Exploring the Electronic and Topological Properties of Bulk and Few-Layer Metastable 1T'-WSe2

Many theoretical calculations have predicted topological behavior in 1T’ monolayer transition metal dichalcogenides (TMDs) and their alloys. Indeed, topological behavior has been observed in two of the compounds: monolayer WTe2, which exhibits a quantum spin Hall insulator state (QSHI), and TaIrTe4, which exhibits a “dual” QSHI state. However, the QSHI states in these two materials exhibit relatively weak mean free paths (<200 nm for complete quantization) and are also limited to temperatures below 100 K. To add to this, both of these materials are exceptionally air-sensitive, making the fabrication of high-quality devices particularly challenging. A potential solution to these challenges comes in the form of metastable 1T' transition metal sulfides or selenides, in particular monolayer 1T'-WSe2 which has been shown by ARPES to have a bulk band gap of 125 meV. Prior results have also shown that 1T'-MoS2 and 1T'-WS2 are relatively air-stable compared to their telluride counterparts, suggesting that the 1T’ transition metal selenides may also retain this robustness. With the potential for high-temperature QSHI and greater air-stability than the tellurides, in this presentation I will talk about our investigations into the topological behavior of metastable bulk and few-layer 1T'-WSe2. Importantly, with improved quality of bulk crystals we find evidence for topological states in the Shubnikov-de Haas oscillations. These oscillations are non-existent in micron-thick crystals, but become increasingly prominent for flakes below 100 nm’s. In the trilayer limit, we begin to see evidence for a bulk gap opening up in the temperature-dependent magnetotransport, that is also gate-tunable. These effects persist down to the bilayer, where I will discuss our measurements of the Landau fan diagram and evolution of the Landau levels under high magnetic fields. If time allows, I’ll also discuss improvements we’ve made in crystal growth of the stable hexagonal TMDs.