Abstract
Combination of low-dimensionality and electron correlation is vital for exotic quantum phenomena such as the Mott-insulating phase and high-temperature superconductivity. Transition-metal dichalcogenide (TMD) 1T-TaS2 has evoked great interest owing to its unique nonmagnetic Mott-insulator nature coupled with a charge-density-wave (CDW). To functionalize such a complex phase, it is essential to enhance the CDW-Mott transition temperature TCDW-Mott, whereas this was difficult for bulk TMDs with TCDW-Mott < 200 K. Here we report a strong-coupling 2D CDW-Mott phase with a transition temperature onset of ~530 K in monolayer 1T-TaSe2. Furthermore, the electron correlation derived lower Hubbard band survives under external perturbations such as carrier doping and photoexcitation, in contrast to the bulk counterpart. The enhanced Mott-Hubbard and CDW gaps for monolayer TaSe2 compared to NbSe2, originating in the lattice distortion assisted by strengthened correlations and disappearance of interlayer hopping, suggest stabilization of a likely nonmagnetic CDW-Mott insulator phase well above the room temperature. The present result lays the foundation for realizing monolayer CDW-Mott insulator based devices operating at room temperature.
Highlights
Combination of low-dimensionality and electron correlation is vital for exotic quantum phenomena such as the Mott-insulating phase and high-temperature superconductivity
We discuss the electronic structure of monolayer 1T-TaSe2 whose monolayer nature was confirmed by our scanning tunneling microscopy (STM) measurement (Supplementary note 1)
The gap size below EF, called here ΔMott, roughly corresponds to a half of the full Mott-gap size 2ΔMott because EF is nearly located at the midpoint between the lower Hubbard band (LHB) and the upper Hubbard band (UHB) as suggested from the comparison of angle-resolved photoemission spectroscopy (ARPES) and tunneling spectroscopy data[20,23,24] (Supplementary note 3)
Summary
The topmost Ta 5d band does not cross the Fermi level (EF) and exhibits an insulating gap of ~0.3 eV below EF at the Γ point This gap is not assigned to a band gap, a substrate-induced gap, or a conventional CDW gap (Supplementary note 2), but to a Mott–Hubbard gap. It is important to note that we could selectively fabricate a pure 1TTaSe2 phase (and 1T-NbSe2 phase, discussed later) with ease by controlling the substrate temperature[18]. This enables observation of a clear hybridization-gap discontinuity in our data as compared with a recent study, where admixture from the 1HTaSe2 phase made it difficult to see the discontinuity[20].
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