Towards Highly Sensitive Capacitance Measurements of a Quantum Anomalous Hall Phase in Van Der Waal Heterostructures

One of the pioneering achievements in condensed matter physics of the 20th century is the observation of the quantum Hall e↵ect (QHE) in which the Hall resistance in a two-dimensional (2D) sample takes on quantized values in the presence of a strong perpendicular magnetic field. The precise quantization of the hall resistance to one part in a billion has provided a practical, worldwide resistance standard. A long-standing goal has been to realize a similar state of matter but without the need of a strong quantizing magnetic field. The quantum anomalous Hall e↵ect (QAHE) is such a state that is predicted to exist in 2D materials with intrinsic magnetism and strong spin orbit coupling. Very few materials have these inherent properties, but new materials can be synthetically engineered by stacking and combining 2D layers into heterostructures with desired characteristics. In this thesis, we work toward combining graphene and few-layer graphene with materials that exhibit strong spin orbit coupling (molybdenum disulfide) with the goal of realizing a robust QAHE. To ascertain the presence of a zero-field gap in the electronic spectrum of the material, a benchmark of the QAHE, we implement a highly sensitive capacitance measurement technique. We present theoretical background on the quantum Hall e↵ects and capacitance measurements to begin. We then present fabrication and measurements of four devices, two incorporating single layer graphene and two with bilayer graphene. Our work opens the door to prospective devices with utility in spintronics and topological quantum computing

Van Der Waals Heterostructure Engineered Quantum Anomalous Hall Effect

The quantum anomalous hall effect (QAHE) is a phase of matter in which a dissipationless current is made to flow around the edge of a two dimensional (2D) material. Making use of this effect for next generation electronics could lead to faster processors and low power devices. There are very few materials that exist in nature that intrinsically possess the QAHE, however by sandwiching target 2D materials together we can establish this highly sought after phase. By using three 2D materials: graphene, molybdenum disulfide (MoS2) and chromium tri-iodide (CrI3) forming a van der Waals heterostructure we can create a proximity induced magnetism effect. Here, we took highly sensitive capacitance measurements of graphene on MoS2 devices at low temperatures and high magnetic fields. By taking measurements of the penetration field capacitance vs charge density and polarization of a graphene and MoS2 device at 2 Kelvin and zero external magnetic field, we are able to see the charge neutrality point in graphene and the conduction band of MoS2. Using this method of capacitance measurements we plan to integrate thin CrI3 flakes into our graphene and MoS2 devices to develop a full device to study the proximity induced QAHE.https://digitalscholarship.unlv.edu/durep_podium/1019/thumbnail.jp

Heated Assembly and Transfer of Van der Waals Heterostructures with Common Nail Polish

Recent advances in the manipulation and control of layered, two-dimensional materials has given way to the construction of heterostructures with new functionality and unprecedented electronic properties. In this study, we present a simple technique to assemble and transfer van der Waals heterostructures using common nail polish. Commercially available nail polish acts as a resilient sticky polymer, allowing for the fabrication of complex multi-material stacks without noticeable fatigue. Directly comparing four commercially available brands of nail polish, we find that one stands out in terms of stability and stacking characteristics. Using this method, we fabricate two top-gated devices and report their electrical properties. Our technique reduces the complexity in assembling van der Waals heterostructures based on the proven van der Waals pick up method