This thesis is devoted to the study of one-dimensional quantum spin chains using matrix product state-based techniques known. We deal mostly with the transverse field Ising model and perturbations. Firstly, we consider an out of equilibrium steady state by applying an energy current. We confirm the phase diagram, the correlations and the entanglement scaling. Subsequently, we introduce two kinds of perturbations. Firstly, we add an interaction that takes the system away from integrability and we study the correlations and the entanglement scaling, determining its phase diagram. Secondly, we weaken the strength of the interaction every nth pair with focus on the simplest case n=2. In every case we compute correlations and central charge (entanglement scaling) and we show that in the presence of an energy current, there is no conservation of energy. Finally, we motivate briefly work on a system that exhibits the Haldane phase.

No description available.

Vibrating sample magnetometry showed that dilute assemblies of unfiltered Fe and Co nanoclusters well above their blocking temperature are superparamagnetic and the magnetisation curves follow the Langevin function with a tight size distribution around a mean of 2.2 nm. Magnetisation curves of blocked non-interacting particles showed a small coercivity and a remanence value consistent with a uniaxial anisotropy in the particles. At low temperature, the films assume an asperromagnetic state and fits to this model indicated an enhanced anisotropy constant by a factor of ten over the bulk. Cluster assembled films with packing densities up to 30 at % concentration are described by the Langevin function with an increased mean particle size. As the packing density is increased further, the behaviour deviates from the super-paramagnetic model towards a correlated spin glass. At low temperatures the anisotropy energy of the particles is greater than the exchange energy and the sample is asperromagnetic. In pure Fe cluster assembled films (~500 A) thick capped by Ag a transition, as the temperature rises from 2 to 300K, from an asperromagnetic state to a correlated spin glass occurs. Uncapped Fe cluster films displayed only correlated spin glass behaviour even at 2K. Giant magnetoresistance has been observed in the cluster assembled systems Fex-Ag1-x, FexCu1-x and CoxAg1-x and investigated for varying cluster packing densities. The results were compared to models formulated for granular alloys and were found to be in good agreement for < 16% cluster films. As the cluster interactions are increased, the peak of the GMR, at H~0, was found to be quenched due to the magnetic correlations among the particles. Co cluster films created with Ag or SiO capping layers displayed a ferromagnetic - antiferromagnetic interaction caused by partial oxidation of the clusters. Magnetometry of an uncapped Co cluster film held in UHV has shown spin glass behaviour at 2K as found for Fe cluster films.

Parts I and II of this thesis have certain common features. Thus both parts are concerned with molecules of the form XHn (including XHn+). Also a theoretical technique appropriate for dealing with molecular systems of the type mentioned --- the single-centre method --- features in Part I and Part II. The united-atom approximation (UA) - an early single-centre approach --- is used in Part I in calculations on A1H4-, NeH+ and AH+. Properties evaluated include electronic energy, force constant, diamagnetic susceptibility and X-ray scattering factor. The performance of the UA technique is discussed in relation to previous applications of the method to ten and eighteen electron systems. In Pert II an analysis of Hartree-Fock-Roothaan (HFR) calculations on CH4 and HF is presented. Single-centre calculations are included and are related to polycentre calculations by means of one-electron expectation values, electron density expansions and density contour maps. The expectation values calculated include multipole moments, [r] and X-ray scattering factors. Where possible comparisons with experimental results are made.

The atomic structures of the c(8x2) and c(4x4) reconstructions of InSb(001) have been determined using surface X-ray diffraction. Large in-plane and out-of-plane data sets were measured for each of the reconstructions, enabling a detailed model of the precise atomic structure to be calculated for the first time. A new model, consisting of chains of indium atoms, separated by parallel pairs of antimony dimers on top of an antimony terminated bulk, has been proposed for the InSb(001)-c(8x2) reconstruction. The model represents a significant departure from any of the models previously suggested for the c(8x2) reconstruction of the III-V (001) surfaces. The InSb(001)-c(4x4) surface is found to be consistent with a previously reported missing dimer model. It is proposed, however, that the groups of dimers are incomplete in approximately one third of the cases. The dimer bond lengths and the corresponding bond angles, have also been determined for the first time. The c(4x4) reconstruction is found to be notably flatter than the c(8x2) reconstruction. A surface phase transition study from the highly ordered c(4x4) phase to a disordered asymmetric(1x3) phase, shows that a fully reversible transition takes place. The results indicate that the transition is a consequence of random desorption of antimony dimer atoms in distinct isolated regions, rather than a general reduction in domain size. Oxide removal from the InSb(001) surface has been achieved using a number of techniques. The quality of the resulting surfaces was determined from the reflected X-ray intensity and the in-plane fractional-order reflections.

This thesis presents applications of molecular quantum electrodynamics (MQED) to the analysis of resonance energy transfer (RET), molecular absorption and emission, and light scattering by molecules. An MQED framework describes such processes as a series of microscopic photonic interaction events. Multi-interaction processes entail intermediate states of the system’s evolution remaining unspecified, requiring careful interpretation. RET, as modified by coupling with the nearest molecule of the surrounding refractive medium, is investigated. Special attention is given to a system geometry where unmodified RET is impossible, so coupling with the third chromophore is essential. Two distinct treatments are given to emission by a multi-chromophore system, distinguished by different ways of framing the quantum system: Either all photons are virtual and chromophores share excitation, or real photons interact with a single unspecified chromophore. Anomalously high fluorescence-anisotropy is explainable with the latter analysis. Off-resonant light is known to modify the absorption behaviour of molecules: This weak-interaction is analysed with an MQED formulation modified by field dressing, modelling advanced media effects in the condensed phase. Within the electric-dipole approximation, hyper-Rayleigh scattering (HRS) is considered forbidden for centrosymmetric molecules: By including higher-multipole interactions, mechanisms enabling conventionally-forbidden HRS are discovered. For each process analysed, the main results are predictions for the efficiency or observable rate. The relative positions and orientations of the molecules and fields are the key variables, so the rate equations are typically complicated functions thereof. Where rate equations depend on molecular orientation, it is often appropriate to calculate the average value over all orientations, giving results applicable to the fluid phase. System geometry may exert very fine control – a process forbidden in one case may become allowed by a minor change of one chromophore’s alignment. This thesis contributes to understanding the precise requirements of molecular geometry that must inform the design of energy-transfer systems.

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This thesis concerns the fundamental scalar and vector attributes of molecular collisions. A translationally relaxed sample of fully state-selected and rotationally anisotropic CN(A² ∏,v = 4, jFɛ) was prepared within a thermal bath (~298 K) of partner gas (either Ar, N2, O2 or CO2) by ns-pulsed laser excitation. The collisional evolution of the prepared polarised rotational angular momentum was monitored using highresolution frequency modulation spectroscopy (FMS). The total removal and depolarisation of oriented or aligned rotational angular momentum was measured in CN(A² ∏,v = 4, j = 2.5, 3.5, 6.5, 11.5, 13.5, and 18.5, F1e). The state-to-state rotational energy transfer (RET) and orientation transfer from CN(A² ∏,v = 4, j = 6.5 F1e or j = 10.5 F2f) to ∆j ≤ |5| was investigated. The results for the CN(A² ∏,+Ar system generally agree very well with complementary exact quantum scattering (QS) calculations on the best available ab initio potential energy surfaces (PESs). For all systems, a three-level multiple-collision kinetic model satisfactorily reproduces the observed removal of population and polarisation. Elastic depolarisation is found to be a relatively minor pathway relative to population removal and inelastic depolarisation, as confirmed by complete master equation (ME) simulations for CN(A² ∏, v=4, j = 6.5 F1e)+Ar. The total removal efficiencies lie in the order CO2 > N2 > O2 > Ar, loosely correlated with long-range attractive forces. O2 and CO2 exhibit rapid removal channels in addition to RET, likely to be electronic quenching to CN(X2∑+). There are substantial parity-dependent alternations with Dj in state-to-state RET and polarisation transfer, sufficient for a striking change in sign of orientation for specific transitions. This is attributed to the near-homonuclear nature of CN(A² ∏) and consequent even character of the PESs. QS calculations indicate that the dynamics of parity-conserving and changing transitions differ fundamentally. A preference for spinorbit conservation, strongest for Ar, comes from the near-Hund’s case-(a) character of CN(A² ∏) at low-j. Despite the additional dimensions available, the qualitatively similar behaviour of the molecular partners with Ar suggests that these systems have comparable interaction potentials with CN(A² ∏). Therefore, small centrosymmetric molecules, such as N2, O2 and CO2, may approximately be treated as spherical targets. This is supported by recent spherically-averaged CN(A² ∏)-N2 PESs and associated QS calculations from the literature.

The method of Configuration Interaction (CI) has been used in a number of studies of small atomic and molecular systems. In Part I, CI wave functions for a series of pseudomolecular ions 2HZ2Z-1 are reformulated in terms of natural orbitals. Changes in the electron density as a function of the nuclear charge Z and bond angle ZHZ are investigated by means of an electron population analysis based on the natural orbitals. Contour diagrams of the electron density in the plane of the molecule are obtained. The total energy for each system is analysed in terms of the kinetic energy, nuclear attraction energy, and the electron and nuclear repulsion energies. In Part II, the CI method is examined in detail. Techniques for obtaining CI wave functions of atoms and molecules are discussed in detail. Computer programs based on these techniques are described, and listed in the Appendices. The Valence Configuration Interaction (VCI) method is also examined as a means of reducing the size of the secular equation, and results of VCI calculations are given for various states of atomicand diatomic oxygen and sulphur. Spectroscopic constants for the states of S2 are given. The methods by which these results can be obtained are also discussed. Finally the results of the VCI calculations are compared with some recent SCF calculations on O2 and S2.

No description available.