Robust Coulomb Gap and Varied-temperature Study of Epitaxial 1T'-WSe${}_2$ Monolayers

Abstract

The transition metal dichalcogenides (TMDCs) with a 1T' structural phase are predicted to be two-dimensional topological insulators at zero temperature. Although the quantized edge conductance of 1T'-WTe${}_2$ has been confirmed to survive up to 100 K, this temperature is still relatively low for industrial applications. Addressing the limited studies on temperature effects in 1T'-TMDCs, our research focuses on the electronic and crystal properties of the epitaxial 1T'-WSe${}_2$ monolayers grown on bilayer graphene (BLG) and SrTiO${}_3$(100) substrates at various temperatures. For the 1T'-WSe${}_2$ grown on BLG, we observed a significant thermal expansion effect on its band structures with a thermal expansion coefficient of $\sim$60$\times$10${}^{-6}$ K${}^{-1}$. In contrast, the 1T'-WSe${}_2$ grown on SrTiO${}_3$(100) exhibits minimal changes with varied temperatures due to the enhanced strain exerted by the substrate. Besides, A significant Coulomb gap (CG) was observed pinned at the Fermi level in the angle-resolved photoemission spectroscopy (ARPES) and scanning tunneling spectroscopy (STS). The CG was founded to decrease with increasing temperatures, and can persist up to 200 K for 1T'-WSe${}_2$/BLG, consistent with our Monte Carlo simulations. The robustness of the CG and the positive fundamental gap endow the epitaxial 1T'-WSe${}_2$ monolayers with huge potential for realizing the quantum spin Hall devices.