14 Externe wissenschaftliche Einrichtungen
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Item Open Access Cohesive properties of bcc and fcc rubidium from ab initio pseudopotentials(1985) Maysenhölder, Waldemar; Louie, Steven G.; Cohen, Marvin L.Total-energy calculations have been performed for Rb at zero temperature using a self-consistent ab initio pseudopotential approach within a local-density-functional scheme. The energy difference between fcc and bcc Rb, and the energy barrier between these structures, are found to be extremely small near the equilibrium volume. Agreement of the calculated cohesive properties of bcc Rb with experimental values is good in view of the softness of the material. A transition from bcc to fcc has been calculated to occur at a pressure of about 52 kbar for T=0 K, which compares favorably with the observed value of 70 kbar for this transition at room temperature.Item Open Access Analytical determination of the group velocity of an arbitrary Lamb wave from its phase velocity(1992) Maysenhölder, Waldemar-Item Open Access Some didactical and some practical remarks on free plate waves(1987) Maysenhölder, WaldemarIt is recalled how wave equations can be generated from dispersion relations. The bending wave equation for thin plates including some higher-order terms is obtained by this general method and compared to the corresponding equation of Timoshenko. Further, the dispersion of free plate waves is derived in a concise and elementary manner without using a scalar potential and a vector potential for the displacements. Finally, as a matter of practical interest, exact and approximate expressions for the ratio of parallel to perpendicular displacements at a plate surface are calculated. Under favourable circumstances measurement of this ratio allows decomposing an observed wave into its quasi-longitudinal and bending components.Item Open Access Calculations of point-defect properties in copper, silver, and gold based on three-body interactions(1982) Bauer, Rolf; Maysenhölder, Waldemar; Seeger, AlfredLong-term trends in the calculation of defect properties in the noble metals are criticezed. On physical grounds it is argued that the model potentials used should allow for many-body interactions. Results based on potentials which include three-body interaction terms are presented for Cu, Ag, and Au.Item Open Access On the determination of interaction parameters from correlations in binary alloys(1987) Maysenhölder, WaldemarThe determination of interaction parameters from correlation functions in binary alloys is discussed by means of high-temperature expansions for the correlations. The inherent nonlinearity of this "inverse problem" of statistical mechanics leading to non-unique solutions is emphasized. The formulation includes pair and triplet interactions. Some results of a specific "inverse Monte Carlo calculation" are criticized. The paper provides a collection of formulae, especially for probabilities and correlations, which should prove useful for clarifying the subject.Item Open Access Körperschallausbreitung in Gebäuden : Untersuchungen mit einem Intensitätsmeßverfahren(1988) Maysenhölder, WaldemarBauakustische Körperschallmessungen haben sich bisher fast ausschließlich auf die Messung von Schwingungsamplituden beschränkt ("Körperschallpegel"). Die für einen wirksamen Schallschutz so wichtige Frage, wie sich die Schwingungsenergie ausbreitet, wird aber durch eine reine Pegelmessung meistens nicht zuverlässig beantwortet. Zu diesem Zweck muß die Körperschallintensität gemessen werden. Unter gewissen Bedingungen läßt sich die Körperschallintensität relativ einfach aus dem Kreuzspektrum zweier Beschleunigungssignale bestimmen. Die Grundzüge dieser in der Bauakustik noch neuen Meßtechnik werden erläutert. Ihr Einsatz bei der Lokalisierung von Schallbrücken und beim Studium der Schalllängsdämmung zeigt die deutliche Überlegenheit gegenüber reinen Körperschallpegelmessungen.Item Open Access Proof of two theorems related to the energy of acoustic Bloch waves in periodically inhomogeneous media(1993) Maysenhölder, WaldemarTwo theorems which are applicable to plane waves in homogeneous elastic media are generalized to the case of periodic inhomogeneity. The first theorem is known as Rayleigh's principle and essentially says that, on average, kinetic energy equals potential energy. The second theorem states that group velocity and velocity of energy transport ("energy velocity") are the same. Both equalities are verified below for periodically inhomogeneous media. As distinguished from the homogeneous case one has to deal with Bloch waves instead of plane waves and has to perform spatial averages over an elementary cell of the lattice. By means of plausible arguments the theorems can also be applied to the modes of infinitely extended plates and beams.Item Open Access Two subroutines for calculating lattice sums and the distortion field due to a point force in hexagonal systems(1981) Maysenhölder, WaldemarThe subroutines ARRANGE and DISPL contained in HEXALAT have been developed for application of solid state theory to hexagonal crystals, in particular to the hexagonal closed packed (hcp) structures. ARRANGE generates the coordinates of a roughly estispherical arrangement of atoms which occupy regular sites of an hcp structure, and thus provides a means for calculating lattice sums very easily.Item Open Access The Eshelby factor for cubic crystals with arbitrary elastic anisotropy(1984) Maysenhölder, WaldemarThe Eshelby factor for cubic crystals has been calculated for arbitrary anisotropy. One of the two integrations needed could be performed analytically. An expansion up to 4th order in the anisotropy and exact results for special cases are also given.Item Open Access Rigorous computation of plate-wave intensity(1990) Maysenhölder, WaldemarExact analytical expressions exist for the displacement fields of elastic waves in homogeneous, isotropic, infinite plates of constant thickness. From these expressions the intensity and the time-averaged energy density of an arbitrary propagating mode at arbitrary frequency is derived without any approximation as a function of the spatial coordinate from surface to surface. Spatial averaging of these quantities across the thickness of the plate is also performed analytically and exactly. Practical application of the results to a particular mode requires the knowledge of its phase and group velocity, which in general have to be computed by numerical methods. Numerous diagrams illustrate the behaviour of the analytical expressions, especially for the quasi-longitudinal mode and the bending mode. Comparison of the exact intensity of the bending mode with the approximate expression from simple bending wave theory leads to a precise determination of the error which enters the conventional method of measuring the bending wave intensity. This offers the possibility of extending the conventional measuring technique beyond the validity of thin plate theory.