Photoelectron holography studies of Bi on Si(111)
Photoelectron holography is applied to Bi atoms adsorbed on Si(111) for a determination of the three-dimensional atomic structure. Instead of measuring the absolute intensity as a function of photon energy as is traditionally done, a differential measurement is carried out to yield the logarithmic derivative of the intensity. This leads to intensity self-normalization, and significantly improves the accuracy. The resulting images show a well-resolved trimer structure for the adsorption geometry.
Principles of energy extension in electron-emission holography.
We have derived the principles of energy extension in image reconstruction of electron-emission holography. We illustrate these principles with examples based on multiple-scattering simulations. The physical origins of small-window characteristics in forward-scattering and backscattering geometries are explained. We show that images reconstructed from backscattering holograms in the energy range 150-300 eV are split for the Ni(001) (1×1)-Cu system, and a method is introduced to remove this splitting. We provide a direct view of a system in which the adsorbate atom occupies an off-centered site
Two-dimensional gravitational anomalies, Schwinger terms and dispersion relations
Abstract We are dealing with two-dimensional gravitational anomalies, specifically with the Einstein anomaly and the Weyl anomaly, and we show that they are fully determined by dispersion relations independent of any renormalization procedure (or ultraviolet regularization). The origin of the anomalies is the existence of a superconvergence sum rule for the imaginary part of the relevant formfactor. In the zero mass limit the imaginary part of the formfactor approaches a δ-function singularity at zero momentum squared, exhibiting in this way the infrared feature of the gravitational anomalies. We find an equivalence between the dispersive approach and the dimensional regularization procedure. The Schwinger terms appearing in the equal time commutators of the energy momentum tensors can be calculated by the same dispersive method. Although all computations are performed in two dimensions the method is expected to work in higher dimensions too.