Killing vectors of vacuum space times and electromagnetic four potentials

A conjecture is formulated about the connection between isometries of vacuum space times and the existence of corresponding space times with electromagnetic fields for which the structure of electromagnetic four potentials is readily predicted. One example is presented showing how this conjecture works, three others are only commented.     

Infrared absorption spectrum of a small-polaron system

Characteristic features and the temperature dependence of the small-polaron absorption spectrum are discussed using a recently published model. The physical interpretation of formal results is introduced, in particular a connection of small-polaron states with oscillator coherent states is emphasized. The continuous change of absorption curves with temperature is illustrated by numerical calculations.     

Physics with and without the equivalence principle

A differential manifold (d-manifold, for short) can be defined as a pair (M, C), where M is any set and C is a family of real functions on M which is (i) closed with respect to localization and (ii) closed with respect to superposition with smooth Euclidean functions; one also assumes that (iii) M is locally diffeomorphic to Rⁿ. These axioms have a straightforward physical interpretation. Axioms (i) and (ii) formalize certain compatibility conditions which usually are supposed to be assumed tacitly by physicists. Axiom (iii) may be though of as a (nonmetric) version of Einstein's equivalence principle. By dropping axiom (iii), one obtains a more general structure called a differential space (d-space). Every subset of Rⁿ turns out to be a d-space. Nevertheless it is mathematically a workable structure. It might be expected that somewhere in the neighborhood of the Big Bang there is a domain in which space-time is not a d-manifold but still continues to be a d-space. In such a domain we would have a physics without the (usual form of the) equivalence principle. Simple examples of d-spaces which are not d-manifolds elucidate the principal characteristics the resulting physics would manifest.on leave of absence from the Institute of Nuclear Physics, Department of Theoretical Physics, ul. Radzikowskiego 152, 31–342 Cracow, Poland.     

Speeding-up and scaling of domain-wall relaxation nearT c of a uniaxial ferromagnet

The dynamic susceptibility () measured between 10 Hz and 10 MHz on different GdCl₃-ellipsoids (T _c=2.21 K) reveals a completely reversible motion of the domain walls. Taking into account the contribution of the fast adiabatic intradomain magnetization to the nearly Debye-shaped (), we determine for the first time the kinetic Onsager coefficientL _d of the domain wall relaxation of a ferromagnet. Approaching the CurietemperatureL _d speeds up critically, which by a novel simple relaxational model can be related to the increasing width (=correlation length) of linear Bulaevskii-Ginzburg domain-walls and to the shrinking domain period. The reduction ofL _d by an external field can be represented by a universal scaling function, and within the same dynamical model, this effect is ascribed to the increase of the domain period, predicted for a bubble phase. However, the effect of sample size onL _d is much smaller than expected.     

Dynamical scattering of X-rays by dislocations in the Bragg and Laue case

The interbranch scattering of X-rays at dislocations is studied experimentally and with the aid of numerical simulations. Narrow beam of a small divergence is used for the study which is made both in the Laue and Bragg case of diffraction. The role of interbranch scattering in the formation of defect contrast in X-ray topography is discussed.     

Valence change of europium in the Eu(Ir1−x Pd x )2Si2 silicides

Eu(Ir_1–x Pd _x )₂Si₂ solid solutions which exist only for 0x0.125 and 0.75x1 crystallize in the tetragonal ThCr₂Si₂-type structure. X-ray diffraction data, magnetic susceptibility and¹⁵¹Eu Mössbauer measurements suggest that these compounds can be characterized as homogeneous mixed valence systems. At room temperature and for 0x0.125, the europium valence decreases asx increases. For 0.75x1, a sharp continuous valence transition from Eu²⁺ to Eu³⁺ occurs near 48 K, 54 K and 78 K forx=0.75, 0.81 and 0.94 respectively. These valence changes are discussed in relation with the Eu–(Ir, Pd) interatomic distance.     

Complementary influence of electric and magnetic fields on the sample voltage oscillations ofP-type ultrapure germanium

We report on a previously not observed behaviour of oscillations of the voltage over a hyperpure germanium sample, when increasing the sample current from 0.05 to 13 A, for a number of transverse magnetic fields ranging from 697 to 1746 gauss. The sample temperature was 7.54 K±0.02 K. For each magnetic field the first observed peaks in the frequency spectrum of the sample voltage go to chaos by the period-doubling route and simultaneously shift to lower frequencies (first scenario). Superimposed upon this chaotic spectrum a new peak emerges, which again shifts to lower frequencies with increasing sample current (second scenario). A complementary effect of the sample current and the magnetic field is detected for four properties: firstly, the sudden onset of high resistance in the current versus voltage diagram, secondly, the transition between the two frequency scenarios described above, thirdly the presence of certain frequencies in the sample voltage spectra and fourthly the occurrence of an amplitude maximum for the principal frequency of the first scenario. It is shown that the first two transitions are interrelated.