Browsing by Subject "Interaction effects"

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    Electronic properties and electron-electron interaction effects in transition metal dichalcogenides
    (2018-08-09) Larentis, Stefano; Tutuc, Emanuel, 1974-; Banerjee, Sanjay K; MacDonald, Allan H; Register, Leonard F; Shi, Li
    Transition metal dichalcogenides (TMDs) are a new class of two-dimensional layered materials characterized by a MX₂ chemical formula, where M (X) stands for a transition metal (chalcogen). MoS₂, MoSe₂ and MoTe₂ are semiconducting TMDs, which at the monolayer limit possess bandgaps >1 eV, rendering them attractive as possible channel material for scaled transistors. The bandstructures of monolayers feature coupled spin and valley degrees of freedom, thanks to large spin-orbit interaction, and large effective masses (m*), suggesting that electron-electron interaction effects are expected to be important in these semiconductors. In this dissertation we discuss the fabrication and electrical characterization of TMD-based electronic devices, with a focus on their electronic properties, including scattering mechanisms contributing to the mobility, carriers' effective mass, band offset in heterostructures, electronic compressibility, and spin susceptibility. We begin studying the four-point field-effect mobilities of few-layers MoS₂, MoSe₂ and MoTe₂ field effect transistors (FETs), in top-contact, bottom-gate architectures. Using hexagonal boron-nitride dielectrics, we fabricate FETs with an improved bottom-contact, dual-gate architecture to probe transport at low temperatures in monolayer MoS₂, and mono- and bilayer MoSe₂. From conductivity and carrier density measurements we determine the Hall mobility, which shows strong temperature dependence, consistent with phonon scattering, and saturates at low temperatures because of impurity scattering. High mobility MoSe₂ samples probed in perpendicular magnetic field, at low temperatures show Shubnikov-de Haas oscillations. Using magnetotransport we probe carriers in spin split bands at the K point in the conduction band and extract their m* = 0.8m [subscript e]; m [subscript e] is the bare electron mass. Quantum Hall states emerging at either odd or even filling factors are explained by a density dependent, interaction enhanced Zeeman splitting. Gated graphene-MoS₂ heterostructures reveal a saturating electron branch conductivity at the onset of MoS₂ population. Magnetotransport measurements probe the graphene electron density, which saturates and decreases as MoS₂ populates, a finding associated with the negative compressibility of MoS₂ electrons, modeled by a decreasing chemical potential, where many-body contributions dominate. Using a multi-gate architecture in monolayer MoTe₂ FETs, that allows for independent contact resistance and threshold voltage tuning, we integrate reconfigurable n- and p-FETs, and demonstrate a complementary inverter.
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    Interaction Effects of Information Technologies and Best Practices on Construction Project Performance
    (ASCE Journal of Construction Engineering and Management, 2013) Kang, Youngcheol; O'Brien, William J.; Dai, Jiukun; Mulva, Stephen P.; Thomas, Stephen P.; Chapman, Robert E.; Butry, David
    Building from considerable empirical research in the general business literature, this paper quantitatively explores the view that the benefits of information technologies manifest themselves through improvement in work processes. In turn, better work processes lead to increased project performance. Using an overall sample of 133 projects (missing data make specific correlation sample sizes smaller) from the Construction Industry Institute Benchmarking and Metrics database, this paper analyzes correlations between technology use and integration, best practices, and project performance measured with cost, schedule, and rework metrics. Data are also used to assess the complementary interaction between technology use, work processes as measured by best practices, and performance. The findings show that there are limited significant beneficial correlations between information technology use and performance, slightly more significant beneficial correlations between best practice use and performance, and several significant correlations between information technology use and application of Best practices. Interaction effects of the combined use of information technologies and best practices against performance are assessed, finding several positive correlations, although limited data availability prevents robust statistical evaluation. Overall, the paper concludes there is evidence that the benefits of information technologies in construction are found through changes in work processes. This paper thus challenges more common approaches that attempt to directly correlate the impact of information technology use on project performance with corresponding implications for both academic and industrial attempts at assessment.