Electronic Properties, Magnetotransport and STM in Quasi-One-and-Two-Dimensional Metals

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This research involves the study of electronic structure due to charge-density waves (CDWs) and spin-density waves (SDWs) in lower dimensional metals, semimetals and semiconductors. The research utilizes scanning tunneling microscopy (STM) and atomic force microscopy (AFM) to detect both the atomic and electronic structure with resolution on the scale of individual atoms and correlates the results with studies of the effects of high magnetic fields on the electronic transport properties of the some materials. The materials to be investigated include transition metal chalcogenides, as well as more complex crystals such as tantalum nickel selenide. This combination of the above techniques provides a unique characterization of the electronic modifications associated with CDWs, SDWs and mixed density waves. They also provide extremely sensitive detection of modifications in the CDW structure as induced by impurity doping. Doping of trichalcogenides produces new electronic phases, metal-insulator transitions and modified CDWs, all of which can be detected by STM spectroscopy, AFM scanning and high magnetic field transport. Results should provide a detailed understanding of the electronic properties of materials as modified by the presence of density- waves in the electron gas as well as changes in the electronic density waves due to impurity doping. %%% The highly anisotropic metals being studied in this research and the unusual electronic properties associated with density-waves provide insight into new electronic modifications of materials. Studies of these properties on an atomic scale by scanning tunneling microscopy (STM) and atomic force microscopy (AFM), combined with macroscopic measurements of the associated electronic conduction phenomena in high magnetic fields, lead to a unique understanding of the overall electronic structure. The STM and AFM studies provide a picture of both the crystal structure and the electronic structure at the atomic level. These methods are extremely sensitive to modifications by impurities and defects and can be used to systematically follow impurity atom locations, ordering and the formation of new electronic phases in the crystals. The accompanying modifications of the electronic and magnetotransport properties will correlate the changes in atomic configuration and local electronic structure with the overall electronic response of the material. The development of these methods should be extremely useful for the design and testing of new electronic materials.