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published_or_final_version / Physics / Master / Master of Philosophy

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Various aspects of the tunneling states problem in metallic glasses are examined. As a first step, a computer model based on the analogy between local environments in crystalline and amorphous states is developed and used to generate and relax structures for two specific materials, namely Ni(,80)P(,20) and Cu(,33)Zr(,67). The model is found to give a physically realistic picture of the systems examined, in particular by properly accounting for short-range order effects. The structures are then analyzed in terms of single-atom tunneling taking place between two metastable minimum-energy positions. The probability of occurence of such "two-level systems" (TLS's) is shown to be strongly correlated with the degree of relaxation and thus with the density of the system. Further, they are seen to be associated with voids in the structure which disappear upon relaxation, suggesting an analogy with the physical process of annealing metallic glasses below their glass transition temperatures. It is therefore concluded that a reduction of the various low-temperature anomalies in these materials should result from the annealing process. As a verification, the change in superconducting transition temperature T(,c) due to the presence of TLS's is evaluated. In leading to a non-negligible enhancement effect, our model is indeed found to provide a reasonable estimate of the observed drop in T(,c), thus corroborating our hypothesis of a reduction of the TLS density of states upon thermal relaxation.

Many low dimensional structures arise from self-assembly when depositing metals on silicon surfaces, including both quantum dots and quantum wires. One class of these objects are rare earth silicide nanowires (RENW) grown on Si(001). In this dissertation, NW thermal stability, control of NW cross section, and associated surface reconstructions are studied by Scanning Tunneling Microscopy (STM). We test thulium and find for the first time that it forms NWs and these NWs are stable against prolonged annealing. We also find that the RENWs nucleate at 2×7 reconstruction domain boundaries. These results pave the way for precise control over NW size, placement, and integration with functional nanostructures and nanodevices.Another type of self-assembled NWs on Si(001) are insulating CaF2 NWs. As an ideal model system for epitaxial growth of an insulator on a semiconductor surface, CaF2 offers unique properties such as simple structure, good lattice match to silicon and congruent evaporation. In this thesis the growth behavior of CaF2 on the Si(001) surface is investigated. At low coverages CaF2 molecules randomly locate on Si(001). Features observed at this stage are explained in terms of dissociated fragments of CaF2 terminating the dangling bonds of Si dimers. Etching is observed after surface is saturated by these features with a 2×1 periodicity. A 2×n phase, grown at 750°C, suggests the dissociation of CaF2, as proved by the simulation of LEED patterns. A c(4×4) phase is observed from 0.5ML to about 1ML with deposition temperature from 600oC to 700oC. At the highest CaF2 deposition coverages studied, a stripe phase and CaF2 NWs are observed by a combination of STM, AFM and SEM. The results provide a significant expansion in the knowledge of CaF2 on Si(001). The common thread that links all these studies is the extent to which nanostructures can be controlled by careful growth conditions, not just by substrate temperature and the amount of material deposited, but also by timing of post-deposition annealing, etc. The grown nanostructures are metastable and result from a balance of energetic considerations and kinetics.