0428同步年報-2021-全

Physics and Materials Science 011 Fig. 1 : Schematics of star-of-David clusters, scanning tunneling microscopy and core-level photoemission spectra of monolayer 1T-TaSe 2 . (a) Schematics of displacement of Ta atoms in the star-of-David cluster. M and X represent transition-metal and chalcogen atoms, respectively. (b) Schematics of crystal structure for monolayer 1T-TaSe 2 and star-of-David clusters with periodicity √13 × √13. (c) STM image in surface area 8 × 8 nm 2 for monolayer 1T-TaSe 2 on bilayer graphene measured at T = 4.8 K. (d) Temperature dependence of EDCs around the Ta-4 f core level measured with h ν = 260 eV for monolayer 1T-TaSe 2 . [Reproduced from Ref. 4] A Monolayer CDW-Mott Phase Robust Against Heat, Light and Doping Angle-resolved photoemission spectroscopy showed that the band structure of a monolayer of the transition-metal dichalcogenide 1T-TaSe 2 is stable against heat, laser excitation and doping. T he combination of low-dimensionality and electron correlation are crucial for exotic quantum phenomena such as the Mott-insulating phase and high-temperature superconductivity. The transition-metal dichalcogenide (TMD) 1T-TaS 2 has attracted enormous interest because of its unique non-magnetic Mott-insulator nature, which is well known to be associated with a charge-density-wave (CDW) transition. In its bulk form, the CDW-Mott transition temperature T CDW-Mott for 1T-TaS 2 is, however, less than 200 K. Attempts to increase T CDW-Mott have not succeeded to date, but enhancement of T CDW-Mott is necessary for use in applications at room temperature and above. Researchers in an international collaboration have now shown that a monolayer of 1T-TaSe 2 exhibits a strong-coupling two-dimensional (2-D) CDW-Mott phase with transition temperature onset ~530 K. Furthermore, the electron-electron correlation-derived lower Hubbard band survives under external perturbations such as carrier doping and laser excitation, in contrast to the bulk counterpart. The layered transition-metal dichalcogenide (TMD) 1T-TaS 2 is considered to be a special example of a bandwidth- controlled Mott-transition material 1,2 in the absence of magnetic order. Bulk 1T-TaS 2 undergoes a Mott transition accompanied by a commensurate CDW characterized by the star-of-David cluster ( Fig. 1(a) ) with a periodicity √13 × √13 ( Fig. 1(b) ), at T CDW-Mott ~200 K. Recent studies have focused on the exploration of unusual properties in the atomic-layer limit in TMD, with the possible emergence of exotic quantum phenomena in the pure 2D limit. 3 The nature of a pure 2D CDW-Mott phase in terms of its stability at high temperatures and under photoexcitation, possibility for magnetism, etc . compared with the 3D bulk case has been scarcely explored. Furthermore, the most important issue regarding the interplay between the Mott phase and dimensionality has yet to be clarified. In this work, the authors grew high-quality single-phase monolayer 1T-TaSe 2 with a CDW-Mott transition temperature onset ~530 K. The samples were first characterized with a scanning tunneling microscope ( Fig. 1(c) ) to confirm the monolayer nature. Core-level photoemission spectra ( Fig. 1(d) ) showed a temperature-dependent splitting of the Ta 4 f core levels, which is well known as a signature of CDW order in TMD materials. Figure 2 (see next page) shows the temperature-dependent angle-resolved photoemission spectroscopy (ARPES) results that confirmed the CDW-Mott insulating phase to be stable up to high temperatures; the authors estimated a transition temperature T CDW-Mott ~530 K. The authors then carried out doping-dependent studies at beamline TLS 21B1 of the Taiwan Light Source. A calibrated dosing of the monolayer 1T-TaSe 2 surface with K atoms was implemented; the corresponding ARPES spectra are shown in Fig. 3 (see next page). The results showed that the lower Hubbard band, a characteristic of a strongly correlated CDW-Mott phase, survived with a large doping content and only shifted to higher binding energies, 4 as seen in Fig. 3 .