Table of contents

We report measurements of the longitudinal magnetoresistance and magnetization of in pulsed magnetic fields of up to 50 T and temperatures down to 400 mK, using samples of different purity. Below 2 K the amplitude of the Shubnikov - de Haas oscillations in is found to decrease dramatically with falling temperature. This effect is shown to coincide with quasipersistent eddy current resonances in the magnetization, which are a signature of the quantum Hall effect. Evidence is provided for the existence of a novel interplane conduction mechanism involving highly metallic edge states with supressed scattering at the surface of the sample (a so-called `chiral Fermi liquid'), operational when the chemical potential is between Landau levels in the bulk of the material.

The ratio of spin-flip to non-flip intensity in low-energy electron energy-loss spectroscopy for multiplicity-changing transitions within the 3d multiplet in NiO and CoO can be found exactly. It is shown to be independent of scattering geometry, incident energy and multiple scattering if well-defined local spins are assumed. For non-multiplicity-changing transitions this ratio can be calculated approximately, using R-matrix techniques. The angular dependence of the scattering processes at low kinetic energies of the incident electron can be understood in terms of the point group symmetry at the target ion and the contribution of dominant angular momentum components to incoming and outgoing electron waves.

Diamond-anvil cell experiments and first-principles theory have been used to investigate the structural stability of uranium up to 1 Mbar in pressure. Experiments and theory agree; there is no phase transition in uranium below 1 Mbar. Previous speculations about a crystallographic phase transition in uranium below this pressure are thus shown to be incorrect. In this regard, uranium is exceptional in the series of light actinides, where pressure-induced phase transitions typically occur at pressure below 1 Mbar. The ground-state crystal structure of uranium is orthorhombic with three structural parameters: the axial ratios b/a and c/a, and an internal parameter y measuring the displacement, along the b-axis, of alternate planes. The experimental and theoretical results reported here indicate that one of these parameters, c/a, is substantially more sensitive to pressure than the other two, changing by as much as 5%, while b/a and y are constant within 1% within the pressure range studied. This flexibility in the structure facilitates this structure over a wide pressure range. Theory suggests that electrostatic contributions to the total energy drive the variation in the c/a ratio as a function of pressure, and a simple model is utilized to show this.

We present experimental results on the field ionization of beryllium acceptors in micrometre AlGaAs/GaAs structures on a nanosecond timescale. At liquid helium temperature, the tunnelling ionization of beryllium was found to switch on effectively at electric fields higher than .

We review recent experimental work falling under the broad classification of colossal magnetoresistance (CMR), which is magnetoresistance associated with a ferromagnetic-to-paramagnetic phase transition. The prototypical CMR compound is derived from the parent compound, perovskite . When hole doped at a concentration of 20 - 40% holes/Mn ion, for instance by Ca or Sr substitution for La, the material displays a transition from a high-temperature paramagnetic insulator to a low-temperature ferromagnetic metal. Near the phase transition temperature, which can exceed room temperature in some compositions, large magnetoresistance is observed and its possible application in magnetic recording has revived interest in these materials. In addition, unusual magneto-elastic effects and charge ordering have focused attention on strong electron - phonon coupling. This coupling, which is a type of dynamic extended-system version of the Jahn - Teller effect, in conjunction with the double-exchange interaction, is also viewed as essential for a microscopic description of CMR in the manganite perovskites. Large magnetoresistance is also seen in other systems, namely and some Cr chalcogenide spinels, compounds which differ greatly from the manganite perovskites. We describe the relevant points of contrast between the various CMR materials.

The structure of the complex perovskite (PFT), which belongs, according to the literature, to the ferroic species , has been refined by single-crystal and powder x-ray diffraction at room temperature, i.e. within the range of stability of the cubic phase. Attention has been paid to the possible presence of tetragonal spontaneous strain, suggested by the anomalous uniaxial birefringence of each pyramidal growth sector. A crystal cut from a single growth sector was measured and compared with an as-grown one. Several models, both cubic and tetragonal (P4/mmm), these latter used in refining the single growth sector, are compared. The main feature of all the refined structures is the positional disorder at the Pb site, the importance of which in connection with the ferroic transitions is briefly discussed.

Silicon carbide often grows in the cubic phase under conditions where this is not the most stable phase. Ab initio calculations are presented which determine the energy of a stacking reversal at the (0001) and surfaces of silicon carbide, and thus whether the cubic form is preferred in the vicinity of a free surface. In these calculations the surfaces are not reconstructed but hydrogen terminated, and care is taken to eliminate the spurious dipole - dipole interaction caused by the imposition of periodic boundary conditions, and also in estimating the amount of residual ionic relaxation. These calculations do show a clear distinction of 13 meV per surface pair between the silicon and carbon surfaces, although the results are not in complete agreement with experiment.

The electronic properties of the stage-2 IBr graphite intercalation compound were investigated by means of the de Haas - van Alphen (dHvA) effect. Thirty dHvA frequencies were found with the magnetic field parallel to the c-axis. These results are explained by the Fermi surface model formed by zone folding resulting from commensurate in-plane superlattice translations which coupled carrier orbits. The unfolded Fermi surface was the primitive two-dimensional Fermi surface of Blinowski et al for graphite intercalation compounds. By adjusting the dimensions of this Fermi surface, all the dHvA frequencies were interpreted with an accuracy of . The Fermi radii of the unfolded Fermi surface were and . The dHvA frequencies (455 and 1106 T) were used to determine the band parameters (the interaction of nearest neighbours) and (the interaction of nearest layers) in Blinowski's theoretical band model for acceptor-type graphite intercalation compounds.

The high-frequency magnetoconductivity for a double-layer electron - hole system is calculated at finite temperature. The formalism rests on the temperature Green's function and Kubo's formula for conductivity. It is found that at frequencies higher than the cyclotron frequencies of electrons and holes, the frequency-dependent electromagnetic absorption exhibits oscillatory behaviour. The maxima in the absorption coefficient correspond to a situation where the photon energy is approximately the sum of the energies of the electron pair and the hole pair which are simultaneously excited by the radiation field.

On the basis of a microscopic scattering theory, for a parabolic quantum well system subjected to crossed magnetic and electric fields, the local field including the third-order nonlinearity is determined. Then the optical response is investigated. It turns out that the magnetic and electric fields produce notable changes in the optical properties.

We introduce a model for the persistent current carried by spinless fermions moving in a ring with a high-dimensional cross section. The effects of both disorder and electron - electron interaction are considered. It is found that the non-interacting system behaves like previously considered low-dimensional models. The more complicated interacting/disordered case is analysed by means of a functional integral which is evaluated in the limit of infinite dimensionality by expanding in the number of transverse channels. To leading order in this expansion scheme, the Coulomb interaction does not affect the persistent current. It is found that the insensitivity to interaction effects is due to the absence of local contributions to the Coulomb vertex in our model (which in turn is a consequence of the neglect of the electron spin). It is argued that the physical mechanism suppressing the interaction in the high-dimensional spinless model applies to the analogous low-dimensional case as well.

Starting with a simple Lagrangian for the electromagnetic field with broken gauge symmetry, we derive an effective circuit Hamiltonian for a superconducting weak-link ring. The energy eigenstates of this Hamiltonian exhibit sensitivity to both external magnetic flux and an applied Faraday law voltage. We show that the flux periodicity of the rf SQUID ring, and the voltage periodic behaviour found in ultra-small-capacitance weak-link rings, can be seen as limiting cases of a more general phenomenon.

The magnetization measurement of systems with the nominal formula (R = Eu, Gd, Tb, Y) has been made. It has been found that and are weak ferromagnets of Dzialoshinsky - Moriya type whereas is an antiferromagnet. The substitution of manganese ions by cobalt ones leads to the appearance of different magnetic states: cluster-spin-glass-like , inhomogeneous ferromagnets with well defined Curie temperatures and again a spin glass state . Spontaneous magnetizations and Curie temperatures reach maxima for compounds with x = 0.5. External magnetic field induces a metamagnetic transition in (R = Gd, Tb, Y). The transition becomes irreversible at low temperature in the Tb- and Y-based compounds. The annealing in vacuum of leads to the increase of the Curie temperature and to the appearance of a metamagnetic transition for the compound. The metamagnetic behaviour is interpreted taking into account the ionic ordering of and ions and possible 3d-orbital ordering in the sublattice.

We discuss the transmission of electromagnetic radiation through a Fabry - Pérot resonator in the microwave or far-infrared frequency ranges where the magneto-optic properties are governed by a gyromagnetic permeability tensor. We restrict our attention to normally incident radiation but consider both Faraday (magnetic field normal to plate) and Voigt (field parallel to plate) geometries for ferromagnets and antiferromagnets. The effect of partially reflecting mirrors is included. It is shown that provided one uses the polarization eigenmodes, circular for Faraday geometry and plane for Voigt geometry, the transmission and reflection can be expressed by general formulae in terms of single-interface reflection and transmission coefficients. The resonator can be regarded as a magnetic-field tunable polarizer and it is shown how inclusion of mirrors sharpens the properties.

phosphors find applications in dosimetry of ionizing radiations. Phosphors having Dy or Tm activators are high-sensitivity thermoluminescence dosimetry materials, while the phosphors with Eu or Sm activators are the proposed materials for radio-photoluminescence dosimetry. Mechanisms of several processes related to various luminescence phenomena, however, are not yet properly understood. Usually, all phosphors are prepared under identical conditions and comparative studies are carried out. It is shown here that the form in which the impurity is incorporated and the nature of defects produced is quite different in than in . The results presented throw light on the mechanism for thermoluminescence, radio-photoluminescence, incorporation of the impurity and conversion.

The magnetic susceptibility and field dependence of the magnetization of pristine and potassium-intercalated nanocrystalline diamond were studied using a SQUID magnetometer. The host material was found to possess paramagnetic properties along with a small irreversible magnetization recorded for zero-field-cooled and field-cooled cycles of susceptibility measurements. Upon intercalation the irreversibility increases, and the magnetization of the material reveals a distinct ferromagnetic loop at 5 K. This is essentially different from the results for potassium intercalation in graphite or fullerene, and shows that bonding promotes magnetic ordering in carbon. The measurements present the first experimental evidence of an intercalation-induced spin-glass state in a nanocrystalline diamond system.