Spin-waves in two-dimensional antiferromagnets of the K2FeF4 type

The spectrum of spin-waves in two-dimensional antiferromagnet of the K₂FeF₄ type is calculated taking into account the total single-ion anisotropy and lattice distortions. The formulas for the AFMR frequencies are obtained and it is shown that the observed values cannot agree with theoretical ones without deformations being taken into account. The temperature dependence of strain tensor is found in the region of the spin- wave approximation validity.     

Orbital order fluctuations in KCuF3

The interaction of orbital momenta of copper in KCuF₃ via the field of elastic deformations is analyzed taking into account the local rotations of electron density on copper ions. It is found that the interaction energy related to local rotations of copper ions is comparable in order of magnitude with the Dzyaloshinskii-Moriya interaction energy. The calculated interaction energy of copper orbitals in the ab plane of the crystal exceeds the energy of their interaction along the c axis. This circumstance explains recently found specific features of orbital melting in LaMnO₃ with increasing temperature.     

Fusion of aligned deformed heavy ions calculated in the surface friction model

The surface friction model for heavy ion induced fusion is extended to cover collisions of aligned deformed nuclei taking into account dynamic quadrupole and octupole vibrations and rotations of the reaction partners. Calculated fusion cross sections for unpolarized beam and the tensor analyzing powerT ₂₀ for fusion of²³Na +²³Na are compared with data. The analyzing power is found to be sensitive to variations of the tangential friction strength.     

Quantum hall effect in an antidot lattice: Macroscopic limit

The quantum Hall effect in a 2D system with antidots is studied. The antidots are assumed to be large compared with the quantum and relaxation lengths. In this approximation the electric field in the system can be described by the continuity equation. It is found that the electric field in a system without conducting boundaries can be expressed in terms of the same system without a magnetic field. Specific problems of the electric field and current in structures containing one or two antidots and in a circular disk with point contacts are solved. The effective Hall and longitudinal conductivities in a sample containing a large number of randomly distributed antidots are found. In the limit of zero local longitudinal conductivity, the effective longitudinal conductivity also vanishes, and the Hall conductivity is equal to the local conductivity. The corrections to the conductivity tensor which are due to the finiteness of the local conductivity are obtained. Breakdown of the quantum Hall effect in a lattice of antidots is studied on the basis of the assumption that a high current density in narrow locations of the system results in overheating of the electrons. Local and nonlocal models of over-heating are studied. The high-frequency effective conductivity of a system with antidots and the shift of the cyclotron resonance frequency are found.     

Time Relativity in the Dynamics of Open Quantum Systems

Various methods for determining the Coulomb logarithm in the kinetic theory of transport and various variants of the choice of the plasma screening constant, taking into account and disregarding the contribution of the ion component and the boundary value of the electron wavevector are considered. The correlation of ions is taken into account using the Ornstein–Zernike integral equation in the hypernetted-chain approximation. It is found that the effect of ion correlation in a nondegenerate plasma is weak, while in a degenerate plasma, this effect must be taken into account when screening is determined by the electron component alone. The calculated values of the electrical conductivity of a hydrogen plasma are compared with the values determined experimentally in the megabar pressure range. It is shown that the values of the Coulomb logarithm can indeed be smaller than unity. Special experiments are proposed for a more exact determination of the Coulomb logarithm in a magnetic field for extremely high pressures, for which electron scattering by ions prevails.     

Effects of Coulomb Collisions on the Dispersion Relation of Longitudinal Plasma Oscillations

The dispersion relation of longitudinal electron-plasma oscillation for a fully ionized plasma with equal electron and ion temperatures in the absence of an external magnetic field has been calculated by taking into account of both the electron-ion and electron-electron collisions. The calculation utilizes the absorptive parts of the collisional conductivity functions derived by DuBois and coworkers for obtaining the polarizability parts via Kramers-Kronig relation. The results are presented in graphical forms and show that the longitudinal plasma oscillations are shifted toward higher frequencies from the resonance of collisionless plasma predicted by the Bohm-Gross dispersion relation.     

High-frequency conductivity of metals in degenerate semiconductors with extrinsic electron states

Spatial dispersion is taken into account in calculations of the dissipative part of the transverse conductivity of weakly alloyed semiconductors. Local and resonant states of the electrons are considered. The maxima of the conductivity are observed for transitions between local, resonant, and band electron states.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 4, pp. 11–14, April, 1987.The author expresses thanks to E. A. Kaner for his discussion of the study results.     

Diffusion of a pulsed field and electromagnetic forces in ferromagnets

Self-similar solutions of the magnetic field distribution in a conducting ferromagnet with a non-linear permeability are considered. The velocity of magnetic field penetration is calculated in various models of ferromagnetic properties of the conductor. Using the Maxwell tensor, equations are derived for bulk forces taking into account the possibility of saturation of the ferromagnet. A numerical solution for the distributions of the magnetic induction and of the density of the bulk force in a ferromagnetic conductor is obtained using the Preisach model taking hysteresis into account.     

Skin effect in metals: Passage to the infinite conductivity limit

The infinite conductivity limit in the classical skin effect in metals is considered. The analysis is carried out with and without taking into account the influence of free electron relaxation, as well as local relationships between the electric field and current density, in the free electron model. Both ultimately nonlocal and real limits are considered for the indefinitely increasing free path.     

Importance of exactb-tensor calculation for quantitative diffusion tensor imaging and tracking of neuronal fiber bundles

Quantitative diffusion tensor imaging (DTI) is a novel method of magnetic resonance (MR) imaging providing information on the brain’s microstructure in vivo. DTI can be effectively measured with modern clinical MR scanners. However, imaging sequence details required for accurateb matrix calculation and for following DTI quantification are normally unknown to the user. In this work, we investigated the accuracy ofb value approximation if theb matrix is calculated without taking into account the effect of imaging gradients. It was found that an error of more than 4% in DTI estimation arises for a quite typical brain imaging protocol. The errors in mean diffusivity and fractional anisotropy index depend on diffusion tensor shape and eigenvectors orientation and exceed noise level in DTI quantification. These errors however have a strong impact on fiber tracking — up to 30% difference was found between the fiber tracks corresponding to exact and approximate calculated DTI data. Since these errors are dependent on imaging parameters and sequence implementation, accurateb matrix calculations are important for adequate comparison between data acquired on different MR scanners and also for data measured with the different imaging protocols.     

Effective electrical and thermal conductivity of multifilament twisted superconductors

The effective electrical and thermal conductivity of composite wire with twisted superconducting filaments embedded into normal metal matrix is calculated using the extension of Bruggeman method. The resistive conductivity of superconducting filaments is described in terms of symmetric tensor, whereas the conductivity of a matrix is assumed to be isotropic and homogeneous. The dependence of the resistive electrical conductivity of superconducting filaments on temperature, magnetic field, and current density is implied to be parametric. The resulting effective conductivity tensor proved to be non-diagonal and symmetric. The non-diagonal transverse–longitudinal components of effective electrical conductivity tensor are responsible for the redistribution of current between filaments. In the limits of high and low electrical conductivity of filaments the transverse effective conductivity tends to that of obtained previously by Carr. The effective thermal conductivity of composite wires is non-diagonal and radius-dependent even for the isotropic and homogeneous thermal conductivities of matrix and filaments.     

A theoretical validation of the B-matrix spatial distribution approach to diffusion tensor imaging

The recently presented B-matrix Spatial Distribution (BSD) approach is a calibration technique which derives the actual distribution of the B-matrix in space. It is claimed that taking into account the spatial variability of the B-matrix improves the accuracy of diffusion tensor imaging (DTI). The purpose of this study is to verify this approach theoretically through computer simulations.Assuming three different spatial distributions of the B-matrix, diffusion weighted signals were calculated for the six orientations of a model anisotropic phantom. Subsequently two variants of the BSD calibration were performed for each of the three cases; one with the assumption of high uniformity of the model phantom (uBSD-DTI) and the other taking into account imperfections in phantom structure (BSD-DTI). Several cases of varying degrees of phantom uniformity were analyzed and the distributions of the B-matrix obtained were used for the calculation of the diffusion tensor of a model isotropic phantom. The results were compared with standard diffusion tensor calculation.The simulations confirmed the improvement of accuracy in the determination of the diffusion tensor after the calibration. BSD-DTI improves accuracy independent of both the degree of uniformity of the phantom and the inhomogeneity of the B-matrix. In cases of a relatively good uniformity of the phantom and minor distortions in the spatial distribution of the B-matrix, the uBSD-DTI approach is sufficient.     

Effect of scattering processes on the dynamics of electrons in narrow energy bands

We have investigated the effect of a magnetic field and the degree of filling and width of the band on the scattering processes on the basis of the Hubbard method. The electrical conductivity tensor was calculated with account taken of damping processes. It is shown that the relaxation times depend essentially on the electron spin orientations. Theordering of the electron spins under the influence of the external magnetic field leads to an increase of the electrical conductivity tensor, that in turn leads to a negative magnetoresistance.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 6, pp. 65–69, June, 1976.     

Propagation of radiation in a plasma situated in a gravitational field

Weak electromagnetic and gravitational fields in a plasma situated in a strong gravitational field, are studied using linearized, general-relativistic, kinetic equations. A tensor operator is constructed for the electrical conductivity of a plasma in a gravitational field, which is a general-relativistic generalization of the electrical conductivity of a homogeneous plasma. Similar tensor operators, which allow one to determine the energy-momentum tensor and the vector current, induced by electromagnetic and gravitational fields in a plasma, are also obtained.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 9, pp. 57–62, September, 1976.     

On Splitting the Conduction Band of Liquid Mercury

Electron characteristics of the contributions to the mercury optical conductivity at room temperature have been calculated based on comparison of the model spectrum of the optical conductivity with the experimental data. It is shown that the experimental spectrum of the liquid-mercury optical conductivity can be described only taking into account the possibility of splitting the conduction band.     

The effect of Zeeman splitting on Shubnikov-de Haas oscillations in two-dimensional systems

A theory of the Shubnikov-de Haas effect is developed for two-dimensional systems in a tilted magnetic field. The conductivity tensor is calculated for an arbitrary ratio r of the Zeeman splitting to the cyclotron splitting. Possible anisotropy of the g factor is taken into account. It is shown that at integer values of r, the main harmonic dominates in the spectrum of Shubnikov-de Haas oscillations and the phase of the oscillations depends on the parity of r. At half-integer values of r, the conductivity oscillations are determined by the harmonics of the second order of smallness.     

Spin-wave resonance spectra in bilayer magnetic films

The anti-Hermitian part of the high-frequency susceptibility tensor components in a bilayer film is calculated by solving the equation of motion for magnetization taking into account damping and using the exchange boundary conditions. The resultant expressions are used for determining the main parameters of the spin-wave resonance spectrum. Comparison of the calculated and experimental results demonstrates the correctness of the proposed method for calculating spectra.     

Electrical Conductivity of Noble Gases at High Pressures

We present experimental and theoretical results for the electrical conductivity of noble gases (He, Ne, Ar, Kr, Xe) up to high pressures where a transition from nonmetallic to metallic‐like conductivities occurs. In addition, we show the behavior of the thermal conductivity and thermopower for xenon as an example. The experiments were performed using explosively driven shock waves. Different geometries allow to probe various parameter regions up to several megabars. Besides single‐shock experiments along the principal Hugoniot curve, also multiple‐shock experiments were performed which follow almost an isentrope. The theoretical calculations were performed within a partially ionized plasma model. The composition is determined by solving a system of mass action laws. The transport coefficients are calculated within linear response theory taking into account the relevant scattering mechanisms of electrons at different ion species, atoms, and other electrons. The general trends of the experimental results can be explained within this theoretical approach. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Nonempirical cluster calculations of the electric field gradient tensor in yttrium-aluminum garnet Y3Al5O12

The electric field gradient (EFG) tensor at the aluminum nucleus sites in yttrium-aluminum garnet Y₃Al₅O₁₂ is calculated using the Hartree-Fock method and the nonempirical cluster approach. It is shown that the EFG tensor at the Al_tetr sites is described well taking into account the nearest neighbors (an [AlO₄]^?5 cluster), whereas for the Al_oct sites the effect of more distant ions is important. The results are compared with the available experimental data and the results of band calculations.     

Quantum electrodynamic corrections for valence electrons in Eka-Hg

The quantum electrodynamic (QED) corrections to the coupling energy of valence electrons in heavy and superheavy nuclei are calculated in the effective local-potential approximation, as well as by the Hartree-Fock-Dirac self-consistent method. It is clearly shown that the contribution from the QED corrections is within the accuracy of modern calculations, which do not take into account QED effects. It is shown that, in certain cases, to exactly calculate the coupling energy of electrons in heavy and superheavy atoms, it is necessary to take into account the self-consistency, which shows that the inaccuracy of the use of the method of the effective local potential in calculations of QED effects can exceed 10%, which is also within the limits of calculations of the coupling energy without taking into account QED effects.