Preserving Historic Qilou Districts in Southern China: Consideration of Western Models

The historic qilou buildings are essential to the people in Southern China. Unfortunately, due to some of the urban renewal policies, many of the qilou districts have been either demolished or abandoned. For years, many scholars had written books and articles about how to preserve these significant historic urban elements. However, these studies failed to include examinations of the historic preservation program in other countries or the existing ones in China. This thesis was written in the hope of filling the gap. This study discusses the historical developments of the qilou buildings and their significances in four selected cities: Guangzhou, Macau, Meizhou, and Kaiping. This study then includes analysis on whether creating the US- based Main Street Program, and using the UNESCO World Heritage Sites model to manage these qilou districts are feasible in the selected cities

Flux quantization effects in disordered normal metal rings and superconducting networks

The effects of the magnetic flux on the properties of disordered normal metal rings and bond or site diluted two-dimensional superconducting networks are investigated theoretically, with an emphasis on the quantum coherence of the electrons and the localization nature in the disordered systems. The conductance of disordered metal rings in magnetic field is obtained via the Landauer\u27s formula through calculations of the localization length L[subscript] c. The important role of the ensemble averaging and the self-averaging to obtain the half-flux-quantum h/2e conductance oscillation is demonstrated unambiguously in both rings of a strictly one-dimensional geometry and rings with a finite width. The amplitude of the localization length oscillation is found to follow a universal relation for all the numerical data: [delta](L[subscript] c/L) = [alpha](L[subscript] c/L)[superscript]2. L is the radius of the ring. The expected universal conductance fluctuations are observed for L[subscript] c/L ~ 1. For L[subscript] c \u3e L, much larger oscillation amplitudes are obtained. In the case of two-dimensional site or bond percolation superconducting networks, the nature of the eigenstates and the effects on the superconducting-to-normal phase boundary is examined by finite-size transfer matrix calculations within the mean-field Ginzburg-Landau theory of second order phase transitions. The fine structures of the T[subscript] c-H phase boundary are found to be washed out immediately when a small fraction of sites or bonds is removed. A rich structure for the mobility edge is obtained. The existence of two different phase boundaries, one for the first eigenstate and the other for the first mobility edge, may have important experimental consequences. ftn*DOE Report IS-T-1388. This work was performed under contract No. W-7405-Eng-82 with the U.S. Department of Energy

Electronic landscape of the P-cluster of nitrogenase as revealed through many-electron quantum wavefunctions

The electronic structure of the nitrogenase metal cofactors is central to nitrogen fixation. However, the P-cluster and iron molybdenum cofactor, each containing eight irons, have resisted detailed characterization of their electronic properties. Through exhaustive many-electron wavefunction simulations enabled by new theoretical methods, we report on the low-energy electronic states of the P-cluster in three oxidation states. The energy scales of orbital and spin excitations overlap, yielding a dense spectrum with features we trace to the underlying atomic states and recouplings. The clusters exist in superpositions of spin configurations with non-classical spin correlations, complicating interpretation of magnetic spectroscopies, while the charges are mostly localized from reorganization of the cluster and its surroundings. Upon oxidation, the opening of the P-cluster significantly increases the density of states, which is intriguing given its proposed role in electron transfer. These results demonstrate that many-electron simulations stand to provide new insights into the electronic structure of the nitrogenase cofactors.Comment: 23 pages, 5 figure