Quantum Manipulation And Spectroscopy Study On Transition Metal Dichalcogenides

Quantum manipulation of materials has been a hot research topic in condense matter physics.Various kinds of measurement technics including doping,intercalation,pressure,etc.has been applied to explore the electron interaction and evolution of energy bands and electronic states.The transition metal dichalcogenides 1 T-TaS2 and 1 T-TaSe2 host abundant electronic states,like charge density wave,Mott insulating,superconducting,quantum spin liquid state etc..In this thesis,we apply low temperature scanning tunneling microscopy measurements on 1 T-TaS2 and 1 T-TaSe2 to study the crystal and electronic structure.We explore the electronic interaction and the evolution in 1T-TaS2 via doping and intercalation,to study the novel phases and related strong-correlation physics.The main contents are as follows:(1)Influence of stacking orders on the electronic state of 1T-TaS2 and 1T-TaSe2.Stacking order plays a crucial role in determining the surface electronic state of the transition metal dichalcogenides 1T-TaS2 and 1T-TaSe2.Applying STM,we acquire precise structure and corresponding electronic state on the upper and lower terraces near the steps.We systematically study the influence of stacking orders on the surface electronic state of 1T-TaS2 and 1T-TaSe2.In 1T-TaS2,the insulating surface is dominate.The corresponding stacking orders of 400 mV large gap are center-to-out corner(AC)and center-to-center(AA)stacking.The insulating gap majorly reflects the Mott insulating state of the single layer 1T-TaS2.Other stacking orders are considered as perturbation,which weaken the electron correlation and induce a 200 mV small gap and metallic surface.It’s more complex in 1T-TaSe2.When the lower terrace is metallic,the electronic state of upper terraces have one-to-one correspondence to the stacking orders.While the lower terrace is metallic,the same electron state of upper terraces may correspond to different stacking order.The complex situation may relate with stronger interlayer couping on metallic surface.After systematical study on stacking orders,we think that the insulating state of 1T-TaS2 and 1T-TaSe2 can be explained by the single band Hubbard model.Stacking orders can be considered as a perturbation,which induce small gap and metallic state.By analyzing the ratio of metallic and insulating surface,we are able to understand the metallic transport behavior on some content.(2)The electronic state evolution on Ti doped 1T-TaS2.We study the electronic evolution at different Ti doping level.At extremely low doping level(x<0.01),Ti doping locally destroys the superlattices unit cell named as Star of David(SD),and induces a clover shape superlattices unit cell.Due to the electronic distribution different of Ti and Ta,Ti doping also introduce spin holes at those doped SDs.We also discuss the influence to the possible quantum spin liquid state.As the doping level increases,we observe the insulator-to-metal transition in transport measurement at x=0.01.The transition occurs in small domains first.At higher doping level(x=0.08),1T-TaS2 exhibits metal-to-insulator transition at low temperature transport measurements.We analysis the spatial evolution of electronic density of state,and study the disorder-induced transition.The insulating state of high doping sample is identified as Anderson insulator.Besides,we identify the twisted domain structures with different crystal orientation.The CDW of the domains exhibit different chirality.Accompanied with band calculation and orbital analysis,we illustrate the origin of the chirality in Ti doped 1T-TaS2.The 1T-TaS2 is treated as a Mott insulator system.we study the electronic state evolution and the novel phase at different doping levels and explore the related strong-correlated physics.(3)Novel electronic state in copper intercalated 1T-TaS2.Intercalating Cu atoms into 1T-TaS2 modifies the interlayer interaction and induces some exotic electronic states.We synthesis Cu intercalated 1 T-TaS2 via CVT method.After characterizing the intercalation content via EDS and XPS,we find intercalating 5%Cu into 1T-TaS2 suppresses the commensurate charge density wave state.Applying ADF-STEM.we locate the Cu atoms are right behind Ta atoms.At low temperature,the topography displays many nano-size metallic domains.which is consistent with the transport result.In metallic domains,we observe clover-shaped SDs due to lacking of central Ta atoms.The coherent peak above the Fermi level in DOS is suppressed at the defect SDs.The surrounding six SDs exhibit strong enhanced state near the Fermi level,which is different from the electronic behavior in pristine samples.There are spatial electronic evolution near the metallic domain walls in insulating domains.The metallic domain wall introduce electron into the insulating domains.The coherent peak above the Fermi level of those SDs near the domain wall split and evolve slowly in space.The spatial evolution is consistent with the electron doping in single band Hubbard model.The intercalated Cu atoms play roles as medium,which enhance the interaction between SDs and induces the enhanced state near the defect SDs in metallic domains.It also increase the perpetrating depth in insulating domains.