Powder Resonance Research Articles

EXPERIMENTAL PROGRESS IN UNDERSTANDING HIGH-TEMPERATURE SUPERCONDUCTIVITY

In this paper first the structure of the newest layered compounds is presented, which do not contain rare-earth ions, instead either Bi- or Tl-oxide layers are sandwiched between the copper-oxide layers. The latter consist of either corner-connected octahedra, pyramids or planar squares: the transition temperature T/sub c/ increases with the density of copper-oxide layers per unit cell and is accounted for by single-electron interlayer tunneling. The Bi- or Tl-oxide layers can either be double or single, with the former having the higher T/sub c/. Of considerable scientific interest is the discovery of a cubic perovskite with a high T/sub c/ of 29.6 {Kappa} which contains no copper. Ba/sub 1-x/K/sub x/BiO/sub 3/. Owing to its doping with K/sup +/ for Ba/sup 2+/, holes must again be present. If they are on the oxygens, this would be a common feature with the cuprates. This paper is aimed at determining the superconducting gap {Delta} and its anisotropy. Some measurements of infrared reflectivity on single crystals and nuclear magnetic resonance in powders indicate that the ratio is 2{Delta}/{kappa}T/sub c/ {ge} 7. Infrared data supporting this ratio in BaY/sub 2/Cu/sub 3/O/sub 7/ have now been extended. Others however yield values near the BCS weak couplingmore » limit of 3.5. Determination of this ratio is important, because it indicates whether the coupling is weak or strong. A Kramer-Kronig analysis of IR data in the normal-conducting regime supports intermediately strong coupling, but weaker than in many fermion systems. It is also pointed out that the tunneling experiments yield gaps which are somewhat too low because {Delta} is depressed at the surface owing to the intrinsic short coherence lengths.« less

Electron paramagnetic resonance in powdered semiconductors and semimetals

The problem of electron paramagnetic resonance (EPR) in powders of highly conducting semiconductors and semimetals is examined. Attention is focused on the fact that the observed EPR signal can be strongly influenced by the conductivity of the powder grains, and particularly by the high degree of anisotropy (gyrotropy) of the conductivity which arises in these materials when a dc magnetic field is present. The interaction of a single powder grain with an electromagnetic wave is analyzed in terms of the recently developed theory of Ford and Werner for electromagnetic excitations of a gyrotropic sphere. Although the general form of this theory can only be handled numerically, we show that in the case of powder grains, which are typically small compared to the outside wavelength, closed analytic expressions can be obtained for the magnetic and electric polarizabilities of the particles. Using these polarizabilities as a point of departure, the description of the powder as a whole is then obtained in the form of an effective powder as a whole is then obtained in the form of an effective powder permeability and permittivity. A detailed analysis of these general results in specific parameter ranges shows that both the EPR line shape and its intensity depend strongly on the value of the particle conductivity, on its tensorial character, and on the particle size. It is demonstrated that when the powder particles are small compared to the skin-depth and internal wavelength, the EPR absorption line shape is Lorentzian, predicted by the imaginary (dissipative) part of the magnetic susceptibility χ. When particles are large compared to the skin depth, the EPR absorption line shape is determined by a mixture of the dispersive and dissipative components of χ, depending on the behavior of the elements of the magnetoconductivity tensor. Specific limiting cases of the electrical parameters are discussed and the effect of particle-size distribution on EPR in powders is also considered. Where available, experimental data are used to illustrate the behavior predicted by the analysis.

Zeeman Quadrupole Resonance in Powders

The Zeeman patterns for spin-32 quadrupole interactions, for powder samples, have been determined for the special case of the perturbing rf magnetic field perpendicular to the small static magnetic field. A four-line Zeeman pattern has been observed experimentally for 35Cl in NaClO3 and HgCl2 using an induction spectrometer, and a theoretical calculation using a zero linewidth quadrupole resonance has provided an adequate explanation. The resonance shapes and positions are insensitive to the asymmetry parameter in the electric field gradient. The technique is useful for the determination of magnetic shift parameters, and a value of 0.5% has been obtained for the parallel component of the paramagnetic shift of 35Cl in K2ReCl6.

Electron Spin Resonance in Powders

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Absolute Values of Absorption Coefficients in Paramagnetic Resonance Absorption

As a special case of Kramers' equation, we have \begin{aligned} \chi'(0){=}\frac{2}{\pi}\int_{0}^{\infty}\frac{\chi''(H)}{H}{dH}, \end{aligned} where χ''( H ) is the absorption coefficient in paramagnetic resonance absorption and χ'(0) is the static susceptibility. We have checked the equation in cases of paramagnetic resonance in powders of MnSO 4 , MnSO 4 ·H 2 O, MnSO 4 ·4H 2 O, MnSO 4 ·5H 2 O and single crystals of CuSO 4 ·5H 2 O. It is found that the relation holds within errors of several percent.