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Register a new userReconcile the Bulk Metallic and Surface Insulating state in 1T-TaSe$_2$
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The transition metal dichalcogenides 1T-TaS$_2$ and 1T-TaSe$_2$ have been extensively studied for the complicated correlated electronic properties. The origin of different surface electronic states remains controversial. We apply scanning tunneling microscopy and spectroscopy to restudy the surface electronic state of bulk 1T-TaSe$_2$. Both insulating and metallic states are identified in different areas of the same sample. The insulating state is similar to that in 1T-TaS$_2$, concerning both the dI/dV spectrum and the orbital texture. With further investigations in single-step areas, the discrepancy of electronic states is found to be associated with different stacking orders. The insulating state is most possibly a single-layer property, modulated to a metallic state in some particular stacking orders. Both the metallic and large-gap insulating spectra, together with their corresponding stacking orders, are dominant in 1T-TaSe$_2$. The connected metallic areas lead to the metallic transport behavior. We then reconcile the bulk metallic and surface insulating state in 1T-TaSe$_2$. The rich phenomena in 1T-TaSe$_2$ deepen our understanding of the correlated electronic state in bulk 1T-TaSe$_2$ and 1T-TaS$_2$.
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We investigate the low-temperature charge-density-wave (CDW) state of bulk TaS$_2$ with a fully self-consistent DFT+U approach, over which the controversy has remained unresolved regarding the out-of-plane metallic band. By examining the innate structure of the Hubbard U potential, we reveal that the conventional use of atomic-orbital basis could seriously misevaluate the electron correlation in the CDW state. By adopting a generalized basis, covering the whole David star, we successfully reproduce the Mott insulating nature with the layer-by-layer antiferromagnetic order. Similar consideration should be applied for description of the electron correlation in molecular solid.
New theoretical proposals and experimental findings on transition metal dichalcogenide 1T-TaS$_2$ have revived interests in its possible Mott insulating state. We perform a comprehensive scanning tunneling microscopy and spectroscopy experiment on different single-step areas in pristine 1T-TaS$_2$. After accurately determining the relative displacement of Star-of-David super-lattices in two layers, we find different stacking orders corresponding to the different electronic states measured on the upper terrace. The center-to-center stacking corresponds to the universal large gap, while other stacking orders correspond to a reduced or suppressed gap in the electronic spectrum. Adopting a unified model, we conclude that the universal large gap is a correlation-induced Mott gap from the single-layer property. Our work provides more evidence about the surface electronic state and deepens our understanding of the Mott insulating state in 1T-TaS$_2$.
Metallization of 1T-TaS2 is generally initiated at the domain boundary of charge density wave (CDW), at the expense of its long-range order. However, we demonstrate in this study that the metallization of 1T-TaS2 can be also realized without breaking the long-range CDW order upon surface alkali doping. By using scanning tunneling microscopy, we find the long-range CDW order is always persisting, and the metallization is instead associated with additional in-gap excitations. Interestingly, the in-gap excitation is near the top of the lower Hubbard band, in contrast to a conventional electron-doped Mott insulator where it is beneath the upper Hubbard band. In combination with the numerical calculations, we suggest that the appearance of the in-gap excitations near the lower Hubbard band is mainly due to the effectively reduced on-site Coulomb energy by the adsorbed alkali ions.
Electron-electron and electron-phonon interactions are two major driving forces that stabilize various charge-ordered phases of matter. The intricate interplay between the two give rises to a peculiar charge density wave (CDW) state, which is also known as a Mott insulator, as the ground state of layered compound 1T-TaS2. The delicate balance also makes it possible to use external perturbations to create and manipulate novel phases in this material. Here, we study a mosaic CDW phase induced by voltage pulses from the tip of a scanning tunneling microscope (STM), and find that the new phase exhibit electronic structures that are entirely different from the Mott ground state of 1T-TaS2 at low temperatures. The mosaic phase consists of nanometer-sized domains characterized by well-defined phase shifts of the CDW order parameter in the topmost layer, and by altered stacking relative to the layer underneath. We discover that the nature of the new phases is dictated by the stacking order, and our results shed fresh light on the origin of the Mott phase in this layered compound.
We calculate Raman response functions on the Fermi surface in metallic cuprates.
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