STM Characterization Of Mott Insulator-Metal In 1T-TaS₂ And Band Structure In SrAs₃

The discovery of high-T_c cuprate superconductors has triggered enormous interests on the study of strongly correlated physics,especially intrinsic and doped Mott insulators.The transition metal chalcogenide 1T-TaS₂ shows the ground state of Mott insulators and multiple charge density waves(CDW)at low temperatures.The metallization has been realized experimentally via heating,chemical substitution,ultrafast laser pulses,current excitation or applying a pulse voltage.It is generally believed that the metallization of 1T-TaS₂ is accompanied by the destruction of the CDW long program,that is,the CDW long-range order is inhibited to generate domain boundaries,and metal states are generated at the same time and appear in domain boundaries.However,the nature of Mott insulator-metal transition in 1T-TaS₂ still remains elusive,partially owing to the complex correlation between the Mott insulator and the CDW state.It is not even clear whether the collapse of the Mott gap is necessarily connected with the destruction of the long-range CDW order.In this paper,we explore the Mott insulator-metal transition of 1T-TaS₂ upon heating and surface doping by alkali K atoms.The CDW and electron local density of states before and after metallization were directly characterized by scanning tunneling microscope(STM)and scanning tunneling spectroscopy(STS).Otherwise,the cluster perturbation theory(CPT)was used to construct a theoretical model of 1T-TaS₂ metallization under the surface alkali metal doping?Here are the main work and results what we obtained:1?By STM and STS,the Mott insulator-metal transition during 1T-TaS₂ heating was studied.In the Fourier transform pattern of the STM image,the temperature corresponding to the occurrence of satellite spots near the CDW point is higher than the temperature at which the zero bias conductance is abruptly changed in the d I/d V spectrum and the temperature at which the UHB spectral weights disappear.We found that the temperature at which the NCCDW phase transition occurred was higher than the temperature at which the sample metallized.2?The surface of 1T-TaS₂ was electron-doped by the method of surface evaporation of alkali metal,and the evolution of electron states during the transition from Mott insulator to metal was characterized by STM.We find the long-range CDW order is always persisting,and the metallization is instead associated with additional in-gap excitations.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.We also used a scanning tunneling microscope to characterize the topological nodal line semimetal(TNLSM)SrAs₃.TNLSM can be regarded as the parent material of other topological materials.By destroying time or space inversion symmetry and introducing effective mass term,TNLSMs may evolve into either Weyl/Dirac semimetals,or topological insulators.Research on semi-metals is particularly important.SrAs₃ as a topological nodal semimetal has been verified in experiments.But the full understanding of the band structure is still lacking,even though which is of significance in understanding their exotic physics.We characterized the band structure of SrAs₃by using QPI and DFT calculations.We demonstrate an unconventional band structure that the valence band is two-dimensional like while the conduction band three-dimensional like.This unconventional band structure of SrAs₃ is expected to facilitate a 2D-3D transition in its properties,such as the transport behavior,via tuning the position of Fermi level.