Structure function of a damped harmonic oscillator

Following the Caldeira-Leggett approach to describe dissipative quantum systems the structure function for a harmonic oscillator with Ohmic dissipation is evaluated by an analytic continuation from euclidean to real time. The analytic properties of the Fourier transform of the structure function with respect to the energy transfer (the ``characteristic function'') are studied and utilized. In the one-parameter model of Ohmic dissipation we show explicitly that the broadening of excited states increases with the state number without violating sum rules. Analytic and numerical results suggest that this is a phenomenologically relevant, consistent model to include the coupling of a single (sub-)nuclear particle to unobserved and complex degrees of freedom.Comment: 23 pages, 5 figures, RevTex4, minor changes following referee's comments and by PRC: the definite article in the original title has been droppe

Ab initio atomistic thermodynamics and statistical mechanics of surface properties and functions

Previous and present "academic" research aiming at atomic scale understanding is mainly concerned with the study of individual molecular processes possibly underlying materials science applications. Appealing properties of an individual process are then frequently discussed in terms of their direct importance for the envisioned material function, or reciprocally, the function of materials is somehow believed to be understandable by essentially one prominent elementary process only. What is often overlooked in this approach is that in macroscopic systems of technological relevance typically a large number of distinct atomic scale processes take place. Which of them are decisive for observable system properties and functions is then not only determined by the detailed individual properties of each process alone, but in many, if not most cases also the interplay of all processes, i.e. how they act together, plays a crucial role. For a "predictive materials science modeling with microscopic understanding", a description that treats the statistical interplay of a large number of microscopically well-described elementary processes must therefore be applied. Modern electronic structure theory methods such as DFT have become a standard tool for the accurate description of individual molecular processes. Here, we discuss the present status of emerging methodologies which attempt to achieve a (hopefully seamless) match of DFT with concepts from statistical mechanics or thermodynamics, in order to also address the interplay of the various molecular processes. The new quality of, and the novel insights that can be gained by, such techniques is illustrated by how they allow the description of crystal surfaces in contact with realistic gas-phase environments.Comment: 24 pages including 17 figures, related publications can be found at http://www.fhi-berlin.mpg.de/th/paper.htm

Radiation damage in reactors

From topical conference on energy; Chicago, Illinois, USA (4 Feb 1974). Fuel, cladding, structural materials, moderators, coolants, pressure vessels, and other components of nuclear reactors are bombarded by fast and thermal neutrons (with fluxes up to and above 10/sup 15/ cm/sup -2/ sec/sup -1/) at elevated temperatures. Fuel elements are also exposed to fission fragments; all components are exposed to high fluxes of gamma rays. As a result the physical properties of materials are modified, usually in undesirable ways. Brittleness, altered creep. and swelling which leads to complex deformations and induced stress are common problems. A broad understanding of these effects in terms of displaced atoms, point defects, dislocations. disordered regions, voids, and transmutation-induced impurities has been developed. Much empirical information is available, and means for lessening certain effects have been found. Knowledge of the details of atomic mechanisms, particularly their quantitative aspects, in the broad range of materials of interest is still inadequate, and the body of empirical data, particularly at the higher levels of exposure, needs to be extended. Bombardment of thin specimens with charged particles from accelerators has been a useful tool for simulating high neutron exposure, but the degree to which the simulation is adequate needs further investigation. (auth

Single crystal surface structure by bragg scattering

X-ray diffraction is becoming an important tool in the measurements of surface structures. Single crystalline samples are used as in Low Energy Electron Diffraction (LEED)-studies. The X-ray technique is somewhat more involved due to the need of bright, collimated photon sources, in general synchrotron X-rays, and of very accurate angular settings in the ultrahigh-vacuum environment of the sample. We present the technique and discuss examples of experimental results