2026 Theses Doctoral

Investigation of correlated states in high mobility tungsten diselenide

Pack, Jordan

Two dimensional semiconductors present many opportunities for designing, studying, and controlling quantum phases. Semiconducting transition metal dichalcogenides (TMDs) — a family of van der Waals materials that are semiconductors in the atomically-thin limit — are among the most promising platforms for developing these experiments. Compared with quantum well structures, ultra-thin van der Waals semiconductors introduce new opportunities for creating devices that leverage designer interfaces such as moiré patterns or proximity effects to produce new functionalities.

However, a number of technical challenges have made it difficult to realize the potential of these semiconducting TMDs. In particular, high levels of disorder have been the dominant influence on electronic properties and barriers to making reliable electrical contacts have prevented their direct characterization.

Here we describe a new contact architecture for studying the electronic transport properties in the monolayer TMD WSe₂ that makes it possible to probe emergent electronic properties at low temperatures, low carrier densities, and in high magnetic fields where electronic interactions are significant. Using this architecture to make contact to high purity WSe₂ crystals, we measure a record-high mobility exceeding 80000 cm²/Vs. Additionally, we identify a low-density metal to insulator transition in a regime where electron interactions are expected to be significant and report the first transport signatures of the fractional quantum Hall effect (FQHE) in a monolayer TMD. With further improved crystal quality, we demonstrate a tunable FQHE in the bilayer where a layer pseudospin influences the sequence of FQH phases. By tuning the pseudospin polarization using a displacement field, new FQH states can be formed with both homo- and hetero-orbital character.

To expand the investigation of these correlated phases to be compatible with scanning probe architectures, we identify a sliding disassembly technique to simply and cleanly prepare van der Waals heterostructures with exposed surfaces. Using this approach, we are able to prepare complex devices for combined transport and scanning probe measurements, and prepare heterostructures of air-sensitive and moiré materials. This sliding approach also enables new approaches to device engineering such as samples with tunable dielectric environment or moiré formation through sliding-based lamination.

Finally, we present an approach to simulate flat-band lattice models in monolayer TMDs using lithographically patterned gates. Using high-resolution electron-beam resist HSQ, we demonstrate approaches to pattern high density dot arrays whose structure can be imprinted on a TMD device. While these structures are still limited by disorder, new approaches utilizing sliding disassembly offer new opportunities for producing ultra-clean nanostructured gates. Altogether, investigations of the transport properties of atomically-thin TMD semiconductors are an exciting avenue for interrogating and manipulating emergent electronic phenomena.