No description available.

Electrical transport measurement of small, irregular shape sample is difficult task. In order to overcome these limitations, this thesis employed a focused ion beam microfabrication. Process of microfabrication is successfully tested on a room temperature antiferromagnet CuMnAs. Temperature dependence resistivity and magnetoresistance of tetragonal bulk CuMnAs single crystal is found to be strongly anisotropic due to layered structure and magnetic order. Anisotropic magnetoresistance shows two fold symmetry and reached a magnitude of 0.12 %. Employing the∼ phenomenological Stoner-Wohlfarth we conclude that the sample has uniaxial magnetic anisotropy with an easy axis not pointing along any principal crystallographic direction and behaves like a two domain antiferromagnet.

No description available.

No description available.

Ph.D. / A comprehensive experimental study of the effects of the spin-density-wave (SDW) on the physical properties of antiferromagnetic , doped with Mn and V, and Cr-Re alloys is reported. The purpose of the study is twofold: c c Al Cr − 1 (i) To gain insight in the anomalous behaviour of the magnetic phase diagram reported for the binary Cr-Al system. (ii) To investigate SDW effects on the anharmonic behaviour of the lattice vibrations of Cr alloys with a member of the transition metals of group-7 in the periodic table. The investigation entails the following measurements: thermal expansion in the temperature range 77 – 450 K for all the specimens, velocity of sound in the temperature range 4 – 300 K for the Cr-Al-V alloys, ultrasonic wave velocity for the Cr-Re alloys as a function of applied pressure (up to 0.242 GPa) at different temperatures and electrical resistivity in the temperature range 77 – 450 K for the Cr-Re alloys. Concentration-temperature magnetic phase diagrams of the (Mn, V) alloy systems were constructed from the measurements. Alloying with Mn, to increase the electron concentration, is observed to drive an incommensurate (I) SDW alloy towards a commensurate (C) SDW state. This results in a triple point, where the ISDW, CSDW and paramagnetic (P) phases coexist on the magnetic phase diagram. A hysteretic first-order ISDW-CSDW/CSDWISDW phase transition line is then observed on the phase diagram for Mn concentrations above the triple point concentration. Adding V, in order to decrease the electron concentration, to an ISDW alloy is found to have the opposite effect. It drives such a system deeper into the region of the ISDW c c Al Cr − 1 c c Al Cr − 1 phase. A CSDW alloy is, on the other hand, driven towards the triple point by addition of V, instead of Mn. c c Al Cr − 1 Theoretical analysis of the magnetic phase diagrams of the (Mn, V) systems confirms a previous suggestion that the Al impurity acts as an electron acceptor in the Cr matrix for c c Al Cr − 1 2 < c at.% Al, as opposed to an electron donor for at.% Al. 2 > c The high-pressure ultrasonic studies on the Cr-Re alloys were used to construct their pressure-temperature ( T p − ) magnetic phase diagrams. Applying hydrostatic pressure to a CSDW Cr-Re alloy induces a hysteretic first-order CSDW-ISDW phase transition at a certain critical pressure, resulting in a triple point on the phase diagram. An interesting aspect of the observations on the Cr-Re alloys is the suggestion of a new phase line, separating pressureinduced and temperature-induced ISDW phases, on the T p − T p − phase diagram. Acoustic-mode Grüneisen parameters, which quantify the lattice anharmonicity, were calculated for the Cr-Re alloys from the high-pressure ultrasonic measurements. The results indicate exceptionally large interactions between the SDW and the long-wavelength longitudinal phonons in Cr-Re alloys. These effects are particularly large in the vicinity of the Néel phase transition temperature. Interactions of the SDW with the shear mode phonons are on the other hand relatively much smaller. The work on the Cr-Re alloys is considered to finally complete studies of the lattice anharmonicity of Cr alloy systems with elements of all the important groups of the periodic table. It now paves the way for developing microscopic theories to explain the unique behaviour of the magneto-elasticity of dilute Cr alloys. / Prof. H.L. Alberts Dr. A.R.E. Prinsloo

Nanomateriais têm sido amplamente estudados, como resultado de suas propriedades físicas e químicas diferenciadas, que oferecem um grande número de possibilidades para aplicações em biomedicina, principalmente na terapia de câncer e no desenvolvimento de estratégias de diagnóstico não invasivo. O óxido de ferro superparamagnético (SPION) é o principal material estudado como agente de contraste para imagem por ressonância magnética, devido à sua capacidade de reduzir o tempo de relaxação transversal (T2) em diferentes tecidos e sua menor toxicidade que os complexos de Gd3+ e Mn2+ usados atualmente. Entretanto, o acumulo de SPIONs pode ser facilmente confundido com sinais referentes à calcificação, depósito de metais pesados e sangramentos, e a alta susceptibilidade magnética do material promove distorções na imagem. Assim, alguns aspectos são desejáveis em material para que este tenha potencial para substituir o SPION, tais como forma nanoparticulada, para fácil modificação de superfície e possibilidade de funcionalização com agentes biosseletivos, e contraste positivo em T1. As nanopartículas (NPs) antiferromagnéticas de MnO atendem a todos os requisitos necessários para substituir o óxido de ferro. As NPs de MnO foram sintetizadas a partir da decomposição térmica do acetilacetonato de manganês(II) em uma variação do método poliol modificado, resultando na formação de NPs com tamanho médio de 21 ± 3,9 nm. Foi realizada a substituição de ligantes de superfície para que se substituísse o ácido oleico adsorvido sobre o material por 3-aminopropiltrimetoxisilano (APTMS) e foi determinada a concentração de grupamentos amino sobre a superfície das NPs. Posteriormente, obteve-se uma estrutura do tipo \"core/shell\" dispersível em meio aquoso e biocompatível pela reação dos grupos amino livres com o carboxilato da carboximetil dextrana (CMDex). O potencial de superfície e a estabilidade coloidal das NPs funcionalizadas foram caracterizados por mobilidade eletroforética e por espalhamento de luz dinâmico em água deionizada e em condições que mimetizavam o sangue. As NPs apresentaram toxicidade em células cancerosas de carcinoma cervical humano (HeLa). Entretanto, não foi observada toxicidade significativa na linhagem de células não cancerosas NCTC clone L929. Tanto as NPs como sintetizadas quanto as recobertas com CMDex apresentaram controle de tamanho e forma, apresentando distribuição de tamanho compatível com o esperado para as aplicações em biomedicina. / Nanomaterials have been widely studied as a result of their interesting physical and chemical properties, which offer a large number of possibilities for applications in biomedicine mainly in cancer therapy and the development of strategies for non-invasive diagnosis. The superparamagnetic iron oxide nanoparticles (SPION) is the main studied material as contrast agent for magnetic resonance imaging (MRI) due to its ability to reduce the transverse relaxation time (T2) in different tissues and lower toxicity than Gd3+ and Mn2+ complexes currently used. However, this SPIONs accumulation can be confused with signals from calcification, bleeding or metal deposits, and the high magnetic susceptibility distorts the background image because its ferromagnetic behavior. Some aspects are desirable to replace SPIONs, such as nanoparticulate form for simple surface modification and labeling with targeting agents, and positive longitudinal T1 relaxation time contrast ability. The antiferromagnetic MnO NPs attend all these requirements and overcome the drawback of using SPION. In our study, MnO NPs were synthesized by the thermal decomposition of Mn(II) acetylacetonate by a variation of the modified polyol process resulting in spherical nanoparticles with average size of 21 ± 3,9 nm. The ligand-exchange step was used to replace the oleic acid adsorbed on the as-synthesized NPs surface by 3-aminopropyltriethoxysilane (APTMS) and the total free amine groups on the NPs surface was determined. After that, a biocompatible and water-dispersible core/shell structure was obtained by coating with carboxymethyl dextran (CMDex) using the free amine-terminal group from APTMS and the carboxylate groups present in the CMDex molecules conjungation. Surface potential and colloidal stability of these functionalized NPs were evaluated by electrophoretic mobility and dynamic light scattering techniques in both water and artificial blood by using the Simulated Body Fluid (SBF) medium. While the water-dispersible NPs have shown toxicity in the human cell line derived from cervical cancer (HeLa), they have not shown significantly cytotoxicity in the healthy fibroblast cells (cell line L929). Both the as-synthesized and coated NPs present controlled size and shape and the final NPs size distribution and magnetic properties are compatible with the expected for biomedical applications.

Li, Kwan Ho = 多體量子系統問題 / 李君豪. / Thesis (M.Phil.)--Chinese University of Hong Kong, 2007. / Includes bibliographical references (leaves [243]-246). / Text in English; abstracts in English and Chinese. / Li, Kwan Ho = Duo ti liang zi xi tong wen ti / Li Junhao. / Abstract --- p.i / Acknowledgment --- p.iv / Chapter I --- Computational Quantum Mechanics and Its Applications --- p.1 / Chapter 1 --- Theoretical Methodology of Electronic Structures --- p.2 / Chapter 1.1 --- The Schrodinger Equation --- p.3 / Chapter 1.2 --- Molecular Hamiltonian and Born-Oppenheimer Approximation --- p.4 / Chapter 1.3 --- The Variational Method --- p.7 / Chapter 1.3.1 --- Rayleigh-Ritz Variation Principle --- p.7 / Chapter 1.3.2 --- Linear Variation --- p.8 / Chapter 1.4 --- Many-Electron Wavfunction --- p.9 / Chapter 1.4.1 --- Hartree-product Wavefunction --- p.9 / Chapter 1.4.2 --- Slater Determinant and the Pauli Exclusion Principle --- p.11 / Chapter 1.5 --- The Expectation Value of Total Electronic Energy --- p.13 / Chapter 1.6 --- Derivation of the Hartree-Fock Equations --- p.16 / Chapter 1.7 --- Orbital Energies and the SCF Total Electronic Energy --- p.19 / Chapter 1.8 --- Koopmans' Theorem --- p.20 / Chapter 1.9 --- The LCAO expansion and Self-Consistent Field Calculation --- p.22 / Chapter 1.10 --- AO Basis Sets --- p.23 / Chapter 1.10.1 --- Slater-Type Orbitals --- p.24 / Chapter 1.10.2 --- Gaussian Functions --- p.24 / Chapter 1.11 --- Hartree-Fock Limit --- p.25 / Chapter 1.12 --- Electron Correlation --- p.26 / Chapter 1.12.1 --- Weakness in the Single Determinantal Wavefunction --- p.26 / Chapter 1.12.2 --- Configuration Interaction --- p.26 / Chapter 1.13 --- Density Functional Theory --- p.27 / Chapter 1.13.1 --- Early approximations --- p.28 / Chapter 1.13.2 --- Hohenberg-Kohn Theorems --- p.31 / Chapter 1.13.3 --- Kohn-Sham (KS) Method --- p.33 / Chapter 1.13.4 --- Exchange-Correlation Energy Functional and B3LYP --- p.36 / Chapter 2 --- Theoretical Investigation of Oxadiazole-Triphenylamine Based Compounds --- p.40 / Chapter 2.1 --- Organic Light Emitting Diode and Oxadiazole-Triphenylamine Based Com- pounds --- p.40 / Chapter 2.2 --- Methodology --- p.42 / Chapter 2.2.1 --- Theoretical Methodology --- p.42 / Chapter 2.2.2 --- Computational Methodology --- p.46 / Chapter 2.3 --- Computational Results --- p.47 / Chapter 2.3.1 --- Molecular Structure --- p.47 / Chapter 2.3.2 --- Electronic Structure --- p.79 / Chapter 2.4 --- Absorption and Emission Energy --- p.94 / Chapter 2.5 --- Reorganization Energy and Mobility --- p.95 / Chapter 2.6 --- Summary --- p.99 / Chapter 3 --- The Transport Properties of Oligoacenes --- p.102 / Chapter 3.1 --- Introduction --- p.102 / Chapter 3.2 --- Computational Details --- p.103 / Chapter 3.3 --- Results and Discussions --- p.103 / Chapter 3.3.1 --- Molecular Configuration --- p.103 / Chapter 3.3.2 --- Reorganization Energy --- p.106 / Chapter 3.3.3 --- Coupling Matrix Element --- p.107 / Chapter 3.4 --- Conclusion --- p.133 / Chapter 4 --- The Transport Properties in Rubrene Thin Film and Crystal --- p.135 / Chapter 4.1 --- Introduction --- p.135 / Chapter 4.2 --- Computational Details --- p.137 / Chapter 4.3 --- Results and Discussions --- p.137 / Chapter 4.3.1 --- Molecular Structure --- p.137 / Chapter 4.3.2 --- Frontier Orbitals --- p.144 / Chapter 4.3.3 --- Reorganization Energy --- p.147 / Chapter 4.3.4 --- Coupling Matrix Element and Mobility --- p.147 / Chapter 4.4 --- Conclusion --- p.175 / Chapter II --- Analytical Studies of Anisotropic Spin-1/2 Heisenberg Anti-ferromagnetic Linear Chains --- p.177 / Chapter 5 --- Coupled-Cluster Approximation for Two Anisotropic Spin-1/2 Heisenberg Antiferromagnetic Linear Chains with Inter-chain Anisotropic Ferromagnetic Interaction --- p.178 / Chapter 5.1 --- Introduction --- p.178 / Chapter 5.2 --- Approximation in the Coupled-Cluster Method --- p.179 / Chapter 5.3 --- Anisotropic spin-1/2 Heisenberg Antiferromagnetic Model in One Dimension --- p.183 / Chapter 5.3.1 --- The Zeroth Level --- p.185 / Chapter 5.3.2 --- The First Level --- p.186 / Chapter 5.3.3 --- The Second Level --- p.189 / Chapter 5.4 --- Two Anisotropic Spin-1/2 Heisenberg Antiferromagnetic Linear Chains with Inter-chain Anisotropic Ferromagnetic Interaction --- p.198 / Chapter 5.4.1 --- The Zeroth Level --- p.199 / Chapter 5.4.2 --- The First Level --- p.201 / Chapter 5.4.3 --- The Second Level --- p.206 / Chapter 5.5 --- Conclusion --- p.242 / Bibliography --- p.243

Recentemente, observamos na comunidade científica o despertar de um interesse pelo estudo de lantanídeos com base em titânio; neste ínterim, as amostras Sm2Ti2O7 e Nd2Ti2O7 se destacam por terem sido pouco exploradas, especialmente a baixas temperaturas. Análises cristalográficas de monocristais de Sm2Ti2O7 e Nd2Ti2O7 nos permitiram confirmar inicialmente características de rede como a geometria cúbica e monoclínica, respectivamente, enquanto as análises magnéticas e calorimétricas forneceram a temperatura de transição, até então inédita no caso do Nd2Ti2O7 (aproximadamente T=0,62K), além de ordenamento do tipo antiferromagnético e presença de anisotropia a altas temperaturas, na amostra de Sm2Ti2O7, e por toda a extensão de temperaturas trabalhadas, na amostra de Nd2Ti2O7. Por fim, identificamos também a entropia esperada para spins do tipo Ising em ambas as amostras. / Recently the scientific community has shown interest for titanium lanthanides, with the titanates Sm2Ti2O7 and Nd2Ti2O7 being poorly approached, specially at low temperatures. Crystallographic analysis were performed on the single crystal samples of Sm2Ti2O7 and Nd2Ti2O7, which allowed us to confirm its expected cubic and monoclinic structures, respectively; Magnetic and calorimetric data displayed the transition temperature, not previously seen in case of Nd2Ti2O7, of T=0.35K and T=0.62K, respectively, and the antiferromagnetic behaviour of both samples; Anisotropy was observed at high temperatures on the Sm2Ti2O7 data and along all the temperature range studied for the Nd2Ti2O7 sample. Finally, we have identified the entropy of Ising spins on both samples.

Recentemente, observamos na comunidade científica o despertar de um interesse pelo estudo de lantanídeos com base em titânio; neste ínterim, as amostras Sm2Ti2O7 e Nd2Ti2O7 se destacam por terem sido pouco exploradas, especialmente a baixas temperaturas. Análises cristalográficas de monocristais de Sm2Ti2O7 e Nd2Ti2O7 nos permitiram confirmar inicialmente características de rede como a geometria cúbica e monoclínica, respectivamente, enquanto as análises magnéticas e calorimétricas forneceram a temperatura de transição, até então inédita no caso do Nd2Ti2O7 (aproximadamente T=0,62K), além de ordenamento do tipo antiferromagnético e presença de anisotropia a altas temperaturas, na amostra de Sm2Ti2O7, e por toda a extensão de temperaturas trabalhadas, na amostra de Nd2Ti2O7. Por fim, identificamos também a entropia esperada para spins do tipo Ising em ambas as amostras. / Recently the scientific community has shown interest for titanium lanthanides, with the titanates Sm2Ti2O7 and Nd2Ti2O7 being poorly approached, specially at low temperatures. Crystallographic analysis were performed on the single crystal samples of Sm2Ti2O7 and Nd2Ti2O7, which allowed us to confirm its expected cubic and monoclinic structures, respectively; Magnetic and calorimetric data displayed the transition temperature, not previously seen in case of Nd2Ti2O7, of T=0.35K and T=0.62K, respectively, and the antiferromagnetic behaviour of both samples; Anisotropy was observed at high temperatures on the Sm2Ti2O7 data and along all the temperature range studied for the Nd2Ti2O7 sample. Finally, we have identified the entropy of Ising spins on both samples.

This thesis describes a diversity of scattering experiments on a number of different systems. Using time-of-flight neutron scattering, a study of polycrystalline sodium in the highmomentum limit known as the impulse approximation has been performed. The purpose of this study was to look for anharmonic effects in the neutron recoil scattering of sodium as the temperature was increased from 30K to 300K. No such effects were detected and the results agreed with an isotropic harmonic solid to an accuracy of about 4%. Two experiments were carried out on antiferromagnetic systems using triple-axis neutron scattering techniques to measure the spin-wave dispersion relations. The first was on CuO to verify its description as a spin 1/2 one-dimensional antiferromagnet. The dispersion relation was measured along the chain direction up to an energy transfer of 8OmeV. This was done above and below the Néel temperature (T_N =240K). However, no evidence was seen to justify the description of CuO as a one-dimensional antiferromagnet, with the spin waves behaving like those in a classical three-dimensional system. The other spin-wave study examined the two-dimensional antiferromagnet KFeF₄ . The measurement of the spin-wave dispersion relation at two temperatures (50K and 100K) below the Néel temperature (T_N =136.75±0.25K), confirmed the description of KFeF₄ as a two-dimensional Heisenberg antiferromagnet with small Ising anisotropy. Studies of the magnetic phase transition in KFeF₄ revealed that below the Néel temperature, the critical behaviour is described by two-dimensional Ising models, and above a crossover to Heisenberg behaviour is seen. This crossover was detected by measuring the order parameter below T_N, and the static and dynamic susceptibilities above T_N using neutron scattering techniques. The results were compared to power-law behaviour and also to theories for the classical Heisenberg antiferromagnet and the more recent quantum Heisenberg antiferromagnetic model. The final study of KFeF₄ involved an x-ray experiment on the structural phase transition around 400K. It has been suggested that there is a second-order transition at 410K to an incommensurate phase, which then undergoes a first-order lock-in transition at 400K to the low-temperature structure. This single crystal x-ray scattering study confirms the existence of the first-order phase transition, but shows no evidence for a higher temperature second-order transition or for the incommensurate phase.