Doctoral Dissertations

Date of Award

5-2001

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Physics

Major Professor

E. Ward Plummer

Committee Members

Larry Pai, David Zehner, Joe Carpinelli

Abstract

Imperfections and defects have a strong influence on phase transitions, especially in systems with lower dimensionality, where fluctuations can be strong enough to prevent long-range order. In quasi-ID or 2D systems that exhibit a Charge Density Wave Transition defects play a very special role. There, due the collective nature of the phenomena, a small proportion of microscopic disorder can control the global properties.Defects would produce pre-transitional effects such as charge oscillations in their vicinity, affecting CDW. On the other hand, the interaction of mobile defects with CDWwould lead to alignment of defects, or Defect Density Waves (DDW). In this dynamic model the distribution of defects would no longer be either random or static. Instead Defects would align their positions to optimize the energy of the pinned CDW.In this thesis I present a new view of the phase transitions occurring in quasi-2d Systems of ultra-thin films of a metal deposited on a semiconductor surface: α-phase ofSn/Ge(111) and α-phase of Sn/Si(111). While most of the previous studies were devoted to the elucidating the nature of the room temperature (RT) and low temperature (LT)phase, that is, electronic and lattice structure at two temperatures, above and below the transition temperature, my Variable Temperature STM observations showed how thesephases evolve into each other. From these observations it has become clear that point defects - substitutional atoms and vacancies - play a crucial role. The perturbation of the lattice and electronic structure induced as a response of the system to a defect in its vicinity has a form of density waves with the symmetry of LT phase. These waves have a short range at RT and decay exponentially with the distance from defects. When the temperature is lowered the range of these waves grows. As characteristic decay length of the perturbation reaches the average distance between defects, the density wave mediated defect-defect interaction becomes strong enough to make defects exchange their positions with their nearest neighbors. This motion brings waves originated on different defects into coherence and defects into partial ordering leading to the disorder-order phase transition in defect distribution. Defects dictate the structure of phases both above and below critical temperature. Moreover their presence smears out the critical temperature and changes the properties of the phase transition. My STM observations and modeling imply that the transition temperature of the pure system without defects should be the temperature at which the decay length of defect-induced waves becomes infinite. ForSn/Ge(lll) this temperature is ~ 70 K. This is about 140 K (!!!) lower than the temperature measured by electron diffraction and still widely cited in literature. Thesharp domain walls are the features that distinguish the low temperature phase. A newmodel is presented that allows to predict the configuration, of domain walls for a given defect distribution.

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