[en] An electron density distribution n(r) which can be represented by that of a single-determinant ground state of noninteracting electrons in an external potential v(r) is called pure-state v -representable (P-VR). Most physical electronic systems are P-VR. Systems which require a weighted sum of several such determinants to represent their density are called ensemble v -representable (E-VR). This paper develops formal Kohn-Sham equations for E-VR physical systems, using the appropriate coupling constant integration. It also derives local density- and generalized gradient approximations, and conditions and corrections specific to ensembles

[en] Intersubband (ISB) plasmons in remotely doped wide quantum wells acquire a linewidth even at zero temperature and in-plane wave vector q_#parallel#=0 by a combination of intrinsic (electron-electron interaction) and extrinsic effects (impurities and interface roughness). We present a quantitatively accurate theory of the linewidth that treats both effects on equal footing and from first principles by a combination of time-dependent density-functional theory with the memory function formalism. Comparison with recent optical absorption experiments shows that the ISB plasmon linewidth has a significant contribution from electron-electron interaction, and is only weakly related to the mobility

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[en] In conventional nuclear technology a lot of experience has been accumulated meanwhile for ferritic alloys, austenitic steels and Ni-based alloys as main component materials during R and D, design, construction and operation. Future Nuclear Power Plants such as Liquid Metal Reactors (LMRs), High Temperature Reactors (HTRs) or Fusion Reactors (FRs) are complex in the design and experiences with coolants/components are limited. Generally materials and their joining are key issues and their performance in components has not yet been demonstrated. Grades commonly used in fission reactors or conventional power systems have been proposed for the Gen IVs due to the extensive amount of work for the qualification and for code application. For high temperature operating conditions in heat exchangers of HTRs conventional alloys such as Alloy 800H or Alloy 617 have been specified in previous European projects (AVR, THTR, PNP,..). Both alloys have been tested already during the extensive German R and D activities for HTR/PNP projects between 1980-90. For the reactors of GEN IV, information gained from these activities and also from the European FBR -development (EFR) can be used and the available data may limit the extend of necessary R and D work. Design relevant properties especially of Alloy 617 from the previous R and D activities are summarized and compared with some recent results. The creep data from investigations for conventional power plants in the range of 700-800 deg. C fit well the data from the previous HTR programme. Problems such as reheat cracking are discussed. Critical issues of materials response to the helium environments are reviewed and the effect of ageing on mechanical properties. (author)

[en] A theory of transport in spin and charge disordered media is developed, with a particular emphasis on dilute magnetic semiconductors. The approach is based on the equation of motion for the current-current response function and considers both spin and charge disorder and electron-electron interaction on an equal footing. The formalism is applied to the specific case of Ga_1-xMn_xAs. Within the single parabolic band approximation it is shown that both spin (p-d exchange) and charge (Coulomb) scattering contributions to the resistivity are of the same order of magnitude and should be treated simultaneously. Positional correlations of charged impurities are shown to significantly increase the Coulomb scattering. In the magnetically ordered phase, the suppression of localized spin fluctuations leads to a sizable reduction of spin scattering, which may contribute to the experimentally observed drop in resistivity below the critical temperature. The developed model allows for a comprehensive treatment of electron-electron interaction, screening and correlation effects by means of time-dependent density-functional theory. It is shown that collective modes and a dynamical treatment of electron-electron interaction are essential for an accurate description of the infrared absorption spectrum.

[en] The evolution of microstructure defects under compression, their mutual interactions at high defect densities and the effect of these phenomena on the hardening were studied in metastable austenitic steel containing 16 wt% Cr, 6 wt% Mn and 9 wt% Ni. At this chemical composition, the estimated martensite start temperature and the stacking fault energy of austenite are about −240 °C and 26 mJ/m"2, respectively. Consequently, the metastable austenite should be thermodynamically stable far below the room temperature and the formation of stacking faults should be retarded. The microstructure analysis, which was performed by means of XRD, ECCI, EBSD and TEM, revealed that the dislocation slip is the dominant deformation mechanism in the initial stages of the deformation process (ε≤0.05). After a critical stress level was reached, stacking faults started to form and widen. In regions with high local stacking fault densities, twinning and the formation of ε-martensite were detected simultaneously. The local deformation mechanism and the related microstructure changes were found to depend on the orientation of the respective grain. In favorably oriented grains, deformation bands consisting of ε-martensite and twins were predominantly formed in the primary slip system. In other grains, the dislocation slip remained the dominant deformation mechanism. The coexistence of faulting, twinning and dislocation slip and the interaction between the dislocations and stacking faults are discussed as the main reasons for the high observed density of microstructure defects and high hardening.

[en] The Hohenberg-Kohn (HK) theorem is one of the most fundamental theorems of quantum mechanics, and constitutes the basis for the very successful density-functional approach to inhomogeneous interacting many-particle systems. Here we show that in formulations of density-functional theory (DFT) that employ more than one density variable, applied to systems with a degenerate ground state, there is a subtle loophole in the HK theorem, as all mappings between densities, wave functions, and potentials can break down. Two weaker theorems which we prove here, the joint-degeneracy theorem and the internal-energy theorem, restore the internal, total, and exchange-correlation energy functionals to the extent needed in applications of DFT to atoms, molecules, and solids. The joint-degeneracy theorem constrains the nature of possible degeneracies in general many-body systems

[en] The requirements for materials and their strength significantly increased with the new generation of coal fired power plants operating at steam temperatures of up to 620 °C. Therefore, new materials were introduced to fulfill the defined needs. During the commissioning process of the first plant many cracks occurred in welds of T24 material. The cracks showed clear characteristics of stress corrosion cracking (SCC). Not knowing the exact parameters that lead to cracking, experiments in high temperature water were carried out. Slow tensile tests in a controlled environment are extremely well suited to generate information about material's SCC sensitivity. In the present paper, the influence of the temperature, the oxygen concentration of water, the pre-treatment of the specimen and the heat treatment to the SCC are investigated. Furthermore critical limits for the cracking are defined where possible.

[en] Highlights: • Microstructure changes in deformed metastable austenitic steel were studied in situ. • The texture formation is affected by deformation-induced phase transformations. • A competition between the deformation and transformation textures was observed. • The microstructure features behind this competition were identified and explained. • Role of stacking faults, orientation relationship and variant selection is discussed. The interplay of microstructural mechanisms controlling the deformation-induced martensitic phase transformations and the texture formation in all phases of a metastable austenitic Cr-Mn-Ni steel was investigated using in situ synchrotron radiation diffraction under uniaxial compression and ex situ electron backscatter diffraction. With increasing deformation, the originally fully austenitic steel transformed to a mixture of γ-austenite, ε-martensite and α´-martensite. The face centred cubic γ-austenite formed a fibre texture {110} with respect to the deformation direction. The texture degree increased progressively with increasing deformation. The hexagonal close packed ε-martensite was preferentially oriented with the reciprocal direction $\left\{10\overline{1}3\right\}$ along the load axis. The texture degree was nearly independent of the deformation extent. The body centred α´-martensite formed a mixed texture {100} & {111} along the deformation direction. The texture component {100} was very strong in the early stages of the α´-martensite formation, but it deteriorated with increasing deformation. The texture evolution is explained by the competition between the transformation texture, several deformation-induced mechanisms, which are highly sensitive to the local orientation of the grains with respect to the acting force, like the stacking fault formation and martensitic transformation in austenite, and the variant selection in both martensites and the twinning of α´-martensite.

[en] In this study, the influence of various nitrogen contents (0.12–0.23 wt.%) on the mechanical properties, especially the strain-induced α'-martensite formation behavior, of the austenitic stainless steel X3CrMnNiMoN17-8-4 was investigated by temperature dependent in situ tensile tests. With the aid of in situ magnetic measurements during tensile test, the correlation between the strain-induced α'-martensite formation and the inflection points in the true stress-strain curve could be verified. In addition, a connection between the in situ measured α'-martensite formation rate and the strain hardening curve was established. As the temperature decreases, the formation of a large strain-induced α'-martensite fraction allows a strong increase in strength accompanied by a simultaneous decrease in elongation. The α'-martensite volume fraction increase and the triggering stress for the strain-induced martensite formation decreases with decreasing nitrogen content from to 0.23 wt.% to 0.12 wt.%. The deformation mechanisms taking place at various temperatures during tensile test were analyzed by microstructure analysis. As expected, a transition from Transformation Induced Plasticity to Twinning Induced Plasticity behavior was observed with increasing temperature. Compared to the other examined steels, the steel with 0.19 wt.% nitrogen has the highest tensile strength of 856 MPa accompanied by an excellent total elongation of 75 % at RT.