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Magnetic properties and proton spin-lattice relaxation in molecular clustersAllalen, Mohammed 06 June 2006 (has links)
In this work we studied magnetic properties of molecular magnets of the new heteropolyanion {Cu20}, dodecanuclear cluster {Ni12}, and the heterometallic {Cr7M} wheels, in which one of the CrIII ions of Cr8 has been replaced by a Fe, Cu, Zn, Ni, ion with this extra-spin acts as local probe for the spin dynamics.Such systems have been synthesized recently and they are well described using the Heisenberg spin Hamiltonian with a Zeeman term of an applied magnetic field along the z-axis. Using the numerical exact diagonalization method, we have calculated the energy spectrum and the eigenstates for different compounds,and we have used them for reexamining the available experimental susceptibility data to determine the values of exchange parameters.We have studied the thermodynamic properties such magnetization, susceptibility, heat-capacity. At low temperature regions molecular magnets act as individual quantum nanomagnets and can display super-paramagnetic phenomena like macroscopic quantum tunneling, ground state degeneracy, level-crossing. A crucial issue for understanding these phenomena is the coupling between magnetic molecular levels and the environment such as nuclear spins. We have modeled the behavior of the proton spin lattice relaxation rate as a function of applied magnetic field for low temperatures as it is measured in Nuclear Magnetic Resonance (NMR) experiments.
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Předpojatost, střed kultur, role ženy, národ a národnost v románech Ying Chen / Prejudice, Cultural Clash, Female Role, Nation and Nationaly in the Novels of Ying ChenNavrátilová, Leona January 2012 (has links)
Ying Chen is a Canadian writer of Chinese origin who writes in French. In her novels, she investigates immigration which is closely connected with displacement and the loss of one's original identity. Her literary work is primarily aimed at the North American readership so she includes a lot of details of historical events and social facts about China. Ying Chen belongs to the group of authors who are labelled as immigrant writers. The majority of her literary work centres around the recurring themes of nationalism, feminism, imagination and immigration, which can lead to a loss of original identity. Ying Chen investigates whether a person can exchange his identity, that which was given to him by his parents, with a new one. In her second published novel, L'Ingratitude, Ying Chen speaks through the character of the dominant mother and says: "A person without parents is miserable, like a people without history." With these words she indicates the impossibility of exchanging one's nationhood, national history, and identity. We need to accept who we are, and she emphasises this fact in her novel, Immobile, saying, "I am myself."
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On Classical and Quantum Mechanical Energy Spectra of Finite Heisenberg Spin SystemsExler, Matthias 16 May 2006 (has links)
Since the synthesis of Mn12, which can be regarded as the birth of the class of magnetic molecules, many different molecules of various sizes and structures have been produced. The magnetic nature of these molecules originates from a number of paramagnetic ions, whose unpaired electrons form collective angular momenta, referred to as spins. The interaction between these spins can often be described in the Heisenberg model. In this work, we use the rotational band model to predict the energy spectrum of the giant Keplerate {Mo72Fe30}. Based on the approximate energy spectrum, we simulate the cross-section for inelastic neutron scattering, and the results are compared to experimental data. The successful application of our approach substantiates the validity of the rotational band model. Furthermore, magnetic molecules can serve as an example for studying general questions of quantum mechanics. Since chemistry now allows the preparation of magnetic molecules with various spin quantum numbers, this class of materials can be utilized for studying the relations between classical and quantum regime. Due to the correspondence principle, a quantum spin system can be described exactly by classical physics for an infinitely large spin quantum number s. However, the question remains for which quantum numbers s a classical calculation yields a reasonable approximation. Our approach in this work is to develop a converging scheme that adds systematic quantum corrections to the classical density of states for Heisenberg spin systems. To this end, we establish a correspondence of the classical density of states and the quantum spectrum by means of spin-coherent states. The algorithm presented here allows the analysis of how the classical limit is approached, which gives general criteria for the similarity of the classical density of states to the quantum spectrum.
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