This doctoral thesis presents density functionalcalculations applied in several domains of interest in solidstate physics and materials science. Non-collinear magnetismhas been studied both in an artificial multi-layer structure,which could have technological relevance as a magnetic sensordevice, and as excitations in 3d ferromagnets. The intricatebulk crystal structure of γ-alumina has been investigated.An improved embedded cluster method is developed and applied tostudy the geometric and electronic structures and opticalabsorption energies of neutral and positively charged oxygenvacancies in α-quartz. Ab initio total energycalculations, based on the EMTO theory, have been used todetermine the elastic properties of Al1-xLixrandom alloys in the face-centered cubiccrystallographic phase. The obtained overall good agreementwith experiment demonstrates the applicability of the quantummechanics formulated within the framework of the DensityFunctional Theory for mapping the structural and mechanicalproperties of random alloys against chemical composition.

<p> The Moore-Read Pfaffian and anti-Pfaffian states have been under scrupulous review as candidates which describe the fractional quantum Hall effect at filling factor 5/2. Quantum states in the universality class of the Moore-Read Pfaffian/anti-Pfaffian have non-trivial intrinsic topological order and support low-energy non-Abelian excitations that have applications in fault-tolerant topological quantum computing schemes. Both states are exact ground states of three-body Hamiltonians that explicitly break particle-hole symmetry. We study the topological order of a competing ansatz state &PSgr;₂ that is the exact ground state of a two-body Hamiltonian that preserves particle-hole symmetry. In particular, we calculate the bipartite entanglement entropy and spectra in the lowest Landau level in the spherical geometry for &PSgr;₂. We perform such calculations for a finite number of electrons up to 14. We then extrapolate to the thermodynamic limit the topological entanglement entropy &gamma; as a measure of the topological order of the ansatz and compare to the known value of the Moore-Read Pfaffian/anti-Pfaffian state. We also study the orbital entanglement spectra for &PSgr;₂ and compare with the Moore-Read Pfaffian and two-body Coulomb ground states. We show that our extrapolation of &gamma; lies within the uncertainty of the known value of &gamma; for the Moore-Read Pfaffian state, and that the orbital entanglement spectra of &PSgr;₂ assumes a similar structure to that of the two-body Coulomb interaction.</p><p>

<p> Superconducting pairs are able to leak into non-superconducting materials when placed in close proximity. In the presence of ferromagnetism pair correlations are modified by the magnetization; singlet Cooper pairs transform into a mixture of singlet and triplet correlations. In this work we analyze how pair correlations are modified in a magnetic Josephson junction in the clean limit, and consider the effect of different magnetic configurations. We use a tight-binding Hamiltonian and the Bogoliubov-de Gennes(BdG) formalism, to describe the proximity system. Applying the Bogoliubov-Valatin transformation we generate the BdG equations in matrix form. We use an iterative process to diagonalize the matrix together with solving the self-consistency relation for the pair potential &Delta; numerically. From the solution we construct Gor'kov functions which are used to describe the pair amplitudes and Josephson current through the junction. Taking the simplest case first we apply our method to a normal metal Josephson junction and match our model to known results. We then apply it to a homogeneous magnetic Josephson junction and investigate how certain parameters such as magnetization and temperature affect the properties of the junction. Finally our methodology is applied to an inhomogeneous magnetic Josephson junction, to study the differing effects between gradual and abrupt changes in the magnetization on the pair correlations.</p>

<p>This thesis aims to build upon the previous work done on the magnetic relaxation of iron(II) phthalocyanine (FePc) thin films by exploring the dynamic aspects of coercive fields in order to determine whether FePc can be classified as a low-dimensional material known as a single chain magnet. In thin films, the chain length is controlled by deposition temperature and therefore systematic studies of the chain-length dependent properties can be made. Hysteresis loops of FePc thin-films with five different chain lengths ranging from approx. 30 nm to 300 nm were measured at a range of sweep speeds from 10.4 mT/s to 1.07 mT/s. Each measurement was repeated at 5 different temperatures in the interval from 2.5 K to 3.8 K, where hysteresis was observed. Significant reductions in coercivity with slower sweep speeds reveal the non-equilibrium behavior of the magnetic states. Mean-field theory based on one-dimensional chains within a Glauber-Ising model suggests a power law behavior of coercivity with sweep rate. Indeed all experimental data is consistent with that behavior. The critical exponent varies from 0.521 to 0.153 for short to long chains. Given the limited observational window, coercivity due to inter-chain coupling cannot fully be ruled out, yet a large dynamic response in the coercivity supports the notion of a single chain magnet.

<p>Organic semiconductors have been studied thoroughly and are well understood, though the effects in which oxygen and humidity have on them are less well known. In this thesis we immerse copper phthalocyanine thin films deposited at two different substrate temperatures and with two different thicknesses to an environment with oxygen pressures from 10 mbar to 210 mbar and water vapor pressures from 5 mbar to 13 mbar. The conductivity of our thin films at atmospheric oxygen and water pressure ranged from as high as 6:7 &times; 10^&minus;4 S/m and as low as 9:6 &times; 10^&minus;5 S/m, about 7 orders of magnitude less than silicon and germanium. At the smallest oxygen doping, the conductivity ranged from as high as 4:9 &times; 10^&minus;5 S/m to as low as 1:1 &times; 10^&minus;5 S/m. We can conclude that the small grain samples are more easily doped by oxygen, the small grain samples have a larger sensitivity gain than the large grain samples, and the intrinsic conductivity increases as both grain size decreases and as film thickness decreases. </p>

Conditions have been established for the epitaxial growth, by vacuum evaporation at 10^-6 torr, of single crystal films of the dihydrides of gadolinium,terbium, dysprosium, holmium and erbium, on the cube, dodecahedral and octahedral faces of rock salt. The film structures have been investigated by electron microscopy and diffraction. Lattice parameter measurements on single crystal and polycrystalline specimens indicate that thin films (less then 200A) are face-centred-cubic dihydrides whereas thick films (>900A) are dominated by hexagonal close-packed metallic structure. Films of intermediate thickness exhibit a mixed (f.c.c. dihydride and h.c.p. metal) phase. The effects,on the structure of Gd and Tb dihydride films, of such parameters as substrate nature and temperature, vacuum environment, annealing and hydrogen treatment have been studied. Some nucleation and decoration studies have been conducted for gadolinium and terbium dihydrides. The electrical resistivity and temperature coefficient of resistance of polycrystalline films of the dihydrides of gadolinium, terbium, dysprosium, holmium and erbium, grown on glass slides, have been measured in vacuo. Resistivity measurements on the dihydride films, made in the temperature range 77K to 573K indicate generally metallic conduction characteristics. Studies of the changes of resistivity on annealing the dihydride films in the presence of a stream of hydrogen gas support the conclusion that the films become converted to semiconducting trihydrides having negative temperature coefficient of resistance.

A sensitivity study is conducted on the polarimetric measurable [delta] = ([delta]p-[delta]s), the differential change of phase on reflection air/metallic p s-film. A computer aided study shows the quantity A to he sensitive to the extinction coefficient k and to film thickness. It is only sensi-tive to the refractive index n in certain ranges of n , k and for certain d values. For films of d 2001, A becomes insensitive to n but maintains high sensitivity to k , while ford &pound;200 A it shows some sensitivity to n as well as k in certain ranges of lower values of k 2.5. An interferometric technique due to Tolansky (1944), is used to determine A as a function of 6 , the angle of incidence for films of Ag , Au and At at different wavelengths in the visible region of the spectrum. It is shown that this interferometric technique, when critically applied, is capable of an accuracy of +/-0.003. The extinction coefficients of films investigated are found from curves of A vs n for constant values of k . They are shown to be accurate to +/-0.03 when A is accurate to +/-0.003 tt thus rendering a simple and inexpensive technique an accurate and effective method for the determination of the extinction coefficient of highly reflecting metallic films. A new optical system to produce multiple beam interference fringes of variable chromaticity in the first order is described. The system enables the surveying of features within the order. It also removes the ambiguity met in conventional monochromatic interferometry concerning the order of interference to which a fringe may belong. White light multiple fringes are obtained first and the orders are recognized. Some outlines are projected for investigating the sensitivity of the optical phase properties of metallic films, namely the phase changes on reflection air/metallic film [beta], dielectric substrate/metallic film [beta] and in transmission [gamma], at normal incidence, to n, k and d . It is proposed to try to explain and link the behaviour of these phase quantities with thickness for d 200. in terms of n and k. Also a compact new monochromator to produce fringes of variable chromaticity for surface microtopography studies is projected.

Measurements of the lattice thermal conductivity of in lium antimonide (n=10¹⁴ to 10¹⁷ and p=10¹⁵ to 10¹⁷ cm^-3) single crystal samples have been performed in the temperature range of 1.2 to 10&deg;K, and for the lower temperatures in magnetic fields up to 40 KG. A new 8-bit microprocessor controlled system was constructed to make fully automatic measurements under optimised conditions. This enabled thermal conductivity measurements to be made to an accuracy of 2% absolute and 0.1% relative. The zero field results are in good agreement with theory and with previous measurements. The thermal conductivity of the p-InSb decreased with increase of magnetic field. The acceptor ground state is four fold degenerate and the thermal phonons could only be scattered by transition within the quartet of energy levels. It was found that Ge doped sample (P = 2.8 x 10¹⁵ cm^-3 at 1.23&deg;K) exhibited a broad minimum when the field reached about 40 KG which was due to the first order Zeeman splitting of the acceptor ground state. Quantum oscillations of thermal conductivity of highly doped n-type samples were observed in a magnetic field. The period of oscillation was the same as the Shubnikov-de liaas oscillation of electrical conductivity and it is shown that the oscillation occurs because the electron-phonon interaction osciliates with magnetic field as the Fermi level crosses successive Landau levels.

Rare earth metal films have been prepared in Ultra-high Vacuum (U.H.V.) and work function measurements made, in situ, by a vibrating capacitor technique. The effects of aging in U.H.V. and of atmospheric contamination have been studied. Measurements have been made on Gd above and below the Curie temperature, before and after annealing at 390K by means of a novel nichrome-film heater (developed for U.H.V. use). The resistivities of the rare earth films were determined by an in situ potential probe method in conjunction with interferometric thickness measurements. Work function measurements have also been made on other metal films and on silicon surfaces to provide data relevant to theoretical work on point-contact detectors for far infra-red radiation.

The static indentation hardness test has been applied to the study of the environment effects on hardness, plastic deformation, and fracture properties of silicon carbide crystals. Measurements of microhardness on the (0001) planes show that the hardness of silicon carbide is significantly lowered by water absorbed from the air. Upon annealing the crystals, at very high temperatures, the hardness is found to increase. A three-dimensional distribution of stress, beneath an indenter, has been evaluated using 'elastic equations'. The mechanics of crack initiation around the contact circle of a spherical indenter with the specimen has been analysed. The crack extension force, for a crack propagating downward from the surface of the specimen, has been evaluated. Evidence of plastic deformation by slip is provided by a number of indentations, specially those with the spherical and double-cone indenters. (1010) is the preferred slip plane.