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Search Results: 1 - 10 of 4947 matches for " Christoph Tegenkamp "
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Organic Molecules on Wide-Gap Insulators: Electronic Excitations
Wei Chen,Christoph Tegenkamp,Herbert Pfnür
Physics , 2011,
Abstract: The electronic excitation of a conjugated molecule-insulator interface, as exemplified by the adsorption of benzoic acid and its phenolic derivative on NaCl(001) surface, is addressed by many-body Green's function methods. By solving the two-particle Bethe-Salpeter equation on top of the $GW$ quasiparticle energies, it turns out that instead of the intramolecular $\pi-\pi^\ast$ transition of the adsorbate, the lowest singlet excited state of the adsorbate system, being a charge transfer excitonic effect, is essentially assigned to the transition from the surface valence band maximum to the $\pi-\pi^\ast$ state of the molecule. An accurate description of this lowest electronic excitations confined at the interface requires the knowledge of a full excitonic Hamiltonian due to the sizable electron-hole exchange interaction.
Anomalous molecular orbital variation upon adsorption on wide band gap insulator
Wei Chen,Christoph Tegenkamp,Herbert Pfnür,Thomas Bredow
Physics , 2010, DOI: 10.1063/1.3431755
Abstract: It is commonly believed that organic molecules are physisorbed on the ideal non-polar surfaces of wide band gap insulators with limited variation of the electronic properties of the adsorbate molecule. On the basis of first principles calculations within density functional theory (DFT) and $GW$ approximation, we show that this is not generally true. We find that the molecular frontier orbitals undergo significant changes when a hydroxy acid (here we chose gluconic acid) is adsorbed on MgSO$_4$$\cdot$H$_2$O(100) surface due to the complex interaction between the molecule and the insulating surface. The predicted trend of the adsorption effect on the energy gap obtained by DFT is reversed when the surface polarization effect is taken into account via the many-body corrections.
Color centers in NaCl by hybrid functionals
Wei Chen,Christoph Tegenkamp,Herbert Pfnür,Thomas Bredow
Physics , 2010, DOI: 10.1103/PhysRevB.82.104106
Abstract: We present in this work the electronic structure and transition energies (both thermodynamic and optical) of Cl vacancies in NaCl by hybrid density functionals. The underestimated transition energies by the semi-local functional inherited from the band gap problem are recovered by the PBE0 hybrid functional through the non-local exact exchange, whose amount is adjusted to reproduce the experimental band gap. The hybrid functional also gives a better account of the lattice relaxation for the defect systems arising from the reduced self-interaction. On the other hand, the quantitative agreement with experimental vertical transition energy cannot be achieved with hybrid functionals due to the inaccurate descriptions of the ionization energies of the localized defect and the positions of the band edges.
Local transport measurements on epitaxial graphene
Jens Baringhaus,Frederik Edler,Christoph Neumann,Christoph Stampfer,Stiven Forti,Ulrich Starke,Christoph Tegenkamp
Physics , 2013, DOI: 10.1063/1.4821364
Abstract: Growth of large-scale graphene is still accompanied by imperfections. By means of a four-tip STM/SEM the local structure of graphene grown on SiC(0001) was correlated with scanning electron microscope images and spatially resolved transport measurements. The systematic variation of probe spacings and substrate temperature has clearly revealed two-dimensional transport regimes of Anderson localization as well as of diffusive transport. The detailed analysis of the temperature dependent data demonstrates that the local on-top nano-sized contacts do not induce significant strain to the epitaxial graphene films.
Direct Measurement of Surface Transport on a Bulk Topological Insulator
Lucas Barreto,Lisa Kühnemund,Frederik Edler,Christoph Tegenkamp,Jianli Mi,Martin Bremholm,Bo Brummerstedt Iversen,Christian Frydendahl,Marco Bianchi,Philip Hofmann
Physics , 2013, DOI: 10.1021/nl501489m
Abstract: Topological insulators are guaranteed to support metallic surface states on an insulating bulk, and one should thus expect that the electronic transport in these materials is dominated by the surfaces states. Alas, due to the high remaining bulk conductivity, surface contributions to transport have so-far only been singled out indirectly via quantum oscillations, or for devices based on gated and doped topological insulator thin films, a situation in which the surface carrier mobility could be limited by defect and interface scattering. Here we present the first direct measurement of surface-dominated conduction on an atomically clean surface of bulk-insulating Bi$_2$Te$_2$Se. Using nano-scale four point setups with variable contact distance, we show that the transport at 30 K is two-dimensional rather than three-dimensional and by combining these measurements with angle-resolved photoemission results from the same crystals, we find a surface state mobility of 390(30) cm$^{2}$V$^{-1}$s$^{-1}$ at 30 K at a carrier concentration of 8.71(7)$\times 10^{12}$ cm$^{-2}$.
Epitaxial graphene on SiC: Modification of structural and electron transport properties by substrate pretreatment
Mattias Kruskopf,Klaus Pierz,Stefan Wundrack,Rainer Stosch,Thorsten Dziomba,Cay-Christian Kalmbach,André Müller,Jens Baringhaus,Christoph Tegenkamp,Franz J. Ahlers,Hans W. Schumacher
Physics , 2015, DOI: 10.1088/0953-8984/27/18/185303
Abstract: The electrical transport properties of epitaxial graphene layers are correlated with the SiC surface morphology. In this study we show by atomic force microscopy and Raman measurements that the surface morphology and the structure of the epitaxial graphene layers change significantly when different pretreatment procedures are applied to nearly on-axis 6H-SiC(0001) substrates. It turns out that the often used hydrogen etching of the substrate is responsible for undesirable high macro steps evolving during graphene growth. A more advantageous type of sub-nanometer stepped graphene layers is obtained with a new method: a high-temperature conditioning of the SiC surface in argon atmosphere. The results can be explained by the observed graphene buffer layer domains after the conditioning process which suppress giant step bunching and graphene step flow growth. The superior electronic quality is demonstrated by a less extrinsic resistance anisotropy obtained in nano-probe transport experiments and by the excellent quantization of the Hall resistance in low-temperature magneto-transport measurements. The quantum Hall resistance agrees with the nominal value (half of the von Klitzing constant) within a standard deviation of 4.5*10(-9) which qualifies this method for the fabrication of electrical quantum standards.
Graphene Grown on Ge(001) from Atomic Source
Gunther Lippert,Jarek Dabrowski,Thomas Schroeder,Yuji Yamamoto,Felix Herziger,Janina Maultzsch,Jens Baringhaus,Christoph Tegenkamp,Maria Carmen Asensio,Jose Avila,Grzegorz Lupina
Physics , 2013, DOI: 10.1016/j.carbon.2014.03.042
Abstract: Among the many anticipated applications of graphene, some - such as transistors for Si microelectronics - would greatly benefit from the possibility to deposit graphene directly on a semiconductor grown on a Si wafer. We report that Ge(001) layers on Si(001) wafers can be uniformly covered with graphene at temperatures between 800{\deg}C and the melting temperature of Ge. The graphene is closed, with sheet resistivity strongly decreasing with growth temperature, weakly decreasing with the amount of deposited C, and reaching down to 2 kOhm/sq. Activation energy of surface roughness is low (about 0.66 eV) and constant throughout the range of temperatures in which graphene is formed. Density functional theory calculations indicate that the major physical processes affecting the growth are: (1) substitution of Ge in surface dimers by C, (2) interaction between C clusters and Ge monomers, and (3) formation of chemical bonds between graphene edge and Ge(001), and that the processes 1 and 2 are surpassed by CH$_{2}$ surface diffusion when the C atoms are delivered from CH$_{4}$. The results of this study indicate that graphene can be produced directly at the active region of the transistor in a process compatible with the Si technology.
Exceptional ballistic transport in epitaxial graphene nanoribbons
Jens Baringhaus,Ming Ruan,Frederik Edler,Antonio Tejeda,Muriel Sicot,Amina Taleb Ibrahimi,Zhigang Jiang,Edward Conrad,Claire Berger,Christoph Tegenkamp,Walt A. de Heer
Physics , 2013, DOI: 10.1038/nature12952
Abstract: Graphene electronics has motivated much of graphene science for the past decade. A primary goal was to develop high mobility semiconducting graphene with a band gap that is large enough for high performance applications. Graphene ribbons were thought to be semiconductors with these properties, however efforts to produce ribbons with useful bandgaps and high mobility has had limited success. We show here that high quality epitaxial graphene nanoribbons 40 nm in width, with annealed edges, grown on sidewall SiC are not semiconductors, but single channel room temperature ballistic conductors for lengths up to at least 16 micrometers. Mobilities exceeding one million corresponding to a sheet resistance below 1 Ohm have been observed, thereby surpassing two dimensional graphene by 3 orders of magnitude and theoretical predictions for perfect graphene by more than a factor of 10. The graphene ribbons behave as electronic waveguides or quantum dots. We show that transport in these ribbons is dominated by two components of the ground state transverse waveguide mode, one that is ballistic and temperature independent, and a second thermally activated component that appears to be ballistic at room temperature and insulating at cryogenic temperatures. At room temperature the resistance of both components abruptly increases with increasing length, one at a length of 160 nm and the other at 16 micrometers. These properties appear to be related to the lowest energy quantum states in the charge neutral ribbons. Since epitaxial graphene nanoribbons are readily produced by the thousands, their room temperature ballistic transport properties can be used in advanced nanoelectronics as well.
Monopole and multipole plasmons in a two-dimensional system
H. Pfnür,T. Langer,J. Baringhaus,C. Tegenkamp
Physics , 2011,
Abstract: Using monolayer graphene as a model system for a purely two-dimensional (2D) electron gas, we show by energy electron loss spectroscopy, highly resolved both in energy and momentum, that there is a significant probability for the excitation of not only one but two dispersing losses. The appearance of both losses is independent of the substrate (we tested graphene on the Si face of 6H-SiC(0001), and on Ir(111) without and with intercalated Na layer), and the ratio of the slope in the dispersion curves varies between 1.4 (SiC) and 2. While the lower dispersion curve can be attributed to the excitation of the monopole plasmon, in agreement with theoretical model calculations, the upper dispersion branch has not been identified before for plasmonic excitations in a 2D electron gas, and we assign it to the excitation of a multipole sheet plasmon.
Tuning of one-dimensional plasmons by Ag-Doping in Ag-$\sqrt{3}$-ordered atomic wires
U. Krieg,Y. Zhang,C. Tegenkamp,H. Pfnür
Physics , 2013, DOI: 10.1088/1367-2630/16/4/043007
Abstract: We generated arrays of silver wires with a height of one atom and an average width of 11 atoms on the Si(557) surface via self assembly with local $\sqrt{3}\times\sqrt{3}$ order, and investigated the 1D plasmon formation in them using a combination of high resolution electron loss spectroscopy with low energy electron diffraction. As it turned out by a series of thermal desorption experiments followed by adding small concentrations of Ag, pure Ag-$\sqrt{3}$ ordered arrays of nanowires, separated by (113) facets, are intrinsically semi-metallic or semiconducting. Added Ag atoms in the range up to few percent of a monolayer result in 1D plasmon formation without any concentration threshold. The quantitative Ag concentration dependence of the plasmonic losses is clearly non-linear and fully compatible with a $\sqrt{n_e}$ dependence of the 1D plasmon. Adsorption of traces of residual gas can have a qualitatively similar doping effect.
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