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Nb3Sn is a promising thin film material for superconducting radio frequency (SRF) applications. Although surface resistivity, critical temperature, and critical field are advantageous in comparison to pure Nb, currently the performance of Nb3Sn is lacking behind due to its complex defect structure and phase inhomogeneities. In this work, the influence of the kinetic energy of the deposited particles on the defect structure in Nb3Sn thin films synthesized at low temperatures is investigated. A combination of extended x-ray absorption fine structure analysis, x-ray absorption spectroscopy mapping, and transmission electron microscopy reveals an improved local order and elemental homogeneity of the Nb3Sn films induced by higher kinetic energies of the elemental species during deposition. Even more, these process conditions lead to suppressed local inhomogeneities at grain boundaries, which can be one of the causes of critically reduced superconducting properties of low temperature sputter-coated Nb3Sn thin films. Finally, we show that the magnetic field-induced reduction of critical currents across weak-links formed at grain boundaries can be eliminated by the suggested materials' synthesis. [ABSTRACT FROM AUTHOR]

Laser polishing of material surfaces is a complex process depending on many variable parameters, such as, e.g., the properties of the used laser and optics (wavelength, pulse duration, fluence, and profile), as well as the processing (spot size, feed rate, and line or point overlap), and the thermodynamical properties of the material to be polished (heat capacity, heat conduction, etc.). For the successful laser polishing of any material, a systematic variation of all the process parameters is required to obtain satisfactorily polished surfaces with an appropriate set of parameters for the material of interest. In order to allow systematic studies of laser polishing processes, a new setup employing a highly stable nanosecond laser with an adjustable wavelength has been realized. The sample is located in a small high-vacuum chamber with the capability of introducing additional gases in a controlled manner, and the entire chamber is scanned in the beam to allow laser polishing of selected spots, lines, or larger areas. The setup is fully remote-controlled and allows in situ inspection of the initiated processes by means of a long-range microscope, electrical measurements, reflected laser light from the sample surface, and an analysis of the vacuum within the process chamber. The main properties of the setup will be presented, and some exemplary results on niobium and molybdenum metal samples will be discussed. [ABSTRACT FROM AUTHOR]

We report on fabrication and detailed characterization of point-type graphene emitters, which can be further used as electron sources in modern vacuum devices. Two-dimensional free-standing and triangular-shaped field emitters with the tip angles of 30°, 60°, and 90° and a height of ∼1 mm were made of thin graphene films of varying thicknesses (2.5-μm and 8-μm). The field emission properties of these emitters were systematically investigated by different measurement techniques. Wider and thicker graphene film emitters exhibited better stability and provided higher emission currents (up to ∼100–420 μA). The short-term current fluctuations stayed within 8%–14%. The graphene film emitter with a tip angle of 90° and 8-μm thick yielded a high field emission current of up to 2.2 mA at 9 V/μm. All emitters reproducible showed a non-linear Fowler–Nordheim behavior, which was correlated with the electron energy spectroscopy results. High-resolved energy spectra showed that up to three discrete peaks can be observed from the graphene edges at currents of <1 μA. The integral energy distributions of electrons at higher currents showed single broad emission spectra with a width of up to 1–2 eV. Additionally, graphene point emitters were characterized in various vacuum environments (Xe, Ar, N2, H2, O2, and air), different pressure levels (2 × 10−4 Pa, 2 × 10−3 Pa, and 2 × 10−2 Pa), and at various distances between the anode and the graphene emitter tip. The results showed that graphene emitters can operate in non-ultrahigh vacuum conditions, and further optimization of the vacuum gap could result in a lower turn-on voltage. [ABSTRACT FROM AUTHOR]

Flat Sb-doped germanium (100) crystals were investigated in the triode configuration under pulsed tunable laser illumination (pulse duration tlaser=3.5 ns and photon energy hv =0.54-5.90 eV) and under DC voltages <104 V. Large bunch charges up to ~1 µC were extracted from the cathodes for laser pulses >1 MW/cm² corresponding to a high quantum efficiency up to 3.3% and cathode currents up to 417 A. This laser-induced explosive electron emission (EEE) from Ge was characterized by its voltage-, laser power- and hv-sensitivity. The analysis of the macroscopic surface damage caused by the EEE is included as well. Moreover, we have carried out first direct measurements of electron energy distributions produced during the EEE from the Ge samples. The measured electron spectra hint for electron excitations to the vacuum level of the bulk and emission from the plasma plume with an average kinetic energy of ~0.8 eV. [ABSTRACT FROM AUTHOR]

We have investigated the properties of black Si field emitter arrays under strong electric fields and laser illumination. A low onset field of 1.8 MV/m for an emission current of 1?nA was obtained. A pronounced saturation region of the dark and photo-enhanced current was observed, which provided a short-term stability of 0.1% at 0.4 μA and 0.7% at 1.0 μA, respectively. As maximum value for the photosensitivity, an on-off current switching ratio of 43 reaching about 13 μA was achieved at a laser power of 15 mW. Electron spectra in the dark and under laser illumination are presented, showing a current and light-sensitive voltage drop across the emitters as well as hints for hot electron emission. [ABSTRACT FROM AUTHOR]

We report on formation of non-crystalline Fe-silicides of various stoichiometries below the amorphized surface of crystalline Si(001) after irradiation with 5 keV Fe+ ions under off-normal incidence. We examined samples prepared with ion fluences of 0.1×1017 and 5×1017 ions cm-2 exhibiting a flat and patterned surface morphology, respectively. Whereas the iron silicides are found across the whole surface of the flat sample, they are concentrated at the top of ridges at the rippled surface. A depth resolved analysis of the chemical states of Si and Fe atoms in the near surface region was performed by combining X-ray photoelectron spectroscopy and X-ray absorption spectroscopy (XAS) using synchrotron radiation. The chemical shift and the line shape of the Si 2p core levels and valence bands were measured and associated with the formation of silicide bonds of different stoichiometric composition changing from an Fe-rich silicides (Fe3Si) close to the surface into a Si-rich silicide (FeSi2) towards the inner interface to the Si(001) substrate. This finding is supported by XAS analysis at the Fe K-edge which shows changes of the chemical environment and the near order atomic coordination of the Fe atoms in the region close to surface. Because a similar Fe depth profile has been found for samples co-sputtered with Fe during Kr+ ion irradiation, our results suggest the importance of chemically bonded Fe in the surface region for the process of ripple formation. [ABSTRACT FROM AUTHOR]

The effect of pulsed laser polishing on rough niobium surfaces was investigated. We created different well-defined roughness profiles with standard emery papers and subsequently remelted random surface areas with a size of about 2 x 2 mm² with nanosecond laser pulses (wavelength of 1064 nm, pulse length of 10 ns). Pristine as well as laser-treated surfaces were investigated using optical profilometry and atomic force microscopy, and the surface topography was described by means of correlation functions. Uniformly rough and highly smooth surface geometries were achieved for fractals above and below 7 µm, respectively. Moreover, the behavior of foreign particles during the laser processing was investigated in detail. The polishing procedure was also monitored point by point by detecting electrical signals, i.e., sample charging, which resulted from the intense laser illumination. The measured electrical charges were found to be correlated with the local surface texture. Thus, regions with initially high roughness profiles and regions with extensive laser-induced defects could be directly identified from the detected electrical signals. [ABSTRACT FROM AUTHOR]

We report on the functional capabilities of our field emission (FE) measurement systems. The samples are prepared and inserted under clean room conditions and can be precisely xyz-positioned, heat-treated, and investigated in different vacuum environments and temperatures. The FE scanning microscope is a unique microscope being used for the localization of FE sites with high lateral resolution by means of extraction voltage U(x, y) or emission current I(x, y) maps over the cathode area of 25 × 25 mm2 and measuring the FE properties of localized emission sites or individual emitters. In contrast, the integral measurement system with the luminescent screen provides real-time integral information about the distribution of emission sites over the whole cathode during long- and short-term current stability measurements, thus allowing us to investigate the properties of various materials and their applications. Commissioning results with the upgraded systems using a silicon emitter array and a point-type graphene emitter will be given to demonstrate that both FE measurement techniques are very useful for an improved understanding and tailoring of materials for applications. The results showed that point-type graphene emitters are more preferable for practical applications. Using simulation software, a suitable triode configuration for point emitters was designed, and a current transmission ratio of ∼100% was obtained. The FE current stability of graphene emitters at pressures >10−3 Pa can be improved by heating the cathode at 100 °C–300 °C for 1 min–30 min. Finally, point-type graphene film emitters were used for transmission-type x-ray sources, and their applications in imaging and fluorescence spectroscopy are presented. [ABSTRACT FROM AUTHOR]

The structure/property relationships of fluorochlorozirconate glass ceramics as a function of divalent and trivalent europium (Eu) co-doping and thermal processing have been investigated; the influence of doping ratio on the formation of barium chloride (BaCl2) nanocrystals therein was elucidated. X-ray absorption near-edge structure spectroscopy shows that the post-thermal annealing changes the Eu valence of the as-poured glass slightly, but during the melting process Eu3+ is more strongly reduced to Eu2+, in particular, when doped as a chloride instead of fluoride compound. The Eu2+-to-Eu3+ doping ratio also plays a significant role in chemical equilibrium in the melt. X-ray diffraction measurements indicate that a higher Eu2+ fraction leads to a BaCl2 phase transition from hexagonal to orthorhombic structure at a lower temperature. [ABSTRACT FROM AUTHOR]

Time-resolved X-ray absorption spectroscopy are combined with UV-vis and Raman spectroscopies to study the in situ reduction of Ce(super 4+) to Ce(super 3+) in ethanolic solution with a time resolution of ca.4-5 s. It is observed that high quality results can be obtained simultaneously, thus giving a detailed insight into the mechanisms of the investigated chemical reaction.

Lithium intercalation in anatase TiO2 results in structural and electronic changes. A two-phase system is completely suited in the atomic structure (EXAFS) and the electronic structure (XANES) regions of the X-ray absorption spectra of anatase TiO2 upon Li intercalation.

The depth distribution of secondary phases in the solar cell absorber material Cu2ZnSnS4 (CZTS) is quantitatively investigated using X-ray Absorption Near Edge Structure (XANES) analysis at the K-edge of sulfur at varying incidence angles. Varying information depths from several nanometers up to the full thickness is achieved. A quantitative profile of the phase distribution is obtained by a self-consistent fit of a multilayer model to the XANES spectra for different angles. Single step coevaporated CZTS thin-films are found to exhibit zinc and copper sulfide secondary phases preferentially at the front or back interfaces of the film. [ABSTRACT FROM AUTHOR]

We report an explicit experimental observation of photostimulated electron field emission from diamond. The electron emission properties of monocrystalline diamond needles were investigated in the dark and under illumination by nanosecond laser pulses. A prominent increase in the electron emission current was detected under illumination by light with photon energies above 5.0 eV. The linear dependence of the photoinduced emission current on the light intensity was observed in the spectral range of 5.0 to 5.9 eV, while its field dependence demonstrated saturation behavior. The remarkable feature of the observed phenomenon was the fact that illuminated and field emission areas of the diamond needles were spatially separated by about 100 lm in the used experimental setup. Possible mechanisms for the observed effects are discussed. [ABSTRACT FROM AUTHOR]

The design and capabilities of a novel Quick scanning Extended X-ray Absorption Fine Structure (QEXAFS) monochromator are presented. The oscillatory movement of the crystal stage is realized by means of a unique open-loop driving scheme operating a direct drive torque motor. The entire drive mechanics are installed inside of a goniometer located on the atmospheric side of the vacuum chamber. This design allows remote adjustment of the oscillation frequency and spectral range, giving complete control of QEXAFS measurements. It also features a real step-scanning mode, which operates without a control loop to prevent induced vibrations. Equipped with Si(111) and Si(311) crystals on a single stage, it facilitates an energy range from 4.0 keV to 43 keV. Extended X-ray absorption fine structure spectra up to k = 14.4 Å-1 have been acquired within 17 ms and X-ray absorption near edge structure spectra covering more than 200 eV within 10 ms. The achieved data quality is excellent as shown by the presented measurements. [ABSTRACT FROM AUTHOR]

Nanocarbon films with upstanding flake-like graphite crystallites of nanometre thickness were fabricated by carbon condensation from a methane-hydrogen gas mixture activated by a direct-current discharge. The nanographite (NG) crystallites are composed of a few graphene layers. The adjacent atomic layers are connected partially at the edges of the crystallites to form strongly curved graphene structures. The extraordinary field emission (FE) properties were revealed for the NG films with an average current density of a few mA/cm², reproducibly obtained at a macroscopic applied field of about 1V/μm. The integral FE current-voltage curves and electron spectra (FEES) of NG cathodes with multiple emitters were measured in a triode configuration. Most remarkably, above a threshold field, two peaks were revealed in FEES with different field-dependent shifts to lower energies. This behaviour evidences electron emission through a dual potential barrier, corresponding to carbon-carbon heterostructure formed as a result of the graphene bending. [ABSTRACT FROM AUTHOR]

We have completed an ultra-high vacuum system for sensitive fast electron spectroscopy from cold cathodes in triode configuration under high electric fields E (<100 MV/m) and pulsed tunable laser illumination (3.5 ns, 10 Hz, hv = 0.5-5.9 eV, and 0.3-17 mJ). The cathodes are prepared and inserted under clean room conditions and can be precisely 3D-positioned, cooled or heated (77-400 K). Commissioning results with the upgraded system are presented. Field emission measurements with a W tip yielded an energy resolution of 14 meV at 4 eV pass energy and a precise determination of the emitter work function, size, and temperature. Photoemission spectroscopy of short electron bunches from a virgin and laser-ablated S-GaP crystal and quantum efficiency measurements revealed surface states, energy relaxation, and band structure effects. In conclusion, this novel system is ready now for the development and characterization of photo-induced field emission cathodes. [ABSTRACT FROM AUTHOR]

A fast X-ray chopper capable of producing ms long X-ray pulses with a typical rise time of few μs was realized. It is ideally suited to investigate the temporal response of X-ray detectors with response times of the order of µs to ms, in particular, any kind of ionization chambers and large area photo diodes. The drive mechanism consists of a brushless DC motor and driver electronics from a common hard disk drive, keeping the cost at an absolute minimum. Due to its simple construction and small dimensions, this chopper operates at home lab based X-ray tubes and synchrotron radiation sources as well. The dynamics of the most important detectors used in time resolved X-ray absorption spectroscopy, namely, ionization chambers and Passivated Implanted Planar Silicon photodiodes, were investigated in detail. The results emphasize the applicability of this X-ray chopper. [ABSTRACT FROM AUTHOR]