Normal skin effect in the polycrystals

The surface impedance γ and the penetration depth δ of the electromagnetic field was calculated for a polycrystalline conductor under the conditions of the normal skin effect. The polycrystal is composed from the single-phase slightly anisotropic single-crystalline grains of arbitrary symmetry. Corrections (γ-γ₀)/γ₀ and (δ-δ₀)/δ₀ due to the polycrystallinity are functions of the parameter ζ=δ₀/a, where γ₀ and δ₀ are the impedance and the penetration depth of the isotropic conductor with the conductivity σ₀=Sp σ_ik/3, σ_ik is the separate grain conductivity tensor, and a is the mean size of a grain.

The effective conductivity of a polycrystal can be obtained either from the impedance γ (σ_ef^(γ)) or from the penetration depth δ (σ_ef^(δ)) It was found that σ_ef^(γ)≠σ_ef^(δ) The corrections (σ-σ_ef^((γ))/σ₀ and (σ₀-σ_ef^(δ))/σ₀ depend on the frequency of the incident wave.

The obtained results are also applicable in the infrared spectrum region. It was shown that, just as for the isotropic conductor, there is the total reflection of the electromagnetic wave from the polycrystal surface.

It was necessary to examine the oblique incidence of an electromagnetic wave onto the surface of an isotropic metal for the substantiation of the set of the boundary conditions used.     

Electromagnetic wave propagation in polycrystalline materials: effective medium approach

A theory for the propagation of electromagnetic waves in inhomogeneous solids made of anisotropic crystallites is developed in the framework of the effective medium approach. The macroscopic dielectric tensor can explicitly be expressed by characteristic integrals containing the radial distribution function and a single anisotropy parameter. The phase mismatch of the waves scattered from misoriented crystallites leads to absorption and refraction effects that are calculated using a self-consistent approach in the sense of the distorted-wave approximation. For higher frequencies resonance structures occur which can be interpreted as an interference effect between disturbed and undisturbed waves in the effective medium.     

Resonance line shape in a magnetically diluted system of an arbitrary dimensionality

The third (∝C²) term in the concentration expansion of free induction decay is calculated for a magnetically diluted system of an arbitrary integer dimensionality d≤3 and for different values of parameter a characterizing the relative intensity of flip-flop processes. The concentration expansion was used to generalize, in a natural way, the Anderson-Weiss-Kubo theory that was originally developed to describe magnetic resonance in ordered systems in terms of the second and fourth moments. The results are compared with recent absorption measurements in quasi-two-dimensional media. The possibility of describing the exchange narrowing is discussed.     

Radiochemical determination of Mo in pure tungsten

A radiochemical neutron activation technique for Mo determination in high purity tungsten, based on some specific properties of Mo and W radionuclides has been developed. Al₂O₃ powder has been used as a sorbent. An estimation of the Mo content was carried out via the selectively separated^99mTc daughter radionuclide. Limit of detection was 10 ng g^–1.     

Spin dynamics in solid dilute paramagnets

The free-induction decay signals for the two-spin clusters, the “matrix,” and the entire spin system as a whole are calculated for a real dipole-dipole interaction on the basis of a method for the cluster expansion of correlation functions previously developed by the authors. The intracluster interactions, which generate a discrete spectrum, are taken into account exactly, and the interactions outside the clusters are taken into account on the basis of the Anderson-Weiss-Kubo (AWK) theory of phase relaxation. The corresponding shape functions of the resonance line are calculated. Before the clusters are separated, an AWK analysis in the limit of slow fluctuations of the local fields is in satisfactory agreement with the exact results obtained in the Anderson model of dipolar interactions, but a similar analysis for realistic fluctuation rates and Dt?1 leads to a significant qualitative difference: exp(?Dt) transforms into $\exp ( - \sqrt {D_1 t} )$ . After the clusters are separated, this difference is obliterated. The free-induction decay is almost exponential up to Dt～10, and nonexponential behavior of the long-time asymptote appears sooner. This finding is not altered significantly when the analysis is complicated by raising the maximum rank of the clusters to 3 or more.     

Thermal conductivity of the spin-Peierls compound CuGeO3

The thermal conductivity of the pure and zinc-doped spin-Peierls compound CuGeO₃ is investigated for the first time. Characteristic features reflecting the changes in the phonon spectrum of the crystals are observed in the k(T) curves at a transition into the dimerized state in CuGeO₃ at T _sp =14.2 K and in Cu_0.98Zn_0.02GeO₃ at T _sp =10.6 K. Near the spin-Peierls transition temperature T _sp the thermal conductivity of Cu_0.98Zn_0.02GeO₃ (k=3 W/m·K) is much less than the thermal conductivity of CuGeO₃ (k=21.5 W/m·K). Pis’ma Zh. éksp. Teor. Fiz. 66, No. 12, 823–826 (25 December 1997)