Jerzy Przystawa

High field magnetisation and hall effect of U3Sb4 single crystals

Abstract We have examined the Hall effect of U 3 Sb 4 (T c = 146 K) in the paramagnetic and ferromagnetic ranges as well as the magnetisation along three principal directions at 4.2 K and in fields up to 35 T. Both ordinary and extraordinary Hall constants are positive, giving the hole concentration of 1.2 × 10 20 cm −3 . The easy magnetic axis was found to be 〈100〉, and the uranium moment was determined to be 2μ B . Magnetisation jumps at 4.9 and 17.6 T in fields along the 〈111〈 and 〈110〉 direction, respectively, were also found. A microscopic spin model including crystal-field and anisotropic-exchange interactions predicts magnetisation behaviour in qualitative agreement with that observed experimentally for U 3 X 4 (X = P, As, Sb) compounds, and it shows that the observed jumps are spin-orientational transitions.

Magnetic field induced spin reorientation transitions in U3Sb4

Abstract Magnetisation curves along the 〈111〉 and 〈100〉 axes have been measured for U 3 Sb 4 at temperatures between 4.2 and 180 K and in fields up to 6 T. The temperature variation of the critical field as well as the value of the magnetisation jump observed at the spin reorientation induced by the field along the 〈111〉 axis, have been determined. The results are discussed in the framework of a microscopic spin model which takes into account both anisotropic crystal field and anisotropic exchange interactions.

On phase transitions with bilinearly coupled order parameters

It is explicitly shown that, contrary to the common assertion, phase transitions driven by two bilinearly coupled order parameters can be continous, i.e., irreducibility of the representation involved is not necessary for the phase transition to be of second order in the Landau sense.

On the order-disorder transition in crystalline adamantane C10H16

Abstract The reported phase transition in crystalline adamantane from a high temperature disordered state with symmetry Fm3m to an ordered structure with space group P 4 2 1 c has been studied by symmetry analysis. This analysis reveals that such a transition requires a complicated order parameter that is composed of four irreducible representations of the Fm3m space group. Although a group-subgroup relationship between the disordered and ordered phase exists, this result is difficult to accept within the Landau theory of phase transitions. Possible implications of this analysis are discussed.

Symmetry and phase transitions

The problem of symmetry changes at phase transitions is discussed in the light of recent developments in the theory of phase transitions, as well as in terms of classical Landau theory. Various group-theoretical criteria, or selection rules, are formulated and discussed from the point of view of how well they comply with the basic requirements of the theory of phase transitions, i.e. the existence of the absolute minimum of the Landau Potential (Landau-Ginzburg-Wilson Hamiltonian) and the continuity of the transition. The physical significance of these criteria is examined.

Violation of interest-rate parity: a Polish example

The mechanism of the so-called “Bagsik Oscillator” is presented and discussed. In essence, it is a repeated exploitation of arbitrage opportunities that resulted from a marked departure from the interest-rate parity relationship between the local Polish currency and the western currencies.

The Bagsik Oscillator without complex numbers

We argue that the analysis of the so-called Bagsik Oscillator, recently published by Piotrowski and Sladkowski, is erroneous due to: (1) the incorrect banking data used and (2) the application of statistical mechanism apparatus to processes that are totally deterministic.

Energy excitations in three-axial magnets with single-ion and exchange anisotropies

Abstract The energy spectra of three-axial ferromagnets like U 3 P 4 and ferrimagnets like U 3 Sb 4 are presented and their properties are discussed. In case of U 3 Sb 4 we find an interesting example of a magnetic structure which, due to the single-ion anisotropy, is unstable in the spin wave approximation. The k 2 dependence of magnon energies requires re-examination of the temperature dependence of the magnetisation and of the specific heat.