giving reversible one-electron transfers even on the cathodic scale. Both oxidation and reduction of TiSc2N@C80 occur at the endohedral cluster changing the valence state of Ti from Ti(II) in anion to Ti(IV) in cation. The unpaired electron in TiSc2N@C80 is largely fixed at the Ti ion as shown by low temperature ESR measurements. Isotropic g-factor 1.9454 points to the significant spin-orbit coupling with an unquenched orbital momentum of the 3d electron localized on Ti. Measurements with the frozen solution also point to the strong anisotropy of the g-tensor. DFT computations show that the cluster can adopt several nearly isoenergetic configurations. DFT-based Born-Oppenheimer molecular dynamics (BOMD) simulations reveal that, unlike in Sc3N@C80, the cluster dynamics in TiSc2N@C80 cannot be described as a 3D rotation. The cluster rotates around the Ti-N axis, while the Ti atom oscillates in one position around the pentagon/hexagon edge. Evolution of the spin populations along the BOMD trajectory has shown that the spin distribution in the cluster is very flexible, and both an intracluster and clustercage spin flows take place. Fourier transformation of the time dependencies of the spin populations results in the spin-flow vibrational spectra, which reveal the major spin-flow channels. It is shown that the cluster-cage spin flow is selectively coupled to one vibrational mode, thus, pointing to the utility of the clusterfullerene for the molecular spin transport. Spin-flow vibrational pectroscopy is thus shown to be a useful method for characterization of the spin dynamics in radicals with flexible spin density distribution. A. A. Popov, Ch. Chen, S. Yang, F. Lipps, L. Dunsch, ACS Nano (2010) Vol. 4, No 8, 4857–4871 URL |
 | Influence of the Cage Size on the Dynamic Behavior of Fullerenes: A Study of 13C NMR Spin-Lattice RelaxationA detailed study on the relaxation mechanisms of higher cage fullerene sizes is done as a prerequisite for studies of the influence of the endohedral structures on fullerene cage carbon relaxation. Recent studies of the dynamic behavior of C60 and C70 in aromatic solvents and CS2 solution show the influence of the shape and the symmetry of the cage to be highly important as well as the influence of the solvent to be negligible. As higher fullerene cages have more than one stable isomer, the isolation of isomeric pure structures is of high importance for a detailed study of the dynamic behavior of such fullerenes. Here we investigated the three higher fullerene cage isomers D2-C76, C2v(3)-C78, and D2-C80 with respect to the relaxation rate of the carbons measured in their temperature dependence. Thus, we study the influence on the relaxation of the carbons and the dynamic behavior of these fullerenes in solution. Besides the diffusion dependence on the shape of the carbon cage, the relaxation behavior at lower temperatures is found to be dependent on the difference in chemical shift anisotropy within the carbon cage. This difference is originated from the changes of symmetry and results in polarization of electron density. Furthermore, the mobility of the carbons is influenced by their pyramidalization. S. Klod, L. Dunsch, ACS Nano vol 4, no. 6 URL |
 | Redox-Tuning Endohedral Fullerene Spin States: From the Dication to the Trianion Radical of Sc3N@C80(CF3)2 in Five Reversible Single-Electron StepsThe first endohedral trianion captured: Sc3N@C80(CF3)2 (see figure) exhibits three reversible reductions and two reversible oxidations and affords the facile generation of the monocation, monoanion, and trianion in solution, which can be characterized by ESR and absorption spectroscopies. This is the first time that such a broad range of charged states of any endohedral fullerene has been spectroscopically characterized. A. A. Popov, N. B. Shustova, A. L. Svitova, M. A. Mackey, C. E. Coumbe, J. P. Phillips, S. Stevenson, S. H. Strauss, O. V. Boltalina, L. Dunsch Chemistry – A European Journal (2010) Vol 16, Issue 16, pages 4721–4724 URL |
 | Saturnene Revealed: X-ray Crystal Structure of D5d-C60F20 Formed in Reactions of C60 with AxMFy Fluorinating Agents (A=Alkali Metal; M=3d Metal)Saturnene has four moons: Reactions of C60 with ternary metal fluorides yielded fluorofullerenes from C60F2 to C60F48, including elusive saturnene, C60F20, which has now been characterized by X-ray crystallography. Four benzene molecules “hover” over this D5d molecule at the corners of a square inscribed in the idealized body-centered-cubic unit cell (see structure; F yellow). The tight unit-cell packing explains the very low solubility of saturnene. N. B. Shustova, Z. Mazej, Y.-S. Chen, A. A. Popov, S. H. Strauss, O. V. Boltalina, Angew. Chem. Int. Ed. 2010, 49, 812 –815 URL |
 | Direct Low-Temperature Nanographene CVD Synthesis over a Dielectric InsulatorGraphene ranks highly as a possible material for future high-speed and flexible electronics. Current fabrication routes, which rely on metal substrates, require post-synthesis transfer of the graphene onto a Si wafer, or in the case of epitaxial growth on SiC, temperatures above 1000 °C are required. Both the handling difficulty and high temperatures are not best suited to present day silicon technology. We report a facile chemical vapor deposition approach in which nanographene and few-layer nanographene are directly formed over magnesium oxide and can be achieved at temperatures as low as 325 °C. M. H. Rümmeli, A. Bachmatiuk, A. Scott, F. Börrnert, J. H. Warner, V. Hoffman, J.-H. Lin, G. Cuniberti, B. Büchner, ACS Nano, 2010, 4 (7), pp 4206–4210 URL |
 | An endohedral redox system in a fullerene cage: the Ce based mixed-metal cluster fullerene Lu2CeN@C80Redox reactions of endohedral fullerenes, and especially their oxidation, usually result in a change of the redox state of the carbon cage. Here we demonstrate that an oxidation of the endohedral species is possible bypassing the fullerene cage in an unprecedented reversible cascade electron transfer under anodic conditions. The first Ce-based non-scandium mixed-metal nitride clusterfullerene (NCF) Lu2CeN@C80(Ih) was synthesized and isolated. The electronic and vibrational properties of Lu2CeN@C80 are characterized by UV-vis-NIR and FTIR spectroscopies and the cage structure of Lu2CeN@C80 is determined by 13C NMR spectroscopy to be C80(/h). At room temperature the NMR peak positions are shifted from the normal values of the diamagnetic M3N@C80 NCFs because of the unpaired f-electron localized on the Ce atom in the Ce3+ state. The variable-temperature NMR study enabled the estimation of the diamagnetic terms in the 13C chemical shifts, which were found to be close to those of diamagnetic M3N@C80 NCFs. The electrochemical properties of Lu2CeN@C80 were investigated by cyclic and square wave voltammetry, revealing two electrochemically irreversible but chemically reversible reduction steps and one reversible oxidation step. As the oxidation potential is significantly less positive than that in all other M3N@C80 NCFs, we conclude that an oxidation of the endohedral Ce occurs with the formation of Lu2CeIVN@C80+, the first endohedral metallofullerene species with the tetra-valent cerium atom. This hypothesis is also supported by DFT calculations. Lin Zhang, Alexey A. Popov, Shangfeng Yang, Sabrina Klod, Peter Rapta and Lothar Dunsch, Phys. Chem. Chem. Phys., 2010, 12, 7840 - 7847, DOI: 10.1039/c002918a URL |
 | Metal Sulfide in a C82 Fullerene Cage: A New Form of Endohedral ClusterfullerenesThe row of endohedral fullerenes is extended by a new type of sulfur-containing clusterfullerenes: the metal sulfide (M2S) has been stabilized within a fullerene cage for the first time. The new sulfur-containing clusterfullerenes M2S@C82-C3v(8) have been isolated for a variety of metals (M = Sc, Y, Dy, and Lu). The UV−vis−NIR, electrochemical, and FTIR spectroscopic characterization and extended DFT calculations point to a close similarity of the M2S@C82 cage isomeric and electronic structure to that of the carbide clusterfullerenes M2C2@C2n. The bonding in M2S@C82 is studied in detail by molecular orbital analysis as well as with the use of quantum theory of atom-in-molecules (QTAIM) and electron localization function (ELF) approaches. The metal sulfide cluster formally transfers four electrons to the carbon cage, and metal−sulfur and metal−carbon cage bonds with a high degree of covalency are formed. Molecular dynamics simulations show that Sc2S cluster exhibits an almost free rotation around the C3 axis of the carbon cage, resulting thus in a single line 45Sc NMR spectrum. L. Dunsch, S. Yang, L. Zhang, A. Svitova, S. Oswald, A. A. Popov, J. Am. Chem. Soc., 2010, 132 (15), pp 5413–5421, URL |
 | Direct Arc-Discharge Assisted Synthesis of C60H2(C3H5N): A cis-1-Pyrrolino C60 Fullerene Hydride with Unusual Redox PropertiesBy direct arc synthesis of C60H2(C3H5N) using a modified Krtschmer−Huffman method, it is demonstrated for the first time that exohedral fullerenes with large side groups can be formed under the arc and reactive gas atmosphere conditions. The thus formed novel pyrrolino fullerene hydride was comprehensively characterized by UV−vis, infrared (IR), Raman and nuclear magnetic resonance (NMR) and studied by means of electrochemistry and in situ electron spin resonance/visible-near infrared (ESR/Vis-NIR) spectroelectrochemistry. The detailed NMR and absorption spectroscopic studies show C60H2(C3H5N) as the fullerene hydride with a pyrrolino ring attached on the vicinal position of two hydrogen atoms. This first cycloaddition adduct of C60H2 gives rise to an unusual reversible dimerization of its anion radicals and a loss of cage hydrogen atoms at the third electron transfer. The spectroelectrochemical study confirms the formation of a [C60H2(C3H5N)]− radical and the diamagnetic state of [C60H2(C3H5N)] 2−. This study reveals a unique function of a carbide structure like Al4C3 and the presence of a reactive gas atmosphere with NH3 in the formation of such an exohedral pyrrolino structure C60H2(C3H5N). N. Chen, S. Klod, P. Rapta, A. A. Popov, L. Dunsch, Chem. Mater., 2010, 22 (8), pp 2608–2615, URL |
 | Surface acoustic wave mediated dielectrophoretic alignment of rolled-up microtubes in microfluidic systemsThe alignment behavior of solution dispersed rolled-up microtubes by surface acoustic waves (SAW) is demonstrated. In contrast to the random alignment of rolled-up insulated silicon oxide tubes, metallic chromium tubes can be effectively aligned and assembled into “tube-chains” parallel to the SAW propagation direction. The experiments suggest that the tube orientation is mainly determined by the dielectrophoresis (DEP) forces acting on the tubes. The DEP forces arise from the induced dipole moment of the tubes in the SAW generated piezoelectric field on the LiNbO3 substrate. X. H. Kong, Ch. Deneke, H. Schmidt, D. J. Thurmer, H. X. Ji, M. Bauer, O. G. Schmid, Applied Physics Letters 96, 134105 (2010) URL |
 | Confinement of Fractional Quantum Number Particles in a Condensed Matter SystemThe idea of confinement states that in certain systems constituent particles can be discerned only indirectly being bound by an interaction whose strength increases with increasing particle separation. Though the most famous example is the confinement of quarks to form baryons and mesons in (3+1)-dimensional Quantum Chromodynamics, confinement can also be realized in the systems of condensed matter physics such as spin-ladders which consist of two spin-1/2 antiferromagnetic chains coupled together by spin exchange interactions. Excitations of individual chains (spinons) carrying spin S=1/2, are confined even by an infinitesimal interchain coupling. The realizations studied so far cannot illustrate this process due to the large strength of their interchain coupling which leaves no energy window for the spinon excitations of individual chains. Here we present neutron scattering experiments for a weakly-coupled ladder material. At high energies the spectral function approaches that of individual chains; at low energies it is dominated by spin 0,1 excitations of strongly-coupled chains. B. Lake, A. M. Tsvelik, S. Notbohm, D. A. Tennant, T. G. Perring,.M. Reehuis, Ch. Sekar, G. Krabbes, B. Büchner Nature Physics, Nature Physics 6, 50 - 55 (2010) URL |
 | Nanoscale Electronic Order in Iron PnictidesThe charge distribution in RFeAsO1-xFx (R ¼ La;Sm) iron pnictides is probed using As nuclear quadrupole resonance. Whereas undoped and optimally doped or overdoped compounds feature a single charge environment, two charge environments are detected in the underdoped region. Spin-lattice relaxation measurements show their coexistence at the nanoscale. Together with the quantitative variations of the spectra with doping, they point to a local electronic order in the iron layers, where low- and highdoping- like regions would coexist. Implications for the interplay of static magnetism and superconductivity are discussed. G. Lang, H.-J. Grafe, D. Paar, F. Hammerath, K. Manthey, G. Behr, J. Werner, B. Büchner, Phys Rev Lett 104, 097001 (2010) URL |
 | Self-assembly of neutral hexanuclear circular copper(II) meso-helicates: topological control by sulfate ionsBis-pyridylimine ligands with different linking elements are capable of forming unique hexanuclear circular Cu(II) mesohelicates; the self-assembly is controlled by coordination of sulfate ions to the metal centres. H. B. Tanh Jeazet, Kerstin Gloe, Th. Doert, O. N. Kataeva, A. Jäger, G. Geipel, G. Bernhard, B. Büchner, Karsten Gloe, Chem. Commun. (2010) 46, 2373–237; URL |
 | Examining Co-based nanocrystals on graphene using low-voltage aberration-corrected transmission electron microscopyWe present a method to produce graphene and few layer graphene sheets using solution phase chemistry, which are used as ultrathin support membranes for enhanced imaging of nanomaterials using transmission electron microscopy. We demonstrate this by decorating the surface of the graphene sheets with Co-based nanocrystals (CoCl2 and hcp Co). Low-voltage aberration-corrected high resolution transmission electron microscopy at 80 kV is used to image the nanocrystals on the thin graphene supports. We show that electron beam irradiation causes the CoCl2 nanocrystals to become mobile on the graphene surface and exhibit both rotational and translational motion. We provide real-time in situ monitoring with atomic resolution of the coalescence of two CoCl2 nanocrystals on the graphene surface, driven by electron beam irradiation. The CoCl2 nanocrystals are then annealed in vacuum and transform into Co nanocrystals with hcp crystal structure. We show that these Co nanocrystals are catalytic and electron beam irradiation leads to the etching of the graphene surface, not observed for the CoCl2 nanocrystals. J. Warner, M. Ruemmeli, A. Bachmatiuk, M. Wilson, B. Buechner, ACS Nano, (2010) 4 (1) pp 470-47, URL |
 | Atomic Resolution Imaging and Topography of Boron Nitride Sheets Produced by Chemical ExfoliationHere, we present a simple method for preparing thin few-layer sheets of hexagonal BN with micrometer-sized dimensions using chemical exfoliation in the solvent 1,2-dichloroethane. The atomic structure of both few-layer and monolayer BN sheets is directly imaged using aberration-corrected high-resolution transmission electron microscopy. Electron beam induced sputtering effects are examined in real time. The removal of layers of BN by electron beam irradiation leads to the exposure of a step edge between a monolayer and bilayer region. We use HRTEM imaging combined with image simulations to show that BN bilayers can have AB stacking and are not limited to just AA stacking. Jamie H. Warner, Mark H. Rümmeli, Alicja Bachmatiuk, Bernd Büchner, ACS Nano 2010 4(3), pp 1299–1304 URL |
 | Graphene Synthesis on Cubic SiC/Si Wafers. Perspectives for Mass Production of Graphene-Based Electronic DevicesThe outstanding properties of graphene, a single graphite layer, render it a top candidate for substituting silicon in future electronic devices. The so far exploited synthesis approaches, however, require conditions typically achieved in specialized laboratories and result in graphene sheets whose electronic properties are often altered by interactions with substrate materials. The development of graphene-based technologies requires an economical fabrication method compatible with mass production. Here we demonstrate for the fist time the feasibility of graphene synthesis on commercially available cubic SiC/Si substrates of >300 mm in diameter, which result in graphene flakes electronically decoupled from the substrate. After optimization of the preparation procedure, the proposed synthesis method can represent a further big step toward graphene-based electronic technologies. V. Yu. Aristov, G. Urbanik, K. Kummer, D. V. Vyalikh, T. Hänke, B. Büchner, I. Vobornik, J. Fujii, G. Panaccione, Y. A. Ossipyan, M. Knupfer, Nano Letters 10, 992–995 (2010) URL |
 | Investigating the outskirts of Fe and Co catalyst particles in alumina-supported catalytic CVD carbon nanotube growthUsing thermal CVD, the synthesis of multi-walled carbon nanotubes exhibiting roots anchored directly onto alpha-alumina supports, rather than the catalyst particle, is reported. At such roots, the alignment of the graphitic planes with the support lattice fringes depends on the support crystal structure and orientation. Surface defects may alter the reactivity of the surface or control the anchoring of supported atoms or nanoparticles. We argue this surface defect is provided by the catalyst particle’s edge interaction with the support, in other words its circumference. The development of oxide-based catalysts is attractive in that they potentially provide an appropriate solution to directly integrate the synthesis of carbon nanotubes and graphene into silicon-based technology. M.H. Ruemmeli, F. Schaeffel, A. Bachmatiuk, D. Adebimpe, G. Trotter, F. Boerrnert, A. Scott, E. Coric, M. Sparing, B. Rellinghaus, P.G. McCormick, G. Cuniberti, M. Knupfer, L. Schultz, B. Buechner, ACS nano 4 (2010) Nr. 2, S. 1146-1152 URL |
 | Addition of Carbene to the Equator of C70 To Produce the Most Stable C71H2 Isomer: 2aH-2(12)a-Homo(C70-D5h(6))[5,6]fullereneUnlike C60, in which all carbon-atom environments are identical, C70 has five distinct carbon-atom environments, which give rise to eight distinct C-C bond types. Hence, the addition chemistry of C70 involves both chemo- and regioselectivity. The synthetic chemistry of C70 is centered on the areas near the poles,[1,2] as these areas have the highest curvature and hence high bond strain.[3, 4] This relatively high bond strain in turn makes the polar regions the most reactive sites of the molecule. The equatorial region of C70, on the other hand, has little curvature and hence lower bond strain. Thus, the carbon atoms at the equator are much less reactive, as there is a much higher activation barrier to be overcome before reactions can occur. For example, carbene (CH2) has been added to the polar region of C70, and several isomers of C71H2 have been synthesized and fully characterized.[5–8] However, the addition of carbene to the equatorial bond of C70 (to form C2v-C71H2) has not been detected. B. Li, C. Shu, X. Lu, L. Dunsch, Z. Chen, T.J.S. Dennis, Z. Shi, L. Jiang, T. Wang, W. Xu, C. Wang, Angew. Chem. Int. Ed. 2010, 49, 962 –966, URL |
 | A Pseudoatom in a Cage: Trimetallofullerene Y3@C80 Mimics Y3N@C80 with Nitrogen Substituted by a PseudoatomY3C80 obtained in the synthesis of nitride clusterfullerenes Y3N@C2n (2n=80-88) by the reactive atmosphere method is found to be a genuine trimetallofullerene, Y3@C80, with low ionization potential and divalent state of yttrium atoms. DFT studies of the electronic structure of Y3@C80 show that this molecule mimics Y3N@C80 with the pseudoatom (PA) instead of the nitrogen atom. Topology analysis of the electron density and electron localization function show that yttrium atoms form Y-PA bonds rather than direct Y-Y bonds. Molecular dynamics simulations show that the Y3PA cluster is as rigid as Y3N and rotates inside the fullerene cage as a single entity. A. Popov, L. Zhang, L. Dunsch, ACS Nano Vol. 4, No. 2 (2010) 795–802 URL |
2009
 | Unravelling the Mechanisms Behind Mixed Catalysts for the High Yield Production of Single-Walled Carbon NanotubesThe use of mixed catalysts for the high-yield production of single-walled carbon nanotubes is well-known. The mechanisms behind the improved yield are poorly understood. In this study, we systematically explore different catalyst combinations from Ni, Co, and Mo for the synthesis of carbon nanotubes via laser evaporation. Our findings reveal that the mixing of catalysts alters the catalyst cluster size distribution, maximizing the clusters’ potential to form a hemispherical cap at nucleation and, hence, form a single-walled carbon nanotube. This process significantly improves the single-walled carbon nanotube yields. S. Tetali, M. Zaka, R. Schönfelder, A. Bachmatiuk, F. Börrnert, I. Ibrahim, J. H. Lin, G. Cuniberti, J. H. Warner, B. Büchner, M, H. Rümmeli ACS Nano, 2009, 3 (12), pp 3839–384, URL |
 | Direct Imaging of Rotational Stacking Faults in Few Layer GrapheneFew layer graphene nanostructures are directly imaged using aberration corrected high-resolution transmission electron microscopy with an electron accelerating voltage of 80 kV. We observe rotational stacking faults in the HRTEM images of 2-6 layers of graphene sheets, giving rise to Moire´ patterns. By filtering in the frequency domain using a Fourier transform, we reconstruct the graphene lattice of each sheet and determine the packing structure and relative orientations of up to six separate sets. Direct evidence is obtained for few layer graphene sheets with packing that is different to the standard AB Bernal packing of bulk graphite. This has implications toward bilayer and few layer graphene electronic devices and the determination of their intrinsic structure. Jamie H. Warner, Mark H. Rümmeli, Thomas Gemming, Bernd Büchner, G. Andrew D. Briggs Nano Lett., 2009, 9 (1), 102-106, URL |
 | Investigating the Graphitization Mechanism of SiO2 Nanoparticles in Chemical Vapor DepositionThe use of SiO2 as a catalyst for graphitic nanostructures, such as carbon nanotubes and graphene, is a new and rapidly developing catalyst system. A key question is whether carbide phases form in the reaction. We show the formation of SiC from SiO2 nanoparticles for the synthesis of graphitic carbon nanostructures via chemical vapor deposition (CVD) at 900 °C. Our findings point to the carbothermal reduction of SiO2 in the CVD reaction. The inclusion of triethyl borate apparently accelerates the process and leads to improved yields. The study helps better understand the growth mechanisms at play in carbon nanotube and carbon nanofiber formation when using SiO2 catalysts. A. Bachmatiuk, F. Boerrnert, M. Grobosch, F. Schaeffel, U. Wolff, A. Scott, M. Zaka, J. H. Warner, R. Klingeler, M. Knupfer, B. Buechner, M. H. Ruemmeli ACS Nano vol 3 (2009) No. 12, 4098-4104 URL |
 | Two-Gap Superconductivity in Ba1-xKxFe2As2: A Complementary Study of the Magnetic Penetration Depth by Muon-Spin Rotation and Angle-Resolved PhotoemissionWe investigate the magnetic penetration depth λ in superconducting Ba1-xKxFe2As2 (Tc≃32 K) with muon-spin rotation (μSR) and angle-resolved photoemission (ARPES). Using μSR, we find the penetration-depth anisotropy γλ=λc/λab and the second-critical-field anisotropy γHc2 to show an opposite T evolution below Tc. This dichotomy resembles the situation in the two-gap superconductor MgB2. A two-gap scenario is also suggested by an inflection point in the in-plane penetration depth λab around 7 K. The complementarity of μSR and ARPES allows us to pinpoint the values of the two gaps and to arrive to a remarkable agreement between the two techniques concerning the full T evolution of λab. This provides further support for the described scenario and establishes ARPES as a tool to assess macroscopic properties of the superconducting condensate. R. Khasanov, D. Yevtushinsky, A. Amato, H.-H. Klauß, H. Luetkens, C. Niedermeier, B. Büchner, G. Sun, C. Lin, J. Park , D. Inosov, V. Hinkov Phys. Rev. Lett. 102, 187005 (2009), URL |
1-(α-Aminobenzyl)-2-naphthol: A New Chiral Auxiliary for the Synthesis of Enantiopure α-Aminophosphonic AcidsA new diastereoselective synthesis of α-aminophosphonates has been developed, based on the reaction, in the presence of trifluoroacetic acid, of trialkyl phosphites with chiral imines derived from (R)- or (S)-1-(α-amino-benzyl)-2-naphthol. The reaction proceeds at room temperature in toluene with high diastereoselectivity. The major diastereomer can be separated by crystallization from an appropriate solvent. The relative configuration of both chiral centers of the major diastereomer was determined by single-crystal X-ray structure analysis. The desired α-aminophosphonic acids can be obtained in enantiopure form by treatment of the corresponding diastereomers with HCl. К. Е. Metlushka, B. A. Kashemirov, V. F. Zheltukhin, D. N. Sadkova, B. Büchner, C. Hess, O. N. Kataeva, C. Е. McKenna, V. А. Alfonsov, Chem-Eur. J. 2009, Vol. 15, n 27, pp.6718-6722 | |
 | Tetranuclear complexes in molecular magnetism: Targeted synthesis, high-field EPR and pulsed-field magnetizationThe present article reviews the synthetic approaches, the structures and the spectroscopic and magnetic properties of three different types of tetranuclear complexes: systems with metal/oximate metalloligands, tetranuclear complexes based on polydentate amine-thiophenolate ligands, and nickel/azido complexes with compartmental pyrazolate scaffolds. In all three cases, well-defined bimetallic entities have been used as building blocks for the controlled synthesis of higher-nuclearity systems. This has allowed one to modulate the magnetic properties of the tetrametallic cores and to specifically target low-spin or high-spin ground states. High-frequency high-field EPR and pulsed-field magnetization measurements have been used to experimentally determine spin Hamiltonian parameters and to probe the magnetic responses at high fields. P. Chaudhuri, V. Kataev, B. Büchner, H.-H. Klauss, B. Kersting, and F. Meyer, Invited review article in: Coordination Chemistry Reviews 253 (2009) 2261–2285 URL |
 | Selective Etching of Thin Single-Walled Carbon NanotubesRaman spectroscopy and in situ Raman spectroelectrochemistry were applied to study the selective etching of thin tubes by lithium vapor in doped single-walled carbon nanotubes (SWCNTs). A strong doping of SWCNTs after the reaction with Li vapor was confirmed by the vanishing of the radial breathing mode (RBM) and by a strong attenuation of the tangential displacement (TG) band in the Raman spectra. The Raman spectra of the Li-vapor-treated SWCNTs after subsequent reaction with water showed changes in the diameter distribution compared with that of a pristine sample (nanotubes with diameters of <1 nm disappeared from the Raman spectra). The samples were tested by the Raman pattern with five different laser lines, and a removal of narrower tubes was confirmed. The remaining wider tubes were not significantly damaged by the treatment with Li, as indicated by the D line in the Raman spectra. Furthermore, the small-diameter tubes are converted not into amorphous carbon but into lithium carbide, which could easily be removed by hydrolysis. The treated samples were further charged electrochemically. It was shown by spectroelectrochemistry that anodic charging may lead to removal of the residual chemical doping from the thicker nanotubes in the sample, but the thin nanotubes did not appear in the spectra. This is a further confirmation of the removal of the small-diameter tubes. M. Kalbac, L. Kavan, L. Dunsch, J. Am. Chem. Soc., (2009), 131 (12), pp 4529–4534, URL |
 | The electronic phase diagram of the LaO1-xFxFeAs superconductorThe competition of magnetic order and superconductivity is a key element in the physics of all unconventional superconductors, for example in high-transition-temperature cuprates1, heavy fermions2 and organic superconductors3. Here superconductivity is often found close to a quantum critical point where long-range antiferromagnetic order is gradually suppressed as a function of a control parameter, for example charge-carrier doping or pressure. It is believed that dynamic spin fluctuations associated with this quantum critical behaviour are crucial for the mechanism of superconductivity. Recently, high-temperature superconductivity has been discovered in iron pnictides, providing a new class of unconventional superconductors4–6. Similar to other unconventional superconductors, the parent compounds of the pnictides show a magnetic ground state7,8 and superconductivity is induced on charge-carrier doping. In this Letter the structural and electronic phase diagram is investigated by means of X-ray scattering, muon spin relaxation and Mössbauer spectroscopy on the series LaO1-xFxFeAs. We find a discontinuous firstorder-like change of the Néel temperature, the superconducting transition temperature and the respective order parameters. Our results strongly question the elevance of quantum critical behaviour in iron pnictides and prove a strong coupling of the structural orthorhombic distortion and the magnetic order both disappearing at the phase boundary to the superconducting state. H. Luetkens, H.-H. Klauss, M. Kraken, F. J. Litterst, T. Dellmann, R. Klingeler, C. Hess, R. Khasanov, A. Amato, C. Baines, M. Kosmala, O. J. Schumann, M. Braden, J. Hamann-Borrero, N. Leps, A. Kondrat, G. Behr, J .Werner, B. Büchner, Nature Materials 8 (2009) Nr. 4, S. 305-309, URL |
 | Strength of the spin-fluctuation-mediated pairing interaction in a high-temperature superconductorTheories based on the coupling between spin fluctuations and fermionic quasiparticles are among the leading contenders to explain the origin of high-temperature superconductivity, but estimates of the strength of this interaction differ widely1. Here, we analyse the charge- and pin-excitation spectra determined by angle-resolved photoemission and inelastic neutron scattering, respectively, on the same crystals of the high-temperature superconductor YBa2Cu3O6:6. We show that a self-consistent description of both spectra can be obtained by adjusting a ingle parameter, the spin–fermion coupling constant. In particular, we find a quantitative link between two spectral features that have been established as universal for the cuprates, namely high-energy spin excitations2–7 and ‘kinks’ in the fermionic band dispersions along the nodal direction8–12. The superconducting transition temperature computed with this coupling constant exceeds 150 K, demonstrating that spin fluctuations have sufficient strength to mediate high-temperature superconductivity. T. Dahm, V. Hinkov, S. V. Borisenko, A. A. Kordyuk, V. B. Zabolotnyy, J. Fink, B. Büchner, D. J. Scalapino, W. Hanke, B. Keimer, nature physics 5 (2009) Nr. 3, S. 217-221, URL |
 | Iron-based Superconductors: Vital clues from a basic compoundInvestigation of the phase diagram of the structurally simple compound FeSe may prove instrumental in raising the transition temperature in Fe-based superconductors and in understanding magnetic-mediated superconductivity. B. Büchner and C. Hess, nature materials- news and views 8 (2009) Nr. 8, S. 615-616, URL |
 | Structural transformations in graphene studied with high spatial and temporal resolutionGraphene has remarkable electronic properties, such as ballistic transport and quantum Hall effects, and has also been used as a support for samples in high-resolution transmission electron microscopy and as a transparent electrode in photovoltaic devices. There is now a demand for techniques that can manipulate the structural and physical properties of graphene, in conjunction with the facility to monitor the changes in situ with atomic precision. Here, we show that irradiation with an 80 kV electron beam can selectively remove monolayers in few-layer graphene sheets by means of electron-beaminduced sputtering. Aberration-corrected, low-voltage, highresolution transmission electron microscopy with suba Angström resolution is used to examine the structural reconstruction occurring at the single atomic level. We find preferential termination for graphene layers along the zigzag orientation for large hole sizes. The temporal resolution can also be reduced to 80 ms, enabling real-time observation of the reconstruction of carbon atoms during the sputtering process.We also report electron-beam-induced rapid displacement of monolayers, fast elastic distortions and flexible bending at the edges of graphene sheets. These results reveal how energy transfer from the electron beam to few-layer graphene sheets leads to unique structural transformations. Jamie H. Warner, Mark H. Rümmeli, Ling Ge, Thomas Gemming, Barbara Montanari, Nicholas M. Harrison, Bernd Büchner, G. Andrew D. Briggs, Nature Nanotechnology, Vol 4 (2009) 500-504, URL |
(pi, pi) electronic order in iron arsenide superconductorsThe distribution of valence electrons in metals usually follows the symmetry of the underlying ionic lattice. Modulations of this distribution often occur when those electrons are not stable with respect to a new electronic order, such as spin or charge density waves. Electron density waves have been observed in many families of superconductors, and are often considered to be essential for superconductivity to exist. Recent measurements seem to show that the properties of the iron pnictides are in good agreement with band structure calculations that do not include additional ordering, implying no relation between density waves and superconductivity in these materials. Here we report that the electronic structure of Ba1-xKxFe2As2 is in sharp disagreement with those band structure calculations, and instead reveals a reconstruction characterized by a ( | |
 | Temperature and Doping-Dependent Renormalization Effects of the Low Energy ElectronicStructure of Ba1-xKxFe2As2 Single CrystalsWe investigate the low energy electronic structure of Ba1-xKxFe2As2 (x ¼ 0; 0.3, Tc ¼ 32 K) single crystals by angle-resolved photoemission spectroscopy with a focus on the renormalization of the dispersion. A kink feature is detected at E ~ 25 meV for the doped compound which vanishes at T ¼ 200 K but stays virtually constant when Tc is crossed. Our experimental findings rule out the magnetic resonance mode as the origin of the kink and render conventional electron-phonon coupling unlikely. They put stringent restrictions on the dominant source of the electronic interaction channel. A. Koitzsch, D. S. Inosov, D.V. Evtushinsky, V. B. Zabolotnyy, A. A. Kordyuk, A. Kondrat, C. Hess, M. Knupfer, B. Buechner, G. L. Sun, V. Hinkov, C. T. Lin, A. Varykhalov, S.V. Borisenko, Phys. Rev. Lett. 102, 167001 (2009), URL |
Two Energy Gaps and Fermi-Surface “Arcs” in NbSe2Using angle-resolved photoemission spectroscopy, we report on the direct observation of the energy gap in 2H-NbSe2 caused by the charge-density waves (CDW). The gap opens in the regions of the momentum space connected by the CDW vectors, which implies a nesting mechanism of CDW formation. In remarkable analogy with the pseudogap in cuprates, the detected energy gap also exists in the normal state (T >T0) where it breaks the Fermi surface into ‘‘arcs,’’ it is nonmonotonic as a function of temperature with a local minimum at the CDWtransition temperature (T0), and it forestalls the superconducting gap by excluding the nested portions of the Fermi surface from participating in superconductivity. S.V. Borisenko, A. A. Kordyuk, V. B. Zabolotnyy, D. S. Inosov, D. Evtushinsky, B. Buechner, A. N. Yaresko, A. Varykhalov, R. Follath, W. Eberhardt, L. Patthey, H. Berger, Phys. Rev. Lett. 102, 166402 (2009) URL | |
Evolution of the Kondo State of YbRh2Si2 Probed by High-Field ESRAn electron spin resonance (ESR) study of the heavy fermion compound YbRh2Si2 for fields up to ~8 T reveals a strongly anisotropic signal in the Kondo state below ~25 K. A similarity between the T dependence of the ESR parameters and that of the specific heat and the 29Si nuclear magnetic resonance data gives evidence that the ESR response is given by heavy fermions. Tuning the Kondo effect on the 4f states with magnetic fields ~2–8 T and temperature 2–25 K yields a gradual change of the ESR g factor and linewidth which reflects the evolution of the Kondo state in this Kondo lattice system. U. Schaufuß, V. Kataev, A. A. Zvyagin, B. Büchner, J. Sichelschmidt, J. Wykhoff, C. Krellner, C. Geibel, F. Steglich, Phys. Rev. Lett. 102, 076405 (2009) URL | |
Electronic Structure and Nesting-Driven Enhancement of the RKKY Interaction at the Magnetic Ordering Propagation Vector in Gd2PdSi3 and Tb2PdSi3Measurements of the low-energy electronic structure in Gd2PdSi3 and Tb2PdSi3 by means of angleresolved photoelectron spectroscopy reveal a Fermi surface consisting of an electron barrel at the point surrounded by spindle-shaped electron pockets originating from the same band. The calculated momentum-dependent RKKY coupling strength is peaked at the 1/2 K wave vector, which coincides with the propagation vector of the low-temperature in-plane magnetic order observed by neutron diffraction, thereby demonstrating the decisive role of the Fermi surface geometry in explaining the complex magnetic ground state of ternary rare earth silicides. D. S. Inosov, D.V. Evtushinsky, A. Koitzsch, V. B. Zabolotnyy, S.V. Borisenko, A. A. Kordyuk, M. Frontzek, M. Loewenhaupt, W. Löser, I. Mazilu, H. Bitterlich, G. Behr, J.-U. Hoffmann, R. Follath, and B. Büchner, Phys. Rev. Lett. 102, 046401 (2009) URL |
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2008
 | Large mixed metal nitride clusters encapsulated in a small cage: the confinement of the C-68-based clusterfullerenesThree novel C68-based mixed metal nitride clusterfullerenes, which comprise of a small non-IPR C68 cage encapsulating large nitride clusters including DySc2N, LuSc2N, and Lu2ScN, have been synthesized and isolated, resulting in overpassing the confinement of the C68 cage. S. Yang, A. Popov, L. Dunsch, Chem. Commun. (2008) 2885–2887 URL |
 | Development of the Tangential Mode in the Raman Spectra of SWCNT Bundles during Electrochemical ChargingThe detailed analysis of the in situ Raman spectroelectrochemical behavior of single walled carbon nanotube (SWCNT) bundles is presented. The Raman modes of metallic SWCNTs exhibit striking changes even before the potential of the first van Hove singularity is achieved. Special attention has been paid to the development of the tangential (TG) mode broadening, which subsequently vanishes if the potential is shifted away from V = 0. The tangential mode band has been fitted by four components. During the electrochemical doping, three components of the tangential mode follow the predictions of a theoretical model for the LO modes of metallic tubes based on the Kohn anomaly. On the other hand, the behavior of the fourth component is consistent with a model based on electron−plasmon coupling. The TO mode of metallic tubes has been identified only at a doping level corresponding to 1.0 V or above. Our results also indicate an asymmetry in the behavior of the TG mode for positive electrode potentials relative to negative ones. M. Kalbac, L. Kavan, L. Dunsch, M. S. Dresselhaus, Nano Lett. (2008) 8 (4), pp 1257–1264 URL |
 | Synthesis, Spectroscopic and Electrochemical Characterization, and DFT Study of Seventeen C70(CF3)n Derivatives (n = 2, 4, 6, 8, 10, 12)Eight new C70(CF3)n derivatives (n=2, 6, 10, 12) have been synthesized and characterized by UV/Vis and 19F NMR spectroscopy, cyclic voltammetry, and quantum chemical calculations at the DFT level of theory. Nine previously known derivatives of C70(CF3)n with n=2-12 were also studied by cyclic voltammetry (and seven of them by UV/Vis spectroscopy for the first time). Most of the 17 compounds exhibited two or three reversible reductions at scan rates from 20 mV s-1 up to 5.0 V s-1. In general, reduction potentials for the 0/- couple are shifted anodically relative to the C700/- couple. However, the 0/- E1/2 values for a given composition are strongly dependent on the addition pattern of the CF3 groups. The data show that the addition pattern is as important, if not more important in some cases, than the number of substituents, n, in determining E1/2 values. An analysis of the DFT-predicted LUMOs indicates that addition patterns that have non-terminal double bonds in pentagons result in derivatives that are strong electron acceptors. A. A. Popov, I. E. Kareev, N. B. Shustova, S. F. Lebedkin, S. H. Strauss, O. V. Boltalina, L. Dunsch, Chem. Eur. J. (2008) 14 (1), 107-121 URL |
 | Hindered Cluster Rotation and 45Sc Hyperfine Splitting Constant in Distonoid Anion Radical Sc3N@C80−, and Spatial Spin−Charge Separation as a General Principle for Anions of Endohedral Fullerenes with Metal-Localized Lowest Unoccupied Molecular OrbitalsDFT calculations of Sc3N@C80 in the neutral and anionic states are performed which revealed that in the neutral state of the nitride clusterfullerene the lowest energy structure has C3 symmetry, while in the anionic and dianionic states the C3v conformer has the lowest energy. Barriers to the cluster rotation inside the cage are also found to increase in the charge states. The 45Sc hyperfine slitting constant, a(Sc), in Sc3N@C80 anion radical is calculated by different theoretical approaches and in different conformations of Sc3N cluster. It is found that a(Sc) is strongly dependent on the cluster orientation with respect to the cage, covering a range form −10 to +25 Gauss at the B3LYP/6-311G*//PBE/TZ2P level of theory. A thorough analysis of the computed values as well as comparison of unrestricted and orbital-restricted calculations revealed that the polarization contribution to a(Sc) is about −10 Gauss and does not depend on the cluster orientation. Dependence of the predicted a(Sc) values on the density functional form (LSDA, BP, PBE, BLYP, OLYP, TPSS, B3LYP, and TPSSh), the basis set, as well as on the scalar-relativistic and spin−orbit corrections were investigated. The analysis of the charge distribution in the Sc3N@C80− anion radical revealed an interesting peculiarity of its electronic structure: while the spin density mostly resides on the cluster, only a slight decrease of its charge is found using both Bader and Mulliken definitions of atomic charges. A set of other endohedral metallofullerenes, including nitride clusterfullerenes Sc3N@C2n (2n = 68, 70, 78) and Y3N@C2n (2n = 78−88), carbide clusterfullerenes Sc2C2@C68, Sc2C2@C82, Sc3C2@C80, Ti2C2@C78, Y2C2@C82, and dimetallofullerenes Sc2@C76, Y2@C82, La2@C2n (2n = 72, 78, 80), was also studied in the neutral and anionic state, and a spatial charge−spin separation is found to be a general rule for all endohedral fullerenes with high contribution of metal atoms to the LUMO. A. Popov, L. Dunsch, J. Am. Chem. Soc., 2008, 130 (52), pp 17726–17742, URL |
Spin-State Polarons in Lightly-Hole-Doped LaCoO3 | |
Linear Plasmon Dispersion in Single-Wall Carbon Nanotubes and the Collective Excitation Spectrum of Graphene | |
High-Field Pauli-Limiting Behavior and Strongly Enhanced Upper Critical Magnetic Fields near the Transition Temperature of an Arsenic-Deficient LaO0.9F0.1FeAs1- Superconductor | |
Optical Study of LaO0.9F0.1FeAs: Evidence for a Weakly Coupled Superconducting State | |
 | Significant Redistribution of Ce 4d Oscillator Strength Observed in Photoionization of Endohedral Ce@C82+ IonsMass-selected beams of atomic Ceq+ ions (q=2, 3, 4), of C82+ and of endohedral Ce@C82+ ions were employed to study photoionization of free and encaged cerium atoms. The Ce 4d inner-shell contributions to single and double ionization of the endohedral Ce@C82+ fullerene have been extracted from the data and compared with expectations based on theory and the experiments with atomic Ce ions. Dramatic reduction and redistribution of the ionization contributions to 4d photoabsorption is observed. More than half of the Ce 4d oscillator strength appears to be diverted to the additional decay channels opened by the fullerene cage surrounding the Ce atom. A. Müller, S. Schippers, M. Habibi, D. Esteves, J. C. Wang, R. A. Phaneuf, A. L. D. Kilcoyne, A. Aguilar, L. Dunsch, Phys. Rev. Lett. 101, 133001 (2008) URL |
Field and Temperature Dependence of the Superfluid Density in LaO1-xFxFeAs Superconductors: A Muon Spin Relaxation Study | |
Commensurate Spin Density Wave in LaOFeAs: A Local Probe StudyWe present a detailed study on the magnetic order in the undoped mother compound LaFeAsO of the recently discovered Fe-based superconductor LaFeAsO1-xFx. In particular, we present local probe measurements of the magnetic properties of LaFeAsO by means of 57Fe Mo¨ssbauer spectroscopy and muon-spin relaxation in zero external field along with magnetization and resistivity studies. These experiments prove a commensurate static magnetic order with a strongly reduced ordered moment of 0:25(5)B at the iron site below TN=138 K, well separated from a structural phase transition at TS=156 K. The temperature dependence of the sublattice magnetization is determined and compared to theory. Using a four-band spin density wave model both, the size of the order parameter and the quick saturation below TN are reproduced. H.-H. Klauss, H. Luetkens, R. Klingeler, C. Hess, F.J. Litterst, M. Kraken, M. M. Korshunov, I. Eremin, S.-L. Drechsler, R. Khasanov, A. Amato, J. Hamann-Borreo, N. Leps, A. Kondrat, G. Behr, J. Werner, B. Büchner, Phys. Rev. Lett. 101, 077005 (2008) URL | |
 | Carbon Pyramidalization in Fullerene Cages Induced by the Endohedral Cluster: Non-Scandium Mixed Metal Nitride ClusterfullerenesEffect of the endohedral species: With the successful isolation of LuY2N§C80 and Lu2YN§C80 - the first mixed metal clusterfullerenes not to involve Sc, the effect of the encaged cluster size on the structure of the carbon cage is addressed: the increase in cluster size is found to result in an increase of the pyramidalization of pyrene-type carbon atoms and the upfield shift of corresponding 13C NMR signals. S.F. Yang, A. Popov, L. Dunsch, Angewandte Chemie-International Edition, Vol. 47, Issue: 43 (2008), pages 8196-8200, URL |
 | The Extended View on the Empty C2(3)-C82 Fullerene: Isolation, Spectroscopic, Electrochemical, and Spectroelectrochemical Characterization and DFT CalculationsAn extended study of the spectroscopic and redox properties of the C82 fullerene is presented. Among the nine isolated-pentagon-rule (IPR) isomers of the C82 fullerene the C82(3) isomer with C2 symmetry is the only stable, empty fullerene structure formed in the arc burning process that can be isolated in an isomerically pure form. Here, its formation and isolation are described and its structure is confirmed by experimental spectroscopic studies as well as time-dependent DFT calculations. The electrochemistry of the C82(3) isomer is studied in detail by cyclic voltammetry and spectroelectrochemistry. The anionic species of C82 with the charge ranging from C82- to C824- were successively generated in o-dichlorobenzene solution at room temperature and characterized by in situ ESR and visible/near-infrared (Vis/NIR) spectroscopy. The data give new insights into the charged states of the C82(3) fullerene. M. Zalibera, A. Popov, M. Kalbac, P. Rapta, L. Dunsch, Chemistry-A European Journal, Vol 14, Issue 32 (2008) pages 9960-9967 URL |
Stepwise Current-Driven Release of Attogram Quantities of Copper Iodide Encapsulated in Carbon NanotubesEncapsulated nanograins of copper iodide have been sequentially discharged from individual carbon nanotubes. Using a high resolution electron microscope equipped with a two-terminal electrical measurements unit, it was possible to manipulate the filling contents with precisions of a few attograms at a time. Changes in electrical resistance and filling ratio were followed in tandem and in real-time. It is shown that the pulsed release of the halide is directly related to the overall conductance of the filled nanotube. Pedro M. F. J. Costa ,D. Golberg, M. Mitome, S. Hampel, A. Leonhardt, B. Büchner, Y. Bando, Nano Letters, Vol. 8, No. 10 (2008) URL | |
75As NMR Studies of Superconducting LaFeAsO0.9F0.1We have performed 75As nuclear magnetic resonance measurements on aligned powders of the new LaFeAsO0.9F0.1 superconductor. In the normal state, we find a strong temperature dependence of the spin shift and Korringa behavior of the spin lattice relaxation rate. In the superconducting state, we find evidence for line nodes in the superconducting gap and spin-singlet pairing. Our measurements reveal a strong anisotropy of the spin lattice relaxation rate, which suggests that superconducting vortices contribute to the relaxation rate when the field is parallel to the c axis but not for the perpendicular direction. H.-J. Grafe, D. Paar, G. Lang, N. J. Curro, G. Behr, J. Werner, J. Hamann-Borrero, C. Hess, N. Leps, R. Klingeler, and B. Büchner, Phys. Rev. Lett. 101, 047003 (2008) URL | |
Pseudogap-Driven Sign Reversal of the Hall Effect | |
Pseudogap and Charge Density Waves in Two DimensionsUsing angle-resolved photoemission spectroscopy we demonstrate that a normal-state pseudogap exists above TN-IC in one of the most studied two-dimensional charge-density wave (CDW) dichalcogenides 2H-TaSe2. The initial formation of the incommensurate CDW is confirmed as being driven by a conventional nesting instability, which is marked by a pseudogap. The magnitude, character, and anisotropy of the 2D-CDW pseudogap bear considerable resemblance to those seen in superconducting cuprates. S. V. Borisenko, A. A. Kordyuk, A. N. Yaresko, V. B. Zabolotnyy, D. S. Inosov, R. Schuster, B. Büchner, R. Weber, R. Follath, L. Patthey, H. Berger, Phys. Rev. Lett. 100, 196402 (2008) URL | |
Electron-Electron Correlation in Graphite: A Combined Angle-Resolved Photoemission and First-Principles StudyThe full three-dimensional dispersion of the p-bands, Fermi velocities, and effective masses of graphite are measured with angle-resolved photoemission spectroscopy and compared to first-principles calculations. The band structure by density-functional theory underestimates the slope of the bands and the trigonal warping effect. Including electron-electron correlation on the level of the GW approximation, however, yields remarkable improvement in the vicinity of the Fermi level. This demonstrates the breakdown of the independent electron picture in semimetallic graphite and points toward a pronounced role of electron correlation for the interpretation of transport experiments and double-resonant Raman scattering for a wide range of carbon based materials. A. Grüneis, C. Attaccalite, T. Pichler, V. Zabolotnyy, H. Shiozawa, S.L. Molodtsov, D. Inosov, A. Koitzsch, M. Knupfer, J. Schiessling, R. Follath, R. Weber, P. Rudolf, L. Wirtz and A. Rubio, Phys. Rev. Lett. 100, 037601 (2008) URL | |
Energy level alignment and injection barriers at spin injection contacts between La0.7Sr0.3MnO3 and organic semiconductorsWe have determined the energy level alignment at interfaces between La0.7Sr0.3MnO3 and two typical organic semiconductors, copper-phthalocyanine and sexithiophene. La0.7Sr0.3MnO3 thin films have been grown using pulsed laser deposition and subsequently ex situ cleaned before the organic materials have been deposited. This procedure is often applied in the fabrication of organic devices. We show that under these conditions the interfaces are free from chemical interaction and characterized by a short range interface dipole and large charge injection barriers. M. Grobosch, K. Dörr, R.B. Gangineni, M. Knupfer, Applied Physics Letters 92 (2008) Nr. 2, S. 23302/1-3 URL |
2007
A nomalous quasiparticle renormalization in Na0.73CoO2: Role of interorbital interactions and magnetic correlationsWe report an angular resolved photoemission study of NaxCoO2 with x~0.73 where it is found that the renormalization of the quasiparticle (QP) dispersion changes dramatically upon a rotation from I´M to I`K. The comparison of the experimental data to the calculated band structure reveals that the quasiparticle renormalization is most pronounced along the I´K direction, while it is significantly weaker along the I´M direction. We discuss the observed anisotropy in terms of multiorbital effects and point out the relevance of magnetic correlations for the band structure of NaxCoO2 with x~0.75 J. Geck, S.V. Borisenko, H. Berger, H. Eschrig, J. Fink, M. Knupfer, K. Koepernik, A. Koitzsch, A.A. Kordyuk, V.B. Zabolotnyy, B. Buechner, Physical Review Letters 99 (2007) Nr. 4, S. 46403/1-4 URL | |
| | Momentum and Energy Dependence of the Anomalous High-Energy Dispersion in the Electronic Structure of High Temperature Superconductors Using high-resolution angle-resolved photoemission spectroscopy we have studied the momentum and photon energy dependence of the anomalous high-energy dispersion, termed waterfalls, between the Fermi level and 1 eV binding energy in several high-Tc superconductors. We observe strong changes of the dispersion between different Brillouin zones and a strong dependence on the photon energy around 75 eV, which we associate with the resonant photoemission at the Cu3p |
| | In Situ Raman Spectroelectrochemical Study of 13C-Labeled Fullerene Peapods and Carbon NanotubesC60 fullerene peapods and double-walled carbon nanotubes (DWCNTs) containing highly 13C enriched C60 and inner tubes, respectively, are studied using Raman spectroscopy and in situ Raman spectroelectrochemistry in order to follow the influence of 13C enrichment on the vibrational pattern of these carbon nanostructures. The Raman response of 13C60 after encapsulation in fullerene peapods differs from that of isotope-natural species, NatC60. The Raman Ag(2) mode of encapsulated 13C60 is upshifted in frequency compared to that of the NatC60 peapods with the same filling factor. The chemical doping of 13C60 peapods (peapod=C60@SWCNT) with K-vapor leads to the downshift of the Ag(2) mode, similar to the case of NatC60 peapods. The 13C60 peapods were successfully transformed into DWCNTs, which confirms high filling of single-walled (SW) CNTs with 13C60. The DWCNTs exhibited distinctly downshifted G and D Raman modes for inner tubes, which proves that only inner tubes were enriched by 13C. The in situ Raman spectroelectrochemistry of NatC60 exhibits strong anodic enhancement, while for 13C60 peapods the enhancement is only weak. On the other hand, the electrochemical charging of the inner-tube-labeled DWCNTs (13Ci-DWCNTs) followed the behavior of ordinary NatCi-DWCNTs as indicated by in situ Raman spectroelectrochemistry. In addition, the spectroelectrochemical behavior of the G mode of inner tubes in 13 Ci-DWCNTs is followed from the start of the electrochemical doping, which was not feasible for NatCi-DWCNTs. M. Kalbá , L. Kavan, M. Zukalová, L. Dunsch, SMALL 3 (2007), 10, 1746-1752 URL |
Ring waveguide resonator on surface acoustic wavesA simple regular electrode structure for surface acoustic wave (SAW) devices is proposed. The structure consists of an interdigital transducer in the form of a ring placed on the Z cut of a hexagonal piezoelectric crystal. Finite thickness electrodes produce the known slowing effect for a SAW in comparison with this SAW on a free surface. The closed “slow” electrode region with the “fast” surrounding region forms an open waveguide resonator structure with the acoustic field concentrated in the electrode region. If the radius of the structure is large enough for a given wavelength, an acceptable level of radiation losses can be reached. The electrical admittance of such resonator does not have sidelobes. S. V. Biryukov, G. Martin, and M. Weihnacht, Applied Physics Letters 90 (2007), Nr. 17, S. 173503/1-2 URL | |
Copper-Filled Carbon nanotubes: Rheostat-like Behavior and Femtogam Copper Mass TransportCopper-filled carbon nanotubes can electrically be probed engineered (see figure) inside a high-resolution transmission electron microscope using a piezo-driven stage. Unique switchlike or rheostatlike electrical behavior and electrical-driven copper femtogram mass transport within nanotubes are shown in the paper. D. Golberg, P. Costa, M. Mitome, S. Hampel, D. Haase, Ch. Müller, A. Leonhardt, Y. Bando, Advanced Mat. 2007, 19, 1937-1942 URL | |
| | Structure, stability, and cluster-cage interactions in nitride clusterfullerenes M3N@C-2n (M=Sc, Y; 2n=68-98): a density functional theory studyExtensive semiempirical calculations of the hexaanions of IPR (isolated pentagon rule) and non-IPR isomers of C68-C88 and IPR isomers of C90-C98 followed by DFT calculations of the lowest energy structures were performed to find the carbon cages that can provide the most stable isomers of M3N@C2n clusterfullerenes (M = Sc, Y) with Y as a model for rare earth ions. DFT calculations of isomers of M3N@C2n (M = Sc, Y; 2n = 68-98) based on the most stable C2n6- cages were also performed. The lowest energy isomers found by this methodology for Sc3N@C68, Sc3N@C78, Sc3N@C80, Y3N@C78, Y3N@C80, Y3N@C84, Y3N@C86, and Y3N@C88 are those that have been shown to exist by single-crystal X-ray studies as Sc3N@C2n (2n = 68, 78, 80), Dy3N@C80, and Tb3N@C2n (2n = 80, 84, 86, 88) clusterfullerenes. Reassignment of the carbon cage of Sc2@C76 to the non-IPR Cs: 17490 isomer is also proposed. The stability of nitride clusterfullerenes was found to correlate well with the stability of the empty 6-fold charged cages. However, the dimensions of the cage in terms of its ability to encapsulate M3N clusters were also found to be an important factor, especially for the medium size cages and the large Y3N cluster. In some cases the most stable structures are based on the different cage isomers for Sc3N and Y3N clusters. Up to the cage size of C84, non-IPR isomers of C2n6- and M3N@C2n were found to compete with or to be even more stable than IPR isomers. However, the number of adjacent pentagon pairs in the most stable non-IPR isomers decreases as cage size increases: the most stable M3N@C2n isomers have three such pairs for 2n = 68-72, two pairs for n = 74-80, and only one pair for n = 82, 84. For C86 and C88 the lowest energy IPR isomers are much more stable than any non-IPR isomer. The trends in the stability of the fullerene isomers and the cluster-cage binding energies are discussed, and general rules for stability of clusterfullerenes are established. Finally, the high yield of M3N@C80 (Ih) clusterfullerenes for any metal is explained by the exceptional stability of the C806- (Ih: 31924) cage, rationalized by the optimum distribution of the pentagons leading to the minimization of the steric strain, and structural similarities of C80 (Ih: 31924) with the lowest energy non-IPR isomers of C766-, C786-, C826-, and C846- pointed out. Alexey A. Popov and Lothar Dunsch, Journal of the American Chemical Society 129 (2007) 38, 11835-11849 URL |
| | Electrochemical, spectrosopic, and DFT study of C-60(CF3)(n) frontier orbitals (n=2-18): The link between double bonds in pentagons and reduction PotentialsThe frontier orbitals of 22 isolated and characterized C60(CF3)n derivatives, including seven reported here for the first time, have been investigated by electronic spectroscopy (n = 2 [1], 4 [1], 6 [2], 8 [5], 10 [6], 12 [3]; the number of isomers for each composition is shown in square brackets) fluorescence spectroscopy (n = 10 [4]), cyclic voltammetry under air-free conditions (all compounds with n <= 12), ESR spectroscopy of C60(CF3)n- radical anions at 25 °C (n = 4 [1] and 10 [1]), and quantum chemical calculations at the DFT level of theory (all compounds including n = 16 [3] and 18 [2]). DFT calculations are also reported for several hypothetical C60(CF3)n derivatives. The X-ray structure of one of the compounds, 1,6,11,16,18,26,36,41,44,57-C60(CF3)10, is reported here for the first time. Most of the compounds with n <= 12 exhibit two or three quasi-reversible reductions at scan rates from 20 mV s-1 up to 5.0 V s-1, respectively. The 18 experimental 0/- E1/2 values (vs C600/-) are a linear function of the DFT-predicted LUMO energies (average E1/2 deviation from the least-squares line is 0.02 V). This linear relationship was used to predict the 0/- E1/2 values for the n = 16 and 18 derivatives, and none of the predicted values is more positive than the 0/- E1/2 value for one of the isomers of C60(CF3)10. In general, reduction potentials for the 0/- couple are shifted anodically relative to the C600/- couple. However, the 0/- E1/2 values for a given composition are strongly dependent on the addition pattern of the CF3 groups. In addition, LUMO energies for isomers of C60(X)n (n = 2, 4, 6, 8, 10, and 12) that are structurally related to many of the CF3 derivatives were calculated and compared for X = CH3, H, Ph, NH2, CH2F, CHF2, F, NO2, and CN. The experimental and computational results for the C60(CF3)n compounds and the computational results for more than 50 additional C60(X)n compounds provide new insights about the frontier orbitals of C60(X)n derivatives. For a given substituent, X, the addition pattern is as important, if not more important in many cases, than the number of substituents, n, in determining E1/2 values. Those addition patterns with double bonds in pentagons having two C(sp2) nearest neighbors result in the strongest electron acceptors. A.A. Popov, I. E. Kareev, N. B. Shustova, E. B. Stukalin, S. F. Lebedkin, K. Seppelt, S. H. Strauss, O. V. Boltalina, L. Dunsch, Journal of the American Chemical Society 129 (2007) 37, 11551-11568 URL |
Frustrated Cuprate Route from Antiferromagnetic to Ferromagnetic Spin-½ Heisenberg Chains: Li2ZrCuO4 as a Missing Link near the Quantum Critical PointFrom thermodynamics, local spin density approximation + Hubbard U studies and exact diagonalizations of a five-band Hubbard model on CuO2 stripes we find that Li2ZrCuO4 (Li2ZuCrO4 in traditional notation) is close to a ferromagnetic critical point. Analyzing its susceptibility χ(T) and specific heat cp (T, H) within a Heisenberg model, we show that the ratio of the 2nd to the 1st neighbor exchange integrals a = – J2/J1 ~ 0.3 is close to the critical value ac = ¼. Comparing with related chain cuprates we explain the rather strong field dependence of cp, the monotonic downshift of the peak of χ(T), and its increase for a -> ac + 0. S.-L. Drechsler, O. Volkova, A.N. Vasiliev, N. Tristan, J. Richter, M. Schmitt, H. Rosner, J. Malek, R. Klingeler, A.A. Zvyagin, B. Buechner Physical Review Letters 98 (2007) Nr. 7, S. 77202/1-4 URL | |
Quasi-One-Dimensional K-O Chain in PTCDA Thin Films: Evidence from First-Principles CalculationsUsing density functional theory calculations we have found that K atoms in a PTCDA (3, 4:9, 10-perylenetetracarboxylic dianhydride) crystal form a quasi-one-dimensional (1D) K-O chain interacting with carboxylic of the terminal anhydride groups of PTCDA. The K-K distance in the chain (3.72 Å) is commensurate to the periodicity of the organic semiconductor. We obtain that the K-O structure is stabilized by charge transfer from K to PCTDA molecules, forming prevalently ionic bonds: the electronic density of the chemistry induced gap states is essentially delocalized on the perylene core of PTCDA, while potassium appears spoiled of its charge. Band dispersion along the direction of molecular stack is evaluated to be 0.2 eV in pure PTCDA crystal and 0.5 eV in the K-doped system, confirming that the interaction occurs between different molecular planes. C. Zazza, S. Meloni, A. Palma, M. Knupfer, G.G. Fuentes, R. Car, Physical Review Letters 98 (2007) Nr. 4, S. 46401/1- URL | |
Exciton Band Structure of Pentacene Molecular Solids: Breakdown of the Frenkel Exciton ModelEmploying inelastic electron scattering we demonstrate here the determination of the exciton dispersion in a molecular solid. The failure of the applied tight-binding description provides strong evidence for a necessary reconsideration of the traditional, Frenkel-exciton based, understanding of the lowest-lying electronic transitions in organic semiconductors. R. Schuster, M. Knupfer, H. Berger, Physical Review Letters 98 (2007) Nr. 3, S. 37402/1- URL | |
One- and Two-Triplon Spectra of a Cuprate LadderWe have performed inelastic neutron scattering on the near ideal spin-ladder compound La4Sr10Cu24O41 as a starting point for investigating doped ladders and their tendency toward superconductivity. A key feature was the separation of one-triplon and two-triplon scattering. Two-triplon is observed quantitatively for the first time and so access is realized to the important strong magnetic quantum fluctuations. The spin gap is found to be 26.4 +- 0.3 meV. The data are successfully modeled using the continuous unitary transformation method, and the exchange constants are determined by fitting to be Jleg = 186 meV and Jrung = 124 meV along the leg and rung, respectively; a substantial cyclic exchange of Jcyc = 31 meV is confirmed. S. Notbohm, P. Ribeiro, B. Lake, D.A. Tennant, K.P. Schmidt, G.S. Uhrig, C. Hess, R. Klingeler, G. Behr, B. Buechner, M. Reehuis, R.I. Bewley, C.D. Frost, P. Manuel, R.S. Eccleston, Physical Review Letters 98 (2007) Nr. 2, S. 27403/1-4 URL | |
Linear Temperature Dependence of the Magnetic Heat Conductivity in CaCu2O3We present experimental results for the thermal conductivity к of the pseudo-two-leg ladder material CaCu2O3. The strong buckling of the ladder rungs renders this material a good approximation to a S = ½ Heisenberg chain. Despite a strong suppression of the thermal conductivity of this material in all crystal directions due to inherent disorder, we find a dominant magnetic contribution кmag along the chain direction. кmag is linear in temperature, resembling the low-temperature limit of the thermal Drude weight Dth of the S = ½ Heisenberg chain. The comparison of кmag and Dth yields a magnetic mean-free path of lmag~22+-5 Å in good agreement with magnetic measurements. C. Hess, H. ElHaes, A. Waske, B. Buechner, C. Sekar, G. Krabbes, F. Heidrich-Meisner, W. Brenig, Physical Review Letters 98 (2007) Nr. 2, S. 27201/1-4 URL | |
Molecular nanostructures: Carbon aheadCarbon nanostructures in zero, one and two dimensions offer great potential in a broad range of applications. The most recent developments in the study of fullerenes, carbon nanotubes, and graphene were highlighted at a conference held in the Austrian Alps T. Pichler, Nature materials 6 (2007), S. 332-333 URL | |
Charge-Injection barriers at realistic metal/organic interfaces: metals become facelessInterfaces between the organic semiconductor a-sexithiophene and sputter cleaned (ideal) metals as well as contaminated (realistic) metals (Ag, Au, Pd, and Pt) were investigated by a combined X-ray and ultra violet photoemission spectroscopy study. The experimental results indicate a substantial impact of the metal surface contamination on the interface electronic properties. In particular, the hole injection barrier is almost independent of the type of the underlying metal. M. Grobosch, M. Knupfer, Advanced Materials 19 (2007) Nr. 5, S. 754-756 URL | |
Violating the Isolated Pentagon Rule (IPR): The Endohedral Non-IPR C70 Cage of Sc3N@C70A "missed" non-IPR C70 cage is found! The first C70-based endohedral clusterfullerene ¾ Sc3N@C70 ¾ was shown to consist of the non-IPR C2v:7854 cage isomer, having three pairs of the adjacent pentagons. They are coordinated to the three Sc atoms of the asymmetrical Sc3N cluster. As a new example of stabilization of the non-IPR cages by an Sc3N cluster, Sc3N@C70 represents the fifth member of the non-IPR endohedral fullerene family. S. Yang, A. Popov, L. Dunsch, Angewandte Chemie- Internationaln Edition 46 (2007) Nr. 8, S. 1256-1259 URL |
3dx2-y2 dispersion results from a strong suppression of the photoemission intensity at the center of the Brillouin zone due to matrix element effects and is, therefore, not an intrinsic feature of the spectral function. This indicates that the new high-energy scale in the electronic structure of cuprates derived from the waterfall-like dispersion may be incorrect. edge. We conclude that the high-energy “waterfall” D. S. Inosov, J. Fink, A. A. Kordyuk,S. V. Borisenko, V. B. Zabolotnyy, R. Schuster, M. Knupfer, B. Büchner, R. Follath, H. A. Dürr, W. Eberhardt, V. Hinkov, B. Keimer, H. Berger, Phys. Rev. Lett. 99, 237002 (2007), 2006
J. Geck, H. von Zimmermann, H. Berger, S.V. Borisenko, H. Eschrig, K. Koepernik, M. Knupfer, B. Buechner
Stripe correlations in Na0.75CoO2, Physical Review Letters 97 (2006) Nr. 10, S. 106403/1-4 URL
H.-J. Grafe, N.J. Curro, M. Huecker, B. Buechner
Nuclear-Magnetic-Resonance Evidence for Charge Inhomogeneity in Stripe Ordered La1.8-xEu0.2SrxCuO4, Physical Review Letters 96 (2006) Nr. 1, S. 17002/1-4 URL
A.A. Kordyuk, S.V. Borisenko, V.B. Zabolotnyy, J. Geck, M. Knupfer, J. Fink, B. Buechner, C.T. Lin, B. Keimer, H. Berger, A.V. Pan,
S. Komiya, Y. Ando
Constituents of the quasiparticle spectrum along the nodal direction of high-Tc cuprates, Physical Review Letters 97 (2006) Nr. 1, S. 17002/1-4 URL
S.V. Borisenko, A.A. Kordyuk, V. Zabolotnyy, J. Geck, D. Inosov, A. Koitzsch, J. Fink, M. Knupfer, B. Buechner, V. Hinkov, C.T. Lin, B. Keimer, T. Wolf, S.G. Chiuzbaian, L. Patthey, R. Follath
Kinks, nodal bilayer splitting, and interband scattering in YBa2Cu3O6+x, Physical Review Letters 96 (2006), S. 117004/1-4 URL
S.V. Borisenko, A.A. Kordyuk, A. Koitzsch, J. Fink, J. Geck, V. Zabolotnyy, M. Knupfer, B. Buechner, H. Berger, M. Falub, M. Shi,
J. Krempasky, L. Patthey
Parity of the pairing bosons in a high-temperature Pb-Bi2Sr2CaCu2O8 bilayer superconductor by angle-resolved photoemission spectroscopy, Physical Review Letters 96 (2006) Nr. 6, S. 67001/1-4 URL
V.B. Zabolotnyy, S.V. Borisenko, A.A. Kordyuk, J. Fink, J. Geck, A. Koitzsch, M. Knupfer, B. Buechner, H. Berger, A. Erb, C.T. Lin,
B. Keimer, R. Follath
Effect of Zn and Ni Impurities on the Quasiparticle Renormalization of Superconducting Bi-2212, Physical Review Letters 96 (2006) Nr. 3, S. 37003/1-4 URL
S. Yang, I. Troyanov, A.A. Popov, M. Krause, L. Dunsch
Deviation from the planarity-a large Dy3N cluster encapsulated in an Ih-C80 cage: an X-ray crystallographic and vibrational spectroscopic study, Journal of the American Chemical Society 128 (2006), S. 16733-16739 URL
C.-R. Wang, Z.-Q. Shi, L.-J. Wan, X. Lu, L. Dunsch, C.-Y. Shu, Y.-L. Tang, H. Shinohara
C64H4: Production, isolation, and structural characterizations of a stable unconventional fulleride, Journal of the American Chemical Society 128 (2006) Nr. 20, S. 6605-6610 URL
A.N. Darinskii, M. Weihnacht
Gap acousto-electric waves in structures of arbitrary anisotropy, IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control 53 (2006) Nr. 2, S. 412-419
L. Shlyk, G. Krabbes, G. Fuchs, C. Mickel, B. Rellinghaus, K. Nenkov
Engineering periodic arrays of nanoscale twin boundaries in bulk YBa2Cu3O7 with RuO2 additions, Applied Physics Letters 88 (2006) Nr. 6, S. 62509/1-3 URL
S. Yang, L. Dunsch
Di- and tridysprosium endohedral metallofullerenes with cages from C94 to C100, Angewandte Chemie- International Edition 45 (2006) Nr. 8, S. 1299-1302 URL
2005
J. Geck, P. Wochner, S. Kiele, R. Klingeler, P. Reutler, A. Revcolevschi, M. von Zimmermann, B. Buechner
Orbital polaron lattice formation in lightly doped La1-xSrxMnO3, Physical Review Letters 95 (2005) Nr. 23, S. 236401/1-4 URL
M.H. Ruemmeli, E. Borowiak-Palen, T. Gemming, T. Pichler, M. Knupfer, M. Kalbac, L. Dunsch, O. Jost, S.R.P. Silva, W. Pompe,
B. Buechner
Novel catalysts, room temperature, and the importance of oxygen for the synthesis of single-walled carbon nanotubes, Nano Letters 5 (2005) Nr. 7, S. 1209-1215 URL
A.N. Darinskii, M. Weihnacht
Acoustic waves in bounded anisotropic media: theorems, estimations and computations, IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control 52 (2005) Nr. 5, S. 792-801
A.N. Darinskii, M. Weihnacht
Resonance reflection of acoustic waves in piezoelectric Bi-crystalline structures, IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control 52 (2005) Nr. 5, S. 904-910
L. Shlyk, G. Krabbes, G. Fuchs, K. Nenkov
Flux pinning enhancement in melt-processed YBa2Cu3O7 with extended nanodefects, Applied Physics Letters 86 (2005) Nr. 9, S. 92503/1-3
M. Krause, L. Dunsch
Gadoliniumnitrid Gd3N in Kohlenstoffkaefigen: der Einfluss von Clustergroesse und Bindungsstaerke, Angewandte Chemie 117 (2005) Nr. 10, S. 1581-1584
M. Wolf, K.-H. Mueller, Y. Skourski, D. Eckert, P. Georgi, M. Krause, L. Dunsch
Magnetic moments of the endohedral cluster fullerenes Ho3N@C80 and Tb3N@C80: the role of ligand fields , Angewandte Chemie- International Edition 44 (2005) Nr. 21, S. 3306-3309 URL
V. Bocharova, A. Kiriy, H. Vinzelberg, I. Moench, M. Stamm
Polypyrrole nanowires grown from single adsorbed polyelectrolyte molecules, Angewandte Chemie 117 (2005) Nr. 39, S. 6549-6552 URL
2004
S.V. Borisenko, A.A. Kordyuk, A. Koitzsch, T.K. Kim, K.A. Nenkov, M. Knupfer, J. Fink, C. Grazioli, S. Turchini, H. Berger
Circular dichroism in angle-resolved photoemission spectra of under- and overdoped Pb-Bi2212, Physical Review Letters 92 (2004) Nr. 20, S. 207001/1-4
A.A. Kordyuk, S.V. Borisenko, A. Koitzsch, J. Fink, M. Knupfer, B. Buechner, H. Berger, G. Margaritondo, C.T. Lin, B. Keimer, S. Ono,
Y. Ando
Manifestation of the magnetic resonance mode in the nodal quasiparticle lifetime of the superconducting cuprates, Physical Review Letters 92 (2004) Nr. 25, S. 257006/1-4
C. Hess, H. ElHaes, B. Buechner, U. Ammerahl, M. Huecker, A. Revcolevschi
Magnon-Hole Scattering and Charge Order in Sr14xCaxCu24O41, Physical Review Letters 93 (2004) Nr. 2, S. 27005/1-4
H. Rauf, T. Pichler, M. Knupfer, J. Fink, H. Kataura
Transition from a Tomonaga-Luttinger Liquid to a Fermi Liquid in Potassium-Intercalated Bundles of Single-Wall Carbon Nanotubes, Physical Review Letters 93 (2004) Nr. 9, S. 96805/1-4
M. Krause, M. Hulman, H. Kuzmany, O. Dubay, G. Kresse, K. Vietze, G. Seifert, C. Wang, H. Shinohara
Fullerene Quantum Gyroscope, Physical Review Letters 93 (2004) Nr. 13, S. 137403/1-4
S.V. Borisenko, A.A. Kordyuk, A. Koitzsch, M. Knupfer, J. Fink, H. Berger, C.T. Lin
Time-reversal symmetry breaking ?, nature, 2 September 2004 431 (2004) URL
G. Martin, H. Schmidt, B. Wall
New SPUDT cell structures, IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control 51 (2004) Nr. 7, S. 859-864
2003
T.K. Kim, A.A. Kordyuk, S.V. Borisenko, A. Koitzsch, M. Knupfer, H. Berger, J. Fink
Doping dependence of the mass enhancement in (Pb,Bi)2Sr2CaCu2O8 at the antinodal point in the superconducting and normal states, Physical Review Letters 91 (2003) Nr. 16, S. 167002/1-4
A.S. Moskvin, J. Malek, M. Knupfer, R. Neudert, J. Fink, R. Hayn, S.-L. Drechsler, N. Motoyama, H. Eisaki, S. Uchida
Evidence for two types of low-energy charge transfer excitations in Sr2CuO3, Physical Review Letters 91 (2003) Nr. 3, S. 37001/1-4
R. Pfeiffer, H. Kuzmany, C. Kramberger, C. Schaman, T. Pichler, H. Kataura, Y. Achiba, J. Kuerti, V. Zolyomi
Unusual high degree of unperturbed environment in the interior of single-wall carbon nanotubes, Physical Review Letters 90 (2003) Nr. 22, S. 225501/1-4
S.V. Borisenko, A.A. Kordyuk, T.K. Kim, A. Koitzsch, M. Knupfer, J. Fink, M.S. Golden, M. Eschrig, H. Berger, R. Follath
Anomalous enhancement of the coupling to the magnetic resonance mode in underdoped Pb-Bi2212, Physical Review Letters 90 (2003) Nr. 20, S. 207001/1-4
K. Franke, M. Ross-Messemer, A. Menck, F.S. Hoeller, H. Schmidt, M. Weihnacht
The highly sensitive optical measurement of absolute SAW amplitudes for power flow analysis, IEEE Transactions on Ultrasonics, Ferroelectrics and frequency control 50 (2003) Nr. 1, S. 77-80
G. Martin, B. Steiner
SAW filters including one-focus slanted finger interdigital transducers , IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control 50 (2003) Nr. 1, S. 94-98
F. Zhang, A. Petr, L. Dunsch
Electrochemically doped polymeric anode for improving the performance of molecular organic light-emitting diodes, Applied Physics Letters 82 (2003) Nr. 25, S. 4587-4589
T. Schwieger, M. Knupfer, W. Gao, A. Kahn
Direct and inverse photoemission spectroscopy studies of potassium intercalated films of two organic semiconductors, Applied Physics Letters 83 (2003) Nr. 3, S. 500-502
H. Peisert, M. Knupfer, F. Zhang, A. Petr, L. Dunsch, J. Fink
Charge transfer and doping at organic/organic interfaces, Applied Physics Letters 83 (2003) Nr. 19, S. 3930-3932