acta physica slovaca

Abstract: The free energy for the harmonic oscillator perturbed by a Gaussian potential is calculated in the framework of Feynman-Kleinert variational method. Two particular systems related to the Gaussian function via limiting procedures are considered, the harmonic oscillator perturbed by a Dirac distribution in the strong coupling limit and the 1D hydrogen atom. The latter is first converted to an infinite superposition of Gaussian potentials via the Gauss quadrature formula and treated in greater detail.

Abstract: The possibility of equilibrium of test particles with spin located in the Schwarzschild-de Sitter spacetimes is investigated. The stationary equilibrium conditions resulting from the equations of motion and the equations of spin dynamics have to be satisfied simultaneously. It is shown that the equilibrium conditions are independent of the spin of the test particles; they are satisfied only at the so called static radius where the gravitational attraction is compensated by the cosmological repulsion.

Abstract: A new approach to derivation of various effective-field approximations for lattice spin models in a unique way is presented. It is shown that it can give a number of methods, including the DMRG method, that can be used to find generally inhomogeneous solutions of 2D classical lattice problems. A method, closely related to the DMRG method but without necessity to perform spin renormalization, is derived, yielding results practically not different from the DMRG ones. The matrix-product wave function of Rommer and Östlund can be constructed from the output of the method. The computational costs of all the derived methods are smaller than those of the DMRG. Most of the results are applicable to the 1D quantum systems, as well.

Abstract: I consider the problem of generalising the Abelian Coulomb gauge condition to the non-Abelian Yang-Mills theory, with an arbitrary compact and semi-simple gauge group. It is shown that a straightforward generalisation exists, which reduces the Gauss law into a form involving the gauge potentials only, but not their time derivatives. The existence and uniqueness of the generalised Coulomb gauge is shown to depend on an elliptic linear partial differential equation for a Lie-algebra valued quantity, which defines the gauge transform by means of which the generalised Coulomb gauge condition is realised. Thus the Gribov problem is actually non-existent in this case.

Abstract: A generation mechanism of the non-Abelian gauge fields in the SU(N) gauge theory is investigated. We show that the SU(N) gauge fields ensuring the local invariance of the theory are generated at the quantum level only due to nonsmoothness of the scalar phases of the fundamental spinor fields. The expressions for the gauge fields are obtained in terms of the nonsmooth scalar phases.

Abstract: The process (IPE), being a natural and unique laboratory for studying the hadron electromagnetic structure in the sub- -threshold time-like region of the virtual-photon ``mass'' , turns out to be also very useful for investigating the nucleon weak structure. A theoretical basis of the methods for extracting practically model-independent values of the electromagnetic hadron form factors in the time-like region and for determining the weak structure of the nucleon in the space-like region from experimental data on IPE at low energies is outlined. The results of extracting, by those methods, the electromagnetic and pseudoscalar GP(t) form-factors of a nucleon are presented, where an indication of the existence of the state in the range 500-800 MeV (possibly the first radial excitation of pion) is obtained, and the coupling constant of this new state with the nucleon is estimated.

Abstract: Open problems in the theoretical formulation and computational implementation of multistep direct preequilibrium reactions are discussed. Recommendations for future research on both theoretical and experimental aspects of the problem are given.

Abstract: The non-degenerate two-quanta Jaynes-Cummings model governed by Milburn equation is studied. This models the decoherence of a quantum system as it evolves through intrinsic mechanisms beyond conventional quantum mechanics governed by the Schrödinger equation. The exact solution is given. Then it is applied to investigate the effects of the intrinsic decoherence on nonclassical effects (population inversion, oscillations of the number distribution, photon antibunching effect, the cross correlation and squeezing of the cavity field). We show how the intrinsic decoherence suppresses the nonclassical behaviour in this model, for the off-resonant case.

Abstract: In this paper we describe and demonstrate a new generation of all-optical devices (Demultiplexer, Routing switch and Clock extractor) based on a recently developed Terahertz Optical Asymmetric Demultiplexer (TOAD). These devices are capable of ultrafast all-optical operation and can be used in the future terabit optical networks.

Abstract: The paper reports results of an analysis of a correlation between the kind of compound (benzene, cyclohexane, n-hexane) and the type of close range molecular configurations (parallel, perpendicular, coplanar) in the liquid phase. The presence of such a correlation was indicated by X-ray diffraction measurements performed at 293 K using CuK radiation . Angular distributions of X-ray scattered intensity were measured, and electron density radial distribution functions were calculated. The mean distances between the neighbouring molecules and the mean coordination numbers were found. The differential functions of radial distribution of electron density were calculated and some intra- and intermolecular distances in liquid benzene, cyclohexane, and n-hexane were determined. The most probable models of local ordering of these molecules were suggested.

Abstract: Electrical conductivity and the electrical response of undoped yttria stabilized zirconia (YSZ) as well as the influence of dopant addition (Fe2O3, Nb2O3 and CuO) on the oxygen sublattice were investigated. The results obtained are discussed and compared by means of various mechanisms of electrical response (impedance/admittance, relative permittivity / electrical modulus). The proposed substitutional model of solid solutions for all investigated systems is in a good agreement with the measured characteristics.

Abstract: Mechanical alloying of elemental powder was used to prepare Fe3Si alloys. Changes in structure, phase transitions and magnetic properties induced by mechanical alloying and subsequent heat treatment of Fe-Si powdered system using XRD, Mössbauer spectroscopy, DSC and magnetic measurements were studied. The development of the atomic ordering at heating is presented.

Abstract: The effect of gamma irradiation on the magnetic properties of powdered Fe-3,3-bis[bis(carboxyethyl)aminoethyl] cresolsulfonephtrane complex has been investigated. It has been shown that the irradiation leads to a decrease of the values of saturation magnetization as well to decrease of the mean value of magnetic moment of superparamagnetic particles.

Abstract: We have studied the electron-quasiparticle interaction in the hexagonal single crystal of antiferromagnetic DyNi5 using the point-contact (PC) spectroscopy above and below TN = 11.6 K. The observed maxima in low energy range up to 10 meV have the CEF origin (the excitations of 4f electrons from ground CEF level to higher excited levels). The broad maximum at about 18 meV is connected with phonons DyNi5. The comparison of calculated CEF level scheme with experimental data is presented.

Abstract: We present the analysis of the symmetrical and the asymmetrical parts of the point-contact (PC) spectra together with thermopower and electrical resistivity measurements on bulk samples of NdCu. The negative asymmetrical part of the derivatives dVas/dI has been observed. The different contributions observed in the spectra are interpreted in terms of the different interactions involved in the system.

Abstract: The measured point-contact (PC) spectra of the heterocontact between oriented hexagonal single crystal of ferromagnetic NdNi5 and Cu in ballistic regime are presented above and below TC. We have observed maxima at characteristic energies 1.5 meV and 4 meV which have the crystalline electric field origin. The small shoulder at 9 meV and a broad maximum at about 18 meV are connected with phonons in NdNi5.

Abstract: A set of Fe73.5GdxNb3Cu1Si13.5B9 samples with Gd concentration x=0-3 at% and the Fe71.5Gd5Cu1Si13.5B9 alloy have been investigated: (i) concerning their ability to form nanocrystalline material after heat treatment and (ii) with respect to the Gd influence on the magnetic properties. All the samples showed a nanocrystalline structure after appropriate heat treatment, except the one with 5 at% of Gd. The observed decrease of the saturation magnetization with increasing Gd content indicates the ferrimagnetic exchange coupling between the Gd and Fe atoms. The Curie temperature was found to be increasing with Gd content up to 3 at% of Gd.

Abstract: Influence of U and Cu on the crystallization process and magnetic properties of the amorphous Fe86Zr6U1CuxB6-x (x =0; 1) after heat treatment were investigated. Structural measurement techniques showed the different course of crystallization process for both alloy systems. The positive influence of Cu on the crystallization of homogeneous nanocrystalline structure was observed as well as on magnetic properties.

Abstract: Magnetic, transport and dilatometric studies of the Ho(Co1-xSix)2 compounds (for 0.15) are presented. The Si substitution for Co yields a dramatic increase of Tc although the room-temperature lattice volume does not change. The first-order magnetic phase transition at Tc and metamagnetism at higher temperatures observed in HoCo2 persists for 0.075, however the resistivity, magnetoresistance and thermal expansion anomalies become gradually suppressed with increasing x.

Abstract: The hexagonal intermetallic compound UNiGa adopts the ZrNiAl structure type, orders antiferromagnetically below 39 K and exhibits several AF phases with lowering temperature. Within narrow temperature range 34 K - 39 K there exist four magnetic phase transitions in zero magnetic field. In all these phases the magnetic structure is highly anisotropic with magnetic moment parallel to the c-axis. The phase just below TN is an incommensurate antiferromagnetic structure characterised by propagation vector q = (0, 0, qz), where qz is in the range 0.36 - 0.37. We present results of magnetic critical scattering study on a UNiGa single crystal. The intensity of critical scattering drops rapidly within a very narow temperature range above TN.

Abstract: Electrical resistivity was measured on a single crystal of UNi2Si2 for T = 4.2 140 K in fields up to 8.4 T. Anomalies in the temperature dependence of resistivity in various fields and on isothermal magnetoresistance curves clearly indicate the magnetic phase transitions. From analysis of these effects a magnetic phase diagram is proposed. Relations between the resistance and types of magnetic phases are considered.

Abstract: An influence of the partial substitution of Fe by Ni on time-temperature stability of amorphous Fe73.5-xNixNb3Cu1Si13.5B9(x=0,5, 10, 15, 20, 25 at.%) alloys was investigated. Stability of the magnetic properties was observed by the magnetic after - effect (MAE) of the reluctivity which is represented by its relative change r/r between two fixed times after demagnetization. The relaxation spectra at different Ni content were studied. The heights of peaks are increasing at x=5, 10 at.% in contrary to Ni concentrations x=20, 25 at.%. The most probable activation energy for MAE was found to be Q=1.7 eV.

Abstract: The study of magnetic properties of the polymeric compounds Cu(C2H8N2)2Ni(CN)4, Cu(C2H8N2)2Ag2(CN)4 and Cu(C10H8N2)2Ag2(CN)4.H2O is the main purpose of this work. Magnetization measurements at 4.2 K in fields up to 6 T confirmed assumptions on weak exchange interactions, resulting from the knowledge of a crystal structure. Specific heat studies performed in the temperature range 50 mK-1.5 K revealed phase transitions into the long-range-order in milikelvin region in all compounds. The deviations from assumed chain-like behaviour are discussed.

Abstract: Magnetotransport measurements have been performed on the superconducting misfit layer compound (BiSe)1.10(NbSe2) with c = 2.36 K down to 100 mK and in fields up to 1 Tesla in order to determine the upper critical field perpendicular as well as parallel to the layer structure. A temperature independent anisotropy has been determined. The temperature dependence of the upper critical field generally follows the Werthamer-Helfand-Hohenberg prediction for classical type - II superconductors. Small discrepancies are ascribed to the inhomogeneity in the sample.

Abstract: In this work we discuss the possibility of contribution of one-site two-magnon bound states (OSTMBS's) to the thermodynamic equilibrium properties in S=1 Heisenberg chain with easy- axis anisotropy. We found that for sufficiently strong anisotropy |D/J|=5 a good agreement between OSTMBS's theoretical contribution to the specific heat using dilute approximation and numerical calculations for the aforementioned system was obtained for temperatures below T=0.12 |D/kB|. The obtained result confirms that OSTMBS's are the lowest energy excitations in S=1 Heisenberg chains with sufficiently strong easy-axis anisotropy. Further, the compound Ni(N2H5)2(SO4)2 previously identified as S=1 antiferromagnetic Heisenberg chain with easy-axis anisotropy with D/kB=-15.6 K, D/J=5, is proposed as an appropriate system for experimental studies of OSTMBS's contribution to thermodynamic properties.

Abstract: Some aspects of theoretical approaches to the magnetoresistance description are reviewed. The conclusions and illustrative remarks are of tutorial character. The role of magnetic field applied to samples is discussed. The density functional technique is considered in the context of its application to the giant and colossal magnetoresistance when the magnetic field is included.