ARPES Study of Strongly Correlated States in Mott-Charge Density Wave Insulator 1T -TaS2

Abstract

Strongly correlated electron materials have been widely studied because electron-electron interactions can give rise to various exotic phenomena, such as metal-insulator transitions and superconductivity. Notably, the Mott insulator, characterized by half-filled electrons with strong electron-electron interactions, serves as a crucial example, especially regarding its potential for the emergence of quantum spin liquid. One of the prototypical quasi-two-dimensional 1T -TaS2 has been considered as a Mott insulator based on the electron correlation. How- ever, due to the two-layer dimerization caused by the overlap of Ta 5d orbitals, it transforms into a Trivial band insulator. This contradicts previous reports of exotic quantum phenomena. In 1T -TaS2, the stacking order arises from this dimerization, leading to surface termination and the manifestation of correlated states dependent on this termination. Additionally, the intermediate tempera- ture Mott phase, emerging in a very narrow temperature range between charge density wave phases, exhibits insulator characteristics without the stacking order due to weakened interlayer interaction. While the origin of this insulator has been proposed as Mott, clear experimental evidence is lacking. In this thesis, we aim to analyze and manipulate the band structure in 1T -TaS2 to gain insights into elec- tron correlation in this material. Firstly, we conducted research on the correlated state that can emerge on the surface of the insulating ground state of 1T -TaS2. We investigate the out-of-plane electron structure, which indicates the interlayer electronic coupling. We distinguish two separate branches within the topmost valence band, clearly associated with the surface and bulk layers, each charac- terized by distinct band gaps. Extensive density functional theory calculations revealed that the bulk band as a band insulator due to interlayer coupling, while the surface band gap is significantly influenced by electron correlation. The dis- tinction between surface and bulk electronic properties consistently encompasses the majority of theoretical and spectroscopic findings so far, has wide implications for van der Waals materials featuring nontrivial interlayer interactions. We also focus on the correlated state emerging in the bulk 2D insulating phase. Through electron doping, we identified and tuned this correlation, illustrating a diverse phase diagram in 1T -TaS2 based on electron correlation and interlayer interac- tion. The evolution of the band structure is examined through surface doping with alkali adsorbates for two distinct phases occurring around 220 K and at 10 K. Contrasting behaviors upon doping are observed, which corroborate the inher- ent difference between two electronic states. While the antibonding state of the spin-singlet insulator at 10 K becomes occupied by an additional electron, leading to the emergence of a Mott insulating state, the presumed Mott insulating state at 220 K undergoes a transition into a correlated metallic state and subsequently into a pseudogap state, which emerge in a quasi two-dimensional electronic sys- tem of 1T -TaS2 through strong electron correlation. The work indicates that the surface doping onto correlated two-dimensional materials can be a powerful tool to systematically engineer a wide range of correlated electronic phases.