We investigate the quantum dynamics of the spins on different Heisenberg antiferromagnetic spin lattice systems. Firstly, we applied the coupled-cluster method to the spin-1/2 antiferromagnetic XXZ model on a square lattice by employing an approximation which contains two-body long-range correlations and high-order four-body local correlations. Improvement is found for the ground-state energy, sublattice magnetization, and the critical anisotropy when comparing with the approximation including the two-body correlations alone. We also obtain the full excitation spectrum which is in good agreement with the quantum Monte Carlo results and the high-order spin-wave theory. Secondly, we study the longitudinal excitations of quantum antiferromagnets on a triangular lattice by a recently proposed microscopic many-body approach based on magnon-density waves. We calculate the full longitudinal excitation spectra of the antiferromagnetic Heisenberg model for a general spin quantum number in the isotropic limit. Similar to the square lattice model, we find that, at the center of the first hexagonal Brillouin zone Γ(q=0) and at the magnetic ordering wavevectors ±[Q= (4π/3,0)], the excitation spectra become gapless in the thermodynamic limit, due to the slow, logarithmic divergence of the structure factor. However, these longitudinal modes on two-dimensional models may be considered as quasi-gapped, as any finite-size effect or small anisotropy will induce a large energy gap, when compared with the counterpart of the transverse spin-wave excitations. We have also investigated the excited states of the quasi-one-dimensional quantum antiferromagnets on hexagonal lattices, including the longitudinal modes based on the magnon-density waves. A model Hamiltonian with a uniaxial single-ion anisotropy is first studied by a spin-wave theory based on the one-boson method; the ground state thus obtained is employed for the study of the longitudinal modes. The full energy spectra of both the transverse modes (i.e., magnons) and the longitudinal modes are obtained as functions of the nearest-neighbor coupling and the anisotropy constants. We have found two longitudinal modes due to the non-collinear nature of the triangular antiferromagnetic order, similar to that of the phenomenological field theory approach by Affleck. The excitation energy gaps due to the anisotropy and the energy gaps of the longitudinal modes without anisotropy are then investigated. We then compares our results for the longitudinal energy gaps at the magnetic wavevectors with the experimental results for several antiferromagnetic compounds with both integer and non-integer spin quantum numbers, and we find good agreements after the higher-order contributions are included in our calculations.

No description available.

The spin-lattice coupling is due to itinerant electrons interacting with both spins of ions and phonons, mediating a coupling between magnetic spin moments and lattice vibrations. In this project, the coupling is studied systematically for finite quantum spin chains of size 2 – 10. The coupling is included in a Hamiltonian model with the Heisenberg exchange interaction and an external magnetic field resulting in an eigenvalue problem which is solved numerically to find phase diagrams of the magnetic moment of the system depending on an external magneticfield and the lattice vibration parameter. The eigenvalue problem is also solved analytically for the 2-ion chain, dimer, and 3-ion chain, trimer, systems. Based on these phase diagrams two propositions are made: the effect of the coupling is larger than an external magnetic field and the behavior of the coupling converges to a common phase diagram for larger spin chains. / Kopplade spinn- och vibrationsfrihetersgrader beror på kringvandrande elektroner som växelverkar med både spin och fononer, vilket förmedlar en koppling mellan magnetiskt spinmoment och gittervibrationer. I detta projekt studeras denna koppling systematiskt för ändliga spin-kedjor av 2–10 joner. Systemet beskrivs av en Hamiltonian med Heisenberg modellen som beskriver spin-spin kopplingen samt ett externt magnetiskt fält. Detta egenvärdesproblem löses analytiskt för dimer- och trimersystem samt numeriskt för längre kedjor. Lösningarna används för att ta fram fasdiagram av de magnetiska momenten av kedjorna beroende på externt magnetfält och spin- och vibrationsfrihetsgradsparametern. Baserat på dessa fasdiagram, framförs två propositioner: kopplingens effekt är större än ett externt magnetfält och kopplingens beteende konvergerar till ett enhetligt fasdiagram för större spinnkedjor. / <p>Subject reader/ämnesgranskare: Anders Bergman</p>

The mobility of N-alkyl nicotinamides and their solubilization equilibria in surfactant micellar solution were investigated using an NMR paramagnetic relaxation method. The spin-lattice relaxation times (Tl) . for protons of these compounds were measured in pure D20 and in cationic surfactant solution in the presence and absence of · a low concentration of paramagnetic Ma2+ ions. The rotational motion of these molecules in aqueous phase became slower when the alkyl group changed from methyl to octyl. The increase of the 1H spinlattice relaxation rate (Rl) of the molecules, when surfactants are added, implies the penetration of these solubilizates into micellar phase. The micelle to water phase distribution coefficient, 1-p, was determined by monitoring the change of Rl of the solubilizates upon addition of paramagnetic ions to the aqueous phase. The mole fraction based distribution coefficient, Kx, as well as the free energy of transfer of N-alkyl nicotinamides from the aqueous phase to the micellar phase were calculated. A model was postulated for the interaction of N -alkyl nicotinamide with micelles. Hydrophobic force between the alkyl chains of the solubilizates and the surfactants accounts for the solubilization of N -alkyl nicotinamides and can force the binding of cationic compounds to cationic micelles · despite charge repulsion. With increasing of the alkyl chain length, the hydrophobic force increases, and the interaction between them becomes stronger, and more N -alkyl nicotinamides are solubilizated into the micellar phase.

The exploration of physical extremes drives technological innovation. Recent decades have seen a push towards materials engineering at the absolute limit of space with electronic systems that are a single atom thick. When electrons are confined to two-dimensional structures, exotic and often unexpected phenomena emerge due to enhanced interaction effects and crystalline anisotropies. The study of such unconventional phenomena offers the opportunity to extend knowledge of fundamental physics with an eye towards advancing the state of the art in control over quantum matter. In this thesis we use scanning tunneling microscopy to study the electronic structure of a collection of novel two-dimensional materials: twisted double-bilayer graphene (TDBG), mirror symmetric twisted trilayer graphene (TTG), small angle twisted double trilayer graphene (TDTG), and the van der Waals heavy Fermion material CeSiI. In TDBG, we directly image spontaneous symmetry breaking of the electronic states as a function of carrier density and attribute this to an intrinsic nematic instability of the metallic Fermi liquid. In TTG, we find evidence for a novel form of lattice relaxation, in which twist angle disorder leads to the formation of moiré lattice defects that can act to lock trilayer devices into a magic angle configuration while strongly modulating the local electronic structure, with implications for the superconducting state. In TDTG, we discover yet another form of lattice relaxation in which a global transformation of the stacking structure creates a net energy reduction, even while the stacking energy density in roughly half of the moiré lattice rises. Lastly, we show through quasiparticle interference spectroscopy and theoretical modeling that CeSiI hosts a nodal hybridization between itinerant conduction electrons and a lattice of local moments, giving rise to a strong angular dependence of the heavy Fermion mass enhancement in this van der Waals material.

Spin-lattice (T1) relaxation studies using solid-state and solution-state :31p nuclear magnetic resonance spectroscopy have proven to be a reliable procedure for determining the onset of a "liquid-like" character of the supported phase in a supported aqueous phase catalyst. It has also been shown that the appearance of the liquid-like character, which can be determined by the length of T b occurs at the onset of maximum catalytic activity in a supported aqueous phase catalyst. Direct sulfonation of 1,2-bis(diphenylphosphino)ethane (DPPE) has yielded 1,2-(bis[di-m-sodiumsulfonato]phenylphosphino)ethane (DPPETS), a new water soluble ligand that has been characterized and used in the synthesis of several new complexes with palladium, rhodium, platinum and nickel centers. T 1 relaxation times and the magnitude of the chemical shift anisotropy of several of the complexes have been determined with solid- and solution-state 31 P NMR and several complexes have been evaluated for their potential in biphasic hydrogenation and hydroformylation catalysis. / Ph. D.

No description available.

A taxa de relaxação magnética nuclear de uma impureza diluída em um meio metálico foi calculada como função da temperatura. Nossos cálculos são aplicados ao modelo de Anderson com degenerescência de spin, originalmente desenvolvido para descrever ligas magnéticas diluídas. Nossos cálculos são aplicados ao modelo de Anderson com degenerescência de spin, originalmente desenvolvido para descrever ligas magnéticas diluídas. Discutimos a relevância a sistemas de férmions pesados, valência flutuante e adsorção química em superfícies metálicas. As taxas de relaxação como função da temperatura exibem picos que concordam qualitativamente com resultados experimentais. No limite de T &#8594 0 as taxas de relaxação são proporcionais a temperatura, mesmo nos casos em que efeitos de muitos corpos invalidam clássica derivação da lei de Korringa. O coeficiente linear é proporcional ao quadrado da suscetibilidade magnética à temperatura zero; isto generaliza a relação derivada por Shiba no limite Kondo. / The nuclear magnetic relaxation rate for an impurity in a metallic environment has been calculated as a function of temperature. Our calculations are based on the spin-degenerate Anderson model originally developed to describe dilute magnetic alloys. The relevance to heavy férmions, Valence fluctuation, and chemisorption on metallic surfaces is discussed. The temperature dependent rates exhibit peaks in qualitative agreement with experimental results. As expected, in the limit T &#8594 0 the rates are proportional to the temperature, even for cases in which many-body effects invalidate the classical derivation of the Korringa Law. The linear coefficient is shown to be proportional to the square of the zero temperature suscetibility; this generalizes a relation derived by Shiba in the Kondo limit.

Neste trabalho, descrevemos um espectrômetro ótico para medidas de Dicroismo Circular Magnético (DCM), utilizado nas seguintes experiências: 1) Medidas de tempo de relaxação spin-rede (T1) para centros F em NaCl, NaBr, CsBr e CsCl, a temperatura de 1,8&#176K em campos magnéticos até 17000Gs. Verificamos a validade da teoria da referência (8) para explicar as diferenças observadas, no comportamento de \'T IND.1\', para halogenetos com diferentes íons alcalinos, bem como diferentes estruturas. Comprovamos que a interação hiperfina ainda continua a ser o mecanismo mais importante para esses centros. Verificamos também que, para temperaturas entre 6&#176K e l5&#176K, os valores de experimentais para T1, em KBr, concordam razoavelmente com a teoria da referência (21). Esta terapia é uma extensão daquela da referência (8). 2) Espectros de DCM para KCl: Co++ e CaF2: Co++ foram obtidos para campos magnéticos ate 56KGs e temperaturas entre 1,8&#176K e 4,2 &#176K. Os resultados obtidos mostraram ser concordantes com a hipótese dos centros Co++ ocuparem sítios intersticiais na rede de KCl. / In this work we describe a Magnetic Circular Dicroism Spectrometer wich was used in the following experiments: 1) We measured the spin-lattice relaxation time (T1) for F centers in NaCl, NaBr, CsBr and CsCl, at 1,8&#176K in magnetic fields up to 15000Gs. We verified the suitability of the theory of ref.(O8) to explain the differences observed for halides of differents alkali ions as well as for different structures. This proves that the hyperfine interaction is the most important mechanism for this kind of centers. We also verified that, for temperatures between 6&#176K and l5&#176K, T1, the T1 experimental values fits the theory of ref.(21) reasonably well ,for F centers in KBr. This theory is an extension of that of ref.(8). 2) We obtained the MCD spectra for KCl: Co++ and CaF2: Co++ in different magnetic fields up to 56KGs, and in temperature range between 1,8 &#176K and 4,2&#176K. Our results are consistent with the assumption that Co++ centers are intersticial in KCl lattice.

In this work I present a 23Na/51V-NMR study of sodium vanadate as pure compound, and the influence of very small amounts of Sodium substitution by Calcium and Lithium is reported and discussed. The measurements of spin-lattice relaxation for 23Na and 51V are also presented. The sodium vanadate is found to have a double phase transition. The two transitions are close together and take place around 34 K. Above the transition temperature there is one V site in the mixed oxidation state 4.5+ and there is one Na site. A consistency for a number of un-doped and very slightly doped samples of three vanadium valences is argued, confirming a charge ordering transition at transition temperature, in good qualitative agreement with Bernert’s model and as well in quantitative and qualitative agreement with Sawa’s monoclinic structure. Below the transition temperature sodium is found to have ten sites. The number of the Na site is continuously developed with decreasing the temperature below transition temperature. This continues development of the Na sites demonstrates that the second transition is continues. The detailed spin-lattice relaxation rate for 23Na in both, the pure and the doped samples, in transition region, provided evidence for a non-symmetric and complex transition peak structure which we relate to the onset of more than one transition occurring at slightly different temperatures. This scales with the transition temperature reduction provided by lithium and calcium doping. The two transitions (i.e. dimerization and charge ordering) are intimately related. We investigated the spin-gap by means of Vanadium and Sodium spin-lattice relaxation temperature dependence well below transition temperature. The analysis of the single crystal data reveals a significant anisotropy in the nature of the gap, which is sensitive to Calcium and Lithium doping, indicating that is constrained to the ladder plane.