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Organic systems provide the opportunity to study physics in lower dimensions. Rather than interactions between atomic sites, organic systems are comprised of molecules with the general shape of flat bricks. The stacking of these bricks produces a wide variety of structures with equally diverse ground states. The tau-phase molecular conductors are comprised of the asymmetric DMEDT-TTF molecule. These donors are arranged into a grid-like pattern to create two-dimensional planes of high conductivity. Materials based on stacks of perylene (Per) donor molecules have a conductivity anisotropy which makes them effectively one-dimensional and therefore susceptible to lattice instabilities. Systematic studies of both materials are reported in this dissertation. The donor molecules of the tau-phase systems, tau-(P-(S,S)-DMEDT-TTF)2(AuBr2)1+y and tau-(EDO-(S,S)-DMEDT-TTF)2(AuBr2)1+y, only differ by the substitution of nitrogen for oxygen yet measurements reveal vastly different results. Measurements of magnetoresistance reveal metallic character in increasing magnetic fields followed by a rapid transition to a bulk insulator, followed by a large hysteresis as the material returns to a metal as the field returns to zero. Comparison between the above mentioned systems for magnetization and pressure dependence suggest a weakly coupled lattice for the nitrogen-based material, which distorts in high fields. With transition temperatures of 8 K and 12 K for (Per)2Pt(mnt)2 and (Per)2Au(mnt)2, respectively, readily available fields are capable of producing large changes in the low temperature, charge density wave (CDW) states of the systems. When the Au metal sites within the anion chains are replaced with Pt (S = ½), magnetism is introduced to the system. When (Per)2Pt(mnt)2 is subjected to increasing fields beyond ~ 20 T, the conventional CDW is suppressed and a new density wave state is formed in high fields. Further measurements have been performed observing the change is the Fermi surface topology with the addition of pressure. We report on the field-induced density wave phase and its agreement with contemporary theories.
Density Waves, Metal-Insulator Transitions, Organic Metals
Date of Defense
June 20, 2005.
A Dissertation submitted to the Department of Physics in partial fulfillment of the requirements for the degree of Doctor of Philosophy.
Includes bibliographical references.
James S. Brooks, Professor Directing Thesis; Naresh Dalal, Outside Committee Member; Nicholas E. Bonesteel, Committee Member; Paul Eugenio, Committee Member; Peng Xiong, Committee Member.
Florida State University
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