We have developed a novel experimental approach for the study of the interaction of sheets of non parallel flux lines in hysteric type II superconductors. We continuously monitor the evolution of the components of the magnetic flux density $\Vert$ and $\perp$ to $H\sb{a}$ (i.e. $\langle B\sb{z}\rangle$ and $\langle B\sb{y}\rangle$) as $H\sb{a}$ is raised to various intensities, and then reduced to zero. In our investigation of a high $T\sb{c}(YBa\sb{2}Cu\sb{3}O\sb{7-x}$) ceramic, $H\sb{a}$ exceeded $H\sb{*}$, the first full penetration field. We applied the phenomenological Clem general critical state model to the analysis of our extensive observations. A computer program was developed to solve the four coupled differential equations of this theory with appropriate physical constraints for the situations prevailing in our experiment. This analysis provides detailed insight into the evolution of the intricate configurations of the magnetic flux density $\vec B$(x), the critical current density, $\vec J$(x), and electric field $\vec E$(x) patterns as the injected and trapped flux lines are made to interact, unpin, migrate and undergo flux cutting processes. The model is seen to generate the variety of complicated measured curves of $\langle B\sb{y}\rangle$ and $\langle B\sb{z}\rangle$ vs $H\sb{a}$ very satisfactorily. (Abstract shortened by UMI.)

The purpose of this study is to investigate the conduction electrons' interaction mechanisms over the wide temperature range in order to find possible deviations from the existing theories. The temperature dependence of the electrical resistivity of two scattering systems, the crystalline and the amorphous, in the range of 1.7--300 K is studied. In the pure crystalline metals, the nearly-free-electron model can qualitatively and quantitatively account for the temperature dependence of the electrical resistivity. In the low-temperature range, the electron-electron scattering is the dominant scattering mechanism. Above the Debye temperature, the phonon-electron scattering is the dominant scattering mechanism. The temperature dependence of the electrical resistivity for the amorphous metallic alloys can be described qualitatively by the Ziman model. The scattering mechanism of the conduction electrons of the samples can be categorized in the weak scattering limit regime. A small, but significant correction must be added to the Ziman model in order to account for the additional scattering contributions that exist in the wide temperature range. In the very-low-temperature range the electron-electron interaction effect is the major correction to the temperature dependence of the electrical resistivity. In the low-temperature range, the weak localization effect is the major correction to the temperature dependence of the electrical resistivity. In the high-temperature range, the temperature dependence of the electrical resistivity is linear with a small but significant correction. For some samples the magnetic contribution is the major correction, for others the multiphonon scattering is the major correction. For the resistivity minima at temperatures below 20 K could be due to the Kondo effect. A more plausible explanation, however, is the electron-electron interaction effect. The Ziman model with the proper corrections is found to be an adequate and successful model in accounting for the temperature dependence of the electrical resistivity of all of the studied amorphous metallic alloys.

The applicability of the weak-localization theory to highly ordered quasicrystals raises the question of whether or not the long-range order in these alloys can be reconciled with the electronic disorder. This study did not detect any unusual structure-induced contribution to the resistivity at low temperatures other than those known for metallic glasses. The temperature dependence of the resistivity in icosahedral quasicrystals of high structural quality showed that the transport behaviour of these alloys at low temperatures can be satisfactorily explained in terms of conventional weak-localization and electron-electron interaction theories. The temperature dependence of the resistivity in weakly disordered (low-resistivity) alloys can be explained qualitatively in terms of the classical theories extended to liquids and disordered alloys. The experimental evidence shows the existence of a close relationship between the icosahedral quasicrystalline structure and the weak localization tendency of the electronic states at the Fermi level. To explain the temperature dependence of the resistivity at high temperatures additional models based on the concept of the band transition and hopping have to be invoked. In the high-resistivity stable icosahedral Al-Cu-Ru alloy the insulating-like (electron hopping) behaviour was found to dominate the electron transport even at low temperatures. This causes the deviations from the weak-localization theory and is due to the enhancement of the density of localized electronic states at the Fermi level. Consequently, the Anderson localization is collapsed in the vicinity of the metal-insulator transition. The values of the correlation gap in the Al-Cu-Fe-Mn icosahedral system suggest a considerably different nature for the pseudogap around the Fermi level in amorphous and icosahedral phases. However, unless an ideal quasicrystal belongs to the metal-insulator transition region (if it exists), the weak-localization theory will be the most appropriate tool to investigate the low temperature electron transport properties in icosahedral phases.

The desirable properties of the paramagnetic substances are discussed and the possibility of using some metallic alloys as well as dielectric crystals doped with proper substances capable of lowering the temperatures is illustrated. For this purpose some low temperature investigations of MgO:Mn and of Mg:Ce alloys is presented. The MgO:Mn doped with 0.0325% Mn gives a final temperature of 0.04°K when demagnetized from a field of 18 Kgauss and approximately 1.3°K. The specific heat measurements resulted in a value of CT 2/R = 6x10-3 for the "high temperature" tail of the specific heat. The entropy determinations performed on the Mg:Ce alloy system on the other hand indicated that the magnetic entropy at about 1°K is approximately of the same value as that of the conduction electrons.Therefore it seems that this particular alloy system is not suitable for appreciable lowering of the temperatures by adiabatic demagnetization. The specific heat of a well behaved pure substance is proposed as a resonably sensitive thermometric standard. Its usefulness is illustrated by calibrating a secondary thermometer using the proposed method. The difficulties encountered in low temperature calorimetry are described and our solution of them is given. A discussion is also included of the difficulties involved in using mechanical heat switches. A double mechanical heat switch is proposed and a design of it is included together with some illustrations of its usefulness.

The ionic and electronic contributions to the total magnetic susceptibility of a non ferromagnetic metal alloy are discussed, and a review of the experimental techniques for measuring susceptibilities is given. The presence of even small amounts of ferromagnetic impurities can have disastrous effects on the measured susceptibilities, and a method is described for correcting for such impurities when the Gouy method is used. This enabled the true susceptibility of the non ferromagnetic part of a substance to be determined with an accuracy of .1% relative to Ge. The room temperature values of magnetic susceptibility of the dilute alloys of Mn and Fe in Mg and Al were measured in view of obtaining information about the electronic configuration of the transition elements in solution. A localized d electron picture rather than a d band picture was found to be adequate. The measured effective Bohr magneton numbers yielded two possible electronic configurations for the solute atoms. The possibility of distinguishing between these is discussed in terms of the corresponding change in population of the conduction band and its influence on the electronic susceptibility.

This thesis reports on the behavior of the n.m.r signal from Al27 in Al and Al Mn alloys. We have obtained values of 0.162 .002% for the Knight shift and of 8.65 .2 oe. for the line width in pure Al, in good agreement with values reported elsewhere. On alloying with Mn, decreases in intensity, line width and Knight shift are observed. This behavior is readily explained by nuclear electric quadrupole interactions. The small effective magnetic moment of the Mn ions seem to have a negligible effect on the line and from that point of view Al Mn behaves essentially like Al Mg and Al Zn which have been extensively studied recently.

This thesis describes the design, assembly, operation and behaviour of a nuclear magnetic resonance spectrometer of the Pound-Knight type built for investigation of Knight shift in metals and alloys. A brief introduction to nuclear magnetic resonance is given followed by a short discussion of the Knight shift. A short description of the various arrangements for observing nuclear magnetic resonance is given and after pointing out the requirements for observing the Knight shift in metals, our choice of the spectrometer is justified. The working of the spectrometer as a whole unit is explained and this is followed by a description of its various components. The technique of determining frequency is described in detail by giving a typical example of A127 signal from a solution of AlCl3. The behaviour of the spectrometer is discussed and its use in detection of various signals is given. The necessary conditions for obtaining good signals are summarised. The temperature variation and also the homogeneity of the magnetic field are studied with the aid of the spectrometer. The accuracy and usefulness of the equipment is tested by measuring the Knight shift in aluminium metal.

Abstract not available.

When studying the high-pressure structural behavior of crystalline materials, it is highly desirable to determine structural changes accurately, preferably at electron density levels. The Maximum Entropy Method (MEM) has already proven to be a very powerful tool for extracting the most probable charge density distributions directly from X-ray diffraction data. This thesis presents high pressure X-ray diffraction studies on two distinct, but structurally similar, classes of guest-host materials: gas clathrate hydrates (M8(H2O)46, with M= Kr, Xe) and silicon clathrate (Ba8Si46). In order to characterize the change of crystalline structure and electron distribution resulting from the increase of density due to the application of high pressure, we have used a recently developed approach wherein the classical Rietveld analysis is complemented iteratively with MEM calculations. It is found that charge density distributions derived from probability maps obtained by MEM provide further, in-depth insights into the structural changes induced by pressure in guest-host compounds. Clathrate hydrates are inclusion compounds, in which guest atoms or molecules are trapped in cages formed by an ice-like host lattice of water molecules. In recent years, large deposits of methane hydrate (a clathrate hydrate) have been found on the oceanic floors, leading to a considerable interest in the physical properties of gas hydrates. In the present study the crystalline structure I of xenon and krypton hydrates was investigated by powder X-ray diffraction at room temperature, over the pressure ranges for which these compounds are stable. Structure I, which has a cubic symmetry with Pm3n space group, is formed by two types of polyhedron, also referred to as small and large cages. The pressure dependence of the structural parameters was determined by applying a Rietveld analysis to the X-ray diffraction data. To further explore the effect of pressure on the guest atoms and the water molecule framework, we used the combined Rietveld/MEM method to derive the most probable charge density distributions at each pressure. Our results show that the charge density distribution of the encaged atoms differs depending on the type of the host cage, small or large, at all pressures. Spherical density distributions were observed for the guest atoms in the small cages, while the atoms in the large cages showed longitudinal elongated electronic distributions. These findings are common to both Kr and Xe hydrates. Along with the observed cage deformations, this is a clear indication that the guest-host interaction differs significantly between the small and large cages at high pressures. A similar behavior has been previously reported in low-temperature studies of methane clathrate hydrate. The combined Rietveld/MEM method was also successfully applied to explore the subtle changes in the electronic density distribution induced in Ba 8Si46 clathrate by the application of high pressure. This compound has been the object of extensive studies since its superconductivity has been discovered. Previous X-ray diffraction, near-edge X-ray absorption, and Raman spectroscopy studies have revealed two iso-structural phase transitions occurring at 5 and 17 GPa in Ba8Si46; their physical origin, however, was still not clearly understood. In our study, the most probable electron density distributions were calculated using the combined Rietveld/MEM method, with the goal to propose possible mechanisms for the two observed transitions. The examination of the electron density maps, and also electron density difference distributions, revealed that the low pressure transition is related to an enhanced charge transfer of Ba atoms to the Si framework, while the 17 GPa transition is a result of a sudden change in the electron density topology of the Si-Si bonds. As the pressure is increased, the electrons in the Si-Si bonds are displaced from the bonding region into the interstitial region, leading to a weakening of the Si-Si bonds, which explains the large volume reduction accompanying this transition.

Measurements of the evolution of the magnetic moment of nonideal (i.e. pinning rich) type II superconductors in stationary magnetic fields H a as a function of time at constant temperatures T < Tc or as the temperature is varied to or from Tc provide insight into the dynamics of flux lines and their interactions with the pinning sites. Three concentric magnetic moments coexist in these materials when the isothermal sweep of Ha causes the locus of the magnetization ⟨M⟩ to migrate along any path joining the upper with the lower envelopes of the major hysteresis curves. The "outermost" magnetic moment is always diamagnetic and is generated by the field opposing Meissner surface current. The two other magnetic moments fill the bulk of the specimen, currents circulating in one direction in the outer annular volume embrace an inner volume occupied by counter-rotating persistent currents. In semi-reversible type II superconductors the magnitude of the Meissner magnetic moment is important compared to that arising from the currents in the bulk whereas in very hysteretic specimens it is negligible. In both types of material it is a fairly straightforward procedure to establish configurations in various static Ha where the opposing magnetic moments exactly cancel each other so that the macroscopic net magnetization is zero. However the internal pattern of persistent currents and magnetic flux density profiles which prevail in these special ⟨M⟩ = 0 states is complicated, fascinating and provides a full crucible for the investigation of flux line behaviour. In this thesis I present the results of our experimental study of the evolution of the magnetization of these initial ⟨M⟩ = 0 states during warming to Tc as a function of the static applied field H a. The phenomena encountered are seen to depend dramatically on the direction of the change of Ha causing ⟨M⟩ to migrate from one envelope of the major hysteresis loop to the opposite. Further we find major differences in the behaviour exhibited by the semi-reversible type II superconductors in comparison with that manifested by the highly hysteretic samples. We also see that the low Tc and high Tc semi-reversible specimens display very similar but highly intricate behaviour. Finally we show that a simple model which exploits the critical state concept and applies the principle of conservation of flux lines can account for the major features of all our observations.