P. Gusin

Magnetic Double Exchange Interaction as a Driving Force of the Coexistence of the Negative Giant Magnetoresistance and the Spin Glass State in the Selected Re-Manganites and the Spinels with Chromium

In this article is pointed out the particularly important role of the double exchange magnetic interaction in the appearance of the phenomenon of the coexistence of the negative giant magnetoresistance and the spin glass state in the selected RE-manganites and the spinels with chromium. To explain this phenomenon, its statistical background and the types of magnetic interactions in the compounds under study are taken into account. The particular importance of the double exchange magnetic interaction “cooperating” with the strong external magnetic induction results here from that: i. its coupling constant is several times greater than the superexchange one, ii. the spin orientation of the hopping electrons is conserved (always ferromagnetic coupling!) and the charge transfer makes the p-type metallic conductance.

Correlations of the electrical and magnetic phase transitions in the Sm1-xSrxMnO3 (x=0.33, 0.45) and RE0.55Sr0.45MnO3 (RE0.55 = Eu0.397Nd0.153,Tb0.25Nd0.30) manganites at the high magnetic stationary fields

Motto The first address of Satan to the human race began most modestly with the word: why? Adam Mickiewicz (1798–1855) The resistance, magnetoresistance and magnetization measurements have been performed in the temperature range of 4.2–300 K and in the external magnetic inductions up to 14 T for the Sm1− x Sr x MnO3 (x = 0.33, 0.45) and RE0.55Sr0.45MnO3 (RE0.55 = Eu0.397Nd0.153, Tb0.25Nd0.30) manganites. In these samples the metal–semiconductor phase transitions as well as two kinds of the magnetic phase transitions the ferromagnetic–paramagnetic and metamagnetic (both spin-flip and spin-flop) have been observed. It was found that correlations between the magnetic and electrical phase transitions appear at high values of the external magnetic inductions. These correlations consist, simultaneously, of the large negative magnetoresistance and metamagnetic phase transitions. The existence and reconstruction of the ferromagnetic and antiferromagnetic clusters under the influence of the external magnetic induction is considered as a driving force of those correlations.

The volume of the black holes — The constant curvature slicing of the spherically symmetric spacetime

We consider the problem of determination of a volume of some bounded space-like hypersurfaces in the case of spherically symmetric spacetimes. In the case when the hypersurface is cut or bounded by a light-like hypersurface, the problem may not be well-defined. In order to define the volume of a hypersurface properly, we introduce the volume forms related to the given foliation (observer) of the considered spacetime. In the case of the constant curvature slice, the volume of the hypersurface cut by the horizon (light-like surface) becomes composed of the two, outer- and inner-horizon parts, treated differently. We compute the corresponding volumes outside and inside of the horizon of the eternal Schwarzschild black hole.

Spin Glass State, Gaussian Distribution, the Stability Magnetic Field and the Fibre Bundle Approach

The fibre bundle approach has been applied to the unified description of the magnetic symmetries of different magnetic structures. In the particular case of the spin glass state the global magnetic coupling constant has been interpreted as a section of the corresponding fibre bundle. The fibre of this bundle makes the space of the Gaussian distributions. Thus one can say that the randomness of the spatial distribution of the magnetic moments in the spin glass state is of the Gaussian-like character.

CAUSAL TRANSFORMATION OF GÖDEL-TYPE SPACETIMES IN CONFORMAL FIELD THEORY

The Godel-type metrics are considered as backgrounds of the sigma-models. In the conformal field theory such backgrounds are deformed by the exactly marginal operators. We examine, how the closed timelike curves (CTCs) transform under such deformations.

On the Relation of the ro/ra versus rt/ra Plot with Direct and RKKY Exchanges in the Compounds Containing Chromium

An attempt was made to explain the physical sense of the special r o /r a vs. r t /r a plot being a phase diagram for the variety of chromium compounds, especially spinels, Krok-Kowalski et al., J. Alloys Compd. 315, 62 (2001). Here r t is the ionic radius of cations in the tetrahedral positions, r o the ionic radius of cations in the octahedral positions, r a the ionic radius of anions. Therefore, a model was proposed in which only the direct and RKKY exchanges were taken into account. The predictions of the presented model are adequate to the experimental data contained in this plot.

D2-brane as the wormhole and the number of the universes

We construct wormhole-like solutions in the type IIA string theory. These solutions represent wormholes in four-dimensions and are given by the D2 branes within appropraited backgrounds fields. We present the conditions on these fields which lead to the four-dimensional wormholes. In the special case we show how the particular solution in the type IIA theory leads to the dynamic wormhole. We also speculate about the number of universes and the cosmological constant.

Spin glass state structure theory and its application to explain experiments

The authors have found the general description of all the eight fundamental magnetic structures and their symmetry groups (including spin glass state - sgs) with the use of the fibre bundle approach [1]. The structure of sgs turns out to consist in the Gaussian-like randomness of the orientations of all the separate atomic magnetic moments in the sample. It means that the magnetization vector M is also subject to Gaussian distribution. Thus the structure of the sgs is represented by the magnetization vector situated along the generatrix of a certain cone whose axis coincides with the direction of an internal spontaneous magnetic field indispensable for stability of the sgs in a sample [2]. The axis of the 3-dimensional Gaussian distribution mentioned above coincides also with the same direction. Every precession angle of M around this field makes the symmetry operation of sgs once the angle between M and this field remains constant. Thus the symmetry group of sgs turns out to be SO(2). The temperature dependence of magnetic susceptibility of sgs in the alloys CuMn [3] corresponds when heating to the gradual degeneration of the Gaussian distribution of the magnetization vector, and when cooling to its gradual recovery with no hysteresis. In both cases an internal spontaneous magnetic field mentioned above plays an essential role. The interrelation of this field, of the Gaussian distribution mentioned above, of the average kinetic energy of magnetic atoms and of the frustration of orientations of the magnetic moments placed on these atoms will be discussed.

THE MODIFICATION OF THE ENERGY SPECTRUM OF CHARGED PARTICLES BY EXOTIC OPEN 4-SMOOTHNESS VIA SUPERSTRING THEORY

In this paper we present a model where the modified Landau-like levels of charged particles in a magnetic field are determined due to the modified smoothness of ℝ4 as underlying structure of the Minkowski spacetime. The standard smoothness of ℝ4 is shifted to the exotic , k = 2p, p = 1, 2, …. This is achieved by superstring theory using gravitational backreaction induced from a strong, almost constant magnetic field on standard ℝ4. The exact string background containing flat ℝ4 is replaced consistently by the curved geometry of SU(2)k × ℝ as part of the modified exact backgrounds. This corresponds to the change of smoothness on ℝ4 from the standard ℝ4 to some exotic . The calculations of the spectra are using the CFT marginal deformations and Wess–Zumino–Witten (WZW) models. The marginal deformations capture the effects of the magnetic field as well as its gravitational backreactions. The spectra depend on even level k of WZW on SU(2). At the same time the WZ term as element of H3(SU(2), ℝ) determines also the exotic smooth . As the consequence we obtain that a nonzero mass-gap induced by the exotic emerges in the spectrum.

Spin Glass State and Other Magnetic Structures with Their Symmetries in Terms of the Fibre Bundle Approach

The fibre bundle approach has been applied to derive the explicit formulas presenting all the eight fundamental magnetic structures and their symmetry groups including the spin glass state. The explanation of the uniqueness of the spin glass state has its roots in the appearance of the probability function p(i, j) in the second term of the assumed Hamiltonian. This term actually describes the random distribution of either dopants or defects in the ferromagnetic matrix under the percolation threshold. On this basis the Gaussian type randomness was derived for both the general global magnetic coupling constant and the magnetization vector, the latter effect bringing to the statistical features of the magnetic structure and the magnetic symmetry group of the spin glass state.