acta physica slovaca

Abstract: Superfluid 3He  is an ideal  system for testing  ideas in other areas  of physics. The superfluid  at low temperatures is almost 100 % condensate and thus described  by a single set  of  simple  equations.  In  this  paper  we discuss the  application  of the  superfluid to  understanding turbulence since  turbulence in  the superfluid  takes the  very simple form of  a tangle of  identical quantized vortices. Various observations and implications are discussed.

Abstract: Many quantum condensed-matter systems, and probably the quantum vacuum of our Universe, are strongly correlated and strongly interacting fermionic systems, which cannot  be treated perturbatively. However, physics which emerges in the low-energy corner does not depend on the complicated details of the system and is relatively simple. It is determined by the nodes in the fermionic spectrum, which are protected by topology in momentum space (in some cases, in combination with the vacuum symmetry). Here we illustrate this universality on some examples of quantum phase transitions, which can occur between the vacua with the same symmetry but with diferent topology of nodes in momentum space.

Abstract: Important recent discoveries in neutrino physics launched a new era in the search for `physics beyond the Standard Model'. In particular, observation of the zenith angle dependence of the atmospheric neutrino flux, of the difference in the deduced flux of solar neutrinos based on their charged and neutral current interactions, and of the reduction in the flux of reactor antineutrinos at large distances convincingly show that neutrinos are massive and mixed. I briefly review the formalism and physics of the neutrino mass and mixing, and then concentrate on discussion of the KamLAND reactor neutrino experiment. That experiment, a continuation of a long tradition of studies involving neutrinos produced in nuclear reactors, is the first one to observe neutrino oscillations with a man-made and well understood source. I describe the detector, physics of the detection reaction, the determination of the reactor antineutrino flux and, naturally, the results and their implications.

Abstract: Ground-state and finite-temperature properties of a special class of exactly solvable Ising-Heisenberg planar models are examined using the generalized decoration-iteration and star-triangle mapping transformations. The investigated spin systems exhibit an interesting quantum behaviour manifested in a remarkable geometric spin frustration, which appears notwithstanding the purely ferromagnetic interactions of the considered model systems. This kind of spin frustration originates from an easy-plane anisotropy in the XXZ Heisenberg interaction between nearest-neighbouring spins that favours ferromagnetic ordering of their transverse components, whereas their longitudinal components are aligned antiferomagnetically.

Abstract: The processes pi pi -> pi pi, K\overline{K}, eta eta at the channel with I^{G}J^{PC} = 0+0++ are analysed using the  analyticity properties of scattering matrix  elements. The  investigation is focussed on the properties of the f0(665), f0(980), f0(1370), f0(1500) and f0(1710) states. The analysis supports  the f0(665) as the very broad resonance. It suggests further to see the f0(980) state as  predominantly the eta eta bound  state with dominant (\overline{q}q)(\overline{q}q) components. The quark content  of other states is inferred and f0(1500) appears as a mixed state with dominant glueball component.

Abstract: A model for the description of the \sqrt{s_NN} dependence of < pi+> ratio at the CERN SPS and upper AGS energies is proposed. It uses hadronic degrees of freedom and the amount of produced strangeness is mainly controlled by the total lifetime of the fireball. Decreasing lifetime with increasing collision energy is conjectured.

Abstract: The requirements on calorimetry for the e+ e- International Linear Collider are formulated. Recent R&D results for the hadron calorimeter from CALICE collaboration are given.

Abstract: Nonstatistical fluctuations of particle production in relativistic collisions have been studied using the method of scaled factorial moments. Such fluctuations may be used to signal the formation of a quark-gluon plasma in the early stage of heavy ion interactions at high energies. The experimental data of 208Pb nuclei at 158 A GeV/c taken with nuclear emulsion have been analysed. The dependences of intermittency parameter lambda_q on the order of factorial moment q have been studied for 16O and 208Pb induced interactions.

Abstract: The improvements of experimental technique in last years give a new possibility for a production of heaviest elements. Experimental limit for \alpha-gamma spectroscopy study was decreased to the order of  several 100 pb which allows detailed spectroscopy experiments for the superheavy elements up to the Seaborgium. Several alpha and alpha-\gamma spectroscopy experiments were recently performed at the velocity filter SHIP in GSI Darmstadt (Germany). As example, the decay properties of 246,247,251Md, 255Lr and 261Sg are presented.

Abstract: Using Radioactive Ion Beams new properties of neutron-rich nuclei near the border of stability  have been investigated. The structure of these exotic nuclei with Z up to 30 near drip lines  has been studied using the elastic scattering of the secondary radioactive neutron-rich beam of 11Li (29 AMeV) on a Si target and the quasi-elastic and inelastic scattering of neutron deficient 7Be and bor beams (40 AMeV) on a 12C target at GANIL, France. The measurements have confirmed the existence  of the neutron halo in 11Li and a proton skin in 7Be and 8B nuclei. The direct measurement of the mass of 31 radioactive nuclei and the derivation of two-neutron separation energies have enabled to establish the new neutron magic numbers N=6 and 16 in the neutron-rich region for the first time instead of normal N=8 and 20 and support the disappearance of the doubly-magic nuclei 10He and 28O. Changes in basic properties of nuclear matter have been confirmed by the novel two step fragmentation in-beam gamma spectroscopy that has established 24O as a new doubly-magic nucleus.

Abstract: Limits on the possible existence of narrow K- nuclear states and on their widths are explored. Deeply bound K- nuclear states are generated dynamically. Substantial polarization of the core nucleus is found for light K- nuclei. The widths of the states are mostly determined by the phase-space suppression on top of the increase provided by the density of the compressed nuclei. The behaviour of the calculated widths as function of the K- binding energy provides useful guidance for the interpretation of recent experimental results.

Abstract: The objective of this work was the study of 6Li diffusion in the Ta and W refractory metals. The samples were prepared by ion implantation of the 380 keV 6Li+ ions into W and Ta thin foils up to the fluence of 10^16 cm^-2 and annealed up to the temperature 1900 C. The depth profiles of 6Li were determined using the Thermal Neutron Depth Profiling (TNDP) technique. The results showed that diffusion of 6Li in both W and Ta is very complex and cannot be described by simple Fick's laws. The Fickian diffusion is affected by the presence of traps and radiation defects in the sample surface layer. Further (and more detailed) TNDP study and computer simulations are therefore necessary.

Abstract: We investigated relativistic (0.6 -1.5 MeV) electron fluxes dynamics in outer radiation belt using MKL device measurements on CORONAS-F satellite within intervals of 22 strong magnetic storms during years 2001 - 2004. We show that in the time of these magnetic storms the maximum of the electron fluxes shifts earthward in to the slot region.

Abstract: The non-thermal plasma technologies based on electrical discharges play an important role in ecological applications. The classical corona discharge is however relatively low power discharge. With the aim to extend its current-voltage range we studied hollow needle-to-plate DC corona discharge enhanced by the flow of a gas through the needle electrode. With this type of the discharge we performed an extensive study of ozone generation and volatile organic compounds (VOCs) decomposition. We found that supply of air through the needle substantially increases current-voltage range of the discharge in comparison with classical pin-to-plate corona discharge. Consequently the ozone generation as well as toluene decomposition efficiency was increased

Abstract: SOFIA laboratory is a test facility for the laser system PALS (Prague Asterix Laser System) and its major task is getting know-how of the OPCPA technique (Optical Parametric Chirped Pulse Amplification), a modern method of the amplification of ultrashort laser pulses. The SOFIA system is a hybrid laser where the oscillator beam is generated in a solid-state optical parametric oscillator (OPO) tuned to 1315 nm (the iodine spectral line) and then amplified in two gaseous iodine amplifiers. The 1315 nm beam is converted to 438 nm in KD*P crystals (4 GW/cm^2) and pumps non-linear crystals (KDP, LBO) as parametric amplifiers. The signal beam for the parametric amplification is produced by Ti:sapphire laser and stretched to 300 ps in a pulse stretcher (single diffraction grating, Offner telescope).

Abstract: Ball milling of melt-spun ribbons and subsequent compaction of the resulting powders in the supercooled liquid region were used to prepare disc shaped bulk amorphous Co-based samples. The several bulk samples have been prepared by hot compaction with subsequent heat treatment (500 oC - 575 oC). The influence of the consolidation temperature and follow-up heat treatment on the magnetic properties of bulk samples was investigated. The final heat treatment leads to decrease of the coercivity to the value between the 7.5 to 9A/m.

Abstract: We have used Mossbauer spectrometry for a study of an iron axe. Two experimental techniques were applied: (a) non-destructive measurements performed in backscattering geometry, and (b) identification of small pieces of surface covering substance which has accidentally disintegrated with the investigated axe. These measurements were performed at room and also at low (77 K) temperature. Presence of corrosion products including goethite, magnetite and lepidocrocite was confirmed.

Abstract: Structural relaxation during strain recovery of the metallic glass Fe40Ni41B19 after longtime stress-annealing at different temperatures below the crystallization temperature was investigated by anisothermal DSC and dilatometric experiments. The structural relaxation effects depend on the stress-annealing temperature of the amorphous metallic alloy. The activation energy spectra were calculated from the anisothermal experimental data. The influence of the annealing temperature on the shape of creep recovery spectra was analyzed.

Abstract: Structural variability of the Ni/C60 thin film composites was observed after ion irradiation and/or thermal annealing. Annealing of the irradiated Ni/C60/Ni multilayers led to micrometer-large rod-type particles, annealing of the non-irradiated specimens resulted in truncated rectangular 'footprints'. Micrometer-large spherical or dodecahedral Ni droplets were synthesized by annealing of the non-irradiated Ni+C60 mixtures. The structural variation of the Ni/C60 systems points out the complex physiochemical processes incited by elevated temperatures. First insight on the microstructural evolution of the Ni/C60 composites after ion irradiation and/or thermal annealing is presented in this paper..

Abstract: The crystal structure of NaX, NaLSX and NaY zeolites (faujasites with various Si/Al) with chemisorbed methyl cations (CH3+ or (CD3+ have been investigated by powder neutron diffraction. The magic angle spinning (MAS) method was used for recording of 13C MAS NMR spectra of adsorbed species. Powder neutron experiments were carried out at 7 K and 294 K. The complete structural parameters were determined by Rietveld analysis of powder neutron patterns. Difference Fourier maps were used to locate the Na+ cations and the chemisorbed CD3 or CH3 groups. The significant effect of chemisorbed methyl cations on the distribution of sodium cations in the lattice of chemisorbed zeolites was detected.

Abstract: Alcohols with nuclear spins highly polarized due to the polarization transfer from stable nitroxyl radicals are suitable materials for polarized nucleon targets. In this work the ethanol-TEMPO system was studied by 1H and 13C high resolution NMR in a liquid phase. The concentration and temperature dependences of spin-lattice relaxation rates, linewidths and chemical shifts were measured. The highest impact of doping was seen for protons of OH group as a result of hydrogen bonds to the oxygen of TEMPO. For  the other nuclei the relaxation enhancement corresponds to the diffusion-controlled regime above 210 K.

Abstract: Spin valve structures of Co/Cu/Co, Co/Au/Co, and Fe/Co/Cu/Co types were fabricated by electron beam deposition. Cr or Cr/M (M=Ag, Au, Cu) layers were used as buffer layers. Layer thicknesses and interface roughnesses were determined by the X-ray reflectivity while internal structure of the layers was analyzed by the X-ray diffraction. Magnetoresistance (MR) data indicate the effect of both buffer and magnetic layer thicknesses on the magnetic field dependence of the MR. We investigated the influence of Cu, Ag and Au layers on the value of magnetic field necessary to reverse the polarization of magnetic layers of different thicknesses. The largest value of such a magnetic field was obtained for 5nm Cr/2.5nm Au/5nm Co/2.2nm Au/2nm Co/1Au multilayer.

Abstract: We investigate changes of magnetic ordering in  Ni3Al in L1_2 structure along tetragonal (Bain's) deformation path. At some degree of deformation, the ferromagnetism of Ni3Al is lost. We have determined phase boundaries between the magnetic and non-magnetic phases along the deformation path. Magnetic behavior of Ni3Al was also investigated by means of Stoner model and  by local environmental analysis, i.e. by investigating the number and type of atoms in the first coordination spheres.

Abstract: Rectangular core-shell structure with an axially magnetized core and the conductive shell has been theoretically analysed in this article. Presented eddy current model exactly calculates the power loss and the velocity of a single domain wall propagating through the ferromagnetic core. It is shown that the thickness as well as the conductivity of the  non-magnetic shell can substantially affect the ferromagnetic core.

Abstract: We present results of electrical resistivity measurements on HoB12, ErB12 and TmB12 single crystalline samples in the temperature range of 1.6 - 30 K and in magnetic field up to 6 T. From the received data, B vs. T phase diagrams of all samples were created. Spin wave scattering of conduction electrons in the antiferromagnetic phase and the scattering of electrons in the paramagnetic phase are analysed and discussed.

Abstract: A combination of small-cluster exact-diagonalization calculations and a well-controlled approximative method is used to study the ground-states of the spin-one-half Falicov-Kimball model (FKM) in two dimensions. The results obtained are used to categorize the ground-state configurations according to common features for weak, intermediate and strong interactions. It is shown that only a few configuration types form the basic structure of the ground-state phase diagram. In particular, the largest regions of stability correspond to phase segregated/separated, n-molecular and axial striped configurations. This opens new route towards to understanding the inhomogeneous charge ordering in strongly correlated electron systems.

Abstract: Temperature dependences of the hyperfine magnetic fields at 57Fe nuclei in five magnetic sublattices of BaFe12O19 hexagonal ferrite were analyzed to obtain parameters characterizing the local exchange interactions. We report theoretical analysis of the temperature dependences of magnetization carried out in the framework of various cluster models based on mean field approaches including the Bethe-Peierls-Weiss approximation. Exchange integrals were extracted from the experimental data reported earlier by G. Albanese et al. 1974  and P. Novak et al. 1989

Abstract: We report experimental investigations of He II turbulence and its decay. Turbulent state was generated by counterflow and pure superflow in channels of circular and square crossection. The steady-state turbulence is generated by applying power to the heater placed either in the dead end of the channel or immersed in He II in a volume adjacent to one of the silver-sintered superleaks having an outlet above the helium bath level.  When this power is switched off, quantum turbulence displays a complex decay. We discuss the steady state data and forms of the observed decays in terms of available models and compare with quantum turbulence generated by towing a grid of bars through a stationary sample of He II.

Abstract: A flexible experimental setup for the investigation of He II flow and turbulence generated by (i) thermal counterflow, (ii) thermally-induced pure superflow in channels of various crossections, (iii) between counterrotating discs with blades and (iv) due to a torsionally oscillating cylinder has been designed and constructed. The flows (iii) and (iv) are driven via a shaft by the PC controlled DC motor placed on the top flange of the  cryostat. Our principal method of probing the flow is second sound attenuation; thermally-induced flows can additionally be probed by measuring the temperature difference along the channel.

Abstract: The current source is designed for NMR application in the low temperature experiments. The current is operating in the range of 0-6.5 A with a setting accuracy of 100 microA at a maximum output voltage of 7 V. The current can be swept linearly in time with various rates and its value is measured with resolution of ~25 microA. This value also corresponds to the time stability of the output current in standby mode. Other characteristics of the source are also presented

Abstract: The layered cobaltite system La1-xSr1+xCoO4 has been prepared in the range from x=0 (pure Co3+) up to  x=0.4 (40 % Co4+). Analogously to the pseudocubic perovskites La1-xSrxCoO3, a ferromagnetic moment is induced with increasing x. Its presence is evidenced by neutron diffraction study performed on sample x=0.4 at low temperatures. The relatively low magnitudes of cobalt moments (0.7 - 0.8 mu_B) suggest that the ferromagnetic ordering is not complete and significant part of the sample remains in a paramagnetic or spin-glass state.

Abstract: The dynamic and viscous properties of dilute solutions of flexible polymers are studied. We come from our hydrodynamic theory of the single polymer kinetics and build the joint Rouse-Zimm model of the polymer behaviour. To take into account the presence of other polymer coils in the solution the Brinkman's porous medium approach is used. Various observable quantities such as the shear viscosity of the solution, the Huggins coefficient and the monomer mean square displacement are calculated. We also give the first description of the hydrodynamic screening as a concentration- and time-dependent effect.

Abstract: The dynamics of polymers in dilute solutions is described within the joint Rouse-Zimm (RZ) theory, in which the Z and R models correspond to infinitely large and small draining parameter. The equation of motion for a "test" polymer in the solution is solved together with Brinkman's hydrodynamic equations for the solvent. The solvent flow is disturbed by other coils and is "freezed" as the concentration of polymers grows, which indicates the hydrodynamic screening. We give the description of the effect of concentration on the motion of the whole coil as well as on the relaxation of its internal modes.