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The state of the art and the project of modernization of the extensive-air-shower array Carpet-2 of the Baksan Neutrino Observatory of the Institute for Nuclear Research, Russian Academy of Sciences are described. The modernized array will allow the performance of detailed study of variations in the cosmic ray intensity, the energy spectra and composition of primary cosmic rays in the energy range 1013–1016 eV, and the anisotropy of primary cosmic rays with energies above 1013 eV.

Magnetic birefringence experiments under static eld are performed on ionic ferrofluid samples based on Fe2O3 . A colloidal size-sorting of the particles allows to obtain narrow size distributions. The optical birefringence of the solutions is found positive and can be described by a Langevin formalism. It scales as H 2 in the low eld limit. In the high eld limit the particles size dependence of the saturation birefringence is compatible with a surface anisotropy constant KS =2 : 810 2 erg cm 2 associated with a particle elongation of 1.25 both coherent with N eel predictions of surface anisotropy in small grains.

Ferromagnetic resonance (FMR) experiments at 9.26 GHz on non-interacting maghemite (γ-Fe2O3) nanoparticles of ferrofluids are performed as a function of temperature (3.5–300 K) and particle diameter (4.8–10 nm). The orientational mobility of the particles inside the fluid is employed to monitor the orientational distribution of the anisotropy axes by solidifying the MF matrix under the external field. On those textured suspensions, angular analysis is performed allowing to determine the anisotropy of the system, which turns out to be uniaxial and positive. The dependence of angular variations with respect to particle size leads to the conclusion that this anisotropy is induced by the particle surface, its density being Ks=2.7×10−2 erg/cm2 at 3.5 K. Extra isotropic contributions to internal fields at low temperature appear as the signature of the disordered magnetic structure of the surface layer.

Signal-to-noise properties of a periodically driven noisy magnetic bistable system—superparamagnetic particles with uniaxial anisotropy—are investigated in the framework of the Fokker-Planck equation solved both analytically and numerically. The system is subject to a constant ~bias! field that imparts even harmonics into its low-frequency magnetic spectrum. A comparative study of the linear and quadratic susceptibilities and stochastic resonances ~SR! is carried out. We show that the quadratic SR is much sharper than the linear one and unlike the latter is essentially frequency dependent. @S1063-651X~97!10411-1#

We study the time-dependent orientational response of a nematic liquid crystal doped with single-domain ferromagnetic particles. A set of coupled macroscopic equations describing evolution of the director texture, magnetization distribution and concentration of the particles under the change of the external field is derived. For the case of a plane layer, expressions for the main rela; .ation rates are obtained and analyzed.

Some results on the EAS neutron component measurements by means of the PRISMA-32 array are presented. The array consists of 32 electron-neutron detectors (en-detectors) capable to detect two main EAS components: electromagnetic one consisting of charged particles, and hadronic one by measuring delayed thermal neutrons accompanying the showers. For thermal neutrons detection, a compound of a well-known inorganic scintillator ZnS(Ag) and LiF, enriched to 90 % with 6Li isotope is used. The setup allows us to record neutron component over the whole array area.

A new theoretical approach to the dynamic field-induced birefringence in magnetic fluids is presented. On equal basis it takes into account both internal (magnetic) and external (mechanical) orientation relaxations of the particles. Besides uniting several former approximate models, already tested experimentally, it covers a variety of intermediate cases which have been never addressed before.

The joint influence of superparamagnetic and powder-pattern effects on the formation of FMR absorption lines in ferrofluids is discussed. Consistent consideration of these mechanisms yields specific contours of the absorption lines and affects substantially the temperature dependence of the linewidth.

EAS array of novel type have been constructed on the base of ARGO-YBJ experiment (Tibet, China). It consists of the four specially designed scintillator en-detectors capable to measure two main EAS components: hadrons through thermal neutrons (n) and electrons (e). The results of simulation for these arrays using CORSIKA and GEANT4 codes are presented. Simulated thermal neutron and electron lateral distributions are compared with experimental data. Obtained distributions are compared with those obtained by other arrays.

A simple approximate equation of motion for the longitudinal component of the magnetic moment of a single-domain uniaxial superparamagnetic particle is proposed. This equation is used to evaluate the spectral density function of the interparticle magnetic motion at finite temperatures and to describe the stochastic resonance effect. The correspondence of the results obtained with those of previous calculations is discussed.

A search for transient point sources of ultra-high-energy (UHE) gamma-rays has been performed, based on the correlation of two extensive air shower arrays, BAKSAN (North Caucasus, 1700 m a.s.l., BAKSAN Neutrino Observatory, Russia) and EAS-TOP (Campo Imperatore, 2005 a.s.l., Laboratori Nazionali del Gran Sasso, Italy), which are located at very similar latitudes (phi approximate to 43 degrees N), and separated in longitude by Delta lambda approximate to 33.7 degrees. The search has been conducted at primary energy E (gamma) greater than or equal to 5 x 10(13) eV on the time scale of a daily source transit over the sky, observable in the northern hemisphere (19 degrees < delta < 69 degrees), and for three sources, namely the Crab Nebula, Markarian 421 and Markarian 501, observed at TeV energies by atmospheric Cherenkov detectors. The stability of the single detectors has been studied and verified up to high significance level. No coincident excess, not compatible with the expectations from statistical fluctuations, has been observed. The obtained upper limits to the rate of transient events, n gamma, at 90% c.l., are concerning the sky survey, in the declination band corresponding to the zenith, n gamma/(Omega t)< 12/(yr sr) with Phi(gamma),(E-gamma > 5 x 10(13) eV)> 2.0 x 10(-11) cm(-2)s(-1) and duration Delta t < 8 h; concerning the candidate sources, e.g., for Markarian 421: n gamma/t < 7.7/yr with Phi(gamma),(E-gamma, > 5 x 10(13) eV) > 1.3 x 10(11) cm(-2) s(-1) and Delta t < 7.5 h. cm A coincident episode from Markarian 421. observed on 15 January, 1994, with expected chance imitation rate n(ch) = 0.01 is discussed. (C) 2000 Elsevier Science B.V. All rights reserved.

Dynamic field-induced birefringence in magnetic fluids is studied with the aid of the generalized Fokker–Planck equation that takes into account both internal (magnetic) and external (mechanical) orientational motions of the particles. The solution found accounts for the variety of cases and unites several former models.

We consider higher harmonic generation in an assembly of noninteracting superparamagnetic particles under DC and AC magnetic fields. In a solid system the only quantity that responds is its magnetization. In a magnetic fluid, uniquely, the field-induced particle re-orientation gives birth to oscillations of birefringence and thus the intensity of the transmitted light. Depending on the ratio of the magnetic field strengths (AC-to-DC) and also on the level of noise (temperature), the higher harmonics display a specific resonance-like behavior.

In a uniaxially anisotropic particle the frequency of intrinsic ferromagnetic resonance depends on the angle deviation of the magnetic moment from its equilibrium position. At low spin–lattice relaxation and in the presence of superparamagnetism, the regime of relaxation in an assembly of such particles closely resembles the Landau damping regime, which is well known in the kinetic theory of plasma. In the high-temperature limit in the case of polydisperse systems, particle volume and surface anisotropy affect the absorption lines differently.

A theory describing magneto-orientational properties of suspensions containing antiferromagnetic nanoparticles is developed. Due to their small size, these particles possess, apart from an anisotropic magnetic susceptibility pertinent to antiferromagnets, a spontaneous magnetic moment caused by sublattice decompensation. In a colloid subjected to a DC field of increasing strength an orientational crossover takes place: the particle magnetic moments, initially aligned along the field, turn to the transverse orientation. This behavior considerably changes the observable characteristics of the system: the spectrum of linear dynamic susceptibility and the integral time of magnetic relaxation under a pulse field.

The general Fokker-Planck equation is derived, which takes into account two simultaneous diffusion processes: rotation of the magnetic moment of the particle around its crystal axes and rotation of the particle together with its magnetic moment with respect to the liquid. The influence of the joint diffusion on additional (rotational) viscosity arising in the flow of magnetic fluid in the presence of magnetic field is studied.

A simple dynamic theory of nonequilibrium phenomena in magnetic fluids is suggested. It is shown that temperature maximum of the magnetic susceptibility and special character of its dispersion (1/ω−noise) are explained by the one-particle effects. A conception which links these phenomena with transition of the magnetic fluid into the spin-glass state is criticized.

On the basis of Brown's kinetic equation a consistent study of the regimes of cyclic magnetization reversal of single-domain particles with a uniaxial anisotropy is performed. The applied field is harmonic and linearly polarized, its amplitude equals the maximal coercive force of a Stoner-Wohlfarth particle. The dynamic magnetic hysteresis loops are obtained for a particle (oriented particle ensemble), whose easy-magnetization axis is tilted to the field direction under an arbitrary angle. It is shown that the Stoner-Wohlfarth regime (often termed as quasistatic) is able to describe the behavior of a nanoparticle only in a quite limited material and external parameter range. The developed approach has at least two major merits: it enables one to consider dynamic magnetic hysteresis in the temperature-frequency domains inaccessible with the aid of approximate methods, and provides a tool to test the accuracy of the latter.

Energy absorption due to viscous friction in a dilute suspension of single-domain ferromagnetic particles subjected to a rotating field is considered. The problem is treated in the framework of kinetic approach. The behavior of specific loss power (SLP) as a function of the field amplitude and frequency is studied. It is shown that for either of these parameters (while the other is kept constant) SLP first grows quadratically and then saturates. The cases of a rotating field and oscillating fields are compared, and the essential differences are revealed. The results obtained enable one to assess the allowable or optimal field parameters for a given magnetic suspension intended for rotational magneto-inductive heating.

On the basis of the rotation-diffusion model the orientational dynamics of a dipolar particle suspension (ferrocolloid, stiff-chain polymer solution, etc.) in an alternating field is studied. The frequency spectrum of the field-induced oscillations of the suspension optical anisotropy is obtained. It is demonstrated that, providing the field strength is sufficiently large (the nonlinear regime), the characteristic extrema appear on the field dependences of the optical signal harmonics. This peculiar amplitude-frequency behavior may be employed for estimation of the dispersed particle parameters.

Derivation of a theory for linear and cubic dynamic susceptibilities of random assemblies of fine magnetic particles is outlined. The origin of differences between the low-frequency magnetic spectra of solid systems and magnetic suspensions (ferrofluids) is discussed.

The GaAs-electrolyte interface has been investigated by low-temperature photoluminescence experiments. An unusual new line, referred to as the S-band, has been found. The band is shown to result from radiative recombination of spatially separated 2D e- and h-layers localized near the surface. To prove the 2D nature of the system diamagnetic properties of it were studied in the magnetic fields up to 6 T. At low magnetic fields (i.e. (Lb)2 ⪢ 1, where L - magnetic length, and b-1 - characteristic width of the system) the system shows well-known 2D system properties. The basic energy characteristics obtained from S-band photoluminescence spectra analysis are in a good agreement with a theoretical description of the system under consideration.

The diamagnetic properties of the 2D e-h system on the GaAs surface are studied detailly by the photoluminescence method at T = 1.9 K. An anomalous value of the ground state energy diamagnetic shift δ∥, of the 2D e-h system in magnetic parallel field is found. As well an unusual effect is observed in magnetic parallel field concerned with the self-consistent origin of the system: a jump of δ∥ versus the excitation level G. It is shown that the effects are concerned with the transformation of the energy structure of the system under high magnetic field and pumping level. [Russian Text Ignored]

The micromagnetic Fokker-Planck equation is solved for a uniaxial particle in the low-temperature limit. Asymptotic series in the parameter that is the inverse barrier height-to-temperature ratio are derived. With the aid of these series, the expressions for the superparamagnetic relaxation time and the odd-order dynamic susceptibilities are presented. The obtained formulas are both quite compact and practically exact in the low (with respect to FMR) frequency range that is proved by comparison with the numerically-exact solution of the micromagnetic equation. The susceptibilities formulas contain angular dependencies that allow to consider textured as well as randomly oriented particle assemblies. Our results advance the previous two-level model for nonlinear superparamagnetic relaxation., 18 pages, 6 Encapsulated PostScript figures, uses REVTex4 style

Dynamic birefringence in a ferrofluid subjected to crossed bias (constant) and probing (pulse or ac) fields is considered, assuming that the nanoparticles have finite magnetic anisotropy. This is done on the basis of the general Fokker-Planck equation that takes into account both internal magnetic and external mechanical degrees of freedom of the particle. We describe the orientation dynamics in terms of the integral relaxation time of the macroscopic orientation order parameter. To account for an arbitrary relation between the bias (external) and anisotropy (internal) fields, an interpolation expression for the integral relaxation time is proposed and justified. A developed description is used to interpret the measurements of birefringence relaxation in magnetic fluids with nanoparticles of high (cobalt ferrite) and low (maghemite) anisotropy. The proposed theory appears to be in full qualitative agreement with all the experimental data available.

As an important example of noisy rotary oscillations, the dynamic magnetization of an assembly of superparamagnetic particles is considered. In the presence of a bias field, there exists a mechanism that causes selective suppression of higher harmonics in the response spectrum of the system. Manifestation of this effect at temperature variation is known as the noise-induced resonance. Its manifestation at the change of excitation intensity, formerly unknown, we term the force-induced resonance.

Micromagnetic Fokker–Planck equation with allowance for the gyration term is solved for a uniaxial particle in the low-temperature limit. The solution is obtained in the form of an asymptotic series in the parameter that is the barrier height-to-temperature ratio, and in these terms the superparamagnetic relaxation time and linear and cubic dynamic susceptibilities are presented. The effect of strong enhancement of cubic susceptibility in the weak precession damping limit, formerly found in numerical simulations, is proven analytically.

A prototype of the PRISMA project array has been deployed and started running on the basis of the NEVOD complex at National Research Nuclear University MEPhI. It consists now of 32 detectors of a novel type (en-detector) sensitive to two main EAS components: electromagnetic (e) and hadronic (through thermal neutrons (n)) ones. The purpose of the array is testing and developing of a new experimental method of EAS study through neutron and electromagnetic components recording, optimization of the detector design and the data acquisition system. First experimental results are presented and compared with first Monte-Carlo simulations.

Forbush decrease was recorded for the first time with a global net of the neutron scintillation detectors (en-detectors) developed in INR RAS. The detectors are located at four geographic points: Baksan (43E, 43N), Gran Sasso, Italy (13E, 42N), Moscow (37E, 56N) and Obninsk (38E, 55N). This experimental fact by itself shows not only the possibility of using the detectors for cosmic ray variations study but also unbiasly supports the nuclear physics method developed by us as a novel instrument both for geophysical researches and for neutron environment monitoring.

The theory of ferromagnetic resonance (FMR) in an assembly of single-domain uniaxially anisotropic particles is being applied to suspensions. The approach used has, as its basis, the independent-grain model once proposed for the description of FMR in polycrystals. We surmise that the basic assumptions of this model are also appropriate for composites of fine magnetic particles. With regard to the specific properties of the systems in question, we extend the model taking into account (i) the thermofluctuational effects (superparamagnetism) and (ii) the orientational mobility of the particles which leads to the change of the orientational distribution of their anisotropy axes under the influence of the applied field. The joint action of these effects imparts some specific features to the FMR spectra of a magnetic suspension. For example, in a certain temperature range two peak absorption lines must occur.

The calculation of the viscosity of liquid crystals based on the rotational diffusion approximation recently reported by Osipov and Terentjev [1 a] is critically examined. It is shown that the main result of their treatment - the evaluation of the activational exponent - arises from the neglect of the second derivative of the distribution function with respect to the azimuthal angle in the Fokker-Planck equation. The correct asymptotic formula for the rotational viscosity coefficient is derived.

A model approach is developed for intuitionistic and classical theories in the language of the simple theory of types. A weak form of completeness theorem is proved, of which the Henkin completeness theorem is a special case. An ultraproduct of Kripke structures is introduced, and an application of the omitting types theorem to theories with infinite rules for deduction is given.Bibliography: 14 titles.

Absorption and modulated transmission in the spectral region of spin-orbit transitions in Ge are measured and compared with calculations which take into account excitonic effects and their changes under high excitation. Measurements at high magnetic fields corroborate the findings. Es werden Absorption und modulierte Transmission im Spektralgebiet der Spin–Bahn-Ubergange von Ge gemessen und mit Rechnungen verglichen, die exzitonische Effekte und deren Veranderung bei hoher Anregung berucksichtigen. Messungen bei hohen Magnetfeldern unterstutzen die Aussagen.

A model of exciton condensation into the liquid phase in thin samples taking into account the surface recombination of embryos sticked to the surface is developed. It predicts a drastic increase of the homogeneous nucleation threshold and its temperature independence in thin crystals. The model explains qualitatively the experimental data for thin samples of germanium presented.