Date of Award

Spring 1-1-2013

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Physics

First Advisor

Kyle P. McElroy

Second Advisor

Daniel S. Dessau

Third Advisor

Dmitry Reznik

Fourth Advisor

Michael A. Hermele

Fifth Advisor

Scott Bunch

Abstract

Strongly correlated electron problems, including high temperature superconductivity, colossal magneto-resistivity, charge-density wave, heavy fermion phenomena, etc., have been major research focuses in condensed-matter physics for the past several decades. Involvements of and competitions between different degrees of freedom and energy scales present considerable difficulties for understanding these problems. In this thesis, I have used home-built variable-temperature high resolution (spatial and energy) spectroscopic imaging scanning tunneling microscopes (SI-STM) to study the topography and the local single-particle tunneling spectroscopy in a family of two-dimensional charge-density wave (2D-CDW) materials (TaSe2, TaS2, and NbSe2) and in a newly discovered 5d Mott insulator Sr2IrO4 with strong spin-orbit coupling. In the 2D-CDW materials, we have shown the strong lattice distortion and weak electron modulation accompanying CDW transition, directly opposite to Peierls mechanism. We have also discovered that the 2D-CDW transitions in TaSe2 and TaS2 show involvement of the orbital degree of freedom, shedding light on a long standing puzzle with their electrical resistivity anomalies. In the novel 5d Mott insulator Sr2IrO4 we have found a large insulating gap of 620 meV, indicating a stronger-than-expected correlation effect for a 5d electron system.

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