Browsing by Subject "Magnetotransport"
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Item 2D impurity scattering of electron in a magnetic field(2020-01-30) Feng, Jingjing, 1990-; Niu, QianMagneto-transport of two-dimensional electrons is an interesting but yet complicated topic in condensed matter physics. While most existing theories in magneto-transport are based on phase space Berry curvature, the mechanism of impurity scattering is less mentioned, but actually plays an important role in magneto-transport. Our theory focuses on how single electron-impurity scattering is influenced by electromagnetic field. In order to address this problem, we consider two different regimes separately in transport: electron transport in weak magnetic field, electron transport in strong magnetic field. Classical electron transport in weak magnetic field: In order to address this difficulty, we introduce a new model recipe based on an abstraction of the actual impurity scattering process to redefine scattering parameters for the single elastic impurity scattering. It can introduce an appropriate set of scattering parameters in the presence of magnetic field to calculate the differential cross section. Specifically, the real scattering process can be abstracted into a sudden switch between the initial asymptotic and final asymptotic trajectory. Two effects emerges under this model: skew scattering and coordinate jump, which will eventually modify the Boltzmann equation. It results in anomalous Hall effect and Negative magnetoresistivity. Classical and Quantum electron transport in strong magnetic field: In strong magnetic field, the electron motion can be generally described by the migration of guiding center. During the electron-impurity scattering, the guiding centers suddenly shift its coordinate. Traditionally, the electric field is not addressed in the impurity scattering process, but only in the drifting process. Because here we consider the electric field effect, the cyclotron radius changes in each individual scattering and the cumulative scattering broadens the cyclotron radius to be multiple values. At the same time, the coordinate shift accumulatively contribute to a longitudinal current due to the symmetry breaking of the energy spectrum by the electric field. From quantum perspective, the broadened cyclotron radius is a classical manifestation of Landau Level broadening and shifting induced by electric field.Item 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, LiTransition 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.Item Magnetotransport studies of tungsten diselenide holes(2018-10-10) Movva, Hema Chandra Prakash; Tutuc, Emanuel, 1974-; Banerjee, Sanjay; Register, Leonard F; Yu, Edward T; Akinwande, Deji; Demkov, Alexander AThis dissertation describes the electronic transport properties of holes in tungsten diselenide (WSe₂), a prototypical transition metal dichalcogenide (TMD) material, probed using low-temperature magnetotransport measurements, and facilitated by a device structure with platinum (Pt) bottom-contacts and hexagonal boron nitride (h-BN) encapsulation. The discovery of graphene has stimulated an intense interest in exploration of materials with stable two-dimensional allotropes, TMDs being one of them. Of the myriad variety of TMDs, sulfides and selenides of molybdenum and tungsten have garnered great attention on account of their semiconducting nature. A major roadblock to investigation of TMDs' electronic transport properties has been the poor quality of electrical contacts. The top-gated device structure with Pt bottom-electrodes presented in this dissertation ensures Ohmic hole contacts to WSe₂ down to a temperature of 0.3 K and permits low-temperature magnetotransport measurements. Encapsulating WSe₂ in h-BN preserves its intrinsic quality, resulting in high hole mobilities at low-temperatures, and thereby enabling observation of Shubnikov-de Haas (SdH) oscillations and quantum Hall states (QHS) in perpendicular magnetic fields. Analysis of the SdH oscillations in monolayer and bilayer WSe₂ reveals two-fold degenerate Landau levels and a hole effective mass of 0.45m [subscript e]; m [subscript e] is the free electron mass. Bilayer data also show carrier localization in the two layers signifying weak interlayer coupling, and negative compressibility of holes in the bottom layer. The QHS data reveal interesting transitions between even and odd filling factors as the hole density is tuned, which can be explained by a Zeeman-to-cyclotron energy ratio that changes as a function of density due to strong electron-electron interactions. Tilted magnetic field measurements reveal that the holes reside in the K valleys, as evinced by their spins which are locked perpendicular to the WSe₂ plane. In trilayer WSe₂, holes are found to populate two subbands with different effective masses, 0.5m [subscript e] and 1.2m [subscript e], associated with the K and Γ valleys, respectively. At a fixed total hole density, the K and Γ occupations are tunable via an applied transverse electric field, an observation which can be explained in terms of an electric field dependent bandstructure.Item van der Waals epitaxy and electronic transport in topological insulators(2017-08) Trivedi, Tanuj Kiranbhai; Banerjee, Sanjay; Neikirk, Dean P., 1957-; Bank, Seth R; Register, Leonard F; Ekerdt, John GTopological insulators (TI) have been demonstrated as a unique electronic phase of matter, possessing topological surface states (TSS) with promising applications in spin-based logic and memory, heterostructures in 2D electronics and exotic physical phenomena such as Majorana quantum computing, axion electrodynamics and topological magnetoelectronics. Since the early stages of discovery, the field of applied research in TIs has evolved. However, demonstration of scalable applications remain challenging due to practical hurdles such as rapid prototyping of new TI compounds, and efficient probing of TSS for device applications. This research work endeavors to take a two-pronged approach: to address the challenges of reliable material growth and to explore TI transport physics. While indirect spectroscopy methods have indisputably shown the presence of TSS, transport in TI devices remains challenging, in part due to parasitic conduction channels. As an alternative to staple binaries, ternary and quaternary compounds (Bi₁₋[subscript y]Sb[subscript y])₂(Te₁₋[subscript x]Se[subscript x])₃ are being explored to reduce unintentional bulk-doping and gain better access to the Dirac point. The sulfur-based ternary Bi₂Te₂S has received little attention, even as its potential as a promising TI is theoretically predicted. We demonstrate first-time van der Waals epitaxial (vdWE) growth of crystalline Bi₂Te₂₋[subscript x]S₁₊[subscript x] (BTS) nanosheets on SiO₂, hBN and mica. We also perform detailed magnetotransport measurements on BTS devices, establishing BTS as a candidate TI with readily accessible TSS and providing a sound picture of multiple transport channels in TI devices. A versatile process for large-area custom-feature TI growth and fabrication is also demonstrated using BTS as the candidate TI, achieved through selective-area modification of surface free-energy on mica. TI features grow epitaxially in large single-crystal trigonal domains, exhibiting armchair or zigzag edges highly oriented with the substrate lattice. Unusual nonlinear thickness dependence on lateral dimensions and denuded zones are observed, explained by semi-empirical two-species surface migration modeling with robust estimates of growth parameters. TSS contribute up to 60% of device conductance at room-temperature, indicating excellent electronic quality. The process is constructed from highly adaptable microfabrication technology, and in conjunction with multi-species modeling, it can be customized for TI and other vdW materials device fabrication processes ranging from rapid prototyping to scalable manufacturing.