Quantum transport in atomically-thin molybdenum disulfide

Abstract

Two-dimensional semiconducting transition metal dichalcogenides (TMDCs) are recently emerged electronic systems with various novel properties, such as spin-valley locking, circular dichroism, valley Hall effect, and Ising superconductivity. In this thesis, magneto transport study was performed to atomically thin molybdenum disulfide (MoS₂), a representative material among TMDCs, to reveal its electronic properties such as valley Zeeman effect and electron-electron interactions. Introduction was given to two-dimensional materials like MoS₂, including the crystal structure, electronic band structure, and basic transport properties. Focus was placed on the K valley conduction band minima of MoS₂, where spin-orbital coupling plays an important role and there are anomalous Landau levels (LLs) due to the massive Dirac dispersion. To carry out the experiment, high-quality encapsulated bilayer MoS₂ samples were fabricated. The fabrication procedures were described in detail. Electrical measurements using low-frequency lock-in technique at cryogenic temperatures and high magnetic fields were also introduced. Facilities like Raman spectroscopy were used to characterize the samples. From the electronic measurements, results were obtained and analyzed in two parts: Chapter 3 and Chapter 4. Chapter 3 focuses on the analysis of the quantum oscillations and the valley Zeeman effect. Rich information about the charge carriers was extracted from the quantum oscillations, such as carrier density and effective mass. The two-to-one degeneracy lifting was determined, attributed to the valley Zeeman splitting of the K valley electrons, which in turn, proofs the bilayer system to be layer-polarized. Chapter 4 focuses on the many-body effects in the LL spectrum. Crossings between two sets of LLs were observed, as the gate voltage varied. A spin/valley-resolved LK formula was developed for MoS₂ to analyze the strange behaviors, which revealed the valley-resolved oscillations and a quantum Hall ferromagnetic-to-ferrimagnetic transition. By mapping the Landau fan diagram, density-dependent valley susceptibility was determined, which was enhanced by electron-electron interaction. Anticrossings and the nearby enhanced resistance were found, related to the Ising quantum Hall ferromagnetism.

Date of Award	2018
Original language	English
Awarding Institution	The Hong Kong University of Science and Technology