Publications of the IFF

Tunable Band Gap in Hydrogenated Quasi-Free-Standing Graphene

We 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

Direct Low-Temperature Nanographene CVD Synthesis over a Dielectric Insulator

Graphene 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@C₈₀

Redox 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 Clusterfullerenes

The 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@C_2n. 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 Properties

By 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 systems

The 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 System

The 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 Pnictides

The charge distribution in RFeAsO_1-xF_x (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 ions

Bis-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 microscopy

We 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 Exfoliation

Here, 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 Devices

The 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 growth

Using 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]fullerene

Unlike 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 Pseudoatom

Y3C80 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

Unravelling the Mechanisms Behind Mixed Catalysts for the High Yield Production of Single-Walled Carbon Nanotubes

The 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 Graphene

Few 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 Deposition

The 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 Ba_1-xK_xFe₂As₂: A Complementary Study of the Magnetic Penetration Depth by Muon-Spin Rotation and Angle-Resolved Photoemission

We 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 Acids

A 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 magnetization

The 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 Nanotubes

Raman 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 superconductor

The 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 superconductor

Theories 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 compound

Investigation 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 resolution

Graphene 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 superconductors

The 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 Ba_1-xK_xFe₂As₂ is in sharp disagreement with those band structure calculations, and instead reveals a reconstruction characterized by a (, ) wavevector. This electronic order coexists with superconductivity and persists up to room temperature (300 K).
V.B. Zabolotnyy, D.S. Inosov, D.V. Evtushinsky, A. Koitzsch, A.A. Kordyuk, G.L. Sun, J.T. Park, D. Haug, V. Hinkov, A.V. Boris, C.T. Lin, M. Knupfer, A.N. Yaresko, B. Buechner, A. Varykhalov, R. Follath, S.V. Borisenko, Nature 457, 569-572 (2009), URL

Temperature and Doping-Dependent Renormalization Effects of the Low Energy Electronic

Structure of Ba_1-xK_xFe₂As₂ Single Crystals

We investigate the low energy electronic structure of Ba_1-xK_xFe₂As₂ (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 NbSe₂

Using 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 ESR

An 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 Tb₂PdSi₃

Measurements 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

Large mixed metal nitride clusters encapsulated in a small cage: the confinement of the C-68-based clusterfullerenes

Three 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 Charging

The 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 Orbitals

DFT 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

Inelastic neutron scattering (INS), electron spin resonance (ESR), and nuclear magnetic resonance (NMR) measurements were employed to establish the origin of the strong magnetic signal in lightly-hole-doped La1-xSrxCoO3, x~0.002. Both INS and ESR low temperature spectra show intense excitations with large effective g factors ~10–18. NMR data indicate the creation of extended magnetic clusters. From the Q dependence of the INS magnetic intensity, we conclude that the observed anomalies are caused by the formation of octahedrally shaped spin-state polarons comprising seven Co ions. The present INS, ESR, and NMR data give evidence for two regimes in the lightly-hole-doped samples: (i) T<35 K dominated by spin polarons; (ii) T>35 K dominated by thermally activated magnetic Co3+ ions.A. Podlesnyak, M. Russina, A. Furrer, A. Alfonsov, E. Vavilova, V. Kataev, B. Büchner, Th. Strässle, E. Pomjakushina, K. Conder, and D. I. Khomskii, Phys. Rev. Lett. 101, 247603 (2008) URL

Linear Plasmon Dispersion in Single-Wall Carbon Nanotubes and the Collective Excitation Spectrum of Graphene

We have measured a strictly linear plasmon dispersion along the axis of individualized single-wall carbon nanotubes, which is completely different from plasmon dispersions of graphite or bundled single-wall carbon nanotubes. Comparative ab initio studies on graphene-based systems allow us to reproduce the different dispersions. This suggests that individualized nanotubes provide viable experimental access to collective electronic excitations of graphene, and it validates the use of graphene to understand electronic excitations of carbon nanotubes. In particular, the calculations reveal that local field effects cause a mixing of electronic transitions, including the “Dirac cone,” resulting in the observed linear dispersion. C. Kramberger, R. Hambach, C. Giorgetti, M. H. Rümmeli, M. Knupfer, J. Fink, B. Büchner, Lucia Reining, E. Einarsson, S. Maruyama, F. Sottile, K. Hannewald, V. Olevano, A. G. Marinopoulos, and T. Pichler Phys. Rev. Lett. 100, 196803 (2008) URL

High-Field Pauli-Limiting Behavior and Strongly Enhanced Upper Critical Magnetic Fields near the Transition Temperature of an Arsenic-Deficient LaO0.9F0.1FeAs1- Superconductor

We report upper critical field Bc2(T) data for disordered (arsenic-deficient) LaO0.9F0.1FeAs1- in a wide temperature and magnetic field range up to 47 T. Because of the large linear slope of Bc2 -5.4 to -6.6 T/K near Tc 28.5 K, the T dependence of the in-plane Bc2(T) shows a flattening near 23 K above 30 T which points to Pauli-limited behavior with Bc2(0) 63–68 T. Our results are discussed in terms of disorder effects within conventional and unconventional superconducting pairings. G. Fuchs, S.-L. Drechsler, N. Kozlova, G. Behr, A. Köhler, J. Werner, K. Nenkov, R. Klingeler, J. Hamann-Borrero, C. Hess, A. Kondrat, M. Grobosch, A. Narduzzo, M. Knupfer, J. Freudenberger, B. Büchner, and L. Schultz, Phys. Rev. Lett. 101, 237003 (2008) URL

Optical Study of LaO0.9F0.1FeAs: Evidence for a Weakly Coupled Superconducting State

We have studied the reflectance of the recently discovered superconductor LaO0.9F0.1FeAs in a wide energy range from the far infrared to the visible regime. We report on the observation of infrared active phonons, the plasma edge, and possible interband transitions. On the basis of this data and the reported in-plane penetration depth L(0)=254 nm [H. Luetkens et al., Phys. Rev. Lett. 101, 097009 (2008)] a disorder sensitive relatively small value of the total electron-boson coupling constant tot= e-ph+ e-sp~0.6±0.35 can be estimated adopting an effective single-band picture. S.-L. Drechsler, M. Grobosch, K. Koepernik, G. Behr, A. Köhler, J. Werner, A. Kondrat, N. Leps, C. Hess, R. Klingeler, R. Schuster, B. Büchner, and M. Knupfer, Phys. Rev. Lett. 101, 257004 (2008) URL

Significant Redistribution of Ce 4d Oscillator Strength Observed in Photoionization of Endohedral Ce@C82+ Ions

Mass-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

We present zero field and transverse field muon spin relaxation experiments on the recently discovered Fe-based superconductor LaFeAsO1-xFx (x ¼ 0:075 and x ¼ 0:1). The temperature dependence of the deduced superfluid density is consistent with a BCS s-wave or a dirty d-wave gap function, while the field dependence strongly evidences unconventional superconductivity. We obtain the in-plane penetration depth of abð0Þ ¼ 254ð2Þ nm for x ¼ 0:1 and abð0Þ ¼ 364ð8Þ nm for x ¼ 0:075. Further evidence for unconventional superconductivity is provided by the ratio of Tc versus the superfluid density, which is close to the Uemura line of high-Tc cuprates. H. Luetkens, H.-H. Klauss, R. Khasanov, A. Amato, R. Klingeler, I. Hellmann, N. Leps, A. Kondrat, C.Hess, A. Köhler, G. Behr, J. Werner, B. Büchner, Phys. Rev. Lett. Phys. Rev. Lett. 101, 097009 (2008), URL

Commensurate Spin Density Wave in LaOFeAs: A Local Probe Study

We 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 Clusterfullerenes

ffect 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 Calculations

An 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 Nanotubes

Encapsulated 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 LaFeAsO_0.9F_0.1

We have performed ⁷⁵As nuclear magnetic resonance measurements on aligned powders of the new LaFeAsO_0.9F_0.1 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

We present a calculation of the Hall coefficient in 2H -TaSe₂ and 2H-Cu_0.2NbS₂ based on their electronic structure extracted from angle-resolved photoemission spectra. The well-known semiclassical approach, based on the solution of the Boltzmann equation, yields the correct value for the normal-state Hall coefficient. Entering the charge density wave state results in the opening of the pseudogap and redistribution of the spectral weight. Accounting for this allows us to reproduce the temperature dependence of the Hall coefficient, including the prominent sign change, with no adjustable parameters. D. V. Evtushinsky, A. A. Kordyuk, V. B. Zabolotnyy, D. S. Inosov, B. Büchner, H. Berger, L. Patthey, R. Follath, S. V. Borisenko, Phys. Rev. Lett. 100, 236402 (2008) URL

Energy level alignment and injection barriers at spin injection contacts between La_0.7Sr_0.3MnO₃ and organic semiconductors

We have determined the energy level alignment at interfaces between La_0.7Sr_0.3MnO₃ and two typical organic semiconductors, copper-phthalocyanine and sexithiophene. La_0.7Sr_0.3MnO₃ 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