Results 1 - 10 of 1618
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[en] A central key to understanding heavy-fermion systems involves revealing how itinerant low-energy excitations emerge from local f moments. An effective way to understand the f-electron behavior is to compare the electronic structure of f-electron compounds with a reference isostructural compound with no f electrons. Here we present a systematic electronic structure study of LaCoIn5, which is a non-f reference compound of the heavy-fermion superconductor CeCoIn5. Our angle-resolved photoemission spectroscopy study of the three-dimensional Fermi surface and band structure of LaCoIn5 highlights the rather three-dimensional electronic character of this compound. The conduction bands of LaCoIn5 are almost identical to that of CeCoIn5, though obvious differences can be found in the low-energy electronic structure. Finally, we give a quantitative analysis of the Fermi pocket change in LaCoIn5 and CeCoIn5, which may be helpful for understanding the important “large” and “small” Fermi surface issue in heavy-fermion compounds.
[en] We present a joint experimental and theoretical study of the electronic properties of the rebonded-step reconstructed Ge/Si(1 0 5) surface which is the main strained face found on Ge/Si(0 0 1) quantum dots and is considered a prototypical model system for surface strain relaxation in heteroepitaxial growth. Using a vicinal surface as a model system for obtaining a stable single-domain film structure with large terraces and rebonded-step surface termination, we realized an extended and ordered Ge/Si planar hetero-junction suitable for direct study with angle-resolved photoemission spectroscopy. At the coverage of four Ge monolayers photoemission spectroscopy reveals the presence of 2D surface and film bands displaying energy-momentum dispersion compatible with the 5 × 4 periodicity of the system. The good agreement between experiment and first-principles electronic structure calculations confirms the validity of the rebonded-step structural model. The direct observation of surface features within 1 eV below the valence band maximum corroborates previously reported analysis of the electronic and optical behavior of the Ge/Si hetero-interface. (paper)
[en] The internal transport barrier (ITB), Alfven Eigenmodes (AEs) and double tearing modes (DTM) have been observed during the off-axis sawteeth oscillation in EAST. The ITB of electron temperature Te is modulated by the sawteeth oscillation, and the formation of ITB can be divided into three stages: (1) the transport produced by sawteeth final crash is suppressed at the first stage with steep gradient of Te; (2) the micro-instability is developed at the second stage for the further increasing of the gradient of Te; (3) the ITB is formed eventually after the transition from BAEs to RSAEs, where the BAEs-RSAEs pair enables the tracking of qmin≤1 from the experiment directly.
[en] X-ray photoelectron spectroscopy (XPS) is a quantitative surface analysis technique used to identify the elemental composition, empirical formula, chemical state, and electronic state of an element. The kinetic energy of the electrons escaping from the material surface irradiated by an x-ray beam produces a spectrum. XPS identifies chemical species and quantifies their content and the interactions between surface species. It is minimally destructive and is sensitive to a depth between 1-10nm. The elemental sensitivity is in the order of 0.1 atomic %. It requires ultra high vacuum ((Formula presented.) Pa) in the analysis chamber and measurement time varies from minutes to hours per sample depending on the analyte. XPS dates back 50 years ago. New spectrometers, detectors, and variable size photon beams, reduce analysis time and increase spatial resolution. An XPS bibliometric map of the 10 000 articles indexed by Web of Science identifies five research clusters: (i) nanoparticles, thin films, and surfaces; (ii) catalysis, oxidation, reduction, stability, and oxides; (iii) nanocomposites, graphene, graphite, and electro-chemistry; (iv) photocatalysis, water, visible light, and TiO2; and (v) adsorption, aqueous solutions, and waste water. (author)
[en] The analysis of progressive failure process is one of the critical issues in the research of composite adhesive joints. In view of this point, acoustic emission (AE) is applied to real-time monitoring of the dynamic damage process of composite adhesive joints in some loading modes such as Mode I (double cantilever beam (DCB)) and Mode II (end notch flexures (ENF)). Furthermore, the high speed camera and scanning electron microscopy are carried to analyses the damage mechanisms of composite bonded joints. Results show that there are significant differences in the load- deflection curves of the specimens under the loads of Mode I and II. The main failure mode of the two types of loading modes is adhesion failure, and the accumulation of damage observed at the edge of the adhesively bonded layer cause defeat of the composite bonded joints. In addition, AE parameters including amplitude, hits, energy, and duration are connected with the fail mechanism. Furthermore, the dynamic characteristics of the AE signal, especially amplitude spectrum distribution, can provide evidences for studying progressive failure behaviors of composite adhesive joints. (paper)
[en] The research on bioactive materials, especially coatings, enables the production of resistant, nontoxic, biodegradable, and suitable antimicrobial surfaces for contact with food or humans. This project combined the effect of different types of chitosan in the manufacture of nanostructured films for antimicrobial coatings on textile substrates and silicon slides. Chitosan-based thin films were assembled using the layer-by-layer technique and the axial composition was accessed by X-ray photoelectron spectroscopy with depth profiling. Chitosan (Chi) samples possessing different average degree of acetylation (DA) and viscosity average molecular weight (Mv) were used in this study as well as two different polyanions, namely sulfonated polystyrene (SPS) and carboxymethyl cellulose (CMC). When chitosan, a positively charged polymer in aqueous acidic medium was combined with a strong polyanion (SPS), the total positive charge of chitosan, directly related to its average degree of deacetylation, was the key factor affecting the film formation and its structure. However, when a weak polyanion (CMC) was combined with chitosan, pH and viscosity average molecular weight of chitosan strongly affected film structure and composition. Following, we studied the molecular interdiffusion in thin films of CMC/Chi. There are few studies in the literature which explore the diffusion of a negatively charged polymer (in our case, the SPS) into a biopolymer thin film. This system illustrated in a simple way how chitosan interacts with different macromolecules (CMC and SPS). Higher binding energy values were observed for the Chi/SPS complex, as a result of the strong electrostatic interactions between the polymers. The CMC/Chi system is complemented with electrostatic interactions of weaker nature (i.e. hydrogen bounds). These changes in the binding energies were verified by the XPS high resolution spectra and provided a better understanding of LbL systems containing chitosan. Finally, we explored the spray LbL approach as a methodology to easy scale-up and capable of reducing the time required for the film build-up in comparison to the traditional dipping method. The variation of chitosan architecture, polyanion pair and pH shows that it is possible to molecularly control the chemical and structural properties of nanostructured coatings at the molecular level, thus opening up new possibilities to adapt them for the desired application, as shown by the antimicrobials tests. (author)
[en] Attosecond interferometry is becoming an increasingly popular technique for measuring the dynamics of photoionization in real time. Whereas early measurements focused on atomic systems with very simple photoelectron spectra, the technique is now being applied to more complex systems including isolated molecules and solids. The increase in complexity translates into an augmented spectral congestion, unavoidably resulting in spectral overlap in attosecond interferograms. Here, we discuss currently used methods for phase retrieval and introduce two new approaches for determining attosecond photoemission delays from spectrally overlapping photoelectron spectra. We show that the previously used technique, consisting in the spectral integration of the areas of interest, does in general not provide reliable results. Our methods resolve this problem, thereby opening the technique of attosecond interferometry to complex systems and fully exploiting its specific advantages in terms of spectral resolution compared to attosecond streaking. (paper)
[en] The electronic states of Au-induced atomic nanowires on Ge(0 0 1) (Au/Ge(0 0 1) NWs) have been studied by angle-resolved photoelectron spectroscopy with linearly polarized light. We have found three electron pockets around the line, where the Fermi surfaces are closed in a surface Brillouin zone (SBZ). The results indicate 2D Fermi surfaces of Au/Ge(0 0 1) NWs whereas the atomic structure is 1D. On the basis of the polarization-dependent spectra, the relation between SBZ and the direction of the atomic NW, and the symmetry of the surface state are clarified. These are very useful for further studies on the atomic structure of NWs. (paper)
[en] The band structures of two-monolayer Bi(110) films on black phosphorus substrates are studied using angle-resolved photoemission spectroscopy. Within the band gap of bulk black phosphorus, the electronic states near the Fermi level are dominated by the Bi(110) film. The band dispersions revealed by our data suggest that the orientation of the Bi(110) film is aligned with the black phosphorus substrate. The electronic structures of the Bi(110) film strongly deviate from the band calculations of the free-standing Bi(110) film, suggesting that the substrate can significantly affect the electronic states in the Bi(110) film. Our data show that there are no non-trivial electronic states in Bi(110) films grown on black phosphorus substrates. (paper)
[en] Noise is a common occurrence in many structural and functional materials during plastic deformation processes. The acoustic signal is hard to obtain for traditional materials due to the one-off nature of the deformation. But for shape memory alloys, this phenomenon can be analyzed systematically during cyclic tensile experiments, owing to the shape memory effect. Acoustic emission (AE) is studied in Ni46Mn28Ga20Co3Cu3 shape memory microwires with various diameters during stress-induced martensitic transformation. The microwires exhibit fully reversible strain larger than 10% and obvious serration behavior at room temperature. The AE waves obtained during tensile processing reflect the phonon softening and lattice vibration during stress-induced phase transformation. Analysis of the spectra shows that AE frequency concentrates in certain selected frequency ranges, and this result reveals the characteristic nature of collective atom movement. The correlations between the amplitude, cut-off frequency and size effect of the microwires were also analyzed, respectively. Focusing on the relationship between the acoustic properties and deformation behavior of shape memory alloys may provide a new perspective for materials science. (paper)