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[en] The charge carrier plasmon excitations of 2H-NbSe2 and related single crystalline systems have been investigated using electron energy-loss spectroscopy. These systems consist of hexagonal layers with weak interlayer van-der-Waals bonding. We compare two possible routes to explain the observed unusual negative plasmon dispersion; charge density wave plasmon interaction and band structure effects. Motivated by the orbital character of the plasmon dispersion in alkali metal doped transition metal dichalcogenides, we demonstrate that the separation of the plasmon from the screening contributions of single particle excitations by a Kramers-Kronig analysis reveals a positive (unscreened) plasmon dispersion.
[en] The electronic excitations of 2H-TaSe_2 and related single crystalline systems have been investigated using electron energy-loss spectroscopy. These systems consist of hexagonal layers with weak interlayer van-der-Waals bonding. We show the influence of alkali metal intercalation on the energy position and the dispersion of the charge-carrier plasmon. Further we demonstrate how the slope of dispersion is affected by the additional charge and the change of charge-carrier density in consequence. Our results indicate a universal evolution of the plasmon dispersion independent of the intercalant (Na, K) and the dichalcogenide (TaSe_2, TaS_2, NbSe_2).
[en] Ordinary bulk metallic glasses (BMG) are characterized by high strength, but are usually very brittle. To combine high strength with some ductility, it is necessary to develop new concepts of alloy designing. Here phase separated metallic glasses are chosen, which involve binary terminal systems with both, negative and positive enthalpy of mixing, whereby the structure consists of two different amorphous phases or of nanocrystalline inclusions in an amorphous matrix. Rods and splats of the BMG model system Cu-Zr-Al with small additions of different elements (Gd, Co or Re) with positive enthalpy of mixing to one of the alloying elements are prepared. Their microstructure and glass forming ability (GFA) in dependence of the sample size are investigated and the consequences for mechanical properties are discussed. The GFA is increased by addition of Gd but reduced by addition of Co or Re. The mechanical properties of the Re containing alloys differ strongly from the other alloys investigated by having higher plastic strain at lower stress.
[en] Highlights: • Hole doping of various surfaces by F_1_6CoPc. • Reduction of Cobalt center. • Potential surface functionalization by F_1_6CoPc in many other cases. - Abstract: Various interfaces with F_1_6CoPc as one of the partners are characterized by a charge transfer across the interface and a concomitant reduction of the Co central atom in these molecules. We summarize recent photoemission spectroscopy results and compare those to the data from equivalent interfaces comprising F_1_6CuPc. In the latter, the Cu center remains unchanged indicating the particular role of Co in F_1_6CoPc as the electron accepting site.
[en] In recent years epitaxial growth of transition metal oxides became possible and exotic phenomena were found at their interfaces. A prominent example is the formation of a high-mobility two-dimensional electron gas at the interface of LaAlO3 grown on TiO2-terminated SrTiO3 substrates. Such heterointerfaces fabricated by pulsed laser deposition under various growth conditions were investigated with soft X-ray photoemission spectroscopy. We focused on stoichiometric aspects as well as in-gap states and differences in the core levels. Deviations from an abrupt interface without defects were found.
[en] Single crystals are a prerequisite to perform angular dependent NMR measurements in solids. However, these were not available in sufficient size for doped LaOFeAs until recently. Here we present such NMR measurements on single crystals of LaOFeAs doped with 3.4%, 4.1%, and 6.2% Co. The underdoped samples show clear signatures of the nematic transition in the As NMR linewidth as well as in the spin lattice relaxation rate, T, and its anisotropy. At lower temperatures, a peak in T signals the onset of long-range magnetic order, whereas the superconducting transition cannot be determined unambiguously in a magnetic field of 7 T, which has been applied for the NMR measurements. In contrast, the optimally doped sample with 6.2% Co shows neither nematic nor long range magnetic order. Instead, T decreases at the superconducting transition, indicating bulk superconductivity at T = 12 K even in an applied field of 7 T. Our NMR results in Co doped LaOFeAs single crystals are broadly consistent with those obtained in Co doped BaFeAs.
[en] Electron energy-loss spectroscopy has been used to investigate the loss-function between 0 and 70 eV of single-crystalline Sr14-xCaxCu24O41, composed of two-leg Cu2O3 ladders and edge-sharing CuO2 chains, with various compositions. We found significant excitations in the low-energy range which are different for momentum transfer q parallel to the a- and c-axes. Comparison with reflectivity data from literature shows a good agreement with our data. Also the dispersions of a charge-carrier plasmon like in Bi2Sr2CaCu2O8 was obeserved for Sr3Ca11Cu24O41.
[en] The effect of small additives of Re on a metallic glass forming alloy (Cu46Zr46Al8)100-xRex (x = 1, 2) was investigated for samples of different sizes. Re possess a positive enthalpy of mixing within the Cu-Re terminal system. Splat quenched foils of ∼40 μm thickness display amorphous structure. Their crystallization temperature increases from Tx = 500 to 510 C with increasing Re fraction at nearly constant glass formation temperature Tg = 440 C. By contrast, injection cast rods consist of B2 - CuZr type metastable phase dendrites, a cubic CuZrAl type phase and randomly distributed small particles of a Re-rich phase. The presence of metastable phases leads to a unique combination of mechanical properties of as-cast rods which display high strength at sizeable plastic deformation up to εp ∼4% and an extended range of work hardening prior to failure.
[en] LiMnPO4 is a promising material for building the cathode of Li-ion batteries due to its high stability and large cation mobility. Yet, the mobility of the Li-ions in this material is not well understood. Here, we present 7Li and 31P Nuclear Magnetic Resonance (NMR) measurements on LixMnPO4 single crystals for different doping levels (x=0.6, x=0.8, and x=1). NMR is a powerful tool to investigate the microscopic ionic jump processes rather than the macroscopic material transport. The nuclear spin lattice relaxation rate, 1/T1, and the linewidth, σ, of the resonance lines show characteristic temperature dependencies related to the ionic jump processes that set in at a certain energy or temperature, respectively. The advantage of the use of single crystals is that the linewidth is not broadened by a distribution of linewidths as in a powder sample, but can show a fine structure that could be related to different sites in the crystal. Furthermore, single crystals allow for an orientation dependent investigation of the Li-ion mobility, where certain crystal orientations are preferred by the Li-cations.