Atomic-layer transition-metal dichalcogenide thin films studied by angle-resolved photoemission spectroscopy

概要

1. Introduction
Atomic-layer transition-metal dichalcogenide (TMD) thin films are currently a target of intensive studies because of the emergence of exotic physical properties distinct from those of bulk. Although the TMDs exhibit various physical properties depending on the composition elements, most of the previous studies concentrated on the TMDs with group-VI transition metals. In order to reveal the physical properties of atomic-layer group- IV and V TMDs, we have established fabrication condition and investigated the electronic states by angle- resolved photoemission spectroscopy (ARPES).

2. ARPES study of monolayer NbSe2 and TaSe2
To reveal the electronic structure and physical property specific to the two-dimensional limit, we have fabricated monolayer NbSe2 and TaSe2 by molecular beam epitaxy, and investigated the electronic structure by ARPES. Main results are the following.

Selective fabrication of 1H- and 1T-phases
To establish the growth condition of monolayer NbSe2 and TaSe2, we have fabricated the monolayer NbSe2 and TaSe2 films by MBE at various substrate temperature (Ts) and performed the ARPES measurements. We have succeeded in selectively fabricating 1H- and 1T-phases by tuning the Ts. From the ARPES measurements of these samples, we found that 1H-phase grown at lower Ts has metallic electronic structure similarly to the case of bulk 2H-phase. On the other hand, it was also revealed that 1T-phase grown at higher Ts has insulating electronic structure which is not reproduced by the first-principles band-structure calculations.

Mott-insulating phase in 1T-phase
To clarify the origin of discrepancy between the ARPES and calculations, we have investigated detailed electronic structure of monolayer 1T-NbSe2 and 1T-TaSe2 by ARPES. From the observation of hybridization gap derived from the √13 × √13 CDW formation and from its similarity to the signature of bulk 1T-TaS2 and 1T-TaSe2, we concluded that the observed insulating gap is derived from the Mott-Hubbard transition associated with the reduction of the band width due to the CDW formation. Also, we found that the Mott-Hubbard transition temperature is quite robust under perterbations in contrast to that of bulk. We concluded that such robust Mott states is derived from the reduction of band width associated with the disappearance of interlayer interaction.

Anisotropic band splitting in 1H-phase
To clarify the electronic structure specific to the two-dimensional limit, we have performed the ARPES measurements of monolayer 1H-NbSe2 and 1H-TaSe2 with high precision. As a result, we observed anisotropic band splitting whose magnitude becomes maximum (zero) along the K (M) cut. Since such anisotropic band splitting was reproduced by a band calculation only when we consider spin-orbit coupling (SOC) and the 1H structure has no inversion center (this is different from the 2H structure), we concluded that the anisotropic band splitting is derived from broken inversion symmetry and SOC. Also, we found that monolayer 1H-TaSe2 has larger spin splitting than that in monolayer 1H-NbSe2 likely because of a larger spin-orbit coupling.

3. Dimensionality control of HfTe2 upon K-deposition
To establish the method to systematically control the dimensionality in TMDs, we have performed ARPES on pristine and K-deposited bulk HfTe2 as a demonstration. Main results are summarized below.

Electronic structure of pristine HfTe2
Electronic structure of pristine HfTe2: To clarify three-dimensional electronic structure of pristine bulk 1T-HfTe2, we have performed ARPES using synchrotron radiation. We found that bulk 1T-HfTe2 has a three-dimensional semimetallic electronic structure with hole and electron pockets at the Γ and M points in the Brillouin zone, despite quasi-two-dimensional crystal structure.

Electronic structure of K-deposited HfTe2
To clarify an influence of K-deposition to the electronic structure, we have investigated an evolution of electronic states upon K deposition. We found that the original three-dimensional electronic structure in pristine bulk 1T- HfTe2 converts into the purely two-dimensional electronic structure upon K deposition. Also, we observed the bilayer/trilayer- and monolayer-like electronic structure in lightly and heavily K-dosed HfTe2, respectively. We concluded that such dimensionality reduction of electronic structure is induced by increasing the layer distance associated with K-intercalation.

4. Summary
To study the evolution of electronic structure associated with dimensionality reduction, we fabricated monolayer NbSe2 and TaSe2 by MBE, and investigated electronic structure by ARPES. We have succeeded in selectively fabricating monolayer 1H- and 1T-phases by accurately tuning Ts during the growth. Also, we revealed unique electronic states specific to the 2D limit such as spin-momentum locking and robust Mott states. Moreover, we established a new method to control dimensionality of the electronic states in a representative bulk TMD system, HfTe2 by utilizing K deposition onto the surface.