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1
Content available remote

Magnetic Field Dependence of Critical Attenuation in Ferromagnet

100%
Pawlak A.
Acta Physica Polonica A
|
2012
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vol. 121
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issue 5-6
1185-1187
EN
The influence of magnetic field on the sound attenuation is investigated in ferromagnets. With the aid of the Nelson method we derive frequency-, magnetic field- and temperature-dependent expression for the sound attenuation coefficient near a critical point in the Ising-type system. The shift of the ultrasonic attenuation peak under the influence of magnetic field towards higher temperatures is discussed.
2
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Critical Sound Attenuation and Spin-Lattice Relaxation

100%
Pawlak A.
Acta Physica Polonica A
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1994
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vol. 85
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issue 2
385-387
EN
The nonasymptotic critical properties of sound propagation are studied in compressible magnetic system. The role of coupling of order parameter and acoustic phonons to spin-energy density is investigated and the ultrasonic attenuation scaling function is found.
3
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A Dilute Magnet with Ferro- and Antiferromagnetic Exchange Interactions. The Structural Fluctuations Analysis

100%
Pikuła R., Kirkiewicz J.
Acta Physica Polonica A
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2008
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vol. 113
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issue 2
669-676
EN
We analysed the influence of the fluctuations in distribution of magnetic atoms on the properties of dilute magnets with mixed ferro- and antiferromagnetic exchange interactions. The high- and low concentration approximations were found to give very close results and the most important impact on the physical quantities origins from the lowest, square fluctuations which play the predominant role in the whole range of concentrations of magnetic atoms.
4
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Magnetic Field Effect on Sound Propagation in Antiferromagnets

100%
Pawlak A., Fechner B.
Acta Physica Polonica A
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2009
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vol. 115
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issue 1
232-234
EN
On the basis of the theory of phase transitions, a model describing anomalies of sound attenuation coefficient close to the antiferromagnet-paramagnet phase transition in magnetic field was developed. The scaling behaviour of the sound velocity and attenuation coefficient was determined. The physical origin of the two-peak structure in the field dependent ultrasound attenuation observed in a number of antiferromagnets was identified. The theoretical results were compared with experimental results obtained for terbium.
5
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Magnetic Field Influence on Critical Sound Attenuation and Velocity Variation in Ferromagnets

100%
Pawlak A.
Acta Physica Polonica A
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2009
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vol. 115
|
issue 1
229-231
EN
The ultrasound velocity and attenuation are investigated in ferromagnets under an application of magnetic field. In the magnetic field six different characteristic regimes are distinguished instead of traditional two (hydrodynamic and critical) found for the case of zero magnetic field. The mean-field Landau-Khalatnikov theory as well as the scaling predictions are given for each regime. Various critical exponents are identified for these regions and compared with the ultrasonic data for MnP. The shift of the ultrasonic attenuation peak under the influence of magnetic field towards higher temperatures is discussed.
6
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Diffusive Regime for Critical Sound Attenuation and Dispersion in Magnets

100%
Pawlak A., Błaszak M.
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EN
We consider a new regime of sound propagation in magnetic systems above the critical point. On the basis of a general statistical model including a flow of energy between spin and lattice subsystems we found that there is a region of parameters where the sound attenuation and dispersion can be expressed in a similar way as for noncritical systems for which only the heat conduction matters. Contrary to the case of the classical heat diffusion in the critical magnetic system both the specific heat, entering the reduced frequency, as well as the effective amplitude are frequency and temperature dependent reflecting the critical singularities present in the system near its critical point.
7
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Critical Sound Propagation in Magnetic Metals and Bottleneck Phenomena

100%
Pawlak A.
|
EN
A semiphenomenological model of sound propagation in a critical magnetic system is proposed, which takes into account a relaxation of energy between the system of localised spins and the conduction electron spins as well as the coupling of the longitudinal phonon to bilinear combination of spin fluctuations and to the energy densities. A general expression for the acoustic self-energy is obtained within the one-loop approximation. The effect of "bottlenecking" and its influence on the critical singularities in the sound characteristics is discussed.
8
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Spin-Lattice Relaxation Effects in Critical Sound Propagation

100%
Pawlak A.
Acta Physica Polonica A
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1997
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vol. 92
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issue 2
449-452
EN
The nonasymptotic critical properties of sound propagation are studied in compressible Ising system above T_{C}. In the present paper we analyse a model where in addition to the coupling to two order-parameter fluctuations the sound mode couples linearly to the fluctuations of spin-energy and lattice-energy densities. Both subsystems exchange energy with the rate determined by the bare spin-lattice relaxation time. The total energy may be conserved or not. The crossover between insulator-like behaviour ~t^{-2α} and metal-like behaviour ~t^{-(zν+α)} in ultrasonic attenuation is investigated according to the value of ultrasonic frequency, the reduced temperature t, bare relaxation times and various coupling constants.
9
Content available remote

Theory of Critical Sound Attenuation in Ising-Type Magnets

100%
Pawlak A.
Acta Physica Polonica A
|
2000
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vol. 98
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issue 1-2
23-39
EN
The critical behaviour of sound attenuation has been studied in an elastically isotropic Ising system above the critical point on the basis of a complete stochastic model including both spin-energy and lattice-energy modes linearly coupled to the longitudinal sound mode. The effect of spin-lattice relaxation on the ultrasonic attenuation is investigated. The crossover between Kawasaki behaviour and Murata-Iro-Schwabl behaviour is studied as dependent on the values of ultrasonic frequency, reduced temperature, relaxation times, etc. A new high-frequency regime is discussed in the magnetic systems. This new regime corresponds to an adiabatic sound propagation and is very similar to the ones in binary mixture and liquid helium. A new frequency-dependent specific heat being the harmonic average of the bare lattice and critical spin specific heats is introduced. It was shown that such specific heat describes the process of equilibration between spin and lattice subsystems and includes the most important features of critical sound attenuation.
10
Content available remote

Annihilation of o-Positronium in the Vicinity of the Critical Mixing Point

100%
Jerie K., Baranowski A., Gliński J., Orzechowski K.
Acta Physica Polonica A
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1999
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vol. 95
|
issue 4
569-574
EN
Positron annihilation spectra were investigated in the critical region close to the consolute point of ethanol + dodecane mixture. The anomalies of the positron lifetimes and intensities were small, nevertheless it was found that the intensity of short-living components of annihilation spectrum seems to increase close to the critical temperature.
11
Publication available in full text mode
Content available

Critical Fluctuations and 1/f^α-Activity of Neural Fields Involving Transmission Delays

80%
Hutt A., Frank T.
Acta Physica Polonica A
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2005
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vol. 108
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issue 6
1021-1040
EN
This work studies the stability and the stochastic properties of neural activity evoked by external stimulation. The underlying nonlocal model describes the spatiotemporal response dynamics of neural populations involving both synaptic delay and axonal transmission delay. We show that the linear model recasts to a set of affine delay differential equations in spatial Fourier space. Besides a stability study for general kernels and general external stimulation, the power spectrum of evoked activity is derived analytically in the case of external Gaussian noise. Further applications to specific kernels reveal critical fluctuations at Hopf- and Turing bifurcations and allow the numerical detection of 1/f^αfluctuations near the stability threshold.
12
Content available remote

Fractals, Log-Periodicity and Financial Crashes

80%
Oświęcimka P., Drożdż S., Kwapień J., Górski A.
Acta Physica Polonica A
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2010
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vol. 117
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issue 4
637-639
EN
Presence of self-similar patterns in the financial dynamics is by now well established and even convincingly quantified within the multifractal formalism. Here we focus attention on one particular aspect of this self-similarity which potentially is related to the discrete-scale invariance underlying the system composition and manifests itself by the log-periodic oscillations cascading self-similarly through various time scales. Such oscillations accumulate at the turning (critical) points that in the financial dynamics are often identified as crashes. This property thus allows us to develop a methodology that may be useful also for prediction. A model Weierstrass-type function is used to illustrate the relevant effects and several examples demonstrating that such effects in the real financial markets take place indeed, are reviewed.
13
Content available remote

Effect of On-Site Coulomb Repulsion on Phase Transitions in Exactly Solved Spin-Electron Model

80%
Gálisová L., Strečka J., Tanaka A., Verkholyak T.
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EN
A hybrid lattice-statistical model on doubly decorated planar lattices, which have localized Ising spins at their nodal lattice sites and two itinerant electrons at each pair of decorating sites, is exactly solved by the use of a generalized decoration-iteration transformation. Our main attention is focused on an influence of the on-site Coulomb repulsion on ground-state properties and critical behavior of the investigated system.
14
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Content available

Magnetization Processes in Dilute Metamagnets with Ferro- and Antiferromagnetic Exchange Interactions

80%
Pikuła R., Kirkiewicz J.
Acta Physica Polonica A
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2006
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vol. 110
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issue 1
25-39
EN
Using the Green function method there has been presented the mathematical description of dilute two sub-lattice metamagnets with ferro- and antiferromagnetic exchange interactions. The Green function averaging procedure is based on the Cumulant Expansion Technique and includes any localized Green function irrespectively of the decoupling algorithm making the chain system of equations self-consistent. The derived form of the averaged Green function is the base for the analysis of magnetization processes and phase transitions occurring in the system under the influence of external magnetic field. It has been found in particular that the tricritical point and the second-order phase transition vanish when concentrations of magnetic atoms coupled with different sub-lattice points become different.
15
Content available remote

Ground-State Properties of the Spin-1/2 Heisenberg-Ising Bond Alternating Chain with Dzyaloshinskii-Moriya Interaction

80%
Strečka J., Gálisová L., Derzhko O.
Acta Physica Polonica A
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2010
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vol. 118
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issue 5
742-744
EN
Ground-state energy is exactly calculated for the spin-1/2 Heisenberg-Ising bond alternating chain with the Dzyaloshinskii-Moriya interaction. Under certain condition, which relates a strength of the Ising, Heisenberg and Dzyaloshinskii-Moriya interactions, the ground-state energy exhibits an interesting nonanalytic behavior accompanied with a gapless excitation spectrum.
16
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On the Exact Solution of the Mixed-Spin Ising Chain with Axial and Rhombic Zero-Field Splitting Parameters

80%
Dančo M., Strečka J.
Acta Physica Polonica A
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2010
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vol. 118
|
issue 5
728-729
EN
Ground-state phase diagram of the mixed spin-1/2 and spin-1 Ising chain with axial and rhombic zero-field splitting parameters is exactly calculated within the framework of the transfer-matrix method. It is shown that the rhombic zero-field splitting parameter prefers the magnetically ordered phase instead of the disordered phase.
17
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Nematic ordering problem as the polymer problem of the excluded volume

61%
Yakunin A.
Open Physics
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2003
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vol. 1
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issue 2
355-362
EN
Based on a solution of the polymer excluded volume problem, a technique is proposed to estimate some parameters at the isotropic-nematic liquid crystal phase transition (the product of the volume fraction of hard sticks and the ratio of the stick length, L, to its diameter, D; the maximum value of this ratio at which one cannot regard the stick as hard). The critical exponents are estimated. The transition of a swelling polymer coil to ideal is revealed as the polymerization degree of a macromolecule increases. The entanglement concentration obtained agrees with experimental data for polymers with flexible chains. The number of monomers between neighbor entanglements is assumed to be the ratio L/D. A comparison of the theory with other ones and recent experimental data is made.
18
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Phase Diagrams of the Spin-1/2 Ising-Heisenberg Model on a Triangle-Hexagon Lattice

61%
Kiššová J., Strečka J.
Acta Physica Polonica A
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2010
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vol. 118
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issue 5
730-731
EN
Ground-state and finite-temperature phase diagrams of a geometrically frustrated spin-1/2 Ising-Heisenberg model on a triangle-hexagon lattice are investigated within the generalized star-triangle mapping transformation. It is shown that the ground state is constituted by two different spontaneously long-range ordered phases and one disordered spin-liquid phase.
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