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 NbSe 2 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-NbSe 2 , have remained unclear because of the difficulty of synthesizing it. Here we report the first experimental realization of a monolayer 1T-NbSe 2 crystal grown epitaxially on bilayer graphene. In contrast with 2H-NbSe 2 , monolayer 1T-NbSe 2 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.

Pseudogap, Fermi arc, and Peierls-insulating phase induced by 3D–2D crossover in monolayer VSe2

“…After evaporating V atoms in Se atmosphere onto the substrate kept at 400 ºC, the RHEED intensity due to graphene and buffer structure is reduced, and instead a new sharp 1×1 streak pattern appears ( Fig. 1(c)) similarly to the case of other monolayer TMD films on bilayer graphene [4,20,21], indicating the formation of monolayer VSe2 on bilayer graphene. As seen in the ex-situ AFM image ( Fig.…”

“…Figure 1(b) shows the reflection high-energy electron diffraction (RHEED) pattern of pristine bilayer graphene grown on 6H-SiC(0001). We clearly observe the 1×1 streak pattern and the 6√3×6√3R30° spots originating from bilayer graphene and underlying buffer structure, respectively [4,20,21]. After evaporating V atoms in Se atmosphere onto the substrate kept at 400 ºC, the RHEED intensity due to graphene and buffer structure is reduced, and instead a new sharp 1×1 streak pattern appears ( Fig.…”

Strain-induced stripe phase in charge-ordered single layer NbSe2

“…Lack of inversion symmetry and van der Waals bonded layers may result in remarkably different behavior with respect to the bulk [39][40][41]. The 2H-1T crystal phase transition at the surface of NbSe 2 [42,43] (the 1T crystal phase is unstable in the bulk [44]), the occurrence of 4 × 4 [3,4] and/or a 2 × 2 [4] modulations, linked to a stripe phase, and the observation of distinct charge density wave structures, determining a factor for the incommensuration character [6], are just a few recent examples. A change from the 3×3 to the 4×4 modulation was previously predicted by ab-initio studies [20,48]; on the other hand, a recent study [5] showed that within a 3 × 3 modulation, uniaxial strain leads to a trigonal-stripe CDW stransition.…”

Charge density wave and spin 1/2 insulating state in single layer 1T-NbS 2

“…As a similar technique has been used for the synthesis of 1T-NbSe 2 (with a different substrate) in Ref. 11 , this makes the synthesis of single layer 1T-NbS 2 even more likely.…”