Born-von Kármán force constants of face-centred cubic metals studied by means of phonon frequency sums

Information concerning the Born-von 'Karmán (BvK) force constants of selected cubic metals are obtained from measured phonon frequencies by means of the gradual form of the frequency sum rule. Published model values of the BvK constants are commented on.     

Total charge screening

Solutions of classical massless scalar electrodynamics with a large external charge which exhibit total charge screening, that is, in which the charge in the field exactly neutralizes the external charge, are found. These solutions have lower energy than the unscreened Coulomb potential. The relation to the color screening problem in QCD is discussed.     

An operator formulation of the parton model

A compact and covariant formulation of the basic assumptions of the parton model is given in terms of field operators. Illustrative examples of the applications of these assumptions to current-induced processes are given.     

Representations of the rotation reflection symmetry group of the four-dimensional cubic lattice

The structure and representations of the rotation reflection symmetry group of the four-dimensional cubic lattice are described. Their connections with the representations of the three-dimensional lattice rotation reflection group, and with the representations of the continuous O(3) and O(4) groups are given.     

Application of interference microscopy to the study of hologram build-up in LiNbO3 crystals

Interference microscopy was applied to direct microscopic observation of temporal evolution of phase holograms in LiNbO₃:Fe photorefractive crystals. First a hologram was recorded in the sample, and diffraction efficiency was monitored during hologram build-up using inactinic laser light. Thus kinetics of hologram build-up could be determined. The initial hologram was erased using white light. Then a series of write-erase cycles were performed with increasing exposure times. Holograms were observed by interference microscope after each exposure. The time elapsed between the exposure and the microscopic observation was negligible compared to the relaxation time of the hologram. The obtained temporal evolution of the grating profile gives a deeper insight into the physical mechanism of hologram formation in photorefractive materials than simple diffraction efficiency measurements. A congruently grown sample of LiNbO₃ doped with 10⁻³ mol/mol Fe in melting was studied by this method. Sample thickness was set to 300 μm to allow correct microscopic observation. Plane-wave holograms were recorded in the samples using an Ar-ion laser at λ = 488.0 nm of grating constants of 3, 6.5 and 8.8 μm.     

A method for estimating glueball-meson mixing in lattice QCD

A systematic expansion of lattice QCD amplitudes based on the replica trick is discussed, the leading term of which is the quenched approximation. A parameter is defined that estimates the mixing between glueball and q$\text{q}$ meson states and provides a test for the reliability of the quenched approximation. The procedure is illustrated by an explicit Monte Carlo calculation for a model system on a one-dimensional lattice.     

Prediction of noise from trams

An improved calculation model for the prediction of levels of noise from trams is presented. The model is based on current calculation models for noise from trams and the measurements of equivalent noise levels from trams carried out in Kos?ice, Slovakia. The data were processed using correlation and regression analysis and besides already known factors (the number of tram sets passing a certain location per hour, tram speed, the type of tram rail, the tramline gradient), several new factors (wavelength of rail corrugations and rail grinding, the age of the tramcars and tramlines and the detailed composition of tram sets) have been included in the presented calculation model for the prediction of levels of noise from trams.     

Electrochemical impedance spectroscopic studies on niobium anodic dissolution in HF

Journal of Solid State Electrochemistry - The dissolution of niobium electrode in anodic regime in hydrofluoric acid was studied using electrochemical techniques. In the active region, complex...     

Quark fragmentation in gauge theories

A theory of quark fragmentation is based on the phenomenon of spontaneous pair creation in a strong Coulomb field, applied to the color field surrounding the leading quarks.