Doctoral Dissertations

Date of Award

5-2009

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Physics

Major Professor

E. Ward Plummer

Abstract

Perovskite manganites are complex systems in which the structural, electronic, and magnetic properties are strongly coupled - in many cases, non-linearly. A small change in one property (e.g., structure) can produce a large change in other properties (e.g., the electronic state). Conceptually, creating a surface by cleaving a single crystal or growing a thin film provides a controlled way to disturb the coupled system by breaking the symmetry without changing the stoichiometry, which may lead to completely new physical properties. The emerging surface electronic phases can be further enriched by surface reconstructions which often occur due to bond breaking at the surface and also by extrinsic carrier doping from adsorbates as well. While the cleaving method is limited to a small amount of cleavable layered materials, the thin-film growth method, such as laser molecular beam epitaxy (Laser MBE), becomes the most desired technique for the study of surfaces in general.As an example, La₁-[subscript x]Ca[subscript x]MnO₃, which is one of the most investigated cubic perovskite manganite systems, has a surface that cannot be created by cleaving the bulk but can be prepared by laser MBE. In this dissertation, La1-[subscript x]Ca[subscript x]MnO₂ (001) (x = 3/8) thin films grown by laser MBE were studied with a combination of in-situ techniques such as low-energy electron diffraction (LEED) and high-resolution scanning probe microscopy (SPM). Two different electronic conductivities were observed using scanning tunneling spectroscopy (STS) on the thin-film surface. The "atomic" resolution scanning tunneling microscopy (STM) images revealed that the two different conductivities come from two reconstructed surfaces, (1 x 1) and ([square root]2 x [square root]2)R45°, respectively.The (1 x 1 ) and ([square root]2 x [square root]2)R45° reconstructions were found to be reversible by oxygen adsorption/desorption, and as a result, the conductivity of the surface can be tuned from metallic to insulating by controlling the oxygen adsorption. Further investigations revealed the existence of a surface structure transition driven by the film thickness. The (1 x 1) surface without oxygen adsorption actually has specific superstructures; it changes from c(2 x 4) to (3[square root]2 x 4[square root]2)R45° with a critical thickness of 14 ML associated with an extensive strain induced by the substrate. The discoveries of the unexpected surface structural and electronic transitions revealed in this dissertation open up a new direction for exploring the functionality relationship at the surfaces of complex transition-metal compound thin films.

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