Proton magnetic relaxation in a disperse “dry water” nanosystem

We have studied the characteristic features of proton magnetic relaxation in a disperse “dry water” nanosystem. We have determined the effect of the microstructure of the “dry water” on its NMR relaxation parameters.     

Einstein’s “Zur Elektrodynamik...” (1905) Revisited, With Some Consequences

Einstein, in his “Zur Elektrodynamik bewegter K?rper”, gave a physical (operational) meaning to “time” of a remote event in describing “motion” by introducing the concept of “synchronous stationary clocks located at different places”. But with regard to “place” in describing motion, he assumed without analysis the concept of a system of co-ordinates.In the present paper, we propose a way of giving physical (operational) meaning to the concepts of “place” and “co-ordinate system”, and show how the observer can define both the place and time of a remote event. Following Einstein, we consider another system “in uniform motion of translation relatively to the former”. Without assuming “the properties of homogeneity which we attribute to space and time”, we show that the definitions of space and time in the two systems are linearly related. We deduce some novel consequences of our approach regarding faster-than-light observers and particles, “one-way” and “two-way” velocities of light, symmetry, the “group property” of inertial reference frames, length contraction and time dilatation, and the “twin paradox”. Finally, we point out a flaw in Einstein’s argument in the “Electrodynamical Part” of his paper and show that the Lorentz force formula and Einstein’s formula for transformation of field quantities are mutually consistent. We show that for faster-than-light bodies, a simple modification of Planck’s formula for mass suffices. (Except for the reference to Planck’s formula, we restrict ourselves to Physics of 1905.)     

Incommensurateness effects in a lattice Ginzburg-Landau model

This paper discusses the effect of magnetic translational symmetry on the vortex structure in superconducting crystals with a large basis in artificial Josephson media (regular lattices of superconducting clusters) prepared with opal as the base material. For external magnetic fields lower than the upper critical field, the lattice Ginzburg-Landau model reduces to the two-dimensional Frenkel’-Kontorova model which in some cases is exactly solvable, in which the crystal lattice plays the role of an “hard sublattice” while the deformable vortex lattice plays the role of a “soft sublattice.” It is shown that static shear waves in the vortex lattice are solutions to the two-dimensional sine-Gordon equation with an additional condition of incompressibility implied by flux quantization. The pinning energy is found as a function of the magnetic field, nearness to the transition line, and the crystal lattice constant. Fiz. Tverd. Tela (St. Petersburg) 39, 1158–1162 (July 1997)     

Effect of infrared illumination on the modulated magnetic structure of the weak ferromagnet FeBO<Subscript>3</Subscript>: Mg

The effect of unpolarized white light on the period and conditions of the existence of the modulated magnetic structure of the FeBO₃: Mg single crystal, which is formed in this easy-plane weak ferromagnet in the low-temperature range during technical magnetization, has been investigated experimentally. It has been revealed that the degree of light action on the magnetic state of the crystal depends on both the irradiation duration and the orientation of the ferromagnetic vector during illumination. It has been established that light with wavelengths in the range 0.8 μm < λ < 0.9 μm has a maximum effect on the parameters of the modulated magnetic structure formed in the FeBO₃: Mg single crystal. The results obtained have been interpreted in the framework of the “magnetic ripple” theory using the model of photosensitivity of anisotropic magnetic centers associated with the presence of Mg impurity ions in the composition of the crystal.     

Reconstruction of the Fermi surface in the pseudogap state of cuprates

Reconstruction of the Fermi surface of high-temperature superconducting cuprates in the pseudogap state is analyzed within a nearly exactly solvable model of the pseudogap state, induced by short-range order fluctuations of the antiferromagnetic (AFM), spin-density wave (SDW), or a similar charge-density wave (CDW) order parameter, competing with the superconductivity. We explicitly demonstrate the evolution from “Fermi arcs” (on the “large” Fermi surface) observed in the ARPES experiments at relatively high temperatures (when both the amplitude and phase of the density waves fluctuate randomly) towards the formation of typical “small” electron and hole “pockets,” which are apparently observed in the de Haas-van Alphen and Hall resistance oscillation experiments at low temperatures (when only the phase of the density waves fluctuate and the correlation length of the short-range order is large enough). A qualitative criterion for the quantum oscillations in high magnetic fields to be observable in the pseudogap state is formulated in terms of the cyclotron frequency, the correlation length of fluctuations, and the Fermi velocity. The text was submitted by the authors in English.     

What can polymer crystal structure tell about polymer crystallization processes?

Contrary to most or all other materials, crystallization of chiral but racemic polymers such as isotactic polypropylene is accompanied by a conformational rearrangement which leads to helical geometries: the building units of the crystal are helical stems, -20nm long, which can be either right-handed or left-handed. Helical hand cannot be reversed within the crystal structure: it is therefore a permanent marker and an indicator of molecular processes (in particular segregation/selection of helical hands) which take place during crystal growth, and more precisely during the crucial step of “efficient” helical stem deposition. The issue of proper helical hand selection during polymer crystal growth is mainly illustrated with isotactic polypropylene. Its various crystalline polymorphs (, , and smectic) display virtually all possible combinations of helical hands, azimuthal settings and even non-parallel orientation of helix axes in space. Furthermore, a specific homoepitaxy which generates a lamellar branching in the phase “quadrites” and composite structures makes it possible a) to determine the helical hand and associated azimuthal setting of every stem in the crystalline entities and b) to determine the impact on the crystal structure and morphology of “mistakes” in helical hand of the depositing stem. Analysis of these morphologies demonstrates that the crystallization of isotactic polypropylene (and by implication of other achiral, helical polymers) is a highly sequential and “substrate-determined” process, i.e. that the depositing stem probes the topography of the growth face prior to attachment. These observations appear difficult to reconcile with crystallization schemes in which molecules (helical segments) are prearranged in a kind of pseudo-crystalline bundle (and as such, are not subjected to the high constraints of crystal symmetry) before deposition as a preassembled entity on the substrate. Received: 5 May 2000     

Thermodynamics of the HMF model with a magnetic field

We study the thermodynamics of the Hamiltonian mean field (HMF) model with an external potential playing the role of a “magnetic field”. If we consider only fully stable states, the caloric curve does not present any phase transition. However, if we take into account metastable states (for a restricted class of perturbations), we find a very rich phenomenology. In particular, the caloric curve displays a region of negative specific heat in the microcanonical ensemble in which the temperature decreases as the energy increases. This leads to ensembles inequivalence and to zeroth order phase transitions similar to the “gravothermal catastrophe” and to the “isothermal collapse” of self-gravitating systems. In the present case, they correspond to the reorganization of the system from an “anti-aligned” phase (magnetization pointing in the direction opposite to the magnetic field) to an “aligned” phase (magnetization pointing in the same direction as the magnetic field). We also find that the magnetic susceptibility can be negative in the microcanonical ensemble so that the magnetization decreases as the magnetic field increases. The magnetic curves can take various shapes depending on the values of energy or temperature. We describe first order phase transitions and hysteretic cycles involving positive or negative susceptibilities. We also show that this model exhibits gaps in the magnetization at fixed energy, resulting in ergodicity breaking.     

A short review of the magnetoelectric effect and related experimental techniques on single phase (multi-) ferroics

Whereas materials with intrinsic magnetoelectric (ME) effects have not yet made inroads in technology, the measurement of their tensor characteristics has become a precious tool for magnetic point group determination. Therefore, it is worthwhile to consider different measurement techniques. In particular techniques for determining the linear and bilinear ME effects will be discussed, essentially the quasi-static and dynamic magnetic field-induced methods will be evaluated. The measurement and application of ME “butterfly" loops for determining (weak) ferromagnetism and internal bias fields will be described. For the bilinear ME effect (with invariant EHH) a particularly sensitive measurement method with amplification effect will be highlighted, permitting, e.g., to detect subtle magnetic phase transitions. At least for the linear ME effect, we will stress that in the future only a dimensionless quantity should be used which is valid in all systems of units. Finally, the linear ME effect of TbPO₄ crystals is reexamined because in a former publication it was not clear which system of units was effectively used (“rationalized” or “not rationalized” Gaussian system of units). Effectively, this crystal has the largest linear ME effect known. At T = 1.50 K, in SI units: α _xy or α _yx = 730 ps/m, i.e., 0.220 in “not rationalized” Gaussian system of units.     

Zone structure and its genesis from the states of sublattices in orthorhombic MgGeN<Subscript>2</Subscript>

It is demonstrated that the crystalline structure of MgGeN₂ can be reduced to the superposition of “approximated” high-symmetry Bravais sublattices by shifting atoms in a unit cell within the range of 5% of the lattice constant. The superimposed Brillouin zones were plotted for crystal lattices and sublattices. The computational results are presented for the electronic structure of MgGeN₂ in terms of the theory of the density function. The influence of “hidden” symmetry in this crystal on zone spectra is described.     

Cellular solid behaviour of liquid crystal colloids 2. Mechanical properties

This paper presents the results of a rheological study of thermotropic nematic colloids aggregated into cellular structures. Small sterically stabilised PMMA particles dispersed in a liquid crystal matrix densely pack on cell interfaces, but reversibly mix with the matrix when the system is heated above . We obtain a remarkably high elastic modulus, , which is a nearly linear function of particle concentration. A characteristic yield stress is required to disrupt the continuity of cellular structure and liquify the response. The colloid aggregation in a “poor nematic” MBBA has the same cellular morphology as in the “good nematic” 5CB, but the elastic strength is at least an order of magnitude lower. These findings are supported by theoretical arguments based on the high surface tension interfaces of a foam-like cellular system, taking into account the local melting of nematic liquid and the depletion locking of packed particles on interfaces. Received 13 March 2000 and Received in final form 6 June 2000     

Influence of one-dimensional “defects” on the new-phase nucleation dynamics near the first-order transition in magnets

The evolution of a new-phase nucleus near the first-order spin-reorientation phase transition in magnets has been investigated. A strong influence of one-dimensional magnetic anisotropy “defects” on the nucleation dynamics has been shown. The conditions of the localization of the new-phase nucleus in the region of the magnetic anisotropy “defect” have been determined.     

On Hide's magnetic analogue of Ertel's vorticity theorem

The relativistic formulation of Hide's “magnetic analogue” of Ertel's potential vorticity theorem is Dirac's “new classical theory of electrons”.     

Magnetic structure of iron-doped UPd2Ge2

Neutron diffraction investigations have shown that a 2% substitution Fe atoms for Pd radically alters the magnetic structure of UPd²Ge². If the magnetic structure in the undoped compound at T>50 K consists of a longitudinal spin density wave (LSDW) with “square” modulation, then in polycrystalline U(Pd^0.98Fe^0.02)²Ge² a “simple” antiferromagnetic (AF) phase is observed below 65 K and a sinusoidally modulated LSDW-AF phase is observed between 65 K and the Neel temperature T ^N =135 K. In the interval 65> T>135 K the magnetic cell is incommensurate with the crystal cell, with the exception of the point T=93 K, where the wave vector of the magnetic structure passes through a “commensurate” value equal to 0.75. Below T ^N the magnetic moments of the uranium atoms are always parallel to the tetragonal axis c of the unit cell. Pis’ma Zh. éksp. Teor. Fiz. 66, No. 9, 615–619 (10 November 1997)     

Photoinduced changes in the space-modulated magnetic order of a FeBO<Subscript>3</Subscript>:Mg single crystal

An effect of nonpolarized white light on the modulated magnetic structure of a FeBO₃:Mg single crystal, which arises in this light-plane weak ferromagnet in the low temperature range during technical magnetization, has been revealed. It has been found that the degree of the light action on the magnetic state of FeBO₃:Mg depends both on its duration and on the orientation of the spontaneous magnetization vector M of the crystal during illumination. Interpretation of the results obtained has been performed in the context of the “magnetic ripple” theory on the assumption that the absorbed light induces additional uniaxial magnetic anisotropy in the easy plane of the crystal and that the anisotropy axis is collinear to the vector M during illumination.     

Destruction of the long-range antiferromagnetic order by defects of the “random local field” type

The behavior of diluted antiferromagnets in an external magnetic field has been considered. It has been shown that, because the “force” of induced defects of the “random local field” type depends on the magnetic induction, the destruction of the long-range antiferromagnetic order by these defects in space of dimension d ≥ 2 is impossible.     

Positive magnetoresistivity of Fe0.95Co0.05Ge2 single crystals inside the region of a magnetic phase transition

We report on the detection of two narrow peaks of positive magnetoresistivity in the temperature dependence of the magnetoresistivity of a Fe_0.95Co_0.05Ge₂ single crystal in the vicinity of a “smeared” first-order phase transition. The position of these peaks correlates with the position of singularities in the temperature dependence of the temperature derivative of the electrical resistivity and magnetic susceptibility. We show that these singularities in the transport and magnetic properties are, probably due to the presence of two percolation transitions with temperature in the magnetic subsystem of the crystal. Zh. éksp. Teor. Fiz. 112, 690–697 (August 1997)     

Coexistence of superconductivity and magnetic order

Experimental work on systems showing evidence of the coexistence of superconductivity and magnetic order is reviewed. The presently known systems are shown to be “superconducting spin glasses” rather than “ferromagnetic superconductors”. In those systems where the magnetic order is expected to be long-ranged (up to now) superconductivity vanishes as soon as the magnetic order appears. But it is not excluded that magnetic order with a correlation length larger than the superconductive coherence length can coexist with superconductivity.     

The Fedosov Manifolds and Non-Lagrangian Monopole-Type Systems

A non-symplectic generalization of Hamiltonian mechanics is considered. It allows include into consideration “non-Lagrangian” systems, such as theory of charged particle in the field of magnetic monopole. The corresponding generalization for the Fedosov manifolds is given. The structure of phase space of “charged particle in the field of magnetic monopole” is studied.     

Spontaneous SU(2) symmetry breaking in one-dimensional quantum antiferromagnet

We present a Bethe Ansatz based investigation of a one-dimensional (1D) Heisenberg spin chain in a real 3D crystal lattice. We have shown that due to an influence of the lattice distortion on a crystalline field of ligands of magnetic ions, a Heisenberg antiferromagnetic spin chain is unstable under the appearance of a magnetic anisotropy of the “easy-plane” type. The effects of an external magnetic field and nonzero temperature onto such a phase transition are studied. Received: 19 January 1998 / Revised: 16 March 1998 / Accepted: 17 March 1998