The direct current size effect and the radio frequency size effect oscillation in the magnetoconductivity have been explained by a semiclassical theory which is based on the matching of the sample thickness and the classical spiral orbit of the electron about a magnetic field.

M.Sc. (Physics) / The way we live has been fundamentally changed by technological innovations based on optical, electronic and magnetic materials. Without the continuous increase of scienti c understanding on phenomena that occur in materials, together with the processing and synthesis of materials, these technological revolutions would be impossible. Thus, the search of new materials is still the key driving force for the continuous blooming of modern technology...

The electron-electron interaction is the origin of many interesting phenomena in condensed matter. These phenomena post challenges to theoretical physics and can lead to important future applications. Transition metal dichalcogenide heterostructures provide excellent platforms to study these phenomena because of the two-dimensional nature, large effective mass and tunable bandwidth with moiré potential. As electron bands become narrower such that the Coulomb interaction energy becomes comparable to the bandwidth, interactions can drive new quantum phases. This dissertation describes the realization of this platform and probing of correlated phenomena with low- temperature transport measurements. As the first step, the electrical contact problem of few-layer transition metal dichalcogenides, which prohibits low-temperature transport measurements, needs to be solved. Two different contact schemes have been used to attack this problem. For p-type transition metal dichalcogenide, prepatterned platinum is used to bottom contact transition metal dichalcogenides. This method prevents channel from deterioration due to electron beam evaporation and the high workfunction platinum can place the Fermi level underneath the material valence band. Alternatively, for n-type transition metal dichalcogenides, a single layer of boron nitride is put on transition metal dichalcogenide before cobalt evaporation. This way, the boron nitride layer protects the transition metal dichalcogenide from the process of evaporation and can decrease the work function of cobalt thus putting Fermi level above the conduction band. With these contact methods, Ohmic contacts can be achieved at cryogenic temperature and probing the transition metal dichalcogenide heterostructures with transport measurements become accessible. Then, the magnetotransport properties of monolayer molybdenum disulphide and bilayer tungsten diselenide encapsulated with boron nitride with graphite dual-gate were measured. There are three unique features underlie this two dimensional electron gas system. First, the system is strong correlated. The Landau level spectrum reveals strong correlated signatures, such as enhanced spin-orbit coupling splitting and enhanced effective g-factor. Second, the longitudinal resistance/conductance at half-filling of Landau levels are found to depend on the spin orientation. The minority spin Landau level become totally localized at higher magnetic field. Third, in bilayer device the two layers are weak coupled and can be independently controlled by two gates. All this features establish transition metal dichalcogenide a unique platform for studying correlated physics. Finally, to achieve higher level of correlation, two layers of tungsten diselenide are stacked together with a small twist angle. With the help of moiré potential and layer hybridization, the bandwidth can be continuously tuned by the twist angle. In the range of 3 degree to 5.1degree, with moderate correlation strength, correlated insulating states are shown at half-filled flatband and are highly tunable with vertical electric field.

Thesis: Elec. E., Massachusetts Institute of Technology, Department of Electrical Engineering and Computer Science, 1981 / Includes bibliographical references. / by Mansour Shayegan. / Elec. E. / Elec. E. Massachusetts Institute of Technology, Department of Electrical Engineering and Computer Science

Highly correlated electron materials are systems in which many new states of matter can emerge. A particular situation which favours the formation of exotic phases of the electron liquid in complex materials is that where a quantum critical point (QCP) is present in the phase diagram. Neighbouring regions in parameter space reveal unusual physical properties, described as non-Fermi liquid behaviour. One of the important problems in quantum criticality is to find out how the Fermi surface (FS) of a material evolves near a QCP. The traditional method for studying the FS of materials is the de Haas van Alphen effect (dHvA). A quantum critical end point (QCEP) has been reported in the highly correlated metal Sr₃Ru₂O₇, which is tuned using a magnetic field high enough to perform the dHvA experiment. It moreover features a new emergent phase in the vicinity of the QCEP, a nematic type of electron ordering. The subject of this thesis is the study of the FS of Sr₃Ru₂O₇ using the dHvA effect. Three aspects were explored. The first was the determination of the FS at fields both above and below that where the QCEP arises. The second was the search for quantum oscillations inside the nematic phase. The third was a reinvestigation of the behaviour of the quasiparticle effective masses near the FS. In collaboration with angle resolved photoemission spectroscopy experimentalists, a complete robust model for the FS of Sr₃Ru₂O₇ at zero fields was determined. Moreover, the new measurements of the quasiparticle masses revealed that no mass enhancements exist anywhere around the QCEP, in contradiction with previous specific heat data and measurements of the A coefficient of the power law of the resistivity. Finally, we report dHvA oscillations inside the nematic phase, and the temperature dependence of their amplitude suggests strongly that the carriers consist of Landau quasiparticles.

published_or_final_version / Physics / Doctoral / Doctor of Philosophy

No description available.

Thesis (Ph. D.)--University of Washington, 1998. / Vita. Includes bibliographical references (leaves [97]-102).

Thesis (Ph. D.)--University of Hong Kong, 1994. / Includes bibliographical references.

Thesis (Ph. D.)--University of Wisconsin--Madison, 1969. / Typescript. Vita. eContent provider-neutral record in process. Description based on print version record. Includes bibliography.