Monolayer 1T-NbSe2 as a Mott insulator

Abstract

The emergence of exotic quantum phenomena is often triggered by a subtle change in the crystal phase. Transition metal dichalcogenides (TMDs) exhibit a wide variety of novel properties, depending on their crystal phases, which can be trigonal prismatic (2H) or octahedral (1T). Bulk NbSe2 crystallizes into the 2H phase, and the charge density wave and the superconductivity emerge simultaneously and interact with each other, thereby creating various anomalous properties. However, these properties and their interplay in another polymorph, 1T-NbSe2, have remained unclear because of the difficulty of synthesizing it. Here we report the first experimental realization of a monolayer 1T-NbSe2 crystal grown epitaxially on bilayer graphene. In contrast with 2H-NbSe2, monolayer 1T-NbSe2 was found to be a Mott insulator, with an energy gap of 0.4 eV. We also found that the insulating 1T and metallic 2H phases can be selectively fabricated by simply controlling the substrate temperature during epitaxy. The present results open a path to crystal-phase engineering based on TMDs. A thin film produced by a team in Japan could lead to field-effect transistors that combine superconductors and semiconductors. When thin monolayers of niobium selenide (NbSe2) are deposited on surfaces, quantum effects that are useful for electronic devices emerge. A team led by Takashi Takahashi from Tohoku University has discovered how to fabricate NbSe2 thin films with two crystal phases that have distinct properties. By growing monolayers of NbSe2 on a bilayer graphene surface and tweaking the reaction temperature, the researchers were able to generate crystalline nanoislands that behaved as either superconductors or Mott insulators — special materials whose conductivities can be tuned by varying parameters such as pressure and disorder. Atomic-resolution imaging confirmed the different island structures, which the team sees as a path towards ultrasmall transistors and switches. We have synthesized a monolayer 1T-NbSe2 on bilayer graphene by molecular-beam-epitaxy method and investigated its electronic states by angle-resolved photoemission spectroscopy. In contrast to metallic 2H-NbSe2, monolayer 1T-NbSe2 was found to show insulating characteristics with a finite energy gap and strong modulation of density of states with periodicity. This suggests the Mott-insulating ground state of monolayer 1T-NbSe2 with the formation of ‘star of David’ clusters. We also found that 1T and 2H phases are selectively fabricated by simply controlling the substrate temperature during epitaxy. The present result opens a pathway toward the crystal-phase engineering based with transition-metal dichalcogenides.